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The Triassic reworking of the Yunkai massif (South China): EMP monazite and U-Pb zircon geochronologic evidence
The Triassic reworking of the Yunkai massif (South China): EMP monazite and U-Pb zircon geochronologic evidence Cheng-Hong Chen, Yung-Hsin Liu, Chi-Yu Lee, Yuji Sano, Han-Wen Zhou, Hua Xiang, Naoto Takahata PII: DOI: Reference:
S0040-1951(16)30540-6 doi: 10.1016/j.tecto.2016.11.022 TECTO 127324
To appear in:
Tectonophysics
Received date: Revised date: Accepted date:
16 May 2016 27 October 2016 14 November 2016
Please cite this article as: Chen, Cheng-Hong, Liu, Yung-Hsin, Lee, Chi-Yu, Sano, Yuji, Zhou, Han-Wen, Xiang, Hua, Takahata, Naoto, The Triassic reworking of the Yunkai massif (South China): EMP monazite and U-Pb zircon geochronologic evidence, Tectonophysics (2016), doi: 10.1016/j.tecto.2016.11.022
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ACCEPTED MANUSCRIPT The Triassic reworking of the Yunkai massif (South China): EMP monazite and U-Pb zircon geochronologic evidence
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Cheng-Hong Chen*1, Yung-Hsin Liu2, Chi-Yu Lee1, Yuji Sano1,3, Han-Wen Zhou4, Hua Xiang5, Naoto Takahata3
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1. Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan 2. Institute of Earth Sciences, Academia Sinica, Taipei 11529, Taiwan 3. Department of Chemical Oceanography, Atmosphere and Ocean Research
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Institute, The University of Tokyo, Kashiwa-shi, Chiba 277-8564, Japan 4. State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences (Wuhan), Wuhan 430074, China 5. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
Revision 2 submitted to Tectonophysics
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(October 25, 2016)
*Corresponding author: Cheng-Hong Chen Mailing address: Department of Geosciences, National Taiwan University, No.1, Roosevelt Road Section 4, Taipei 106, Taiwan
Tel.: (+886)-2-3366-5872 Fax: (+886)-2-2363-6095 E-mail: [email protected]
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Highlights:
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1. Yunkai massif is dated by using monazite (EMP and Nano-SIMS) and zircon
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(LA-ICPMS).
2. Monazite inclusions in the platy minerals exclusively yield Triassic ages. 3. Deformations (NW- and NE-trending) in Yunkai massif occurred at ~245 Ma
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and ~230 Ma.
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4. South China-Indochina collision and mafic magma emplacement may cause
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these deformations.
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ACCEPTED MANUSCRIPT Abstract Geohistory of the Yunkai massif in South China Block is important in
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understanding the geodynamics for the build-up of this block during the Phanerozoic
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orogenies. To investigate this massif, we conduct EMP monazite and U-Pb zircon
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geochronological determinations on mineral inclusions and separates for seventeen samples in four groups, representing metamorphic rocks from core domain, the Gaozhou Complex (amphibolite facies, NE-striking) and the Yunkai Group
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(greenschist facies, NW-striking) of this massif and adjacent undeformed granites. Monazite
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Some EMP monazite ages are consistent with the NanoSIMS results.
inclusions, mostly with long axis parallel to the cleavage of platy and elongated hosts,
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give distinguishable age results for NW- and NE-trending deformations at 244-236
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Ma and 236-233 Ma, respectively. They also yield ages of 233-230 Ma for core domain gneissic granites and 232-229 Ma for undefomed granites.
Combining U-Pb
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zircon ages of the same group, ~245 Ma and ~230 Ma are suggested to constrain the time of two phases of deformation.
Aside from ubiquity of Triassic ages in studied
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rocks, ages of detrital monazite in the meta-sandstone match the major U-Pb zircon age clusters of the metamorphic rock that are largely concentrated at Neoproterozoic (1.0-0.9 Ga) and Early Paleozoic (444-431 Ma).
Based on these geochronological
data, Triassic is interpreted as representing the time for recrystallization of these host minerals on the Early Paleozoic protolith, and the also popular Neoproterozoic age is probably inherited. With this context, Yunkai massif is regarded as a strongly reactivated Triassic metamorphic terrain on an Early Paleozoic basement which had incorporated sediments with Neoproterozoic provenances.
Triassic tectonic
evolution of the Yunkai massif is suggested to have been controlled by converging geodynamics of the South China and Indochina Blocks as well as mafic magma emplacement related to the Emeishan large igneous province (E-LIP). 3
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(Keywords: Yunkai massif, EMP monazite age, Indosinian orogeny, Emeishan large
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igneous province)
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ACCEPTED MANUSCRIPT 1. Introduction South China and Indochina blocks are two continental pieces welded together
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after the closure of the Paleo-Tethys Ocean (e.g. Roger et al., 2014 and references
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therein). This geodynamic event took place during the Early to Middle Triassic
close to the suture zone.
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resulting in the formation of metamorphic rocks and generation of magmatic rocks The entire process of amalgamation is commonly referred
to as Indosinian orogeny in both the blocks.
Various age dating systems have been
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applied to know the timing of syn-tectonic metamorphism and post-orogenic Although the U-Pb
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magmatism for elucidating the geodynamics of this orogeny.
zircon age dating becomes popular, its application to orogenic metamorphic and
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magmatic rocks may encounter difficulties when recrystallization or melting of Yunkai
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pre-existing crustal materials is highly involved with inherited zircons. massif in the southern South China may be one of the cases.
China Block.
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Yunkai massif has long been regarded as a Precambrian basement in the South Recently, based on a large number of age results concentrating in the
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Early Paleozoic for granitic and metamorphic rocks in this massif, a concept of the Early Paleozoic Wu-Yun metamorphic belt (orogen) that spreads from the Wuyishan massif in the northeast, through the Baiyun massif, and further extends southwesterly to the Yunkai massif (Fig. 1a), was established (Lin et al., 2008; Faure et al., 2009; Li et al., 2010; Yang et al., 2010; Wang et al., 2011; Yao et al., 2012; Chu et al., 2012b; Wang et al., 2013).
The most significant evidence came from a reconnaissance on
gneissoid granite samples that gives rise to similar zircon U-Pb dates of 457-410 Ma for Wuyishan and 450-415 Ma for Yunkai massifs, respectively (Wang et al., 2011). Along with some extrusive rocks in the Baiyun massif (ca. 435 Ma), the importance of the Early Paleozoic event for the development of this orogen was reinforced (Yao et al., 2012). 5
ACCEPTED MANUSCRIPT However, 40Ar/39Ar mineral dating (mostly biotite) shows different results. For example, thirteen biotite analyses on granite, gneiss and migmatite over the entire
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Yunkai massif, although mostly mylonitized, all yielded Triassic ages (Wang et al.,
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2007a). Further by compiling multi-system geochronology on the Yunkai massif,
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Chu et al. (2009) indicated that ages obtained from the 40Ar/39Ar mineral dating (biotite and muscovite) for high-grade metamorphic rocks are again exclusively Triassic.
The Triassic age given by these minerals may be meaningful to account for
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the complete reset of mineral ages by the mylonitization near shear zones, but difficult
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to apply to a cooling history for the transformation from the Early Paleozoic prograde- to the Triassic retrograde-metamorphism in the Yunkai area (e.g. Wang et
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al., 2012). One more concern is the existence of thin Triassic-aged rim on the Early
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Paleozoic zircon core in some gneissic granites (Wang et al., 2007b; Chen et al., 2012) and supracrustal gneisses (Wan et al., 2010) within the massif.
To date the only
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explanation is ascribed to “overprinting of the Indosinian orogeny”, yet details of overprinting are not known.
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The use of some other dating methods would be helpful to provide more age constraints, and the EMP monazite age determination is an easy and efficient one for this purpose (e.g. Catlos, 2013).
This technique has been applied to granitic plutons
in the neighboring Darongshan area and high-grade metamorphic rocks (granulite and charnockite) near the Gaozhou Reservoir previously, by which large amounts of the EMP monazite ages of ~230 Ma and ~440 Ma were obtained (Chen et al., 2011; 2012). For this study, we chose some representative metamorphic and granitic rocks from Yunkai massif to demonstrate the powerfulness of using the EMP monazite dating to tackle with the petrographic relationship and age results. U-Pb zircon ages are also determined for same samples to provide constraints from direct comparisons.
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ACCEPTED MANUSCRIPT 2. Geological backgrounds and samples South China Block is composed of Yangtze and Cathaysia sub-blocks (Fig. 1a)
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with the characterization of Phanerozoic granitic rocks exposing in the Cathaysia.
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Ages for these granitic rocks are concentrated at Early Paleozoic, Late Permian-Early
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Triassic, Jurassic and Cretaceous time, marking the episodic thermal events on this terrain (e.g. Zhou and Li, 2000). The Yunkai massif is bounded by the WuchuangSihui fault in the east and the Wuzhou-Bobai fault in the west with a total area of It is a conventional thought that this massif is built up by
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about 2,500 km2 (Fig. 1b).
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the Paleo- to Mesoproterozoic high-grade basement rocks that had been modified by Neoproterozoic and Early Paleozoic thermal events, the Late Neoproterozoic to Early
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Paleozoic low-grade basement rocks, and the cover rocks composed of slightly
1996).
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metamorphosed to unmetamorphosed Cambrian to Devonian strata (Zhong et al., Lithologies of the exposed basement rocks in this massif mainly include
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gneissic granites, migmatites and orthogneisses in the center, and supracrustal paragneiss, amphibolite, schist, quartzite and marble (roughly the metapelite
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assemblage) relatively in the peripheral of the massif. Some of these rocks have later been intruded by the Triassic, Jurassic and Cretaceous granitoids.
Recently, a
short time span (443 to 430 Ma) was suggested for the formation of these gneissic granites and supracrustal rocks as constrained by the youngest zircon U-Pb age clusters, and the also abundant older ages (mainly 1.1 to 0.8 Ga) were attributed to the inherited ones (Wan et al., 2010). The Yunkai massif is composed of two tectonic units: the Gaozhou Complex and Yunkai Group. Although the Yunkai Group is loosely defined, we retain the usage of this term same as those in Wan et al. (2010). The former includes the predominant weakly-deformed gneissic granite and accompanying migmatite, orthogneiss and even granulite facies rocks in the core domain, and amphibolite facies metamorphic rocks 7
ACCEPTED MANUSCRIPT in the surroundings, whereas the latter consists mainly of greenshist to amphibolite facies paragneiss and schist (Fig. 1b). The Yunkai massif has recorded four phases
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of deformation at Early Paleozoic, Early Mesozoic, Jurassic-Cretaceous and Late
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Mesozoic (Lin et al., 2008), in which the first two phases are concerned in this study.
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According to these authors, the main deformation is the second one that the NE-directed shearing occurred at the Early Mesozoic leading to the NNE-trending lineation for quartz, biotite, muscovite and sericite basically in the mica schist, Prior to this, the deformation is
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paragneiss and orthogneiss of the Yunkai Group.
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characterized by a NW-trending mineral lineation, represented by crystallization of muscovite, biotite, amphibole, epidote and quartz, in few gneissic rocks of the core
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domain and Gaozhou Complex. However, this deformation event is not well dated,
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although the Early Paleozoic ages have been obtained from some weakly foliated orthogneisses and undeformed charnockites (Wang et al., 2007a; Chen et al., 2012).
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Therefore, dating of monazite enclosed by rock-forming minerals subjected to these two phases of deformation is envisaged.
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In this study, four groups of sample on the basis of rock type and metamorphic grade were collected from the Yunkai massif (Fig. 1b): gneissic granites and associated meta-sedimentary rocks in the massif’s center or core domain (Group 1), various types of gneiss (amphibolite facies) in the Gaozhou Complex (Group 2), gneisses and schists (greenschist facies rocks in the Xinyi area near the major Luoding-Yuocheng shear zone) in the Yunkai Group (Group 3), and undeformed Triassic granites adjacent to this massif (Group 4) (Tables 1 and 2). The Triassic S-type Darongshan granites (Chen et al., 2011) to the west of the Wuzhou-Bobai fault (Fig. 1b) are used as references of undeformed granites.
Basic information for
sample localities is described in Table 1.
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ACCEPTED MANUSCRIPT 3. Analytical methods Rock samples were crushed and sieved for collecting zircon and monazite grains
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through the conventional magnetic and heavy liquid separation procedures, in the
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hope of distinguishing detrital and metamorphic zircon and monazite populations.
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About 50 grains of randomly selected zircon and monazite were casted in epoxy resin and polished for U-Pb zircon and EMP monazite age determinations.
In order to
characterize the zonings of zircon and internal domains of monazite, cathode
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luminescence (CL) images for zircon were obtained using a JEOL JSM-6360LV
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scanning electron microscope attached with a panchromatic CL imaging system (Gatan Mini-CL) at National Taiwan University (NTU), and secondary electron (SE)
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and back-scattered electron (BSE) images for monazite were taken using a JEOL
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electron microscope model JSM-7100F (FE-SEM) equipped with an independent energy dispersive spectrometer at the Institute of Earth Science (IES), Academia
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Sinica (Taipei). X-ray mapping of monazite grains were conducted by using a JEOL electron microprobe analyzer model JXA-8900R equipped with four channels of wavelength dispersive spectrometer at IES.
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U-Pb isotopic ratios of zircons were determined on ~40 m diameter spot by an excimer laser ablation-induced couple plasma mass spectrometer (LA-ICPMS) at NTU. The ICPMS is an Agilent 7500s coupled with the UP213 laser ablation system (New Wave Research/Merchantek). 208
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Pb/206Pb, 206Pb/238U, 207Pb/235U and
Pb/232Th ratios were calculated using the GLITTER 4.0 software (Macquarie
University) and the GJ-1 (Jackson et al., 2004) as an external standard, and common lead corrections were carried out using the procedure described by Andersen (2002). This dating system has been tested by running with the Harvard zircon 91500 and Mud Tank zircon (10614.0 Ma and 7313.9 Ma, respectively; Jackson et al., 2004), and results are in good agreement with the recommended ages (Chiu et al., 2009). 9
ACCEPTED MANUSCRIPT The U-Pb ages (478 in total number, Appendix 1), concordia diagrams, and age frequency curves for the age population were obtained by using the ISOPLOT In this study, weighted mean 206Pb/238U ages of <1000 Ma
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program (Ludwig, 2007).
Statistics of U-Pb zircon ages of dated grains are shown in Appendix 2.
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al., 1992).
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and 207Pb/206Pb ages of >1000 Ma are adopted for concordant zircons (Compston et
Polished slides (one inch diameter) were also made for petrographic observations and mineral composition analysis.
In fact, in-situ EMP monazite age determinations
We focused more on monazites enclosed by different
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during mineral separation.
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were mainly relied on these slides to avoid the problem of age bias due to grain size
host minerals than those setting in the interstices (or heavy concentrates), because age
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interpretations from metamorphic monazite usually require good control of its High resolution BSE images are also
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paragenesis (e.g. Kohn and Malloy, 2004).
referred to minimize analyses of overlapping domain by the electron beam and to
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reveal possible compositional variations between domains from which main age populations for these monazites were calculated.
The instrument used for the EMP
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monazite dating is a Shimadzu-ARL electron microprobe model EMX-SM equipped with four channels of wavelength-dispersive spectrometers at NTU. This allows simultaneous analysis of Th, U, Pb and Y at the same spot. Apparent ages are calculated by using the scheme of Suzuki and Kato (2008), and results of 1158 spot analysis of monazite, along with the detailed analytical procedures (Chen et al., 2006), are listed in Appendix 3. Few discrete monazites with multiple EMP ages were cast into a separate epoxy resin disk for both the EMP and NanoSIMS analyses on the same BSE domain. After EMP measurements (Appendix 4), the monazite mounts were slightly polished and re-coated with gold to dissipate charge during SIMS analysis.
Monazites were
dated by a high lateral resolution secondary ion mass spectrometer, the CAMECA 10
ACCEPTED MANUSCRIPT NanoSIMS NS50 at the University of Tokyo, using a standard monazite with the TIMS U-Pb age of 524.9±3.1 Ma (Sano et al., 2006).
Pb+, 238U16O+, and 238U16O2+ ions were analyzed first by multi-collector system for
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For each spot, 140Ce+, 204Pb+,
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Counting time was 10 sec for
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numbers 204, 206, and 207 to measure Pb isotopes.
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300 sec. Then the magnet was cyclically peak-stepped through a series of mass
Pb+, 2 sec for 206Pb+ and 10 sec for 207Pb+, respectively, in a single cycle.
Using a
critical illumination mode, a ~1 nA mass filtered O-primary beam was used to sputter
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a 5~7 μm diameter crater and secondary positive ions were extracted for mass Apparent ages for fifty EMP and
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analysis using a Mattauch-Herzog geometry.
twenty-five 238U/206Pb and six 207Pb/206Pb spot analyses of monazite were obtained
4. Petrography
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from this disk (Appendix 4).
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Group 1 samples (11GD01, 11GDGZA1 and 11GD02) are mainly the variably deformed gneissic granites collected from the metamorphosed core domain. Among
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them, sample 11GDGZA1 still preserves a granular texture of granitic protolith and the interstitial biotites are mostly replaced by chlorites (Fig. 2a); whereas the other two samples exhibit gneissic banding defined by orientated biotite in different grain sizes (Fig. 2b). Monazite grains in these samples, sometimes containing apatite inclusions, are commonly enclosed by biotite, quartz and plagioclase.
In addition, a
meta-sandstone (sample 23GX11) near Gaozhou, mainly containing quartz with interstitial K-feldspar, chlorite, epidote and few monazite (Chen et al., 2012), is also included for constraining monazite ages of detrital origin in this area. Group 2 samples include biotite gneisses (23GX04 and 23GX05-2), two-mica gneisses (23GX15 and 11GD06), and gneissic granite (11GD07) collected from the Gaozhou Complex.
These samples are spatially associated with amphibolites, on 11
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Sample
23GX05-2, containing equigranular biotite, plagioclase and quartz (Fig. 2c), is a
Two other samples (23GX04 and 23GX15) also show strong foliations
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(N18E).
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gneissoid enclave enclosed by the gneissic granite with a NE-trending foliation
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(N18E and N10E, respectively) in a way consistent with the regional ductile shearing. The highly sutured quartz grains in all gneiss samples and kink-band muscovite in gneissic granite (11GD07) are considered as the results of ductile deformation Especially, sample 23GX15 is
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corresponding to the high-grade metamorphism.
apatite relicts.
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characterized by blasto-porphyroblastic textured anhedral plagioclase (Fig. 2d) and Monazites are basically absent in these pre-existing minerals, and
This indicates that monazite formation is closely
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gneissic banding (Figs. 3a-d).
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always occurred as inclusions within biotite, muscovite and quartz that define the
related to the ductile shearing developed under the amphibolite facies metamorphism.
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For Group 3 rocks, biotite gneiss and garnet-tourmaline schist are the commonest rock types in the Yunkai Group. They apparently experienced slightly lower Also the NW-trending
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metamorphic grade compared to the Group 2 samples.
foliation (N32W) of sample 23GX13 site (for samples 23GX13-1 and -2) is different from the strain directions recorded by the Group 2 samples. Biotite gneisses show relatively minor extent of shearing deformation.
Sample 11GD04 is characterized
by the granoblastic texture, and the constituent minerals are absent of preferred orientation.
Samples 11GD05 and 23GX13-2 contain plagioclase, ± K-feldspar and
quartz porphyroblasts (Fig. 2e), which are surrounded by fine-grained and orientated minerals (quartz, plagioclase, K-feldspar and chlorite in sample 11GD05 and biotite in sample 23GX13-2).
Abundant chlorites and retrogressive veinlets filled with calcite,
K-feldspar and synchysite (a rare earth carbonate mineral) are evidences of intensive effect of the greenschist facies overprint.
In the garnet-tourmaline schists (samples 12
ACCEPTED MANUSCRIPT 11GD03, 23GX13-1 and 23GX14), muscovite and chlorite are the predominant minerals (Ms= 25% and Chl= 15-25%; Table 2) in samples 11GD03 and 23GX14 to Garnet forms anhedral grains in sample 23GX13-1,
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define the major foliation.
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whereas poikiloblasts (up to ~5 mm) containing abundant quartz inclusions and As for
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biotite and chlorite crosscuts in samples 11GD03 and 23GX14 (Fig. 2f).
monazite occurrence, it can be generalized that monazites are present as unaltered or slightly altered inclusions in biotite, muscovite, quartz or plagioclase in biotite
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gneisses, and as unaltered to highly altered monazite grains in retrogressive muscovite
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and chlorite in garnet-tourmaline schists (Figs. 3e-f).
For the three groups of metamorphic rocks mentioned above, the petrographic
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relationships indicate that formation of monazite involve consumption of apatite and
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preceded REE-rich phases, most probably allanite (Wing et al., 2003; Tomakins and Pattison, 2007; Janots et al., 2008).
Absence of relic allanite in the analyzed samples
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may be due to complete equilibration after the allanite to monazite transition in the middle amphibolite facies (525-600 °C) for a metapelitic basement (Spear, 2010).
In
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addition, aluminosilicates (Al2SiO5) that have been considered as an essential silicate phase for producing monazite in the pelitic schists (e.g., Wing et al., 2003) are absent either.
On the other hand, monazites are commonly associated with aligned
muscovite and biotite or matrix plagioclase in the studied samples (Fig. 3).
These
indicate that the reactions responsible for monazite formation are probably similar to those proposed by Tomakins and Pattison (2007) and Janots et al. (2008) for pelitic rocks: Allanite + Apatite = Monazite + Plagioclase + Magnetite, or Allanite + Apatite + Al-Fe-Mg phases 1 = Monazite + Anorthite + Al-Fe-Mg phases 2 (e.g., muscovite, biotite or chlorite, and garnet). Group 4 granites (28GX20 and 28GX22) are coarse-grained and undeformed as compared to the Group 1 gneissic granites.
Sample 28GX20 mainly consists of 13
ACCEPTED MANUSCRIPT granular quartz, plagioclase, and biotite with interstitial K-feldspar (Fig. 2g), and sample 28GX22 is characterized by abundance of coarse-grained K-feldspar and Monazites
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altered plagioclase, and trace amounts of garnet and rutile (Fig. 2h).
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occur as inclusions evenly distributed in these major constituent minerals.
5. Results 5.1. U-Pb zircon ages
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Here we summarize the statistics of the dating results of zircon (Appendix 2).
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For Group 1 gneissic granites, consistent weighted mean ages of 437.4±5.5 Ma (n=22), 440.6±4.1 Ma (n=23), and 439.6±4.5 Ma (n=18) are obtained for samples
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11GD01, 11GDGZA1, and 11GD02, respectively (Figs. 4a-c). As for the
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meta-sandstone (sample 23GX11), an Early Paleozoic age of 443.5±8.9 Ma (n=5) is also obtained from the youngest age cluster (Fig. 4d) although Neoproterozoic is the
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largest population (21 out of 41 analyses). Group 2 rocks are characterized by a high population of Meso- to Neoproterozoic However, various age clusters are obtained: 229±10 Ma (n=7) for sample
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zircons.
23GX04 (Fig. 4e), 989±27 Ma (n=16) and 950±38 Ma (n=9) for samples 23GX05-2 and 23GX15 (Figs. 4f and g), and 434.9±5.1 Ma (n=12) for sample 11GD06 (Fig. 4h). Although ages close to 450 Ma (n=8) and 250 Ma (n=9) are present in sample 11GD07, they are not counted because of the large discordance (Fig. 4i). Group 3 schists and gneisses have even larger age distribution of Meso- to Neoproterozoic zircons than Group 2 rocks. Age clusters of 1004±20 Ma (n=20) and 904±32 Ma (n=10) are obtained for samples 23GX13-1 and 23GX14, respectively (Fig. 4j and k), and no particular age group is available for sample 11GD03 among 30 wide spreading apparent ages (Fig. 4l).
Like Group 2 samples, younger age clusters
are also obtained: 245.1±5.4 Ma (n=5) for sample 11GD04 (Fig. 4m) and 431.3±5.0 14
ACCEPTED MANUSCRIPT Ma (n= 10) for sample 11GD05 (Fig. 4n). For Group 4 rocks, the Phanerozoic population yields a major age cluster at
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250.9±3.3 Ma (n=16) and a minor cluster at ~440 Ma (n=5) for sample 28GX20 (Fig.
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4o), but only one age cluster at 247.7±2.3 Ma (n=18) for sample 28GX22 (Fig. 4p).
5.2. EMP monazite ages
Representative monazite grains with BSE image are shown in Figure 4, in which
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the apparent ages (in Ma) and wt% ThO2 contents (in the parenthesis) of spot analysis Basically, each contrast BSE domain observable in the grain has been
analyzed.
Information of monazite inclusions and their host phases are included in
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are given.
Group 1 rocks
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Table 2.
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Sample 11GD01: Euhedral to anhedral monazite inclusions with sizes ranging from 80x60 to 15x10 m are common, and their ThO2 contents mostly fall in 4-6 wt% Point analyses on these grains yield
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for homogeneous grains under BSE images.
close apparent ages which give a weighted mean age of 233±13 Ma (n=58).
Grains
in the heavy concentrate are larger (120x100 to 80x60 m) and show more obvious patchy zoning (Fig. 5a) than inclusions.
Point analyses on these grains yield similar
apparent ages and give a weighted mean age of 231±15 Ma (n=38).
The weighted
mean age for all the grains is 232.8±4.3 Ma (n=96; Fig. 6a). Sample 11GDGZA1: Some inclusions in biotite are somewhat altered as reflected by a stronger contrast in BSE images (Fig. 5b).
Compositions of unaltered monazite
are similar to the sample 11GD01, and point analyses yield close ages which give a weighted mean age of 230±18 Ma (n=30).
Discrete monazites are larger (110x90 to
80x60 m) and display dark and bright patchy BSE zonings with the latter slightly 15
ACCEPTED MANUSCRIPT higher in Th (ThO2= 7.7 wt%) (Fig. 5c).
Likewise, the unaltered portion yields
similar ages giving a weighted mean age of 234±16 Ma (n=38).
The weighted mean
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age is calculated as 232.8±5.2 Ma for this sample (n=68; Fig. 6b).
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Sample 23GX-11: Our previous study based on monazite inclusions revealed three
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sets of age, 434±18 Ma (n=11 on 5 grains), ~370 Ma (n=3 on 1 grain) and 237±13 Ma (n=11 on 4 grains; Fig. 6c), indicating multi-sources for the constituent minerals (Chen et al., 2012).
In this study, we provide a large variety of EMP monazite ages,
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ranging from Paleoproterozoic to Triassic with major clusters at 1.8 Ga, 860 Ma, 540
heavy concentrate (Fig. 6d).
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Ma, 440 Ma and 250 Ma, as obtained from different domains on 28 grains in the They obviously represent detrital sources, and details
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will be discussed later.
Group 2 rocks
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Sample 23GX04: Inclusions are large and abundant, most are subhedral grains in biotite (Fig. 5d) and euhedral grains in quartz (Fig. 5e). Although compositions of
AC
these monazite grains are variable (ThO2= 5.4-8.3 wt%; UO2= 0.3-1.6 wt%, Y2O3= 0.7-5.0 wt%), apparent ages obtained from inclusions are uniform, with a weighted mean age of 235.5±5.5 Ma (n=67). Monazite grains from the heavy concentrate are slightly lower in ThO2 (4.4-7.9 wt%), higher in UO2 (>0.8 wt%) and Y2O3 (>3 wt%) contents.
Few grains show Early Paleozoic age in the core but Triassic age in the
outmost zone (Fig. 5f).
Some other grains are altered and display a huttonite-rich
outer zone, however, the age is exclusively Triassic in the unaltered portion (Fig. 5g). Thus two ages, 236.8±8.7 Ma (n=53) and 440.7±10.0 Ma (n=12) are revealed from discrete grains.
By putting together the Triassic age data from slide and sand, the
weighted mean age is calculated as 236.6±3.0 Ma (n=120; Fig. 6e).
16
ACCEPTED MANUSCRIPT Sample 23GX05-2: Unlike monazite occurrence in other samples, grains in this sample have irregular shapes in a constant size of ~30 m (Fig. 5h).
Patchy
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compositional zoning is common, the darker domain has lower ThO2 (3.5-6.8 wt%)
IP
and the brighter domain has higher ThO2 contents (up to 12.1 wt%), though the U and
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Y contents are rather constant (UO2= 0.5-1.0 wt% and Y2O3= 1.4-2.6 wt%). However, different domains yield same apparent ages around 230 Ma, giving the weighted mean age of 234.1±5.4 Ma for the sample (n=58; Fig. 6f).
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Sample 23GX15: Inclusions are abundant and often in association with zircon in Mingling of the host phase and monazite inclusions is a
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the same host mineral.
common textural feature (Fig. 5i), suggesting a co-genetic relationship.
Contrasting
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BSE domains are randomly distributed, revealing lower Th and U (ThO2= 3.9-6.3
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wt% and UO2= 0.5-1.1 wt%) in the darker domain and higher Th and U contents (ThO2= 7.1-12.1 wt% and UO2= 0.9-1.3 wt%) in the brighter domain (Fig. 5j) but
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having constant Y contents (Y2O3= 2.7-3.8 wt%). However, EMP monazite dating on the different domain of the datable grains (no severe interference from the mingled
AC
phase) all yield apparent ages around 230 Ma, giving the weighted mean age of 235.5±4.1 Ma (n=102; Fig. 6g). Sample 11GD06: Monazite inclusions are large euhedral to subhedral grains (60x20 to 100x 45 m).
Although concentric zoning is observable in some grains
(Fig. 5k), compositional variations are small. All analyses yield apparent ages around 230 Ma, giving the weighted mean age of 234±11 Ma (n=17).
Monazites in
the heavy concentrate display same zoning pattern, but two out of 16 grains show Early Paleozoic vs. Triassic ages in a core-rim relationship (Fig. 5l).
Combining all
the monazites measured in both the slide and heavy concentrate, two weighted mean ages of 239.7±4.1 Ma (n=83) and 441±28 Ma (n=5) are given (Fig. 6h).
17
ACCEPTED MANUSCRIPT Sample 11GD07: The great majority of inclusions was subjected to alteration producing huttonite grains in the rim (Fig. 5m).
Large variations of ThO2 (2.9-10.3
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wt%) and low but constant concentrations of UO2 (0.02-0.42 wt%) and Y2O3 (0.3-2.0
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weighted mean age of 233±11 Ma (n=14; Fig. 6i).
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wt%) are present in the unaltered cores. Apparent ages from these cores yield a
Group 3 rocks
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Sample 23GX13-2: Only three inclusions in biotite could be observed in the slide,
(15x6 m) grains.
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including a euhedral (50x30 m), a large elongate (100x20 m), and a small anhedral Large part of these grains has altered into an assembly of allanite, Moderate variations of
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chlorite and huttonite (±apatite, epidote and synchysite).
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ThO2 (4.2-8.5 wt%), UO2 (0.24-0.65 wt%) and Y2O3 (0.9-2.5 wt%) are present in the unaltered cores which yield apparent ages of ~240 Ma, ~360 Ma and ~430 Ma, from Such age patterns resemble those obtained from
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three grains, respectively (Fig. 6j).
monazite inclusions in the meta-sandstone (sample 23GX11).
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Sample 23GX14: Usually, schist is considered difficult for age dating because zircon is small and monazite is rare. biotites.
In this case, inclusions are mostly preserved in
Their Th contents are rather constant (4-6 wt%), and U and Y contents are
variable (UO2 =0.6-1.4 wt% and Y2O3 =0.45-2.4 wt%). Available grains all give apparent ages of ~230 Ma, yielding the weighted mean age of 235.9±8.2 Ma (n=27; Fig. 6k). Sample 11GD03: Most inclusions have elongated habit with long-axis parallel to the cleavage of layered host minerals (Fig. 5n).
One special feature is the relatively
high U (UO2 >0.8 wt%) and low Y (Y2O3 <1.5 wt%) contents of these monazites as compared with other samples.
Apparent ages are exclusively ~240 Ma, giving the
weighted mean age of 244±11 Ma (n=14; Fig. 6l). 18
ACCEPTED MANUSCRIPT Sample 11GD04: Both inclusion and interstitial monazites are abundant and few grains appear to be rims of hexagonal apatite enclosed in K-feldspar (Fig. 5o).
They
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are relatively low in Th (ThO2= 1.7-3.3 wt%) and U (UO2 mostly ≦0.2 wt %).
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Generally, concentric zoning is seen (Fig. 5p), but apparent ages from core and rim
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are indistinguishable, giving a weighted mean age of 236.0±4.6 Ma (n=105; Fig. 6m). Sample 11GD05: Monazites are mostly interstitial grains having concentric zoning with a bright rim enclosing the dark irregular core (Fig. 5q). Rims are often
Those which lack the distinct zoning
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against Early Paleozoic ages in the core.
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marked by the increase in Th and U but decrease in Y, and have Triassic apparent ages
basically show single Triassic age. Two sets of weighted mean age obtained are: Monazites in the heavy concentrate
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236.1±7.2 Ma (n=21) and 436±16 Ma (n=15).
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show no discernable differences in zoning pattern, grain size and chemical composition as compared with grains in the slide.
Likewise, two weighted mean Combining all
CE P
ages are obtained: 244.5±9.3 Ma (n=18) and 438.1±8.8 Ma (n=51).
measured monazites in this sample, two resultant weighted mean ages obtained are:
AC
239.2±5.7 Ma (n-39) and 440.6±7.1 Ma (n=66; Fig. 6n).
Group 4 rocks
Sample 28GX20: In the slide, rims of interstitial grain and inclusions in biotite are somewhat altered into allanite and apatite ±chlorite (Fig. 5r), but inclusions in quartz are unaffected (Fig. 5s).
Few monazites in K-feldspar appear to be a phase
surrounding apatite (similar to that of Fig. 5o).
Basically, compositions of monazite
are rather homogeneous, giving the weighted mean age of 233.1±6.2 Ma (n=50). Combining the result from the grain analysis on the heavy concentrates which yielded an age of 230±16 Ma (n=34) (Chen et al., 2011), the resultant weighted mean age is 232.1±4.4 Ma (n=84; Fig. 6o). 19
ACCEPTED MANUSCRIPT Sample 28GX22: Sub- to euhedral unaltered grains of inclusion are found in the slide.
Grain size (~100x80 m) and chemical compositions (ThO2= 6.1-8.7 wt%,
Yttrium content
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present in the S-type Darongshan granites (Chen et al., 2011).
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UO2= 0.2-2.1 wt% and Y2O3= 0.2-4.3 wt%) of these monazites are similar to those
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shows dramatic decrease toward the rims in discrete grains, indicating a co-genetic relationship between monazite and garnet (Table 2).
Apparent ages of these
monazites are exclusively Triassic, yielding a weighted mean age of 229.2±8.4 Ma
5.3. NanoSIMS monazite ages
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(n=21; Fig. 6p).
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Using the nanoSIMS method, Sano et al. (2006) demonstrated the applicability of
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U-Pb and Pb-Pb dating of detrital monazite grains from sedimentary rocks in Taiwan. There are three main age groups, 230 Ma, 440 Ma and 1850 Ma, and these ages are Taking the
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comparable with the U-Th-Pb chemical ages obtained on same grains.
same method and apparatus, we conducted 238U/206Pb and 207Pb/206Pb age
AC
determinations on the same spot as used for EMP analysis (Table 3). Results show that EMP ages, which mainly rely on the Th-Pb decay system, can be correlated to the 238
U/206Pb ages for the Triassic population and to the 207Pb/206Pb ages for the Early
Paleozoic or even Late Neoproterozoic populations (Fig. 7). Therefore, we are confident that EMP monazite age is able to reflect the evolution history of this mineral through various stages of thermal events, particularly to distinguish the Triassic from the Early Paleozoic ages, in the Yunkai area.
20
ACCEPTED MANUSCRIPT 6. Discussion 6.1. Age interpretations
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6.1.1. Variability of U-Pb zircon ages
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Basically, most of the studied samples show a wide spread of zircon ages along or
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near the concordia, and the age cluster for an estimation of the time of thermal event registered in the rock does not define the youngest age in many samples (Fig. 4). So the application of U-Pb zircon ages to each sample needs to be considered thoroughly.
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For Group 1 rocks (Figs. 5a-d), all three gneissic granites (samples 11GD01,
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11GDGZA1 and 11GD02) and the meta-sandstone (sample 23GX11) show major age clusters concentrated at the Early Paleozoic (444-437 Ma).
It would be easy to
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imply an eventual thermal effect imposed on these rocks at the Early Paleozoic if no
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other age constrain is given. Previously, we reported U-Pb zircon ages for two orthogneisses (430 Ma for sample 23GXGZ05 and 436 Ma for sample 23GXGZ08)
CE P
near the Gaozhou Reservoir, the place close to where samples 11GD01 and 11GDGZA1 were collected.
A younger age of 238 Ma, calculated from 8 out of 25
AC
total populations, was obtained from the white CL rim of zircon grains of sample 23GXGZ08.
Also the associated charnockites (436-433 Ma) commonly exhibit
penetrations of Triassic veinlet (Chen et al., 2012).
Similarly, we stress that the
existence of few Triassic records beyond the Early Paleozoic age cluster in Group 1 samples (Figs. 5a and c) can not be overlooked. For Group 2 rocks (Figs. 5e-i), two gneisses (23GX05-2 and 23GX15) show the age cluster at the Neoproterozoic, three others show the youngest age cluster either at the Early Paleozoic (435 Ma for sample11GD06) or at the Triassic (229 Ma for sample 23GX04 and age unavailable for sample 11GD07 due to discordance).
For
Group 3 rocks (Figs. 5j-n), schists have either ages spreading largely over Meso- to Neoproterozoic without a cluster (11GD03) or an age cluster at the Neoproterozoic 21
ACCEPTED MANUSCRIPT (23GX13-1 and 23GX14), whereas gneisses display the youngest age cluster either at the Early Paleozoic (431 Ma for sample 11GD05) or at the Triassic (245 Ma for Such results basically resemble those obtained from Group 2
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sample 11GD04).
The large variability of U-Pb zircon ages with high discordant proportions in
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rocks.
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these two groups of rock makes this dating system not totally useful to indicate the time of ultimate metamorphism for samples composed of abundant detritus. The Group 4 undeformed granites (samples 28GX20 and 28GX22) exhibit ages
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(251-248 Ma; Figs. 5o-p) close to those obtained from zircon cores (Chen et al., 2011)
granites.
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but significantly older than their rims (Deng et al., 2004) of the Darongshan S-type Since core-rim variability on the U-Pb zircon geochronology is not
D
available for these two samples, whether these age really stand for the magma
case of Darongshan.
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emplacing time will be discussed in combination with the monazite age data like the
CE P
In sum, U-Pb zircon ages may be applicable to reflect the records of complicated multi-stage thermal activities on the studied samples. by 207Pb/206Pb ages are discordant.
Many records >1 Ga given
The discordance is possibly arising from the fact
AC
that 207Pb/206Pb ages are calculated assuming recent Pb-loss, which is incorrect if the Pb-loss was ancient (Guitreau and Blichert-Toft, 2014). 206
On the other hand,
Pb/238U ages reveal two major clusters in Neoproterozoic (1.0-0.9 Ga, n=70) and
Early Paleozoic (443-431 Ma, n=116), with the presence of Triassic ages (245Ma and 229 Ma) in spite of the scarcity in numbers (n=15) for all Group 2 and 3 rocks.
Due
to the difficulty to know the paragenesis of dated zircon, no good explanation can be deployed for the youngest U-Pb ages mainly obtained from zircon rims.
22
ACCEPTED MANUSCRIPT 6.1.2. Consistency of EMP monazite ages Monazite ages reflected by discrete grains from heavy concentrate and inclusion
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in rock-forming minerals are concerned. Here we deal with discrete grains first.
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For Group 1 rocks, monazite is abundant in two gneissic granites (samples 11GD01
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and 11GDGZA1) and gives similar weighted mean ages of 231 Ma and 234 Ma, respectively.
For Group 2 and 3 rocks, monazites generally exhibit an Early
Paleozoic core-Triassic rim relationship in some biotite and two-mica granites.
Monazite ages for the Group 4 undeformed granites are solely
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sliced samples.
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Same feature of this mineral has only been observed from those in the matrix of their
Triassic. The common presence of Triassic ages reflected by matrix monazite in all
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four groups of rock in the Yunkai massif is an indication of regional reworking
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without a strong juvenile crustal growth from mantle. Alteration of monazite, presumably a consequence of interaction between
CE P
monazites and fluids, is a common feature. The influence of fluid can generally lead to the alteration sequence of (1) pristine monazite, (2) secondary monazite, (3)
AC
secondary apatite with thorite/huttonite (ThSiO4) inclusions (allanite and epidote), and (4) finally clay minerals (e.g. Finger et al., 1998; Budzyń et al., 2010).
In such
corona-textured alteration zones, the unaltered monazite core usually can retain the age of its formation (Suzuki, 2009; Chen et al., 2012). However, whether the secondary monazite in the intermediate zones can preserve information of alteration is still debatable. Previous investigators have suggested that secondary monazite when subjected to dissolution-reprecipitation by fluid overprinting basically can retain the age of alteration (e.g. Harlov et al., 2011; Williams et al., 2011). Experimental works demonstrate that the alteration zone with ThSiO4-rich phase in the monazite can be induced from fluids especially with alkali-bearing compositions (Harlov and 23
ACCEPTED MANUSCRIPT Hetherington, 2010).
Recently, Seydoux-Guillaume et al. (2012) reported few cases
of low-temperature alteration of large monazite grains (mnz 1) from different places The alteration zone is reflected by patchy domains (BSE images) and
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of the world.
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characterized by the secondary, Th-U(Y)-depleted, high-Th/U monazites (mnz 2) in
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association with thorite/huttonite, thorianite (ThO2) and xenotime (YPO4) as a consequence of reaction between mnz 1 and reactant.
Due to the presence of
nano-scaled Th-U silicates (visible only under TEM) in mnz 2, its chemistry may be
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somewhat disturbed and thus erroneous chemical age dating is concerned.
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In the present case, some interstitial monazites show similar texture of alteration but slightly differ in zonation. They exhibit unaltered inner zone (mnz1), outer zone
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where m-scaled huttonite, (Th,U)SiO4 and (Th,U)O2 phases are common, with or The
TE
without a homogeneous middle zone (mnz 2) under BSE imaging (Fig. 8).
mineral assemblage of the outer zone is similar to those subjected to low-temperature
CE P
alteration probably affected by a mechanism of fluid-mediated coupled dissolutionprecipitation (Seydoux-Guillaume et al., 2012).
Consistency of ThO2 content and
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apparent age between inner and middle zones (Fig. 8) and resemblance of chemistry and age to the pristine monazites enclosed in quartz and feldspars of the same sample are evidences to exclude the existence of exsolved tiny Th- and U-rich phases in mnz 2. We suggest that formation of mnz 2, mainly presented in the discrete grains, is controlled by dissolution-reprecipitation, a common phenomenon in magmatic and metamorphic rocks.
The Yunkai massif, although experienced fluid overprinting,
might not involve large extent of chemical exchange and resetting (mnz 2) could have taken place shortly after monazite crystallization (mnz 1), making them indistinguishable in the EMP ages.
24
ACCEPTED MANUSCRIPT Monazites occurred as inclusions in host minerals with a framework structure (e.g. quartz and feldspars) are generally fresh (Fig. 9-1), whereas those in the sheeted The former
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biotite and muscovite show various degrees of alteration (Fig. 9-2).
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inclusions more likely represent contemporaneous primary species captured by the
to accommodate the secondary monazites.
SC R
growing host crystals because they do not contain sufficient amounts of P and REEs Neither the REEs required for the
secondary monazite inclusions were transported from overprinting fluid, for there is In fact, such monazite inclusions are
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no sign of fluid attack on most host minerals.
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mainly present near the rim of individual quartz or feldspar crystal (Fig. 9), their ages would be nearly simultaneous to the host minerals.
On the other hand, ages of
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monazite inclusions in the sheeted hosts are used to indicate the time of rock
TE
deformation.
Monazites appear to be included in biotite, quartz and plagioclase for the Group 1
CE P
gneissic granites (samples 11GD01 and 11GDGZA1), in biotite, muscovite and quartz (±plagioclase and K-feldspar) for Groups 2 and 3 gneisses, and dominantly in
AC
muscovite and subordinately in biotite and quartz for the Group 3 garnet tourmaline schists (samples 11GD03 and 23GX14). define foliation in these rocks.
Biotite, muscovite and quartz actually
Among all the inclusion grains dated, they give the
exclusive Triassic ages except one grain in biotite that yields ~430 Ma.
Therefore,
the EMP monazite age dating clearly indicates that Triassic is the time not only the great majority of host phases crystallized but also rims of discrete monazite grains overgrew in representative metamorphic rocks of the Yunkai massif.
25
ACCEPTED MANUSCRIPT 6.1.3. Reconciliation of two dating systems Significant age clusters derived from the U-Pb zircon geochronology in this study
T
can be summarized : 444-437 Ma with few Triassic rims (252-246 Ma) for Group 1
IP
rocks; 990-950 Ma, 435-433 Ma and 229 Ma for Group 2 rocks; 1004-904 Ma, 431 In other
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Ma and 245 Ma for Group 3 rocks; and 251-248 Ma for Group 4 rocks.
words, all these groups of rock present Triassic records although the number may be small.
On the other hand, all the studied samples solely display Triassic ages given
Yttrium variations of discrete monazite, as
MA
K-feldspar, biotite and muscovite.
NU
by monazite inclusions in major constituent phases, such as quartz, plagioclase,
manifested by Y decreases in monazite rims for garnet-bearing rocks (samples
D
11GD05 and 28GX22; Table 2) and Y annuli in monazites for garnet tourmaline
TE
schists (sample 23GX14; Appendix 3), also indicate that modification of monazite, particularly its rim, was contemporaneous with garnet that largely controls the Thus, modification of monazite in the
CE P
expense of yttrium during crystallization.
discrete grains and growth of inclusions are considered to have taken place All these
AC
simultaneously with recrystallization of major constituents in the rocks.
features can distinguish an overwhelming within-grain growth of monazite during the Triassic recrystallization versus a weak mesostasis growth in the Early Paleozoic assemblage. It has been suggested that monazite may provide better age control than zircon in some metamorphic rocks with a range in metamorphic grade from greenschist to upper amphibolite facies (Kingsbury et al., 1993).
Because the effective closure
temperature of monazite is approximately 725±25 °C (Parrish, 1990), a monazite age could represent its initial crystallization in many granitic and high-grade metamorphic rocks. Contrarily, zircon, with its high closure temperature ≧900 °C (Lee et al.,
26
ACCEPTED MANUSCRIPT 1997), can be stable throughout the sedimentary cycle and during metamorphism. Our geochronological results provide more evidences to support this viewpoint in
The major factor causing age
IP
obtained for gneiss and schist of the Yunkai massif.
T
view of large variability of U-Pb zircon age versus consistency of EMP monazite age
SC R
bias between these two minerals may be due to the fact that large interstitial grains could be recycled and only newly grown rims can give more reliable age information, and unfortunately, zircon rims are mostly too thin to be datable.
Combining these
NU
two dating systems, ages of ca. 440-430 Ma given predominantly by zircon and
MA
subordinately by monazite (Table 4) can be regarded as the Early Paleozoic relicts being unmodified by the Triassic thermal event.
If two dating systems give
D
consistent results, the U-Pb zircon age is adopted for its better accuracy.
TE
Conclusively, it is suggested that a strong Triassic reactivation had imposed over the entire Yunkai massif, conforming the popular Triassic 40Ar/39Ar biotite ages of major
CE P
metamorphic rocks in this massif (Wang et al., 2007a, 2012; Chu et al., 2009). Two biotite gneiss samples (23GX04 and 11GD04) with U-Pb zircon ages of 229±10 Ma
AC
and 245.1±5.4 Ma are used to constrain the main age for Group 2 and 3 rocks, respectively.
6.2. Multi-phase metamorphism in the Yunkai massif 6.2.1. Inherited monazite and zircon ages It is well known that the Cathaysia block had endured repeated orogeneses, records of poly-phase thermal event may be unveiled by monazite and zircon ages in this study.
Sample 23GX11 provides a wide range of monazite ages varying from
Paleoproterozoic to Triassic on 24 datable grains (Appendix 3).
Many grains show
signs of multiple events, of which two kinds of monazite are recognized: one takes ~430 Ma and the other shows ~240 Ma as the youngest age.
The former (4 grains) 27
ACCEPTED MANUSCRIPT records some older ages, such as ~1750 Ma, ~970 Ma, 880-840 Ma and ~540 Ma, yet the latter (20 grains) records older ages including only ~540 Ma and ~430 Ma.
Two
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examples of respective monazites, with apparent ages (plus ThO2 and Y2O3 contents)
Although chemistry and thus the age generally matches the BSE domain in each
SC R
10.
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of different BSE domains and X-ray maps of Ce, Th, U and Y, are depicted in Figure
grain, resorbed (Fig. 10a) and patchy (Fig. 10f) zoning textures suggest a complicate history of monazite growth and dissipation when compared to monazite inclusions
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that mostly show simple core-rim or homogeneous zoning (Fig. 5). Combining the
MA
in-situ monazite ages, this rock could have been metamorphosed at ~240 Ma as reflected by the youngest high peak in the total age frequency distribution of monazite
D
(Fig. 11a).
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The total age frequency of U-Pb zircon dates for all samples from the Yunkai massif mimics the EMP monazite age curve of the meta-sandstone if discordant Pb/206Pb ages are excluded (Fig. 11b). Hence, a common provenance of these two
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207
minerals is postulated.
Abundance of Neoproterozoic and Early Paleozoic zircons in
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the Yunkai samples (inset of Fig. 11b) supports the thought that this massif is the major supplier for constructing Permian to Late Triassic sedimentary successions in the Qinfang Basin to the west based on the similarity of U-Pb zircon age patterns (Hu et al., 2014). Furthermore, age clusters of 1.0-0.9 Ga in gneissic rocks and schists (Figs. 5f, g, j and k), with such zircons of magmatic origin (Th/U >0.5, Appendix 1) (Hoskin and Schaltegger, 2003), favor a Neoproterozoic arc system in the northern Yunkai Domain (Zhang et al., 2012). Regarding the idea on the tectonic evolution of South China since Neoproterozoic, monazite ages of ~970 Ma and 880-840 Ma found in this meta-sandstone suggest that the episode of Neoproterozoic magmatism probably had involved crustal reworking to some extents.
Detrital monazite ages ~540 Ma of this sample (Appendices 3 and 4) 28
ACCEPTED MANUSCRIPT probably reflect traces of the Pan-African orogeny in the Cathaysia block, which is complementary to the observation that Pan-African age is one of the dominant
These strengthen the propositions that
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in the Yangtze block (Duan et al., 2012).
T
populations in Paleozoic detrital zircon samples from three Lower Devonian sections
SC R
South China block was a part of East Gondwana in Early Paleozoic time (Xu et al., 2013, 2014a, b) and Yunkai massif is a place reflecting the Gondwana property (Wu,
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2000). Detail of the Early Paleozoic age records is discussed in the following.
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6.2.2. Metamorphism and magmatism on the Yunkai protolith Gneissic granites (Group 1) in the core domain exhibit an overwhelming
D
population of Early Paleozoic zircon age (Fig. 4), reflecting a stage of magma However, this zircon age
TE
intrusion during the Early Paleozoic orogenesis.
population is not so obvious for Groups 2 and 3 gneissic rocks and schists, and only
CE P
restricted to few specific samples (11GD05 and 11GD06) that the Early Paleozoic monazite relicts have also been detected (Table 4).
These gneissic rocks and schists
AC
actually possess a popular Neoproterozoic U-Pb zircon age cluster, supporting the development of abundant Neoproterozoic mafic magmatism in the Cathaysia interior (Wang et al., 2012).
The high and sharp age peaks of ~230 Ma and ~430 Ma in both
zircon and monazite age frequency diagrams (Figs. 11a and b), mainly contributed from gneissic granites and undeformed granites, are thus interpreted as an indication of strong Triassic influences on a terrain characterized by a pre-existing Early Paleozoic granitic mass and abundant detrital sediments (or metapelite protoliths) probably derived from the Neoproterozoic magmatic arc and Early Paleozoic domains nearby.
Extent of the Triassic modification on the Yunkai massif may be better
elucidated by monazites.
29
ACCEPTED MANUSCRIPT Usually, the shielding effect of host mineral with a framework structure (feldspar in the present case) can prevent monazite inclusions from fluid attack during the later Their ages can thus be considered as Likewise, ages
IP
representing the formation time of feldspar crystals in the rock.
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thermal events (Montel et al., 2000).
SC R
derived from monazite inclusions in major primary phases of the undeformed granites shall represent magma emplacing time of these intrusives.
On the other hand,
muscovite, biotite and quartz are regarded as phases generated in the course of ductile
NU
deformation (or syn-kinematic minerals), EMP monazite age results suggest that these
MA
minerals in the same sample were recrystallized at the Early Mesozoic time. Further based on the Early Paleozoic-Triassic core-rim relationship on monazite in few
D
samples, the record of Early Paleozoic deformation could be largely overwritten.
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It is noted that contradictory ages for Group 4 granitic rocks are given by the U-Pb zircon dating system because they were not deformed yet having older ages (251-248 Considering the
CE P
Ma) than metamorphic rocks (245-229 Ma) in the same area.
SHRIMP U-Pb dating on the zircon rims yielded characteristic younger ages of ~230
AC
Ma for the neighboring S-type Darongshan granites (Deng et al., 2004), similar ages (231-224 Ma) obtained from the large population of monazite on five major plutons in Darongshan were interpreted as the time of magma emplacement (Chen et al., 2011). In lacking of age constraints from zircon rims, we follow same line of thought by adopting the monazite ages, 232-229 Ma, to represent the time of magmatism in the Yunkai area. In sum, monazite inclusion ages for the Group 3 (greenschist facies) rocks (244-236 Ma) and the slightly younger Group 2 (amphibolites facies) rocks (236-233 Ma) coincide nicely with the results reflected by the youngest U-Pb zircon age cluster for the Group 3 (245 Ma) and the Group 2 (229 Ma) rocks (Table 4 Age-1).
Along
with our monazite age data of Group 1 gneissic granites (233-230 Ma) and intrusion 30
ACCEPTED MANUSCRIPT of Group 4 S-type granites (232-229 Ma) as well as Triassic U-Pb zircon rim ages for the orthogneiss near Gaozhou (Chen et al., 2012), a temporal scheme of progressive
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metamorphism and hotly-pursuing granitic magmatism seems to enlist Yunkai massif
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a terrain that can outline the Triassic geodynamic processes in the southwestern South
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China. This, in general, agrees with the 250-230 Ma syntectonic metamorphism and the 225-205 Ma post-orogenic plutonism in the eastern Indochina (Faure et al., 2014). For elucidating the Yunkai massif in the framework of orogenic evolution, major
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Triassic tectonic units in South China are compared as followed.
7. Tectonic implications
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Yunkai massif, in the middle of the Early Paleozoic Wuyi-Baiyun-Yunkai-Song
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Chay metamorphic belt (Lin et al., 2008), is important in knowing the Early Mesozoic evolution in the southern margin of South China Block. Materials for the Early
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Paleozoic metamorphosis in the Yunkai massif were mainly the Neoproterozoic detritus. They were accreted by a large amount of simultaneous granitic intrusions Based on the
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to construct the highly advocated Early Paleozoic Yunkai protolith.
metamorphic grade, the greenschist facies Yunkai Group and amphibolite facies Gaozhou Complex were differentiated. The South China Block had experienced strong Triassic tectonothermal events mainly by the interactions with Indochina Block in the west and North China Block in the north.
Boundary between the Indochina and the South China Blocks is basically
along the Ailaoshan-Song Ma suture zone although complications exist (e.g. Metcalfe, 2011; Faure et al., 2014, 2016b) (Fig. 1a).
Consequences of such events to this Early
Paleozoic metamorphic belt were somewhat different.
The Song Chay massif close
to the Song Ma suture is a strongly deformed body bounded by a mylonite zone on its southern frank (Lepvrier et al., 2011).
This body is made of an orthogneiss in the 31
ACCEPTED MANUSCRIPT center and mica schist in the peripheral with a NW- or N-trending stretching lineation (Maluski et al., 2001; Yan et al., 2006). Based on the U-Pb zircon geochronological
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data summarized by Chen et al. (2014) that Triassic ages often appear in rims of Early
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Paleozoic zircon for orthogneisses, they were considered to represent reworking of the
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Paleozoic protolithic granites. SIMS monazite U-Th-Pb determinations on garnet mica schist show Early Paleozoic ages for interstitial grain and Triassic ages for inclusions in garnet.
In addition, 40Ar/39Ar mineral ages are Triassic for muscovite
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in gneiss and mica schist, and amphibole in mylonites.
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Wuyishan terrain has long been considered to be exposures of the Precambrian basement composed of Paleoproterozoic and Neoproterozoic-Early Paleozoic Reliable dating information revealed from zircon U-Pb
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metamorphic sequences.
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geochronology led to a consensus that an Early Paleozoic tectonothermal event had caused immense greenschist-amphibolite facies (Wan et al., 2007), minor granulite
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facies metamorphism (Yu et al., 2005) and anatexis and granitic magmatism (Zeng et al., 2008; Liu et al., 2010; Li et al., 2011; Xu et al., 2011). Moreover, EMP monazite
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ages obtained were all Early Paleozoic from basement rocks of Wuyishan (Chen et al., 2008; Charvet et al., 2010).
Recently, an Early Mesozoic metamorphic reworking is
considered to have affected the entire Cathaysia block based on popular discordia intercepts of U-Pb zircon ages at Early Paleozoic (upper intercept) and Triassic (lower intercept), respectively, for high grade metapelitic rocks in Zhejiang and Fujian (Zhao et al., 2015). However, considering 40Ar/39Ar dating of biotite (235.3±2.8 Ma) and muscovite (238.5±2.8 Ma) separated from two mylonite samples and U-Pb zircon age for an undeformed granitic dyke (229.8±2.2 Ma) intruded into the shear zone, Triassic ages in the Wuyishan would be more likely reflecting the time of dextral shearing (Xu et al., 2011).
32
ACCEPTED MANUSCRIPT In the Yunkai massif, gneissic granites in the massif’s core domain show Early Paleozoic ages based on the U-Pb zircon, but Triassic ages on the EMP monazite
Age similarities between Song
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massif (Chen et al., 2014 and references therein).
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geochronologies. This is almost a case similar to the orthogneiss in Song Chay
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Chay and Yunkai massifs commonly include Early Paleozoic-Triassic zircon core-rim and monazite interstitial grain-inclusion relationships as well as dominant Triassic 40
Ar/39Ar mineral ages for a large number of metamorphic rocks.
These results
Moreover, the Permo-Triassic Emeishan-related intraplate
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Paleozoic ages.
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deviate from the Wuyishan massif in which monazites, in particular, retain the Early
magmatism and Late Triassic S-type association adjacent to the Song Chay massif
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(Chen et al., 2014; Faure et al., 2016a) provide a clue for a consideration for the
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influence of mafic magma emplacement. The Darongshan S-type granites and few plutons in their eastward extension (the Group 4 rocks) relative to the Yunkai massif
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are almost the equivalents (see later sections). From the spatial relationship linking the Wuyishan, Yunkai and Song Chay
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massifs, we thus favor that the strong Triassic reworking on the Yunkai massif, like Song Chay massif, was mainly controlled by the collision of Indochina and South China Blocks. The Indochina Block here includes Simao (Fig. 1c), as they were united before collided with the South China Block (Wang et al., 2016 and references therein in).
Current views on the South China-Indochina collisional tectonics have
been reviewed by Faure et al. (2014).
Their geodynamic evolution model described
the Permian subduction of paleo-Tethys underneath Indochina and the subsequent Triassic South China-Indochina collision and separated syntectonic metamorphism and postorogenic plutonism at 230-225 Ma.
Important Triassic tectonic units in the
South China Block may shed light for understanding of the evolution of the Yunkai massif. 33
ACCEPTED MANUSCRIPT The Xuefengshan Belt in the center of this Block (Fig. 12), characterized by large-scale fold and thrust structures and widespread granites, is a unit concerned.
A
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basal decollement zone is exposed in the core of anticlines of this Belt, in which the
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Neoproterozoic pelite and sandstone, and the intruding Early Paleozoic granites were
(Chu et al., 2012a).
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strongly deformed and metamorphosed into mylonites and orthogneiss, respectively SIMS zircon U-Pb dating on these granites reveal two magmatic
episodes: the Early Paleozoic (438-411 Ma) and the Early Mesozoic (225-217 Ma),
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and zircon Hf(t) values indicate a common crust-derived source for the two EMP monazite and SIMS zircon U-Pb
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generations of granites (Chu et al., 2012b).
dating provide age constraints of the ductile shearing at 243-226 Ma (Chu et al., Thus, the Xuefengshan Belt is considered to be a Late Triassic
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2012c).
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intracontinental orogen dominated by the NW-directed shearing and thrusting (Chu et al., 2012a, c; Faure et al., 2016a).
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The Group 4 rocks are considered to represent postorogenic plutonism to the metamorphosis of the Yunkai massif, synchronous with immense Triassic Darongshan
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S-type granitoids and perhaps unfoliated Qiongzhong and adjacent granitic batholiths (234-226 Ma; Yu et al., 2014) in the Hainan Island (Fig. 1a).
As mentioned, this is
accomplished by adopting the zircon core ages to be inherited (or mixing) records, while the monazite age, the time of granitic intrusion.
In this context, the tectonic
architecture of such Triassic metamorphic massifs and magmatic belts composed of a large spread of post-orogenic plutonism in the South China Block would be more likely to be accommodated with an intracratonic magmatism (Chu 2012a, c; Faure et al., 2016b) rather than an early-stage, NW-directed, flat-slab subduction of the Paleo-Pacific plate (Li and Li, 2007).
34
ACCEPTED MANUSCRIPT The process of two phases of Triassic deformation in the Yunkai area, with NW-trending greenschist facies metamorphism at ~245 Ma and NE-trending
From a mass balance perspective, expense
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metamorphism on metapelitic basements.
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amphibolite facies metamorphism at ~230 Ma, is first considered by the prograde
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of muscovite and chlorite to form biotite and garnet via prograde metamorphism commonly involve formation of monazite with observable overgrowths (Kohn and Malloy, 2004). This is sometimes accompanied by the presence of precursor
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minerals such as allanite, apatite and Th oxides (Catlos et al., 2002).
Insignificant
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monazite overgrowths in biotite (Fig. 9), the lack of precursor minerals and different orientations of monazite-bearing muscovite developed in two phases of rock are
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difficult to be explained by the prograde metamorphism within the same massif.
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Here we draw attention to the tendency that the amphibolite facies metamorphic (Group 2) rocks may be correlated with the Xuefengshan Belt in time (e.g. the Such metamorphic
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monazite ages of 243-226 Ma) and the sense of deformation.
features imply that the late phase deformation in the Yunkai massif may be controlled
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by the same collisional tectonism as the Xuefengshan Belt. As for the early phase deformation, we suggest that intrusion of the Emeishan Large Igneous Province (E-LIP at 255-251 Ma; Shellnutt, 2014) may play the role (Fig. 12). This intrusion had triggered not only the Late Permian crustal-derived granites and accompanying peralkaline, peraluminous and metaluminous A-type granites in southwestern China (Shellnutt et al., 2007, 2009, 2010, 2011a, b), but also the contemporaneous metaluminous and peraluminous silicic volcanic rocks in the Song Da magmatic suite (Tran et al., 2011) and alkaline silicic rocks in the Phan Si Pan-Tu Le region in northwestern Vietnam (Hoa and Anh, 2011; Hieu et al., 2013; Usuki et al., 2015; Tran et al., 2015) as well as rifting-related, Late Permian ferrosyenites in the same region (Shellnutt et al., 2008).
Intrusion of the E-LIP 35
ACCEPTED MANUSCRIPT (~0.3x106 km2) could have caused high temperature metamorphism of lower crust of the Yunkai massif by the picritic magma with a mantle potential temperature >1540 o
The presence of abundant high-temperature Permo-Triassic
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C (Shellnutt, 2014).
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granulite enclaves in the Darongshan granites (Chen et al., 2011; Zhao et al., 2012)
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provides a clue that the neighboring Yunkai massif could have been subjected to an elevated geothermal gradient in the upper crust level by the same heat source.
Mafic
granulite xenoliths (230-220 Ma) captured by Jurassic basalts in Daoxian (Li et al.,
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2016) is an indication for the heat pulse lasting until Late Triassic (Fig. 12).
Thus,
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we favor a combined influence from both the continental collision and the E-LIP migration would be responsible for the formation of two phases of metamorphic rock The relative strength of these two affecting factors determines
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of the Yunkai massif.
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the metamorphic grade and striking structure and also explains why two sets of deformation were observed in the Gaozhou Complex.
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To the west of the Song Ma suture, the Truong Son Belt and Kontum massif (Fig. 1a) are the most important Triassic tectonic units that defined the Indosinian orogeny The EMP monazite
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in Vietnam (Lepvrier et al., 2004; 2008; Ferrari et al., 2008).
geochronology has been applied for a large number of rocks in these units (sixty in total, Nakano et al., 2013), in which Kontum Massif gneisses record two distinct thermal episodes (460-430 Ma and 245-230 Ma), whereas Truong Son Belt gneisses show only single dominant event (240-220 Ma) synchronous with the granitoids in these terrains (Roger et al, 2014).
Kontum massif, the largest continuous exposure
of crystalline basement of the Indochina craton (Nam et al., 2001), is worth noting for similarities to the Yunkai massif in the metamorphic grade (amphibolite-granulite, amphibolite and greenschist-amphibolite facies in different complexes; Osanai et al., 2008) and geochronology.
Ages of specific lithologies, such as the high-temperature
granulite-charnockite association, are good examples.
Granulites in the eastern 36
ACCEPTED MANUSCRIPT Kontum massif show Early Paleozoic ages based on the U-Pb monazite dating (Roger et al., 2007) and 40Ar/39Ar biotite dating (Maluski et al., 2005). However, granulites
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and associated charnockites in the western Kontum massif yield Triassic ages on the
This would be an analogy
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al., 2007) and 40Ar/39Ar biotite dating (Nagy et al., 2001).
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U-Pb zircon dating (Nam et al., 2001; Nagy et al., 2001; Carter et al., 2001; Roger et
in the Indochina when compared with similar rocks that exhibit Early Paleozoic ages (440-430 Ma) in the Gaozhou Complex and Permian ages (~260 Ma) in the adjacent
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Darongshan. Therefore, both the Yunkai and Kontum massifs may mark reactivated
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metamorphic terrains, with strong Triassic deformations on the Early Paleozoic basement in South China and Indochina Blocks, respectively.
As both blocks had
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derived from Gondwana landmass (Metcalfe, 2013; Usuki et al., 2013; Xu et al.,
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2013), more comprehensive studies for these two massifs concerning the Triassic rework on Early Paleozoic assemblies would be of help to better understand the
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Conclusions
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so-called Song Ma-Poko (Kontum) system (Ferrari et al., 2008; Wang et al., 2016).
1. This study demonstrates the usefulness of the in-situ EMP monazite analysis which can provide age control particularly for the time of deformations on metamorphic rocks in the Yunkai massif. Monazite inclusions in the principal constituents (muscovite, biotite and quartz), exhibiting long axis orientated sympathetically with these platy or elongated minerals, define both the earlier NW-trending, greenschist facies deformation and the later NE-trending, amphibolite facies deformation occurred during the Triassic time in the Yunkai massif.
This
phenomenon can hardly be reflected by the zircon age alone from same samples mainly because zircons are largely interstitial grains representing un-reset relicts of the preceding rocks. 37
ACCEPTED MANUSCRIPT 2. Combining our new EMP monazite and U-Pb zircon age results, we suggest that greenschist-facies metamorphism of the Yunkai Group with severe ductile
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deformation occurred at the earlier time (~245 Ma) and amphibolite facies
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metamorphism of the Gaozhou Complex later (~230 Ma). They were followed by
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the intrusion of S-type granites probably extended from Darongshan. This framework supports that Yunkai massif, like Song Chay massif to the southwest, is a terrain highly affected by the Triassic thermal events and clearly distinguishes from
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heavily retained (Faure et al., 2016a).
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the Wuyishan massif to the northeast in which trace of the Early Paleozoic rework is
3. The prograde metamorphism as a consequence of the continental collision at
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the Triassic, although may be used to account for the metamorphic facies and age
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results of the Gaozhou Complex, is difficult to explain the difference of the stress patterns imposed on the metamorphic rocks of the Yunkai Group.
A heat source
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contributed by the E-LIP mafic underplating is considered as an additional controlling factor to cause, in particular, the NW-trending deformation for the greenschist facies Combination of these two controlling factors,
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metamorphism in the Yunkai massif.
with the dominance of E-LIP mafic underplating in the early phase and the collisional tectonism in the later phase, may be responsible for the deformation patterns in the Yunkai massif. 4. Remarkable similarities of geochronological data have been observed from some terrains in South China and in Vietnam (roughly along the coastal area of the Indochina Block) revealing the common presence of Early Paleozoic and Triassic ages in almost all kinds of metamorphic rocks in these terrains.
Yunkai massif in
South China and Kontum massif in Indochina are the examples.
Regarding the
collisional tectonism at Triassic, i.e., the essence of the Indosinian orogeny, we suggest that ages used to define the syn-kinematic and post-orogenic stages of this 38
ACCEPTED MANUSCRIPT orogeny may be better constrained by inclusion minerals like monazite.
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Acknowledgments
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Drs. S.L. Chung and Y. Iizuka are thanked for arranging LA-ICPMS U-Pb zircon
imaging at the Academia Sinica, respectively.
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analyses at the National Taiwan University and monazite X-ray mapping, BSE and SE The authors are highly indebted to Dr.
V. Walia (Taipei) who read the manuscript, Dr. M. Faure (Orleans) and one
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anonymous reviewer who gave valuable comments.
This work was supported by
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research grants from National Science Council, Taiwan, ROC
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(NSC97-2116-M-002-019/Chen and NSC100-2116-M-002-008/Lee).
39
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Excel. Berkeley Geochronology Center Special Publication, Berkeley, California, Special Publication 4, 74 pp. Maluski, H., Lepvrier, C., Jolivet, L., Carter, A., Roques, D., Beyssac, O., Tang, Ta T., Thang, N.D., Avigad, D., 2001. Ar–Ar and fission-track ages in the Song Chay massif: Early Triassic and Cenozoic tectonics in northern Vietnam. Journal of
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Yu, J., Mao, J., Chen, F., Wang, Y., Che, L., Wang, T., Liang, J., 2014. Metallogeny of the Shilu Fe-Co-Cu deposit, Hainan Island, South China: Constraints from fluid inclusions and stable isotopes. Ore Geology Reviews 57, 351-362. Zhang, A., Wang, Y., Fan, W., Zhang, Y., Yang, J., 2012. Earliest Neoproterozoic (ca. 1.0 Ga) arc-back-arc basin nature along the northern Yunkai Domain of the Cathaysia Block: Geochronological and geochemical evidence from the metabasite. Precambrian Research 220-221, 217-233. Zeng, W., Zhang, L., Zhou, H.W., Zhong, Z.Q., Xiang, H., Liu, R., Jin, S., Lu, X.Q. and Li, C.Z. 2008 Caledonian reworking of Paleoproterozoic basement in the
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Cathaysia Block: Constraints from zircon U-Pb dating, Hf isotopes and trace elements. Chinese Science Bulletin 53, 895-904. Zhao, L., Guo, F., Fan, W., Li, C., Qin, X., Li, H., 2012. Origin of the granulite enclaves in Indo-Sinian peraluminous granites, South China and its implication for crustal anatexis. Lithos 150, 209-226.
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Zhong, Z.Q., You, Z.D., Zhou, H.W., Han, Y.J., 1996. The evolution and basic structural framework of the basement of the Yunkai uplift. Regional Geology of China 56, 36-43 (in Chinese with English abstract). Zhou, H.W., 1995. Petrological and geochemical studies on the basement rock series of the Yunkai uplift. Unpublished PhD thesis, China University of Geosciences (Wuhan), 130pp with 5 plates (In Chinese with English abstract). Zhou, X.M., Li, X.W., 2000. Origin of Late Mesozoic rocks in southeastern China: implications for lithosphere subduction and underplating of mafic magmas. Tectonophysics 326, 269-287.
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ACCEPTED MANUSCRIPT Figure captions Fig. 1.
(a) Relevant tectonic units in the Yangtze and Cathaysia sub-blocks of the
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South China Block, including Yunkai massif (YKM), Song Chay massif (SCM),
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Darongshan (DRS), Xuefengshan Belt (XFSB), Hainan Island (HN), Wuyishan
Truong Son Belt (TSB) in the Indochina Block.
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(WYS)-Biyunshan (BYS) massif in the South China, and Kontum massif (KTM) and The formerly proposed Early
Paleozoic units are marked by blue and Triassic units, by pink colors. Ailaoshan
Indochina (+Simao) Blocks.
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(ALS)-Song Ma (SM) zone is the collision boundary between the South China and (b) The simplified geological map of the Yunkai massif
tectonic blocks in East Asia.
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(modified from Zhou, 1995) and sample localities.
(c) Relative position of major
Names of fault are 1: the Wuzhou-Bobai Fault, 2: the
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Luoding-Yuocheng Fault (one of the major shear zones), 3: the Wuchuang-Sihui Fault,
Back-scattered electron (BSE) images showing some special petrographic
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Fig. 2.
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4: Chenzhou-Linwu Fault, and 5: Poko Zone (Ferrari et al., 2008).
features of four sample groups.
The gneissic granites in Group 1 show (a) granular
texture (sample 11GDGZA1) and (b) relatively high extent of deformation (sample Biotite gneiss in Group 2 (sample 23GX05-2) is a gneissoid enclave
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11GD01).
consisting mainly of (c) granular quartz, plagioclase and biotite, and a two-mica gneiss in Group 2 (sample 23GX15) shows (d) relict plagioclase grain.
Biotite
gneiss in Group 3 (sample 11GD05) contains (e) K-feldspar and plagioclase porphyroblasts, and garnet-tourmaline schist in Group 3 (sample 23GX14) generally shows (f) garnet poikiloblast with abundant quartz inclusions surrounded by platy muscovite and chlorite.
Group 4 undeformed granites show (g) quartz, plagioclase
and biotite grains surrounded by interstitial K-feldspar (samples 28GX20), and (h) minor amounts of garnet and rutile (sample 28GX22).
Abbreviation of mineral
names follows those described by Whitney and Evans (2010). 50
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BSE images showing mineral phases that enclose monazite grain (Mnz) are
consistent with biotite, muscovite, chlorite, quartz and ilmenite based on energy Monazites in muscovite (a-b), in biotite contacting with
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dispersive X-ray spectra.
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quartz (c), in biotite contacting with muscovite (d), in biotite penetrated by muscovite
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veinlets (e), and in muscovite associated with ilmenite (f) are generally parallel to the foliation of the NE-trending Group 2 (a-d) and the NW-trending Group 3 (e-f) rocks.
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Fig. 4. Concordia diagrams and weighted mean 206Pb/238U age (2σ) for zircons
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from Yunkai samples in four groups: (a-d), (e-i), (j-n) and (o-p). Note that high population of Neoproterozoic zircons is a common feature in Groups 2 and 3 samples.
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data are listed in Appendix 1.
Source
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N= number of analyses and MSWD= mean square of weighted deviates.
BSE images of monazite in the polished section (R) and heavy concentrate
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Fig. 5.
(S) of the representative samples, with the EMP age results (Ma) and ThO2 contents (wt% in the parenthesis).
Although different BSE domain and zonation,
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morphological feature, and degree of alteration are shown, they invariably present Triassic ages in all samples, including the overgrowing rim of two grains (d and o) that exhibit an Early Paleozoic core.
Fig. 6.
EMP monazite isochron and the weighted mean age results for four groups of
rocks in the Yunkai massif (a-d for Group 1, e-i for Group 2, j-n for Group 3 and o-p for Group 4). deviates.
N= number of analyses and MSWD= mean square of weighted
Source data are listed in Appendix 3 (slide is equal to polished section and
sand is equal to heavy concentrate in the text).
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BSE images of discrete monazite grains of some representative samples on
which the EMP ages, with ThO2 and Y2O3 contents (wt% in the parenthesis), and Z: zircon in monazite.
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NanoSIMS U-Pb and Pb-Pb ages are compared.
BSE images of monazite alteration zonations including 1: unaltered core
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Fig. 8.
(mnz 1), 2: intermediate monazite (mnz 2), and 3: the outer zone (mnz 3).
The mnz
3 is characterized by the presence of abundant huttonite (the Th- and U-rich silicate The mnz 2 may be present (a-b) or absent (c-d) when the
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with a bright reflection).
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pristine monazite (mnz 1) was subjected to alteration probably involving fluid attacks.
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Numbers in the parenthesis are EMP ages (Ma) with ThO2 and Y2O3 contents (wt%).
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Fig. 9-1. Monazite inclusions with the EMP age results (Ma) and ThO2 contents (wt% in the parenthesis) in K-feldspar and plagioclase from a biotite gneiss (sample
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11GD04).
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Fig. 9-2. Monazite inclusions with the EMP age results (Ma) and ThO2 contents (wt% in the parenthesis) in biotite and muscovite from a gneissic granite (sample 11GD01).
Fig. 10.
BSE images with results of spot analysis of EMP apparent ages (Ma) and
ThO2 and Y2O3 contents (wt% in the parenthesis), and CeLα, ThMα, UMα and YLα X-ray compositional maps of two representative monazites from sample 23GX11 (S denotes samples from heavy concentrates). (a-e) The presence of ~1750, ~540, and ~440 Ma domains (BSE) in grain M03; (f-j) ~450, ~260 and ~230 Ma domains in grain M24, in which the ~450 Ma domain is relatively low in Ce and Th.
The color
bar in (e) is equally applied for all X-ray maps. 52
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Fig. 11.
Cumulative age probability curves of studied rocks from the Yunkai massif.
Resemblance of two curves with
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zircon ages with histograms of four groups of rock.
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(a) EMP monazite ages of a meta-sandstone (sample 23GX11), and (b) total U-Pb
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peaks at the Neoproterozoic and Early Paleozoic indicates the common provenance for monazite and zircon.
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Fig. 12. Schematic sketch showing the now-exposed Early Paleozoic metamorphic
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terrains (dark) and Triassic reactivated belts (hatched) in the South China and Indochina Blocks in a position relative to the distribution of Permo-Triassic Emeishan An inferred boundary of E-LIP
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large igneous province (E-LIP; Shellnutt, 2014).
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influence in the deep crust (the dashed line in red) is constrained by the Triassic high-temperature granulite xenoliths and enclaves in Daoxian (DX) basalts (Li et al.,
respectively.
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2016) and Darongshan (DRS) granitoids (Chen et al., 2011; Zhao et al., 2012), Metamorphism triggered by the same heat source form E-LIP and
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collision of these two Blocks along the Ailaoshan-Song Ma suture (in green) might have affected the Triassic rework of Song Chay Massif (SCM) and Yunkai massif (YKM), and build-up of the Xuefengshan Belt (XFSB) in South China Block, and Kontum massif (KTM) and Truong Son Belt (TSB) in Indochina Block at various extents.
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List of Tables
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Table 1. Description of studied samples from the Yunkai massif
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Table 2. Percentage of constituent minerals and information of monazite inclusions of the studied rocks Table 3. Comparison of EMP and NanoSIMS ages on same BSE domain in monazites
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Table 4. Record of monazite and zircon ages on rocks from the Yunkai massif.
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List of Appendices (designated to be included in the data repository)
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Appendix 1. LA-ICPMS zircon U-Pb dating results for samples from Yunkai massif.
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Appendix 2. Statistics of U-Pb zircon ages of dated grains from the studied samples.
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Appendix 3. EMP monazite ages of rock samples from Yunkai Massif.
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Appendix 4. Original EMP monazite ages used for comparison with NanoSIMS ages
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Fig. 1.
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Fig. 1. (a) Relevant tectonic units in the Yangtze and Cathaysia sub-blocks of the South China Block, including Yunkai massif (YKM), Song Chay massif (SCM), Darongshan (DRS), Xuefengshan Belt (XFSB), Hainan Island (HN), Wuyishan (WYS)-Biyunshan (BYS) massif in the South China, and Kontum massif (KTM) and Truong Son Belt (TSB) in the Indochina Block. The formerly proposed Early Paleozoic units are marked by blue and Triassic units, by pink colors. Ailaoshan (ALS)-Song Ma (SM) zone is the collision boundary between the South China and Indochina (+Simao) Blocks. (b) The simplified geological map of the Yunkai massif (modified from Zhou, 1995) and sample localities. (c) Relative position of major tectonic blocks in East Asia. Names of fault are 1: the Wuzhou-Bobai Fault, 2: the Luoding-Yuocheng Fault (one of the major shear zones), 3: the Wuchuang-Sihui Fault, 4: Chenzhou-Linwu Fault, and 5: Poko Zone (Ferrari et al., 2008).
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(a)
Pl
Bt Pl
Ms
Ch Qt z
500 µm
Qt
Kfs
z
Ap
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(c)
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Qt
Bt
500 µm
(d)
Bt Pl
Qtz Pl
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Qt
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Ms
z
500 µm
500 µm
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(e) Kfs
(f) Qt
Kfs
Ms
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Qtz
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Gr
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Kfs
Bt Kfs
z
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500 µm
Ch
Qt
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Ms
t
Bt
Pl 500 µm
Pl Rt
Qt z
500 µm
Fig. 2.
Fig. 2. Back-scattered electron (BSE) images showing some special petrographic features of four sample groups. The gneissic granites in Group 1 show (a) granular texture (sample 11GDGZA1) and (b) relatively high extent of deformation (sample 11GD01). Biotite gneiss in Group 2 (sample 23GX05-2) is a gneissoid enclave consisting mainly of (c) granular quartz, plagioclase and biotite, and a two-mica gneiss in Group 2 (sample 23GX15) shows (d) relict plagioclase grain. Biotite gneiss in Group 3 (sample 11GD05) contains (e) K-feldspar and plagioclase porphyroblasts, and garnet-tourmaline schist in Group 3 (sample 23GX14) generally shows (f) garnet poikiloblast with abundant quartz inclusions surrounded by foliated muscovite and chlorite. Group 4 undeformed granites show (g) quartz, plagioclase and biotite grains surrounded by interstitial K-feldspar (samples 28GX20), and (h) minor amounts of garnet and rutile (sample 28GX22). Abbreviation of mineral names follows those described by Whitney and Evans (2010).
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(a)
Ms Chl
(b)
Bt
Qtz Zrn
Chl
T
Ms Mnz
Mnz
IP
Ms
SC R
Fe-Mn-Ti oxide
Chl
(c)
Mnz
Qtz
Bt
(d)
Qtz
NU
Bt
Ms
Bt Mnz
Zrn
MA
Ms
Ms
Qtz
D
Qtz
CE P
Bt
TE
Ms
Mnz
AC
Qtz
Chl
Chl
Ms Qtz
Ms
(e)
(f) Chl
Ilm
Ms
Mnz
Ilm Sercite Chl
Ms Chl
Ilm
Fig. 3.
Fig. 3. BSE images showing mineral phases that enclose monazite grain (Mnz) are consistent with biotite, muscovite, chlorite, quartz and ilmenite based on energy dispersive X-ray spectra. Monazites in muscovite (a-b), in biotite contacting with quartz (c), in biotite contacting with muscovite (d), in biotite penetrated by muscovite veinlets (e), and in muscovite associated with ilmenite (f) are generally parallel to the foliation of the NE-trending Group 2 (a-d) and the NW-trending Group 3 (e-f) rocks.
57
AC
CE P
TE
D
206
Pb/238U
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
207
Pb/235U Fig. 4.
58
AC
CE P
TE
D
206
Pb/238U
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
207 235 207Pb/235U
Pb/
U
Fig. 4 (Cont.)
Fig. 4. Concordia diagrams and weighted mean 206Pb/238U age (2σ) for zircons from Yunkai samples in four groups: (a-d), (e-i), (j-n) and (o-p). Note that high population of Neoproterozoic zircons is a common feature in Groups 2 and 3 samples. N= number of analyses and MSWD= mean square of weighted deviates. Source data are listed in Appendix 1.
59
ACCEPTED MANUSCRIPT
(d) 23GX04R M09 (e) 23GX04R M15 (a)11GD01S M10 (c) 11GDGZA1S (b)11GDGZA1R 233 (5.48) 228 (7.78) 231 (7.67) 231 (6.77) 248 (8.27) 231 (4.44) 246 M09 232 (6.38) M08 236 M (5.84) (7.23) 247 1 (7.15) 237 (4.39)
259 (11.91) 265 (12.05)
243 (7.14) 233 (6.99) 240 (5.80) 435 (7.29) 427 (7.13) 437 (4.47) 441 (7.28)
D
TE
232 (5.59) 234 (5.87)
246 (1.99)
232 (2.24)
235 (8.32) 433 (6.80) 229 (8.22) 443 (6.87) (r) 28GX20R M03 (q) 11GD05R M06 237 Ap (7.95) 249 (7.29)
431 (5.40) 438 (5.42)
239 (6.85) 228 (6.02) 238 (6.74) 234 (6.10) 236 (6.13)
238 (5.76) 241 (6.42) 249 (6.60)
247 (s) 28GX20R (6.46) 224 M07 (5.26)
230 (5.71) 221 (5.67)
Allanite
Fig. 5.
AC
CE P
228 (5.81)
253 224 (5.23) (6.08) 234 (5.28)
(p) 11GD04R (o) 11GD04R 242(2.23) Kfs M03 M40
245 (1.80) 240 (1.78) 234 (1.90)
Huttonite
(l) 11GD06S M11 (m) 11GD07R M07 (n) 11GD03R M01 Huttonite 250 (5.55) 235 (9.06) 441 (7.37)
(k) 11GD06R M04 236 (7.67)
234 (6.05) 238 (6.49)
Ap
244 (4.61)
226 (5.82)
MA
(j) 23GX15R 240 (5.59) M54
SC R
445 (8.11) 440 (10.61) 439 (4.38) 437 (4.24) 445 (5.93)
250 (7.94)
230 (5.39) (f) 23GX04S M31 (g) 23GX04S M15 (i) 23GX15R M25 (h) 23GX05-1R M08 233 (4.45) 239 (10.43) 439 (21.45)
NU
235 (8.25)
IP
225 (5.90)
T
0
Fig. 5. BSE images of monazite in the polished section (R) and heavy concentrate (S) of the representative samples, with the EMP age results (Ma) and ThO2 contents (wt% in the parenthesis). Although different BSE domain and zonation, morphological feature, and degree of alteration are shown, they invariably present Triassic ages in all samples, including the overgrowing rim of two grains (d and o) that exhibit an Early Paleozoic core.
60
CE P
Fig. 6. EMP monazite isochron and the weighted mean age results for four groups of rocks in the Yunkai massif (a-d for Group 1, e-i for Group 2, j-n for Group 3 and o-p for Group 4). N= number of analyses and MSWD= mean square of weighted deviates. Source data are listed in Appendix 3 (slide is equal to polished section and sand is equal to heavy concentrate in the text).
AC
ig. 6.
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
61
ACCEPTED MANUSCRIPT
(a) 11GD05 NM19
(b) 11GD06 NM03 232 Ma (5.53, 1.68) 246 Ma (5.45, 1.67)
T
427 Ma (5.31, 2.23) 417 Ma (4.63, 2.67) 413 Ma (4.61, 2.72) 421 Ma (5.95, 2.42)
Z
IP
409 Ma (Pb-Pb)
(c) 23GX11 NM08
(d) 23GX04 NM06
MA
379 Ma (U-Pb) 379 (U-Pb) 435 Ma Ma (6.29, 1.18) 424 Ma (6.31, 1.16)
TE
CE P
553 Ma (14.41, 1.17) 563 Ma (13.11, 1.31)
D
545 Ma (11.00, 1.30)
503 Ma (U-Pb) 562 Ma (Pb-Pb)
(e) 11GD01 NM14
AC
429 Ma (6.67, 1.06) 443 Ma (6.92, 1.28)
NU
234 Ma (7.17, 1.11) 236 Ma (7.18, 1.14)
SC R
393 Ma (U-Pb) 425 Ma (Pb-Pb)
Z
Z
236 Ma (6.12, 1.58) 237 Ma (6.14, 1.57)
238 Ma (U-Pb) (f) 11GD01 NM15
222 Ma (4.97, 2.37) 209 Ma (4.91, 2.41) 210 Ma (4.85, 2.09) 226 Ma (5.00, 2.45)
229 Ma (4.04, 2.16)
224 Ma (U-Pb)
219 Ma (U-Pb)
229 Ma (5.51, 2.48)
218 Ma (U-Pb)
228 Ma (5.77, 1.85)
214 Ma (U-Pb) Fig. 7.
Fig. 7. BSE images of discrete monazite grains of some representative samples on which the EMP ages, with ThO2 and Y2O3 contents (wt% in the parenthesis), and NanoSIMS U-Pb and Pb-Pb ages are compared. Z: zircon in monazite.
62
ACCEPTED MANUSCRIPT (a) 23GX04S M15
237 Ma (6.75, 3.80) 231 Ma (6.74, 3.96) 246 Ma (6.83, 3.70)
(c) 23GX04R M22 232 Ma (6.31, 4.59) (Th,U)O4 227 Ma (6.30, 4.67) 230 Ma (5.53, 3.48) 224 Ma (5.48, 3.54)
2
T
1
IP
1
244 Ma (4.61, 3.42)
233 Ma (4.45, 3.73)
SC R
3
(Th,U)SiO4
3
(d) 11GD07R M01
(b) 11GD04R M30
1
MA
2
NU
Huttonite
231Ma (2.09, 3.68)
D
3
1 242 Ma (7.69, 1.90) 239 Ma (7.44, 1.85) 239 Ma (7.05, 1.43)
Fig. 8.
CE P
TE
221 Ma Ma (1.99, (1.93, 3.24) 3.22) 223 223 Ma (1.99, 3.24)
3
AC
Fig. 8. BSE images of monazite alteration zonations including 1: unaltered core (mnz 1), 2: intermediate monazite (mnz 2), and 3: the outer zone (mnz 3). The mnz 3 is characterized by the presence of abundant huttonite (the Th- and U-rich silicate with a bright reflection). The mnz 2 may be present (a-b) or absent (c-d) when the pristine monazite (mnz 1) was subjected to alteration probably involving fluid attacks. Numbers in the parenthesis are EMP ages (Ma) with ThO2 and Y2O3 contents (wt%).
63
ACCEPTED MANUSCRIPT Ab
(a) 11GD04
Bt Chl
IP
M10
T
Synchysite
Bt
SC R
M12
Kfs
Mag
Ab Kfs
Ilm (c)
M11 Ab Ms
(d) M12
D
(b) M10
MA
Ms
NU
Bt
Zr
246Ma (2.20)
230Ma (1.95) 227Ma (1.88)
CE P
227Ma (1.93) 242Ma (1.50)
TE
M11
229Ma (2.45) 238Ma (2.19) 243Ma (1.92)
Fig. 9-1.
AC
Fig. 9-1. Monazite inclusions with the EMP age results (Ma) and ThO2 contents (wt% in the parenthesis) in K-feldspar and plagioclase from a biotite gneiss (sample 11GD04).
64
ACCEPTED MANUSCRIPT Zr Pl
(a) 11GD01
Ilm
ap
Zr
Ms
Bt
T
Qtz
Ms
Ilm
M05
M08
Pl
M03
SC R
M04
M07 M06
Qtz
MA
NU
Bt
CE P
(d) M07
AC 239 Ma (6.03)
Pl
224 Ma (5.68)
223 Ma (5.22)
TE
225 Ma (5.78)
Kfs
212 Ma (6.80)
D
(b) M03
250 Ma (5.81)
Pl
IP
Zr
(e) M06
(c) M04
224 Ma (6.45) 235 Ma (5.13)
(f) M05
249 Ma (5.88)
241 Ma (4.80)
224 Ma (7.02) 228 Ma (6.08)
Fig. 9-2. Fig. 9-2. Monazite inclusions with the EMP age results (Ma) and ThO2 contents (wt% in the parenthesis) in biotite and muscovite from a gneissic granite (sample 11GD01).
65
(f) 23GX11S M24
(c)
Ce (d)
IP
(b)
SC R
1763Ma (5.68, 1.36) 1746Ma (5.63, 1.00)
538Ma (4.71, 3.54) 540Ma (4.67, 3.57) 541Ma (4.87, 3.89) 541Ma (4.85, 2.72) 536Ma (4.78, 3.67) 533Ma (6.46, 2.60) 543Ma (5.23, 3.49) 1755Ma (6.59, 0.85) 1728Ma (6.73, 0.92) 1748Ma (6.24, 0.79) 443Ma (5.00, 3.69) 446Ma (5.40, 1.39) 436Ma (5.15, 4.29) 438Ma (6.28, 2.58) 1743Ma (5.82, 0.76) 1769Ma (5.73, 0.94)
U
Th (e)
Y
NU
(a) 23GX11S M03
T
ACCEPTED MANUSCRIPT
(g)
(h)
Ce
Th
(i)
(j)
U
Y
MA
220Ma (7.29, 1.22) 232Ma (7.31, 1.18) 235Ma (8.36, 1.84)
TE
D
458Ma (6.44, 2.09) 458Ma (6.08, 2.96) 459Ma (6.44, 2.30)
CE P
257Ma (13.81, 1.10) 251Ma (14.32, 1.19)
Fig. 10.
AC
Fig. 10. BSE images with results of spot analysis of EMP apparent ages (Ma) and ThO2 and Y2O3 contents (wt% in the parenthesis), and CeLα, ThMα, UMα and YLα X-ray compositional maps of two representative monazites from sample 23GX11 (S denotes samples from heavy concentrates). (a-e) The presence of ~1750, ~540, and ~440 Ma domains (BSE) in grain M03; (f-j) ~450, ~260 and ~230 Ma domains in grain M24, in which the ~450 Ma domain is relatively low in Ce and Th. The color bar in (e) is equally applied for all X-ray maps.
66
Fig. 11.
AC
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
Fig. 11. Cumulative age probability curves of studied rocks from the Yunkai massif. (a) EMP monazite ages of a meta-sandstone (sample 23GX11), and (b) total U-Pb zircon ages with histograms of four groups of rock. Resemblance of two curves with peaks at the Neoproterozoic and Early Paleozoic indicates the common provenance for monazite and zircon.
67
Fig. 12
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
Fig. 12. Schematic sketch showing the now-exposed Early Paleozoic metamorphic terrains (dark) and Triassic reactivated belts (hatched) in the South China and Indochina Blocks in a position relative to the distribution of Permo-Triassic Emeishan large igneous province (E-LIP; Shellnutt, 2014). An inferred boundary of E-LIP influence in the deep crust (the dashed line in red) is constrained by the Triassic high-temperature granulite xenoliths and enclaves in Daoxian (DX) basalts (Li et al., 2016) and Darongshan (DRS) granitoids (Chen et al., 2011; Zhao et al., 2012), respectively. Metamorphism triggered by the same heat source form E-LIP and collision of these two Blocks along the Ailaoshan-Song Ma suture (in green) might have affected the Triassic rework of Song Chay Massif (SCM) and Yunkai massif (YKM), and build-up of the Xuefengshan Belt (XFSB) in South China Block, and Kontum massif (KTM) and Truong Son Belt (TSB) in Indochina Block at various extents.
68
ACCEPTED MANUSCRIPT Table 1. Description of studied samples from the Yunkai massif Sample No. Latitude
Longitude
Locality
Lithology
Remarks
22°25’44.6” 110°31’08.1” Longsun-Gaozhou highway
Biotite gneiss
D*= N18E
23GX05-2
22°25’44.6” 110°31’08.1” Longsun-Gaozhou highway
Biotite gneiss
D*= N18E
23GX11
220031.3” 1110350.4” Near the Gaozhou reservoir
23GX13-1
222754.0” 1105652.1” Highway 207 at 3355.8 km
Meta-sandstone Garnet-tourmaline schist
23GX13-2
222754.0” 1105652.1” Highway 207 at 3355.8 km
23GX14
223257.4” 1105646.5” Highway 207 at 3344.9 km
23GX15 28GX20
IP
D*= N32W No monazite D*= N32W
221635.0” 1101043.7” Near the Lutou reservoir
Two-mica gneiss
D*= N10E
2316’02.0” 11106’06.1” Road 187 near the 10 km mark 2254’38.1” 11047’10.5” Dahe village in Yunrong Town (south to Xindi)
Undeformed granite Yin-nan batholith
SC R
Biotite gneiss Garnet-tourmaline schist
NU
28GX22
T
23GX04
Undeformed granite Beiliu batholith
11GDGZA1 22°00’36.2” 111°04’10.2” Longshou village in Yunlu town Gneissic granite 22°02’06.4” 111°03’04.0” Road between Shilong-Changpo Gneissic granite
11GD02
22°12’40.0” 111°07’18.7” Shuikou village
11GD03
22°32’56.8” 110°56’45.8” Highway 207 at 3344 km
Gneissic granite Garnet-tourmaline schist
11GD04
22°29’57.2” 110°56’13.2” Highway 207 at 3350 km
Biotite gneiss
11GD05
22°27’41.5” 110°57’08.3” Highway 207 at 3355-56 km
Biotite gneiss
11GD06
22°13’19.1” 110°35’44.9” Baoxu-Liujin
Two-mica gneiss
11GD07
22°18’26.4” 110°33’07.5” Liujin village
Gneissic granite
No monazite
CE P
TE
D
MA
11GD01
AC
D*: orientation of deformation in the field.
69
ACCEPTED MANUSCRIPT Table 2. Percentage of constituent minerals and information of monazite inclusions of the studied rocks.
Sample No.
Qtz
Pl
Kfs
Bt
Chl
11GD01
35
25
10
20
3
Ap
Grt
Ep
Tur
11GD02
40
30
10
11GDGZA1
38
30
12
(1)
(2)
(1)
23GX11
60
(3)
(6)
11GD07
40
12
(1)
(4)
(1)
35
15
20
25
(10)
(3)
(3)
(13)
35
25
31
(6)
(11)
(15)
23GX15
40
10
(4)
(2)
11GD06
35
Group 2 Gneissic granite
Bt gneiss
23GX04
TE
(6)
23GX05-2
CE P
(6)
AC
Two-mica gneiss
Grt tur schist
30
(2)
Bt gneiss
6
<3
<1
9
<3
<2
10
<1
<1
<2
15
<1
15
5
NU 30
(10)
2
3 (3)
<3
<3
<3
23
2
20
5
(32)
(1)
(18)
8
20
5
15
<2
(1)
(2)
<2
<2
25
25
<1
8
<2
<1
5
4
<2
12
<2
(1)
(4)
23GX13-1
30
25
5
20
5
23GX14
22
8
<2
12
15
(1)
(1)
11GD04
<3
30
45
12
(16)
(1)
(8)
(10)
(6)
11GD05
35
20
15
6
23GX13-2
<3
(1)
(2)
(9)
2
(7)
25
(4)
11GD03
16
MA
Meta-sandstone
5
(20)
D
(1)
IP
Gneissic granite
SC R
Group 1
Group 3
Ms
T
Lithology
(3)
25 (8)
5
4
10
15
<2
18
3
<1
<1
<3
(2)
40
30
<2
(3)
70
ACCEPTED MANUSCRIPT Group 4 15
18
25
(7)
(3)
(4)
(5)
(4)
28GX22
40
10
30
14
(1)
(2)
(1)
(1)
(4)
<3
<2
<2
<1
<3
<1
<1
T
34
IP
Undeformed granite 28GX20
SC R
1. Abbreviation of mineral names follows those described by Whitney and Evans (2010).
AC
CE P
TE
D
MA
NU
2. Number in the parenthesis (italic) indicates grains of monazite inclusions enclosed by this mineral or in the matrix of this sample.
71
ACCEPTED MANUSCRIPT Table 3. Comparison of EMP and NanoSIMS ages on same BSE domain in monazites#. EMP
NanoSims
UO2 Y2O3 PbO ThO2* (wt %) (wt %) (wt %) (wt %)
Age
U-Pb Error (2s)
206* 238
/
Age
IP
(Ma)
Error (2s)
207 206*
/
Error (2S)
0.0549
0.0005
409
20
(Ma)
0.84 0.95 0.71 0.87
2.51 1.13 2.30 0.59
0.140 0.102 0.159 0.092
7.867 10.236 8.569 9.258
419.4 234.8 437.3 235.3
30.8 17.3 30.6 22.7
0.0569 0.0011
356.8
6.7
0.0579 0.0011 0.0325 0.0006
362.8 206.1
6.8 3.9
6.33 5.34 6.80 5.49
1.06 0.82 0.53 0.68
0.65 2.22 1.17 1.68
0.100 0.083 0.158 0.078
9.765 7.998 8.538 7.693
242.4 245.7 436.0 238.6
13.9 23.7 31.6 20.3
0.0473 0.0319 0.0629 0.0621
0.0010 0.0006 0.0015 0.0013
298.2 202.6 393.1 388.2
6.2 4.0 9.1 8.2
0.0575
0.0006
513
24
0.0553
0.0005
425
20
13.76 11.00 6.30 6.21 6.86
1.00 0.69 0.22 0.18 0.14
1.24 1.30 1.17 1.39 1.65
0.410 0.308 0.128 0.127 0.133
17.063 13.288 7.023 6.781 7.320
557.9 544.7 429.5 441.3 428.3
31.5 40.0 31.6 32.4 31.5
0.0812 0.0019
503.1
11.5
0.0589
0.0005
562
19
0.0606 0.0012 0.0589 0.0012 0.0598 0.0012
379.1 369.1 374.4
7.6 7.2 7.3
2.80 5.44 6.13 NM06 11GDGZA1 4.61 NM09-1 8.57 NM09-2 5.37 NM11
0.49 1.19 0.91
1.54 1.63 1.58
0.044 0.093 0.091
4.387 9.295 9.062
234.8 237.3 236.5
22.7 22.9 21.5
0.0277 0.0007 0.0268 0.0006 0.0376 0.0008
175.8 170.4 238.2
4.6 4.0 5.0
0.0516
0.0010
267
42
0.26 0.43 0.29 0.16 0.45 0.66
2.25 1.77 1.82 1.60 1.49 2.29
0.053 0.105 0.062 0.044 0.132 0.126
5.431 9.964 6.309 4.678 13.208 7.083
231.2 249.4 230.9 222.8 236.6 418.5
21.0 18.3 22.5 21.5 17.4 31.8
0.0309 0.0006 0.0314 0.0006 0.0300 0.0007
195.9 199.1 190.8
4.0 4.0 4.2
0.0336 0.0007 0.0597 0.0013
212.8 373.7
4.3 8.2
0.0548
0.0006
403
24
0.54 0.57 0.51 0.56 0.61 0.61
0.061 0.071 0.055 0.073 0.069 0.074
6.687 7.278 5.691 7.583 7.164 7.684
216.9 228.6 228.9 228.0 228.1 228.1
20.9 21.9 22.1 22.0 22.0 22.0
0.0346 0.0344 0.0353 0.0337 0.0330 0.0327
219.2 217.8 223.6 213.4 209.4 207.6
11.5 11.1 11.4 4.9 4.4 4.9
NM19-1 NM19-2 NM21-1 NM21-2
NM02-1 NM02-2 NM03-1 NM03-2
NM08-2 NM08-3 NM16-1 NM16-2
D
NM08-1
NM04-2
4.16
11.75 4.93
NM18-3
11GD01 NM14-1 NM14-2 NM15-1 NM15-2 NM20-1 NM20-2 #
AC
NM18-1 NM18-2
4.93 5.51 4.04 5.77 5.19 5.71
2.33 2.48 2.16 1.85 2.23 2.22
TE
23GX04 NM04-1
MA
23GX11
NU
11GD06
SC R
5.43 7.18 6.25 6.44
CE P
11GD05
Pb-PbA ge Error (2s) (Ma)
Error (2s)
T
Smpl. no. ThO2
0.0018 0.0018 0.0018 0.0008 0.0007 0.0008
Examples showing two age groups and BSE domains in one single grain see Fig. 6.
EMP compositions listed here are the average of 1-4 spot analysis of the original data (Appendix 4). 206*/238=
206
Pb*/238U ratio and 207/206*= 207Pb/206Pb* ratio, in which 206Pb*= radiogenic 206Pb.
Pb-Pb ages with error >20% are not included.
72
ACCEPTED MANUSCRIPT Table 4. Record of monazite and zircon age clusters on rocks from the Yunkai massif. Sample no.
Adopted Age (Ma)
EMP Monazite Age-1 (Ma)
MS WD
EMP Monazite Age-2 (Ma)
MS WD
LA-ICPMS Zircon Age (Ma)
MS WD
N
11GD01
232.84.3
23313 (n=58)
0.025
23115 (n=38)
0.029
437.45.5 (n=22)
1.70
27
11GDGZA1
232.85.2
23018 (n=30)
0.038
23416 (n=38)
440.64.1 (n=23)
0.41
27
11GD02
n.a..
0.54
28
23814
0.030
(No mnz) ~1760, ~860, ~540, ~440, ~250
439.64.5 (n=18)
23GX11
(No mnz) 23814 (n=10) ~437, ~370
443.58.9 (n=5)
0.10
41
22910 (n=7)
5.3
30
98927 (n=16)
5.1
22
95038 (n=9)
5.1
22
435.05.1 (n=12)
0.79
31
22910
235.55.5 (n=67)
0.026
23GX05-2
234.1±5.4
234.1±5.4 (n=58)
0.16
23GX15
235.54.1 239.74.1 435.05.1
235.54.1 (n=102)
23311
23311 (n=14)
23GX13-1
n.a.
23GX13-2
23815
(No mnz) 23815 (n=9) ~433, ~362
23GX14
235.98.2
11GD03 11GD04
11GD07
23411 (n=17)
0.073
n.d.
n.d. 240.84.4 (n=66) 44128 (n=5)
0.17 0.021
n.d.
0.12
n.d.
235.98.2 (n=27)
0.097
n.d.
90432 (n=10)
24411
24411 (n=14)
0.035
(No mnz)
No age cluster
245.15.4 239.25.7 435.05.1
236.04.6 (n=105) 236.17.2 (n=21) 43616 (n=15)
0.14 0.24 0.047
(No mnz) 244.59.3 (n=18) 438.18.8 (n=51)
245.15.4 (n=5)
0.26
42
0.22 0.069
431.35.0 (n=10)
0.66
30
28GX20
232.14.4
233.16.2 (n=50)
0.27
23016 (n=34)
0.032
250.93.3 (n=16)
1.6
25
28GX22
229.28.4
229.28.4 (n=21)
0.069
n.d.
247.72.3 (n=18)
0.43
23
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Group 4
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11GD05
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Group 3
0.091
0.065 0.065
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11GD06
0.21
0.026
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236.88.7 (n=53) 440.710.0 (n=12)
23GX04
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Group 2
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Group 1
No age cluster
(No mnz)
100420 (n=20)
Monazite age-2: obtained from monazite in heavy concentrates. Zircon age: obtained from zircon in heavy concentrates (N= total number of zircon spot analysis). MSWD= mean square of weighted deviates.
9.1
30
4.6
33
n.d.
Monazite age-1: obtained from interstitial monazite and inclusions in major phases from petrographic slide.
35
n.a.: not available; n.d.: not determined.
73
30
S0040-1951(16)30540-6 doi: 10.1016/j.tecto.2016.11.022 TECTO 127324
To appear in:
Tectonophysics
Received date: Revised date: Accepted date:
16 May 2016 27 October 2016 14 November 2016
Please cite this article as: Chen, Cheng-Hong, Liu, Yung-Hsin, Lee, Chi-Yu, Sano, Yuji, Zhou, Han-Wen, Xiang, Hua, Takahata, Naoto, The Triassic reworking of the Yunkai massif (South China): EMP monazite and U-Pb zircon geochronologic evidence, Tectonophysics (2016), doi: 10.1016/j.tecto.2016.11.022
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ACCEPTED MANUSCRIPT The Triassic reworking of the Yunkai massif (South China): EMP monazite and U-Pb zircon geochronologic evidence
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Cheng-Hong Chen*1, Yung-Hsin Liu2, Chi-Yu Lee1, Yuji Sano1,3, Han-Wen Zhou4, Hua Xiang5, Naoto Takahata3
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1. Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan 2. Institute of Earth Sciences, Academia Sinica, Taipei 11529, Taiwan 3. Department of Chemical Oceanography, Atmosphere and Ocean Research
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Institute, The University of Tokyo, Kashiwa-shi, Chiba 277-8564, Japan 4. State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences (Wuhan), Wuhan 430074, China 5. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
Revision 2 submitted to Tectonophysics
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(October 25, 2016)
*Corresponding author: Cheng-Hong Chen Mailing address: Department of Geosciences, National Taiwan University, No.1, Roosevelt Road Section 4, Taipei 106, Taiwan
Tel.: (+886)-2-3366-5872 Fax: (+886)-2-2363-6095 E-mail: [email protected]
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ACCEPTED MANUSCRIPT
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Highlights:
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1. Yunkai massif is dated by using monazite (EMP and Nano-SIMS) and zircon
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(LA-ICPMS).
2. Monazite inclusions in the platy minerals exclusively yield Triassic ages. 3. Deformations (NW- and NE-trending) in Yunkai massif occurred at ~245 Ma
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and ~230 Ma.
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4. South China-Indochina collision and mafic magma emplacement may cause
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these deformations.
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ACCEPTED MANUSCRIPT Abstract Geohistory of the Yunkai massif in South China Block is important in
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understanding the geodynamics for the build-up of this block during the Phanerozoic
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orogenies. To investigate this massif, we conduct EMP monazite and U-Pb zircon
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geochronological determinations on mineral inclusions and separates for seventeen samples in four groups, representing metamorphic rocks from core domain, the Gaozhou Complex (amphibolite facies, NE-striking) and the Yunkai Group
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(greenschist facies, NW-striking) of this massif and adjacent undeformed granites. Monazite
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Some EMP monazite ages are consistent with the NanoSIMS results.
inclusions, mostly with long axis parallel to the cleavage of platy and elongated hosts,
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give distinguishable age results for NW- and NE-trending deformations at 244-236
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Ma and 236-233 Ma, respectively. They also yield ages of 233-230 Ma for core domain gneissic granites and 232-229 Ma for undefomed granites.
Combining U-Pb
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zircon ages of the same group, ~245 Ma and ~230 Ma are suggested to constrain the time of two phases of deformation.
Aside from ubiquity of Triassic ages in studied
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rocks, ages of detrital monazite in the meta-sandstone match the major U-Pb zircon age clusters of the metamorphic rock that are largely concentrated at Neoproterozoic (1.0-0.9 Ga) and Early Paleozoic (444-431 Ma).
Based on these geochronological
data, Triassic is interpreted as representing the time for recrystallization of these host minerals on the Early Paleozoic protolith, and the also popular Neoproterozoic age is probably inherited. With this context, Yunkai massif is regarded as a strongly reactivated Triassic metamorphic terrain on an Early Paleozoic basement which had incorporated sediments with Neoproterozoic provenances.
Triassic tectonic
evolution of the Yunkai massif is suggested to have been controlled by converging geodynamics of the South China and Indochina Blocks as well as mafic magma emplacement related to the Emeishan large igneous province (E-LIP). 3
ACCEPTED MANUSCRIPT
(Keywords: Yunkai massif, EMP monazite age, Indosinian orogeny, Emeishan large
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igneous province)
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ACCEPTED MANUSCRIPT 1. Introduction South China and Indochina blocks are two continental pieces welded together
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after the closure of the Paleo-Tethys Ocean (e.g. Roger et al., 2014 and references
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therein). This geodynamic event took place during the Early to Middle Triassic
close to the suture zone.
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resulting in the formation of metamorphic rocks and generation of magmatic rocks The entire process of amalgamation is commonly referred
to as Indosinian orogeny in both the blocks.
Various age dating systems have been
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applied to know the timing of syn-tectonic metamorphism and post-orogenic Although the U-Pb
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magmatism for elucidating the geodynamics of this orogeny.
zircon age dating becomes popular, its application to orogenic metamorphic and
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magmatic rocks may encounter difficulties when recrystallization or melting of Yunkai
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pre-existing crustal materials is highly involved with inherited zircons. massif in the southern South China may be one of the cases.
China Block.
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Yunkai massif has long been regarded as a Precambrian basement in the South Recently, based on a large number of age results concentrating in the
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Early Paleozoic for granitic and metamorphic rocks in this massif, a concept of the Early Paleozoic Wu-Yun metamorphic belt (orogen) that spreads from the Wuyishan massif in the northeast, through the Baiyun massif, and further extends southwesterly to the Yunkai massif (Fig. 1a), was established (Lin et al., 2008; Faure et al., 2009; Li et al., 2010; Yang et al., 2010; Wang et al., 2011; Yao et al., 2012; Chu et al., 2012b; Wang et al., 2013).
The most significant evidence came from a reconnaissance on
gneissoid granite samples that gives rise to similar zircon U-Pb dates of 457-410 Ma for Wuyishan and 450-415 Ma for Yunkai massifs, respectively (Wang et al., 2011). Along with some extrusive rocks in the Baiyun massif (ca. 435 Ma), the importance of the Early Paleozoic event for the development of this orogen was reinforced (Yao et al., 2012). 5
ACCEPTED MANUSCRIPT However, 40Ar/39Ar mineral dating (mostly biotite) shows different results. For example, thirteen biotite analyses on granite, gneiss and migmatite over the entire
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Yunkai massif, although mostly mylonitized, all yielded Triassic ages (Wang et al.,
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2007a). Further by compiling multi-system geochronology on the Yunkai massif,
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Chu et al. (2009) indicated that ages obtained from the 40Ar/39Ar mineral dating (biotite and muscovite) for high-grade metamorphic rocks are again exclusively Triassic.
The Triassic age given by these minerals may be meaningful to account for
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the complete reset of mineral ages by the mylonitization near shear zones, but difficult
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to apply to a cooling history for the transformation from the Early Paleozoic prograde- to the Triassic retrograde-metamorphism in the Yunkai area (e.g. Wang et
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al., 2012). One more concern is the existence of thin Triassic-aged rim on the Early
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Paleozoic zircon core in some gneissic granites (Wang et al., 2007b; Chen et al., 2012) and supracrustal gneisses (Wan et al., 2010) within the massif.
To date the only
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explanation is ascribed to “overprinting of the Indosinian orogeny”, yet details of overprinting are not known.
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The use of some other dating methods would be helpful to provide more age constraints, and the EMP monazite age determination is an easy and efficient one for this purpose (e.g. Catlos, 2013).
This technique has been applied to granitic plutons
in the neighboring Darongshan area and high-grade metamorphic rocks (granulite and charnockite) near the Gaozhou Reservoir previously, by which large amounts of the EMP monazite ages of ~230 Ma and ~440 Ma were obtained (Chen et al., 2011; 2012). For this study, we chose some representative metamorphic and granitic rocks from Yunkai massif to demonstrate the powerfulness of using the EMP monazite dating to tackle with the petrographic relationship and age results. U-Pb zircon ages are also determined for same samples to provide constraints from direct comparisons.
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ACCEPTED MANUSCRIPT 2. Geological backgrounds and samples South China Block is composed of Yangtze and Cathaysia sub-blocks (Fig. 1a)
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with the characterization of Phanerozoic granitic rocks exposing in the Cathaysia.
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Ages for these granitic rocks are concentrated at Early Paleozoic, Late Permian-Early
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Triassic, Jurassic and Cretaceous time, marking the episodic thermal events on this terrain (e.g. Zhou and Li, 2000). The Yunkai massif is bounded by the WuchuangSihui fault in the east and the Wuzhou-Bobai fault in the west with a total area of It is a conventional thought that this massif is built up by
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about 2,500 km2 (Fig. 1b).
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the Paleo- to Mesoproterozoic high-grade basement rocks that had been modified by Neoproterozoic and Early Paleozoic thermal events, the Late Neoproterozoic to Early
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Paleozoic low-grade basement rocks, and the cover rocks composed of slightly
1996).
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metamorphosed to unmetamorphosed Cambrian to Devonian strata (Zhong et al., Lithologies of the exposed basement rocks in this massif mainly include
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gneissic granites, migmatites and orthogneisses in the center, and supracrustal paragneiss, amphibolite, schist, quartzite and marble (roughly the metapelite
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assemblage) relatively in the peripheral of the massif. Some of these rocks have later been intruded by the Triassic, Jurassic and Cretaceous granitoids.
Recently, a
short time span (443 to 430 Ma) was suggested for the formation of these gneissic granites and supracrustal rocks as constrained by the youngest zircon U-Pb age clusters, and the also abundant older ages (mainly 1.1 to 0.8 Ga) were attributed to the inherited ones (Wan et al., 2010). The Yunkai massif is composed of two tectonic units: the Gaozhou Complex and Yunkai Group. Although the Yunkai Group is loosely defined, we retain the usage of this term same as those in Wan et al. (2010). The former includes the predominant weakly-deformed gneissic granite and accompanying migmatite, orthogneiss and even granulite facies rocks in the core domain, and amphibolite facies metamorphic rocks 7
ACCEPTED MANUSCRIPT in the surroundings, whereas the latter consists mainly of greenshist to amphibolite facies paragneiss and schist (Fig. 1b). The Yunkai massif has recorded four phases
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of deformation at Early Paleozoic, Early Mesozoic, Jurassic-Cretaceous and Late
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Mesozoic (Lin et al., 2008), in which the first two phases are concerned in this study.
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According to these authors, the main deformation is the second one that the NE-directed shearing occurred at the Early Mesozoic leading to the NNE-trending lineation for quartz, biotite, muscovite and sericite basically in the mica schist, Prior to this, the deformation is
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paragneiss and orthogneiss of the Yunkai Group.
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characterized by a NW-trending mineral lineation, represented by crystallization of muscovite, biotite, amphibole, epidote and quartz, in few gneissic rocks of the core
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domain and Gaozhou Complex. However, this deformation event is not well dated,
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although the Early Paleozoic ages have been obtained from some weakly foliated orthogneisses and undeformed charnockites (Wang et al., 2007a; Chen et al., 2012).
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Therefore, dating of monazite enclosed by rock-forming minerals subjected to these two phases of deformation is envisaged.
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In this study, four groups of sample on the basis of rock type and metamorphic grade were collected from the Yunkai massif (Fig. 1b): gneissic granites and associated meta-sedimentary rocks in the massif’s center or core domain (Group 1), various types of gneiss (amphibolite facies) in the Gaozhou Complex (Group 2), gneisses and schists (greenschist facies rocks in the Xinyi area near the major Luoding-Yuocheng shear zone) in the Yunkai Group (Group 3), and undeformed Triassic granites adjacent to this massif (Group 4) (Tables 1 and 2). The Triassic S-type Darongshan granites (Chen et al., 2011) to the west of the Wuzhou-Bobai fault (Fig. 1b) are used as references of undeformed granites.
Basic information for
sample localities is described in Table 1.
8
ACCEPTED MANUSCRIPT 3. Analytical methods Rock samples were crushed and sieved for collecting zircon and monazite grains
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through the conventional magnetic and heavy liquid separation procedures, in the
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hope of distinguishing detrital and metamorphic zircon and monazite populations.
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About 50 grains of randomly selected zircon and monazite were casted in epoxy resin and polished for U-Pb zircon and EMP monazite age determinations.
In order to
characterize the zonings of zircon and internal domains of monazite, cathode
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luminescence (CL) images for zircon were obtained using a JEOL JSM-6360LV
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scanning electron microscope attached with a panchromatic CL imaging system (Gatan Mini-CL) at National Taiwan University (NTU), and secondary electron (SE)
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and back-scattered electron (BSE) images for monazite were taken using a JEOL
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electron microscope model JSM-7100F (FE-SEM) equipped with an independent energy dispersive spectrometer at the Institute of Earth Science (IES), Academia
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Sinica (Taipei). X-ray mapping of monazite grains were conducted by using a JEOL electron microprobe analyzer model JXA-8900R equipped with four channels of wavelength dispersive spectrometer at IES.
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U-Pb isotopic ratios of zircons were determined on ~40 m diameter spot by an excimer laser ablation-induced couple plasma mass spectrometer (LA-ICPMS) at NTU. The ICPMS is an Agilent 7500s coupled with the UP213 laser ablation system (New Wave Research/Merchantek). 208
207
Pb/206Pb, 206Pb/238U, 207Pb/235U and
Pb/232Th ratios were calculated using the GLITTER 4.0 software (Macquarie
University) and the GJ-1 (Jackson et al., 2004) as an external standard, and common lead corrections were carried out using the procedure described by Andersen (2002). This dating system has been tested by running with the Harvard zircon 91500 and Mud Tank zircon (10614.0 Ma and 7313.9 Ma, respectively; Jackson et al., 2004), and results are in good agreement with the recommended ages (Chiu et al., 2009). 9
ACCEPTED MANUSCRIPT The U-Pb ages (478 in total number, Appendix 1), concordia diagrams, and age frequency curves for the age population were obtained by using the ISOPLOT In this study, weighted mean 206Pb/238U ages of <1000 Ma
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program (Ludwig, 2007).
Statistics of U-Pb zircon ages of dated grains are shown in Appendix 2.
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al., 1992).
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and 207Pb/206Pb ages of >1000 Ma are adopted for concordant zircons (Compston et
Polished slides (one inch diameter) were also made for petrographic observations and mineral composition analysis.
In fact, in-situ EMP monazite age determinations
We focused more on monazites enclosed by different
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during mineral separation.
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were mainly relied on these slides to avoid the problem of age bias due to grain size
host minerals than those setting in the interstices (or heavy concentrates), because age
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interpretations from metamorphic monazite usually require good control of its High resolution BSE images are also
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paragenesis (e.g. Kohn and Malloy, 2004).
referred to minimize analyses of overlapping domain by the electron beam and to
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reveal possible compositional variations between domains from which main age populations for these monazites were calculated.
The instrument used for the EMP
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monazite dating is a Shimadzu-ARL electron microprobe model EMX-SM equipped with four channels of wavelength-dispersive spectrometers at NTU. This allows simultaneous analysis of Th, U, Pb and Y at the same spot. Apparent ages are calculated by using the scheme of Suzuki and Kato (2008), and results of 1158 spot analysis of monazite, along with the detailed analytical procedures (Chen et al., 2006), are listed in Appendix 3. Few discrete monazites with multiple EMP ages were cast into a separate epoxy resin disk for both the EMP and NanoSIMS analyses on the same BSE domain. After EMP measurements (Appendix 4), the monazite mounts were slightly polished and re-coated with gold to dissipate charge during SIMS analysis.
Monazites were
dated by a high lateral resolution secondary ion mass spectrometer, the CAMECA 10
ACCEPTED MANUSCRIPT NanoSIMS NS50 at the University of Tokyo, using a standard monazite with the TIMS U-Pb age of 524.9±3.1 Ma (Sano et al., 2006).
Pb+, 238U16O+, and 238U16O2+ ions were analyzed first by multi-collector system for
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For each spot, 140Ce+, 204Pb+,
204
Counting time was 10 sec for
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numbers 204, 206, and 207 to measure Pb isotopes.
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300 sec. Then the magnet was cyclically peak-stepped through a series of mass
Pb+, 2 sec for 206Pb+ and 10 sec for 207Pb+, respectively, in a single cycle.
Using a
critical illumination mode, a ~1 nA mass filtered O-primary beam was used to sputter
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a 5~7 μm diameter crater and secondary positive ions were extracted for mass Apparent ages for fifty EMP and
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analysis using a Mattauch-Herzog geometry.
twenty-five 238U/206Pb and six 207Pb/206Pb spot analyses of monazite were obtained
4. Petrography
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from this disk (Appendix 4).
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Group 1 samples (11GD01, 11GDGZA1 and 11GD02) are mainly the variably deformed gneissic granites collected from the metamorphosed core domain. Among
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them, sample 11GDGZA1 still preserves a granular texture of granitic protolith and the interstitial biotites are mostly replaced by chlorites (Fig. 2a); whereas the other two samples exhibit gneissic banding defined by orientated biotite in different grain sizes (Fig. 2b). Monazite grains in these samples, sometimes containing apatite inclusions, are commonly enclosed by biotite, quartz and plagioclase.
In addition, a
meta-sandstone (sample 23GX11) near Gaozhou, mainly containing quartz with interstitial K-feldspar, chlorite, epidote and few monazite (Chen et al., 2012), is also included for constraining monazite ages of detrital origin in this area. Group 2 samples include biotite gneisses (23GX04 and 23GX05-2), two-mica gneisses (23GX15 and 11GD06), and gneissic granite (11GD07) collected from the Gaozhou Complex.
These samples are spatially associated with amphibolites, on 11
ACCEPTED MANUSCRIPT which two sets of shearing fabric (NE and NW) are occasionally observed.
Sample
23GX05-2, containing equigranular biotite, plagioclase and quartz (Fig. 2c), is a
Two other samples (23GX04 and 23GX15) also show strong foliations
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(N18E).
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gneissoid enclave enclosed by the gneissic granite with a NE-trending foliation
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(N18E and N10E, respectively) in a way consistent with the regional ductile shearing. The highly sutured quartz grains in all gneiss samples and kink-band muscovite in gneissic granite (11GD07) are considered as the results of ductile deformation Especially, sample 23GX15 is
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corresponding to the high-grade metamorphism.
apatite relicts.
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characterized by blasto-porphyroblastic textured anhedral plagioclase (Fig. 2d) and Monazites are basically absent in these pre-existing minerals, and
This indicates that monazite formation is closely
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gneissic banding (Figs. 3a-d).
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always occurred as inclusions within biotite, muscovite and quartz that define the
related to the ductile shearing developed under the amphibolite facies metamorphism.
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For Group 3 rocks, biotite gneiss and garnet-tourmaline schist are the commonest rock types in the Yunkai Group. They apparently experienced slightly lower Also the NW-trending
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metamorphic grade compared to the Group 2 samples.
foliation (N32W) of sample 23GX13 site (for samples 23GX13-1 and -2) is different from the strain directions recorded by the Group 2 samples. Biotite gneisses show relatively minor extent of shearing deformation.
Sample 11GD04 is characterized
by the granoblastic texture, and the constituent minerals are absent of preferred orientation.
Samples 11GD05 and 23GX13-2 contain plagioclase, ± K-feldspar and
quartz porphyroblasts (Fig. 2e), which are surrounded by fine-grained and orientated minerals (quartz, plagioclase, K-feldspar and chlorite in sample 11GD05 and biotite in sample 23GX13-2).
Abundant chlorites and retrogressive veinlets filled with calcite,
K-feldspar and synchysite (a rare earth carbonate mineral) are evidences of intensive effect of the greenschist facies overprint.
In the garnet-tourmaline schists (samples 12
ACCEPTED MANUSCRIPT 11GD03, 23GX13-1 and 23GX14), muscovite and chlorite are the predominant minerals (Ms= 25% and Chl= 15-25%; Table 2) in samples 11GD03 and 23GX14 to Garnet forms anhedral grains in sample 23GX13-1,
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define the major foliation.
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whereas poikiloblasts (up to ~5 mm) containing abundant quartz inclusions and As for
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biotite and chlorite crosscuts in samples 11GD03 and 23GX14 (Fig. 2f).
monazite occurrence, it can be generalized that monazites are present as unaltered or slightly altered inclusions in biotite, muscovite, quartz or plagioclase in biotite
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gneisses, and as unaltered to highly altered monazite grains in retrogressive muscovite
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and chlorite in garnet-tourmaline schists (Figs. 3e-f).
For the three groups of metamorphic rocks mentioned above, the petrographic
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relationships indicate that formation of monazite involve consumption of apatite and
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preceded REE-rich phases, most probably allanite (Wing et al., 2003; Tomakins and Pattison, 2007; Janots et al., 2008).
Absence of relic allanite in the analyzed samples
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may be due to complete equilibration after the allanite to monazite transition in the middle amphibolite facies (525-600 °C) for a metapelitic basement (Spear, 2010).
In
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addition, aluminosilicates (Al2SiO5) that have been considered as an essential silicate phase for producing monazite in the pelitic schists (e.g., Wing et al., 2003) are absent either.
On the other hand, monazites are commonly associated with aligned
muscovite and biotite or matrix plagioclase in the studied samples (Fig. 3).
These
indicate that the reactions responsible for monazite formation are probably similar to those proposed by Tomakins and Pattison (2007) and Janots et al. (2008) for pelitic rocks: Allanite + Apatite = Monazite + Plagioclase + Magnetite, or Allanite + Apatite + Al-Fe-Mg phases 1 = Monazite + Anorthite + Al-Fe-Mg phases 2 (e.g., muscovite, biotite or chlorite, and garnet). Group 4 granites (28GX20 and 28GX22) are coarse-grained and undeformed as compared to the Group 1 gneissic granites.
Sample 28GX20 mainly consists of 13
ACCEPTED MANUSCRIPT granular quartz, plagioclase, and biotite with interstitial K-feldspar (Fig. 2g), and sample 28GX22 is characterized by abundance of coarse-grained K-feldspar and Monazites
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altered plagioclase, and trace amounts of garnet and rutile (Fig. 2h).
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occur as inclusions evenly distributed in these major constituent minerals.
5. Results 5.1. U-Pb zircon ages
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Here we summarize the statistics of the dating results of zircon (Appendix 2).
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For Group 1 gneissic granites, consistent weighted mean ages of 437.4±5.5 Ma (n=22), 440.6±4.1 Ma (n=23), and 439.6±4.5 Ma (n=18) are obtained for samples
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11GD01, 11GDGZA1, and 11GD02, respectively (Figs. 4a-c). As for the
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meta-sandstone (sample 23GX11), an Early Paleozoic age of 443.5±8.9 Ma (n=5) is also obtained from the youngest age cluster (Fig. 4d) although Neoproterozoic is the
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largest population (21 out of 41 analyses). Group 2 rocks are characterized by a high population of Meso- to Neoproterozoic However, various age clusters are obtained: 229±10 Ma (n=7) for sample
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zircons.
23GX04 (Fig. 4e), 989±27 Ma (n=16) and 950±38 Ma (n=9) for samples 23GX05-2 and 23GX15 (Figs. 4f and g), and 434.9±5.1 Ma (n=12) for sample 11GD06 (Fig. 4h). Although ages close to 450 Ma (n=8) and 250 Ma (n=9) are present in sample 11GD07, they are not counted because of the large discordance (Fig. 4i). Group 3 schists and gneisses have even larger age distribution of Meso- to Neoproterozoic zircons than Group 2 rocks. Age clusters of 1004±20 Ma (n=20) and 904±32 Ma (n=10) are obtained for samples 23GX13-1 and 23GX14, respectively (Fig. 4j and k), and no particular age group is available for sample 11GD03 among 30 wide spreading apparent ages (Fig. 4l).
Like Group 2 samples, younger age clusters
are also obtained: 245.1±5.4 Ma (n=5) for sample 11GD04 (Fig. 4m) and 431.3±5.0 14
ACCEPTED MANUSCRIPT Ma (n= 10) for sample 11GD05 (Fig. 4n). For Group 4 rocks, the Phanerozoic population yields a major age cluster at
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250.9±3.3 Ma (n=16) and a minor cluster at ~440 Ma (n=5) for sample 28GX20 (Fig.
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4o), but only one age cluster at 247.7±2.3 Ma (n=18) for sample 28GX22 (Fig. 4p).
5.2. EMP monazite ages
Representative monazite grains with BSE image are shown in Figure 4, in which
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the apparent ages (in Ma) and wt% ThO2 contents (in the parenthesis) of spot analysis Basically, each contrast BSE domain observable in the grain has been
analyzed.
Information of monazite inclusions and their host phases are included in
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are given.
Group 1 rocks
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Table 2.
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Sample 11GD01: Euhedral to anhedral monazite inclusions with sizes ranging from 80x60 to 15x10 m are common, and their ThO2 contents mostly fall in 4-6 wt% Point analyses on these grains yield
AC
for homogeneous grains under BSE images.
close apparent ages which give a weighted mean age of 233±13 Ma (n=58).
Grains
in the heavy concentrate are larger (120x100 to 80x60 m) and show more obvious patchy zoning (Fig. 5a) than inclusions.
Point analyses on these grains yield similar
apparent ages and give a weighted mean age of 231±15 Ma (n=38).
The weighted
mean age for all the grains is 232.8±4.3 Ma (n=96; Fig. 6a). Sample 11GDGZA1: Some inclusions in biotite are somewhat altered as reflected by a stronger contrast in BSE images (Fig. 5b).
Compositions of unaltered monazite
are similar to the sample 11GD01, and point analyses yield close ages which give a weighted mean age of 230±18 Ma (n=30).
Discrete monazites are larger (110x90 to
80x60 m) and display dark and bright patchy BSE zonings with the latter slightly 15
ACCEPTED MANUSCRIPT higher in Th (ThO2= 7.7 wt%) (Fig. 5c).
Likewise, the unaltered portion yields
similar ages giving a weighted mean age of 234±16 Ma (n=38).
The weighted mean
T
age is calculated as 232.8±5.2 Ma for this sample (n=68; Fig. 6b).
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Sample 23GX-11: Our previous study based on monazite inclusions revealed three
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sets of age, 434±18 Ma (n=11 on 5 grains), ~370 Ma (n=3 on 1 grain) and 237±13 Ma (n=11 on 4 grains; Fig. 6c), indicating multi-sources for the constituent minerals (Chen et al., 2012).
In this study, we provide a large variety of EMP monazite ages,
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ranging from Paleoproterozoic to Triassic with major clusters at 1.8 Ga, 860 Ma, 540
heavy concentrate (Fig. 6d).
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Ma, 440 Ma and 250 Ma, as obtained from different domains on 28 grains in the They obviously represent detrital sources, and details
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will be discussed later.
Group 2 rocks
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Sample 23GX04: Inclusions are large and abundant, most are subhedral grains in biotite (Fig. 5d) and euhedral grains in quartz (Fig. 5e). Although compositions of
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these monazite grains are variable (ThO2= 5.4-8.3 wt%; UO2= 0.3-1.6 wt%, Y2O3= 0.7-5.0 wt%), apparent ages obtained from inclusions are uniform, with a weighted mean age of 235.5±5.5 Ma (n=67). Monazite grains from the heavy concentrate are slightly lower in ThO2 (4.4-7.9 wt%), higher in UO2 (>0.8 wt%) and Y2O3 (>3 wt%) contents.
Few grains show Early Paleozoic age in the core but Triassic age in the
outmost zone (Fig. 5f).
Some other grains are altered and display a huttonite-rich
outer zone, however, the age is exclusively Triassic in the unaltered portion (Fig. 5g). Thus two ages, 236.8±8.7 Ma (n=53) and 440.7±10.0 Ma (n=12) are revealed from discrete grains.
By putting together the Triassic age data from slide and sand, the
weighted mean age is calculated as 236.6±3.0 Ma (n=120; Fig. 6e).
16
ACCEPTED MANUSCRIPT Sample 23GX05-2: Unlike monazite occurrence in other samples, grains in this sample have irregular shapes in a constant size of ~30 m (Fig. 5h).
Patchy
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compositional zoning is common, the darker domain has lower ThO2 (3.5-6.8 wt%)
IP
and the brighter domain has higher ThO2 contents (up to 12.1 wt%), though the U and
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Y contents are rather constant (UO2= 0.5-1.0 wt% and Y2O3= 1.4-2.6 wt%). However, different domains yield same apparent ages around 230 Ma, giving the weighted mean age of 234.1±5.4 Ma for the sample (n=58; Fig. 6f).
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Sample 23GX15: Inclusions are abundant and often in association with zircon in Mingling of the host phase and monazite inclusions is a
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the same host mineral.
common textural feature (Fig. 5i), suggesting a co-genetic relationship.
Contrasting
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BSE domains are randomly distributed, revealing lower Th and U (ThO2= 3.9-6.3
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wt% and UO2= 0.5-1.1 wt%) in the darker domain and higher Th and U contents (ThO2= 7.1-12.1 wt% and UO2= 0.9-1.3 wt%) in the brighter domain (Fig. 5j) but
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having constant Y contents (Y2O3= 2.7-3.8 wt%). However, EMP monazite dating on the different domain of the datable grains (no severe interference from the mingled
AC
phase) all yield apparent ages around 230 Ma, giving the weighted mean age of 235.5±4.1 Ma (n=102; Fig. 6g). Sample 11GD06: Monazite inclusions are large euhedral to subhedral grains (60x20 to 100x 45 m).
Although concentric zoning is observable in some grains
(Fig. 5k), compositional variations are small. All analyses yield apparent ages around 230 Ma, giving the weighted mean age of 234±11 Ma (n=17).
Monazites in
the heavy concentrate display same zoning pattern, but two out of 16 grains show Early Paleozoic vs. Triassic ages in a core-rim relationship (Fig. 5l).
Combining all
the monazites measured in both the slide and heavy concentrate, two weighted mean ages of 239.7±4.1 Ma (n=83) and 441±28 Ma (n=5) are given (Fig. 6h).
17
ACCEPTED MANUSCRIPT Sample 11GD07: The great majority of inclusions was subjected to alteration producing huttonite grains in the rim (Fig. 5m).
Large variations of ThO2 (2.9-10.3
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wt%) and low but constant concentrations of UO2 (0.02-0.42 wt%) and Y2O3 (0.3-2.0
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weighted mean age of 233±11 Ma (n=14; Fig. 6i).
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wt%) are present in the unaltered cores. Apparent ages from these cores yield a
Group 3 rocks
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Sample 23GX13-2: Only three inclusions in biotite could be observed in the slide,
(15x6 m) grains.
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including a euhedral (50x30 m), a large elongate (100x20 m), and a small anhedral Large part of these grains has altered into an assembly of allanite, Moderate variations of
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chlorite and huttonite (±apatite, epidote and synchysite).
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ThO2 (4.2-8.5 wt%), UO2 (0.24-0.65 wt%) and Y2O3 (0.9-2.5 wt%) are present in the unaltered cores which yield apparent ages of ~240 Ma, ~360 Ma and ~430 Ma, from Such age patterns resemble those obtained from
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three grains, respectively (Fig. 6j).
monazite inclusions in the meta-sandstone (sample 23GX11).
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Sample 23GX14: Usually, schist is considered difficult for age dating because zircon is small and monazite is rare. biotites.
In this case, inclusions are mostly preserved in
Their Th contents are rather constant (4-6 wt%), and U and Y contents are
variable (UO2 =0.6-1.4 wt% and Y2O3 =0.45-2.4 wt%). Available grains all give apparent ages of ~230 Ma, yielding the weighted mean age of 235.9±8.2 Ma (n=27; Fig. 6k). Sample 11GD03: Most inclusions have elongated habit with long-axis parallel to the cleavage of layered host minerals (Fig. 5n).
One special feature is the relatively
high U (UO2 >0.8 wt%) and low Y (Y2O3 <1.5 wt%) contents of these monazites as compared with other samples.
Apparent ages are exclusively ~240 Ma, giving the
weighted mean age of 244±11 Ma (n=14; Fig. 6l). 18
ACCEPTED MANUSCRIPT Sample 11GD04: Both inclusion and interstitial monazites are abundant and few grains appear to be rims of hexagonal apatite enclosed in K-feldspar (Fig. 5o).
They
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are relatively low in Th (ThO2= 1.7-3.3 wt%) and U (UO2 mostly ≦0.2 wt %).
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Generally, concentric zoning is seen (Fig. 5p), but apparent ages from core and rim
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are indistinguishable, giving a weighted mean age of 236.0±4.6 Ma (n=105; Fig. 6m). Sample 11GD05: Monazites are mostly interstitial grains having concentric zoning with a bright rim enclosing the dark irregular core (Fig. 5q). Rims are often
Those which lack the distinct zoning
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against Early Paleozoic ages in the core.
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marked by the increase in Th and U but decrease in Y, and have Triassic apparent ages
basically show single Triassic age. Two sets of weighted mean age obtained are: Monazites in the heavy concentrate
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236.1±7.2 Ma (n=21) and 436±16 Ma (n=15).
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show no discernable differences in zoning pattern, grain size and chemical composition as compared with grains in the slide.
Likewise, two weighted mean Combining all
CE P
ages are obtained: 244.5±9.3 Ma (n=18) and 438.1±8.8 Ma (n=51).
measured monazites in this sample, two resultant weighted mean ages obtained are:
AC
239.2±5.7 Ma (n-39) and 440.6±7.1 Ma (n=66; Fig. 6n).
Group 4 rocks
Sample 28GX20: In the slide, rims of interstitial grain and inclusions in biotite are somewhat altered into allanite and apatite ±chlorite (Fig. 5r), but inclusions in quartz are unaffected (Fig. 5s).
Few monazites in K-feldspar appear to be a phase
surrounding apatite (similar to that of Fig. 5o).
Basically, compositions of monazite
are rather homogeneous, giving the weighted mean age of 233.1±6.2 Ma (n=50). Combining the result from the grain analysis on the heavy concentrates which yielded an age of 230±16 Ma (n=34) (Chen et al., 2011), the resultant weighted mean age is 232.1±4.4 Ma (n=84; Fig. 6o). 19
ACCEPTED MANUSCRIPT Sample 28GX22: Sub- to euhedral unaltered grains of inclusion are found in the slide.
Grain size (~100x80 m) and chemical compositions (ThO2= 6.1-8.7 wt%,
Yttrium content
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present in the S-type Darongshan granites (Chen et al., 2011).
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UO2= 0.2-2.1 wt% and Y2O3= 0.2-4.3 wt%) of these monazites are similar to those
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shows dramatic decrease toward the rims in discrete grains, indicating a co-genetic relationship between monazite and garnet (Table 2).
Apparent ages of these
monazites are exclusively Triassic, yielding a weighted mean age of 229.2±8.4 Ma
5.3. NanoSIMS monazite ages
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(n=21; Fig. 6p).
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Using the nanoSIMS method, Sano et al. (2006) demonstrated the applicability of
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U-Pb and Pb-Pb dating of detrital monazite grains from sedimentary rocks in Taiwan. There are three main age groups, 230 Ma, 440 Ma and 1850 Ma, and these ages are Taking the
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comparable with the U-Th-Pb chemical ages obtained on same grains.
same method and apparatus, we conducted 238U/206Pb and 207Pb/206Pb age
AC
determinations on the same spot as used for EMP analysis (Table 3). Results show that EMP ages, which mainly rely on the Th-Pb decay system, can be correlated to the 238
U/206Pb ages for the Triassic population and to the 207Pb/206Pb ages for the Early
Paleozoic or even Late Neoproterozoic populations (Fig. 7). Therefore, we are confident that EMP monazite age is able to reflect the evolution history of this mineral through various stages of thermal events, particularly to distinguish the Triassic from the Early Paleozoic ages, in the Yunkai area.
20
ACCEPTED MANUSCRIPT 6. Discussion 6.1. Age interpretations
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6.1.1. Variability of U-Pb zircon ages
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Basically, most of the studied samples show a wide spread of zircon ages along or
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near the concordia, and the age cluster for an estimation of the time of thermal event registered in the rock does not define the youngest age in many samples (Fig. 4). So the application of U-Pb zircon ages to each sample needs to be considered thoroughly.
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For Group 1 rocks (Figs. 5a-d), all three gneissic granites (samples 11GD01,
MA
11GDGZA1 and 11GD02) and the meta-sandstone (sample 23GX11) show major age clusters concentrated at the Early Paleozoic (444-437 Ma).
It would be easy to
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imply an eventual thermal effect imposed on these rocks at the Early Paleozoic if no
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other age constrain is given. Previously, we reported U-Pb zircon ages for two orthogneisses (430 Ma for sample 23GXGZ05 and 436 Ma for sample 23GXGZ08)
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near the Gaozhou Reservoir, the place close to where samples 11GD01 and 11GDGZA1 were collected.
A younger age of 238 Ma, calculated from 8 out of 25
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total populations, was obtained from the white CL rim of zircon grains of sample 23GXGZ08.
Also the associated charnockites (436-433 Ma) commonly exhibit
penetrations of Triassic veinlet (Chen et al., 2012).
Similarly, we stress that the
existence of few Triassic records beyond the Early Paleozoic age cluster in Group 1 samples (Figs. 5a and c) can not be overlooked. For Group 2 rocks (Figs. 5e-i), two gneisses (23GX05-2 and 23GX15) show the age cluster at the Neoproterozoic, three others show the youngest age cluster either at the Early Paleozoic (435 Ma for sample11GD06) or at the Triassic (229 Ma for sample 23GX04 and age unavailable for sample 11GD07 due to discordance).
For
Group 3 rocks (Figs. 5j-n), schists have either ages spreading largely over Meso- to Neoproterozoic without a cluster (11GD03) or an age cluster at the Neoproterozoic 21
ACCEPTED MANUSCRIPT (23GX13-1 and 23GX14), whereas gneisses display the youngest age cluster either at the Early Paleozoic (431 Ma for sample 11GD05) or at the Triassic (245 Ma for Such results basically resemble those obtained from Group 2
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sample 11GD04).
The large variability of U-Pb zircon ages with high discordant proportions in
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rocks.
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these two groups of rock makes this dating system not totally useful to indicate the time of ultimate metamorphism for samples composed of abundant detritus. The Group 4 undeformed granites (samples 28GX20 and 28GX22) exhibit ages
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(251-248 Ma; Figs. 5o-p) close to those obtained from zircon cores (Chen et al., 2011)
granites.
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but significantly older than their rims (Deng et al., 2004) of the Darongshan S-type Since core-rim variability on the U-Pb zircon geochronology is not
D
available for these two samples, whether these age really stand for the magma
case of Darongshan.
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emplacing time will be discussed in combination with the monazite age data like the
CE P
In sum, U-Pb zircon ages may be applicable to reflect the records of complicated multi-stage thermal activities on the studied samples. by 207Pb/206Pb ages are discordant.
Many records >1 Ga given
The discordance is possibly arising from the fact
AC
that 207Pb/206Pb ages are calculated assuming recent Pb-loss, which is incorrect if the Pb-loss was ancient (Guitreau and Blichert-Toft, 2014). 206
On the other hand,
Pb/238U ages reveal two major clusters in Neoproterozoic (1.0-0.9 Ga, n=70) and
Early Paleozoic (443-431 Ma, n=116), with the presence of Triassic ages (245Ma and 229 Ma) in spite of the scarcity in numbers (n=15) for all Group 2 and 3 rocks.
Due
to the difficulty to know the paragenesis of dated zircon, no good explanation can be deployed for the youngest U-Pb ages mainly obtained from zircon rims.
22
ACCEPTED MANUSCRIPT 6.1.2. Consistency of EMP monazite ages Monazite ages reflected by discrete grains from heavy concentrate and inclusion
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in rock-forming minerals are concerned. Here we deal with discrete grains first.
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For Group 1 rocks, monazite is abundant in two gneissic granites (samples 11GD01
SC R
and 11GDGZA1) and gives similar weighted mean ages of 231 Ma and 234 Ma, respectively.
For Group 2 and 3 rocks, monazites generally exhibit an Early
Paleozoic core-Triassic rim relationship in some biotite and two-mica granites.
Monazite ages for the Group 4 undeformed granites are solely
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sliced samples.
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Same feature of this mineral has only been observed from those in the matrix of their
Triassic. The common presence of Triassic ages reflected by matrix monazite in all
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four groups of rock in the Yunkai massif is an indication of regional reworking
TE
without a strong juvenile crustal growth from mantle. Alteration of monazite, presumably a consequence of interaction between
CE P
monazites and fluids, is a common feature. The influence of fluid can generally lead to the alteration sequence of (1) pristine monazite, (2) secondary monazite, (3)
AC
secondary apatite with thorite/huttonite (ThSiO4) inclusions (allanite and epidote), and (4) finally clay minerals (e.g. Finger et al., 1998; Budzyń et al., 2010).
In such
corona-textured alteration zones, the unaltered monazite core usually can retain the age of its formation (Suzuki, 2009; Chen et al., 2012). However, whether the secondary monazite in the intermediate zones can preserve information of alteration is still debatable. Previous investigators have suggested that secondary monazite when subjected to dissolution-reprecipitation by fluid overprinting basically can retain the age of alteration (e.g. Harlov et al., 2011; Williams et al., 2011). Experimental works demonstrate that the alteration zone with ThSiO4-rich phase in the monazite can be induced from fluids especially with alkali-bearing compositions (Harlov and 23
ACCEPTED MANUSCRIPT Hetherington, 2010).
Recently, Seydoux-Guillaume et al. (2012) reported few cases
of low-temperature alteration of large monazite grains (mnz 1) from different places The alteration zone is reflected by patchy domains (BSE images) and
T
of the world.
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characterized by the secondary, Th-U(Y)-depleted, high-Th/U monazites (mnz 2) in
SC R
association with thorite/huttonite, thorianite (ThO2) and xenotime (YPO4) as a consequence of reaction between mnz 1 and reactant.
Due to the presence of
nano-scaled Th-U silicates (visible only under TEM) in mnz 2, its chemistry may be
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somewhat disturbed and thus erroneous chemical age dating is concerned.
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In the present case, some interstitial monazites show similar texture of alteration but slightly differ in zonation. They exhibit unaltered inner zone (mnz1), outer zone
D
where m-scaled huttonite, (Th,U)SiO4 and (Th,U)O2 phases are common, with or The
TE
without a homogeneous middle zone (mnz 2) under BSE imaging (Fig. 8).
mineral assemblage of the outer zone is similar to those subjected to low-temperature
CE P
alteration probably affected by a mechanism of fluid-mediated coupled dissolutionprecipitation (Seydoux-Guillaume et al., 2012).
Consistency of ThO2 content and
AC
apparent age between inner and middle zones (Fig. 8) and resemblance of chemistry and age to the pristine monazites enclosed in quartz and feldspars of the same sample are evidences to exclude the existence of exsolved tiny Th- and U-rich phases in mnz 2. We suggest that formation of mnz 2, mainly presented in the discrete grains, is controlled by dissolution-reprecipitation, a common phenomenon in magmatic and metamorphic rocks.
The Yunkai massif, although experienced fluid overprinting,
might not involve large extent of chemical exchange and resetting (mnz 2) could have taken place shortly after monazite crystallization (mnz 1), making them indistinguishable in the EMP ages.
24
ACCEPTED MANUSCRIPT Monazites occurred as inclusions in host minerals with a framework structure (e.g. quartz and feldspars) are generally fresh (Fig. 9-1), whereas those in the sheeted The former
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biotite and muscovite show various degrees of alteration (Fig. 9-2).
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inclusions more likely represent contemporaneous primary species captured by the
to accommodate the secondary monazites.
SC R
growing host crystals because they do not contain sufficient amounts of P and REEs Neither the REEs required for the
secondary monazite inclusions were transported from overprinting fluid, for there is In fact, such monazite inclusions are
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no sign of fluid attack on most host minerals.
MA
mainly present near the rim of individual quartz or feldspar crystal (Fig. 9), their ages would be nearly simultaneous to the host minerals.
On the other hand, ages of
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monazite inclusions in the sheeted hosts are used to indicate the time of rock
TE
deformation.
Monazites appear to be included in biotite, quartz and plagioclase for the Group 1
CE P
gneissic granites (samples 11GD01 and 11GDGZA1), in biotite, muscovite and quartz (±plagioclase and K-feldspar) for Groups 2 and 3 gneisses, and dominantly in
AC
muscovite and subordinately in biotite and quartz for the Group 3 garnet tourmaline schists (samples 11GD03 and 23GX14). define foliation in these rocks.
Biotite, muscovite and quartz actually
Among all the inclusion grains dated, they give the
exclusive Triassic ages except one grain in biotite that yields ~430 Ma.
Therefore,
the EMP monazite age dating clearly indicates that Triassic is the time not only the great majority of host phases crystallized but also rims of discrete monazite grains overgrew in representative metamorphic rocks of the Yunkai massif.
25
ACCEPTED MANUSCRIPT 6.1.3. Reconciliation of two dating systems Significant age clusters derived from the U-Pb zircon geochronology in this study
T
can be summarized : 444-437 Ma with few Triassic rims (252-246 Ma) for Group 1
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rocks; 990-950 Ma, 435-433 Ma and 229 Ma for Group 2 rocks; 1004-904 Ma, 431 In other
SC R
Ma and 245 Ma for Group 3 rocks; and 251-248 Ma for Group 4 rocks.
words, all these groups of rock present Triassic records although the number may be small.
On the other hand, all the studied samples solely display Triassic ages given
Yttrium variations of discrete monazite, as
MA
K-feldspar, biotite and muscovite.
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by monazite inclusions in major constituent phases, such as quartz, plagioclase,
manifested by Y decreases in monazite rims for garnet-bearing rocks (samples
D
11GD05 and 28GX22; Table 2) and Y annuli in monazites for garnet tourmaline
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schists (sample 23GX14; Appendix 3), also indicate that modification of monazite, particularly its rim, was contemporaneous with garnet that largely controls the Thus, modification of monazite in the
CE P
expense of yttrium during crystallization.
discrete grains and growth of inclusions are considered to have taken place All these
AC
simultaneously with recrystallization of major constituents in the rocks.
features can distinguish an overwhelming within-grain growth of monazite during the Triassic recrystallization versus a weak mesostasis growth in the Early Paleozoic assemblage. It has been suggested that monazite may provide better age control than zircon in some metamorphic rocks with a range in metamorphic grade from greenschist to upper amphibolite facies (Kingsbury et al., 1993).
Because the effective closure
temperature of monazite is approximately 725±25 °C (Parrish, 1990), a monazite age could represent its initial crystallization in many granitic and high-grade metamorphic rocks. Contrarily, zircon, with its high closure temperature ≧900 °C (Lee et al.,
26
ACCEPTED MANUSCRIPT 1997), can be stable throughout the sedimentary cycle and during metamorphism. Our geochronological results provide more evidences to support this viewpoint in
The major factor causing age
IP
obtained for gneiss and schist of the Yunkai massif.
T
view of large variability of U-Pb zircon age versus consistency of EMP monazite age
SC R
bias between these two minerals may be due to the fact that large interstitial grains could be recycled and only newly grown rims can give more reliable age information, and unfortunately, zircon rims are mostly too thin to be datable.
Combining these
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two dating systems, ages of ca. 440-430 Ma given predominantly by zircon and
MA
subordinately by monazite (Table 4) can be regarded as the Early Paleozoic relicts being unmodified by the Triassic thermal event.
If two dating systems give
D
consistent results, the U-Pb zircon age is adopted for its better accuracy.
TE
Conclusively, it is suggested that a strong Triassic reactivation had imposed over the entire Yunkai massif, conforming the popular Triassic 40Ar/39Ar biotite ages of major
CE P
metamorphic rocks in this massif (Wang et al., 2007a, 2012; Chu et al., 2009). Two biotite gneiss samples (23GX04 and 11GD04) with U-Pb zircon ages of 229±10 Ma
AC
and 245.1±5.4 Ma are used to constrain the main age for Group 2 and 3 rocks, respectively.
6.2. Multi-phase metamorphism in the Yunkai massif 6.2.1. Inherited monazite and zircon ages It is well known that the Cathaysia block had endured repeated orogeneses, records of poly-phase thermal event may be unveiled by monazite and zircon ages in this study.
Sample 23GX11 provides a wide range of monazite ages varying from
Paleoproterozoic to Triassic on 24 datable grains (Appendix 3).
Many grains show
signs of multiple events, of which two kinds of monazite are recognized: one takes ~430 Ma and the other shows ~240 Ma as the youngest age.
The former (4 grains) 27
ACCEPTED MANUSCRIPT records some older ages, such as ~1750 Ma, ~970 Ma, 880-840 Ma and ~540 Ma, yet the latter (20 grains) records older ages including only ~540 Ma and ~430 Ma.
Two
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examples of respective monazites, with apparent ages (plus ThO2 and Y2O3 contents)
Although chemistry and thus the age generally matches the BSE domain in each
SC R
10.
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of different BSE domains and X-ray maps of Ce, Th, U and Y, are depicted in Figure
grain, resorbed (Fig. 10a) and patchy (Fig. 10f) zoning textures suggest a complicate history of monazite growth and dissipation when compared to monazite inclusions
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that mostly show simple core-rim or homogeneous zoning (Fig. 5). Combining the
MA
in-situ monazite ages, this rock could have been metamorphosed at ~240 Ma as reflected by the youngest high peak in the total age frequency distribution of monazite
D
(Fig. 11a).
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The total age frequency of U-Pb zircon dates for all samples from the Yunkai massif mimics the EMP monazite age curve of the meta-sandstone if discordant Pb/206Pb ages are excluded (Fig. 11b). Hence, a common provenance of these two
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207
minerals is postulated.
Abundance of Neoproterozoic and Early Paleozoic zircons in
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the Yunkai samples (inset of Fig. 11b) supports the thought that this massif is the major supplier for constructing Permian to Late Triassic sedimentary successions in the Qinfang Basin to the west based on the similarity of U-Pb zircon age patterns (Hu et al., 2014). Furthermore, age clusters of 1.0-0.9 Ga in gneissic rocks and schists (Figs. 5f, g, j and k), with such zircons of magmatic origin (Th/U >0.5, Appendix 1) (Hoskin and Schaltegger, 2003), favor a Neoproterozoic arc system in the northern Yunkai Domain (Zhang et al., 2012). Regarding the idea on the tectonic evolution of South China since Neoproterozoic, monazite ages of ~970 Ma and 880-840 Ma found in this meta-sandstone suggest that the episode of Neoproterozoic magmatism probably had involved crustal reworking to some extents.
Detrital monazite ages ~540 Ma of this sample (Appendices 3 and 4) 28
ACCEPTED MANUSCRIPT probably reflect traces of the Pan-African orogeny in the Cathaysia block, which is complementary to the observation that Pan-African age is one of the dominant
These strengthen the propositions that
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in the Yangtze block (Duan et al., 2012).
T
populations in Paleozoic detrital zircon samples from three Lower Devonian sections
SC R
South China block was a part of East Gondwana in Early Paleozoic time (Xu et al., 2013, 2014a, b) and Yunkai massif is a place reflecting the Gondwana property (Wu,
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2000). Detail of the Early Paleozoic age records is discussed in the following.
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6.2.2. Metamorphism and magmatism on the Yunkai protolith Gneissic granites (Group 1) in the core domain exhibit an overwhelming
D
population of Early Paleozoic zircon age (Fig. 4), reflecting a stage of magma However, this zircon age
TE
intrusion during the Early Paleozoic orogenesis.
population is not so obvious for Groups 2 and 3 gneissic rocks and schists, and only
CE P
restricted to few specific samples (11GD05 and 11GD06) that the Early Paleozoic monazite relicts have also been detected (Table 4).
These gneissic rocks and schists
AC
actually possess a popular Neoproterozoic U-Pb zircon age cluster, supporting the development of abundant Neoproterozoic mafic magmatism in the Cathaysia interior (Wang et al., 2012).
The high and sharp age peaks of ~230 Ma and ~430 Ma in both
zircon and monazite age frequency diagrams (Figs. 11a and b), mainly contributed from gneissic granites and undeformed granites, are thus interpreted as an indication of strong Triassic influences on a terrain characterized by a pre-existing Early Paleozoic granitic mass and abundant detrital sediments (or metapelite protoliths) probably derived from the Neoproterozoic magmatic arc and Early Paleozoic domains nearby.
Extent of the Triassic modification on the Yunkai massif may be better
elucidated by monazites.
29
ACCEPTED MANUSCRIPT Usually, the shielding effect of host mineral with a framework structure (feldspar in the present case) can prevent monazite inclusions from fluid attack during the later Their ages can thus be considered as Likewise, ages
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representing the formation time of feldspar crystals in the rock.
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thermal events (Montel et al., 2000).
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derived from monazite inclusions in major primary phases of the undeformed granites shall represent magma emplacing time of these intrusives.
On the other hand,
muscovite, biotite and quartz are regarded as phases generated in the course of ductile
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deformation (or syn-kinematic minerals), EMP monazite age results suggest that these
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minerals in the same sample were recrystallized at the Early Mesozoic time. Further based on the Early Paleozoic-Triassic core-rim relationship on monazite in few
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samples, the record of Early Paleozoic deformation could be largely overwritten.
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It is noted that contradictory ages for Group 4 granitic rocks are given by the U-Pb zircon dating system because they were not deformed yet having older ages (251-248 Considering the
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Ma) than metamorphic rocks (245-229 Ma) in the same area.
SHRIMP U-Pb dating on the zircon rims yielded characteristic younger ages of ~230
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Ma for the neighboring S-type Darongshan granites (Deng et al., 2004), similar ages (231-224 Ma) obtained from the large population of monazite on five major plutons in Darongshan were interpreted as the time of magma emplacement (Chen et al., 2011). In lacking of age constraints from zircon rims, we follow same line of thought by adopting the monazite ages, 232-229 Ma, to represent the time of magmatism in the Yunkai area. In sum, monazite inclusion ages for the Group 3 (greenschist facies) rocks (244-236 Ma) and the slightly younger Group 2 (amphibolites facies) rocks (236-233 Ma) coincide nicely with the results reflected by the youngest U-Pb zircon age cluster for the Group 3 (245 Ma) and the Group 2 (229 Ma) rocks (Table 4 Age-1).
Along
with our monazite age data of Group 1 gneissic granites (233-230 Ma) and intrusion 30
ACCEPTED MANUSCRIPT of Group 4 S-type granites (232-229 Ma) as well as Triassic U-Pb zircon rim ages for the orthogneiss near Gaozhou (Chen et al., 2012), a temporal scheme of progressive
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metamorphism and hotly-pursuing granitic magmatism seems to enlist Yunkai massif
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a terrain that can outline the Triassic geodynamic processes in the southwestern South
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China. This, in general, agrees with the 250-230 Ma syntectonic metamorphism and the 225-205 Ma post-orogenic plutonism in the eastern Indochina (Faure et al., 2014). For elucidating the Yunkai massif in the framework of orogenic evolution, major
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Triassic tectonic units in South China are compared as followed.
7. Tectonic implications
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Yunkai massif, in the middle of the Early Paleozoic Wuyi-Baiyun-Yunkai-Song
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Chay metamorphic belt (Lin et al., 2008), is important in knowing the Early Mesozoic evolution in the southern margin of South China Block. Materials for the Early
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Paleozoic metamorphosis in the Yunkai massif were mainly the Neoproterozoic detritus. They were accreted by a large amount of simultaneous granitic intrusions Based on the
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to construct the highly advocated Early Paleozoic Yunkai protolith.
metamorphic grade, the greenschist facies Yunkai Group and amphibolite facies Gaozhou Complex were differentiated. The South China Block had experienced strong Triassic tectonothermal events mainly by the interactions with Indochina Block in the west and North China Block in the north.
Boundary between the Indochina and the South China Blocks is basically
along the Ailaoshan-Song Ma suture zone although complications exist (e.g. Metcalfe, 2011; Faure et al., 2014, 2016b) (Fig. 1a).
Consequences of such events to this Early
Paleozoic metamorphic belt were somewhat different.
The Song Chay massif close
to the Song Ma suture is a strongly deformed body bounded by a mylonite zone on its southern frank (Lepvrier et al., 2011).
This body is made of an orthogneiss in the 31
ACCEPTED MANUSCRIPT center and mica schist in the peripheral with a NW- or N-trending stretching lineation (Maluski et al., 2001; Yan et al., 2006). Based on the U-Pb zircon geochronological
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data summarized by Chen et al. (2014) that Triassic ages often appear in rims of Early
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Paleozoic zircon for orthogneisses, they were considered to represent reworking of the
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Paleozoic protolithic granites. SIMS monazite U-Th-Pb determinations on garnet mica schist show Early Paleozoic ages for interstitial grain and Triassic ages for inclusions in garnet.
In addition, 40Ar/39Ar mineral ages are Triassic for muscovite
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in gneiss and mica schist, and amphibole in mylonites.
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Wuyishan terrain has long been considered to be exposures of the Precambrian basement composed of Paleoproterozoic and Neoproterozoic-Early Paleozoic Reliable dating information revealed from zircon U-Pb
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metamorphic sequences.
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geochronology led to a consensus that an Early Paleozoic tectonothermal event had caused immense greenschist-amphibolite facies (Wan et al., 2007), minor granulite
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facies metamorphism (Yu et al., 2005) and anatexis and granitic magmatism (Zeng et al., 2008; Liu et al., 2010; Li et al., 2011; Xu et al., 2011). Moreover, EMP monazite
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ages obtained were all Early Paleozoic from basement rocks of Wuyishan (Chen et al., 2008; Charvet et al., 2010).
Recently, an Early Mesozoic metamorphic reworking is
considered to have affected the entire Cathaysia block based on popular discordia intercepts of U-Pb zircon ages at Early Paleozoic (upper intercept) and Triassic (lower intercept), respectively, for high grade metapelitic rocks in Zhejiang and Fujian (Zhao et al., 2015). However, considering 40Ar/39Ar dating of biotite (235.3±2.8 Ma) and muscovite (238.5±2.8 Ma) separated from two mylonite samples and U-Pb zircon age for an undeformed granitic dyke (229.8±2.2 Ma) intruded into the shear zone, Triassic ages in the Wuyishan would be more likely reflecting the time of dextral shearing (Xu et al., 2011).
32
ACCEPTED MANUSCRIPT In the Yunkai massif, gneissic granites in the massif’s core domain show Early Paleozoic ages based on the U-Pb zircon, but Triassic ages on the EMP monazite
Age similarities between Song
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massif (Chen et al., 2014 and references therein).
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geochronologies. This is almost a case similar to the orthogneiss in Song Chay
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Chay and Yunkai massifs commonly include Early Paleozoic-Triassic zircon core-rim and monazite interstitial grain-inclusion relationships as well as dominant Triassic 40
Ar/39Ar mineral ages for a large number of metamorphic rocks.
These results
Moreover, the Permo-Triassic Emeishan-related intraplate
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Paleozoic ages.
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deviate from the Wuyishan massif in which monazites, in particular, retain the Early
magmatism and Late Triassic S-type association adjacent to the Song Chay massif
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(Chen et al., 2014; Faure et al., 2016a) provide a clue for a consideration for the
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influence of mafic magma emplacement. The Darongshan S-type granites and few plutons in their eastward extension (the Group 4 rocks) relative to the Yunkai massif
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are almost the equivalents (see later sections). From the spatial relationship linking the Wuyishan, Yunkai and Song Chay
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massifs, we thus favor that the strong Triassic reworking on the Yunkai massif, like Song Chay massif, was mainly controlled by the collision of Indochina and South China Blocks. The Indochina Block here includes Simao (Fig. 1c), as they were united before collided with the South China Block (Wang et al., 2016 and references therein in).
Current views on the South China-Indochina collisional tectonics have
been reviewed by Faure et al. (2014).
Their geodynamic evolution model described
the Permian subduction of paleo-Tethys underneath Indochina and the subsequent Triassic South China-Indochina collision and separated syntectonic metamorphism and postorogenic plutonism at 230-225 Ma.
Important Triassic tectonic units in the
South China Block may shed light for understanding of the evolution of the Yunkai massif. 33
ACCEPTED MANUSCRIPT The Xuefengshan Belt in the center of this Block (Fig. 12), characterized by large-scale fold and thrust structures and widespread granites, is a unit concerned.
A
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basal decollement zone is exposed in the core of anticlines of this Belt, in which the
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Neoproterozoic pelite and sandstone, and the intruding Early Paleozoic granites were
(Chu et al., 2012a).
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strongly deformed and metamorphosed into mylonites and orthogneiss, respectively SIMS zircon U-Pb dating on these granites reveal two magmatic
episodes: the Early Paleozoic (438-411 Ma) and the Early Mesozoic (225-217 Ma),
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and zircon Hf(t) values indicate a common crust-derived source for the two EMP monazite and SIMS zircon U-Pb
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generations of granites (Chu et al., 2012b).
dating provide age constraints of the ductile shearing at 243-226 Ma (Chu et al., Thus, the Xuefengshan Belt is considered to be a Late Triassic
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2012c).
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intracontinental orogen dominated by the NW-directed shearing and thrusting (Chu et al., 2012a, c; Faure et al., 2016a).
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The Group 4 rocks are considered to represent postorogenic plutonism to the metamorphosis of the Yunkai massif, synchronous with immense Triassic Darongshan
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S-type granitoids and perhaps unfoliated Qiongzhong and adjacent granitic batholiths (234-226 Ma; Yu et al., 2014) in the Hainan Island (Fig. 1a).
As mentioned, this is
accomplished by adopting the zircon core ages to be inherited (or mixing) records, while the monazite age, the time of granitic intrusion.
In this context, the tectonic
architecture of such Triassic metamorphic massifs and magmatic belts composed of a large spread of post-orogenic plutonism in the South China Block would be more likely to be accommodated with an intracratonic magmatism (Chu 2012a, c; Faure et al., 2016b) rather than an early-stage, NW-directed, flat-slab subduction of the Paleo-Pacific plate (Li and Li, 2007).
34
ACCEPTED MANUSCRIPT The process of two phases of Triassic deformation in the Yunkai area, with NW-trending greenschist facies metamorphism at ~245 Ma and NE-trending
From a mass balance perspective, expense
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metamorphism on metapelitic basements.
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amphibolite facies metamorphism at ~230 Ma, is first considered by the prograde
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of muscovite and chlorite to form biotite and garnet via prograde metamorphism commonly involve formation of monazite with observable overgrowths (Kohn and Malloy, 2004). This is sometimes accompanied by the presence of precursor
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minerals such as allanite, apatite and Th oxides (Catlos et al., 2002).
Insignificant
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monazite overgrowths in biotite (Fig. 9), the lack of precursor minerals and different orientations of monazite-bearing muscovite developed in two phases of rock are
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difficult to be explained by the prograde metamorphism within the same massif.
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Here we draw attention to the tendency that the amphibolite facies metamorphic (Group 2) rocks may be correlated with the Xuefengshan Belt in time (e.g. the Such metamorphic
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monazite ages of 243-226 Ma) and the sense of deformation.
features imply that the late phase deformation in the Yunkai massif may be controlled
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by the same collisional tectonism as the Xuefengshan Belt. As for the early phase deformation, we suggest that intrusion of the Emeishan Large Igneous Province (E-LIP at 255-251 Ma; Shellnutt, 2014) may play the role (Fig. 12). This intrusion had triggered not only the Late Permian crustal-derived granites and accompanying peralkaline, peraluminous and metaluminous A-type granites in southwestern China (Shellnutt et al., 2007, 2009, 2010, 2011a, b), but also the contemporaneous metaluminous and peraluminous silicic volcanic rocks in the Song Da magmatic suite (Tran et al., 2011) and alkaline silicic rocks in the Phan Si Pan-Tu Le region in northwestern Vietnam (Hoa and Anh, 2011; Hieu et al., 2013; Usuki et al., 2015; Tran et al., 2015) as well as rifting-related, Late Permian ferrosyenites in the same region (Shellnutt et al., 2008).
Intrusion of the E-LIP 35
ACCEPTED MANUSCRIPT (~0.3x106 km2) could have caused high temperature metamorphism of lower crust of the Yunkai massif by the picritic magma with a mantle potential temperature >1540 o
The presence of abundant high-temperature Permo-Triassic
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C (Shellnutt, 2014).
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granulite enclaves in the Darongshan granites (Chen et al., 2011; Zhao et al., 2012)
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provides a clue that the neighboring Yunkai massif could have been subjected to an elevated geothermal gradient in the upper crust level by the same heat source.
Mafic
granulite xenoliths (230-220 Ma) captured by Jurassic basalts in Daoxian (Li et al.,
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2016) is an indication for the heat pulse lasting until Late Triassic (Fig. 12).
Thus,
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we favor a combined influence from both the continental collision and the E-LIP migration would be responsible for the formation of two phases of metamorphic rock The relative strength of these two affecting factors determines
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of the Yunkai massif.
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the metamorphic grade and striking structure and also explains why two sets of deformation were observed in the Gaozhou Complex.
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To the west of the Song Ma suture, the Truong Son Belt and Kontum massif (Fig. 1a) are the most important Triassic tectonic units that defined the Indosinian orogeny The EMP monazite
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in Vietnam (Lepvrier et al., 2004; 2008; Ferrari et al., 2008).
geochronology has been applied for a large number of rocks in these units (sixty in total, Nakano et al., 2013), in which Kontum Massif gneisses record two distinct thermal episodes (460-430 Ma and 245-230 Ma), whereas Truong Son Belt gneisses show only single dominant event (240-220 Ma) synchronous with the granitoids in these terrains (Roger et al, 2014).
Kontum massif, the largest continuous exposure
of crystalline basement of the Indochina craton (Nam et al., 2001), is worth noting for similarities to the Yunkai massif in the metamorphic grade (amphibolite-granulite, amphibolite and greenschist-amphibolite facies in different complexes; Osanai et al., 2008) and geochronology.
Ages of specific lithologies, such as the high-temperature
granulite-charnockite association, are good examples.
Granulites in the eastern 36
ACCEPTED MANUSCRIPT Kontum massif show Early Paleozoic ages based on the U-Pb monazite dating (Roger et al., 2007) and 40Ar/39Ar biotite dating (Maluski et al., 2005). However, granulites
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and associated charnockites in the western Kontum massif yield Triassic ages on the
This would be an analogy
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al., 2007) and 40Ar/39Ar biotite dating (Nagy et al., 2001).
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U-Pb zircon dating (Nam et al., 2001; Nagy et al., 2001; Carter et al., 2001; Roger et
in the Indochina when compared with similar rocks that exhibit Early Paleozoic ages (440-430 Ma) in the Gaozhou Complex and Permian ages (~260 Ma) in the adjacent
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Darongshan. Therefore, both the Yunkai and Kontum massifs may mark reactivated
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metamorphic terrains, with strong Triassic deformations on the Early Paleozoic basement in South China and Indochina Blocks, respectively.
As both blocks had
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derived from Gondwana landmass (Metcalfe, 2013; Usuki et al., 2013; Xu et al.,
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2013), more comprehensive studies for these two massifs concerning the Triassic rework on Early Paleozoic assemblies would be of help to better understand the
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Conclusions
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so-called Song Ma-Poko (Kontum) system (Ferrari et al., 2008; Wang et al., 2016).
1. This study demonstrates the usefulness of the in-situ EMP monazite analysis which can provide age control particularly for the time of deformations on metamorphic rocks in the Yunkai massif. Monazite inclusions in the principal constituents (muscovite, biotite and quartz), exhibiting long axis orientated sympathetically with these platy or elongated minerals, define both the earlier NW-trending, greenschist facies deformation and the later NE-trending, amphibolite facies deformation occurred during the Triassic time in the Yunkai massif.
This
phenomenon can hardly be reflected by the zircon age alone from same samples mainly because zircons are largely interstitial grains representing un-reset relicts of the preceding rocks. 37
ACCEPTED MANUSCRIPT 2. Combining our new EMP monazite and U-Pb zircon age results, we suggest that greenschist-facies metamorphism of the Yunkai Group with severe ductile
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deformation occurred at the earlier time (~245 Ma) and amphibolite facies
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metamorphism of the Gaozhou Complex later (~230 Ma). They were followed by
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the intrusion of S-type granites probably extended from Darongshan. This framework supports that Yunkai massif, like Song Chay massif to the southwest, is a terrain highly affected by the Triassic thermal events and clearly distinguishes from
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heavily retained (Faure et al., 2016a).
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the Wuyishan massif to the northeast in which trace of the Early Paleozoic rework is
3. The prograde metamorphism as a consequence of the continental collision at
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the Triassic, although may be used to account for the metamorphic facies and age
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results of the Gaozhou Complex, is difficult to explain the difference of the stress patterns imposed on the metamorphic rocks of the Yunkai Group.
A heat source
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contributed by the E-LIP mafic underplating is considered as an additional controlling factor to cause, in particular, the NW-trending deformation for the greenschist facies Combination of these two controlling factors,
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metamorphism in the Yunkai massif.
with the dominance of E-LIP mafic underplating in the early phase and the collisional tectonism in the later phase, may be responsible for the deformation patterns in the Yunkai massif. 4. Remarkable similarities of geochronological data have been observed from some terrains in South China and in Vietnam (roughly along the coastal area of the Indochina Block) revealing the common presence of Early Paleozoic and Triassic ages in almost all kinds of metamorphic rocks in these terrains.
Yunkai massif in
South China and Kontum massif in Indochina are the examples.
Regarding the
collisional tectonism at Triassic, i.e., the essence of the Indosinian orogeny, we suggest that ages used to define the syn-kinematic and post-orogenic stages of this 38
ACCEPTED MANUSCRIPT orogeny may be better constrained by inclusion minerals like monazite.
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Acknowledgments
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Drs. S.L. Chung and Y. Iizuka are thanked for arranging LA-ICPMS U-Pb zircon
imaging at the Academia Sinica, respectively.
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analyses at the National Taiwan University and monazite X-ray mapping, BSE and SE The authors are highly indebted to Dr.
V. Walia (Taipei) who read the manuscript, Dr. M. Faure (Orleans) and one
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anonymous reviewer who gave valuable comments.
This work was supported by
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research grants from National Science Council, Taiwan, ROC
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(NSC97-2116-M-002-019/Chen and NSC100-2116-M-002-008/Lee).
39
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ACCEPTED MANUSCRIPT Figure captions Fig. 1.
(a) Relevant tectonic units in the Yangtze and Cathaysia sub-blocks of the
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South China Block, including Yunkai massif (YKM), Song Chay massif (SCM),
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Darongshan (DRS), Xuefengshan Belt (XFSB), Hainan Island (HN), Wuyishan
Truong Son Belt (TSB) in the Indochina Block.
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(WYS)-Biyunshan (BYS) massif in the South China, and Kontum massif (KTM) and The formerly proposed Early
Paleozoic units are marked by blue and Triassic units, by pink colors. Ailaoshan
Indochina (+Simao) Blocks.
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(ALS)-Song Ma (SM) zone is the collision boundary between the South China and (b) The simplified geological map of the Yunkai massif
tectonic blocks in East Asia.
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(modified from Zhou, 1995) and sample localities.
(c) Relative position of major
Names of fault are 1: the Wuzhou-Bobai Fault, 2: the
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Luoding-Yuocheng Fault (one of the major shear zones), 3: the Wuchuang-Sihui Fault,
Back-scattered electron (BSE) images showing some special petrographic
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Fig. 2.
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4: Chenzhou-Linwu Fault, and 5: Poko Zone (Ferrari et al., 2008).
features of four sample groups.
The gneissic granites in Group 1 show (a) granular
texture (sample 11GDGZA1) and (b) relatively high extent of deformation (sample Biotite gneiss in Group 2 (sample 23GX05-2) is a gneissoid enclave
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11GD01).
consisting mainly of (c) granular quartz, plagioclase and biotite, and a two-mica gneiss in Group 2 (sample 23GX15) shows (d) relict plagioclase grain.
Biotite
gneiss in Group 3 (sample 11GD05) contains (e) K-feldspar and plagioclase porphyroblasts, and garnet-tourmaline schist in Group 3 (sample 23GX14) generally shows (f) garnet poikiloblast with abundant quartz inclusions surrounded by platy muscovite and chlorite.
Group 4 undeformed granites show (g) quartz, plagioclase
and biotite grains surrounded by interstitial K-feldspar (samples 28GX20), and (h) minor amounts of garnet and rutile (sample 28GX22).
Abbreviation of mineral
names follows those described by Whitney and Evans (2010). 50
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BSE images showing mineral phases that enclose monazite grain (Mnz) are
consistent with biotite, muscovite, chlorite, quartz and ilmenite based on energy Monazites in muscovite (a-b), in biotite contacting with
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dispersive X-ray spectra.
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quartz (c), in biotite contacting with muscovite (d), in biotite penetrated by muscovite
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veinlets (e), and in muscovite associated with ilmenite (f) are generally parallel to the foliation of the NE-trending Group 2 (a-d) and the NW-trending Group 3 (e-f) rocks.
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Fig. 4. Concordia diagrams and weighted mean 206Pb/238U age (2σ) for zircons
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from Yunkai samples in four groups: (a-d), (e-i), (j-n) and (o-p). Note that high population of Neoproterozoic zircons is a common feature in Groups 2 and 3 samples.
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data are listed in Appendix 1.
Source
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N= number of analyses and MSWD= mean square of weighted deviates.
BSE images of monazite in the polished section (R) and heavy concentrate
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Fig. 5.
(S) of the representative samples, with the EMP age results (Ma) and ThO2 contents (wt% in the parenthesis).
Although different BSE domain and zonation,
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morphological feature, and degree of alteration are shown, they invariably present Triassic ages in all samples, including the overgrowing rim of two grains (d and o) that exhibit an Early Paleozoic core.
Fig. 6.
EMP monazite isochron and the weighted mean age results for four groups of
rocks in the Yunkai massif (a-d for Group 1, e-i for Group 2, j-n for Group 3 and o-p for Group 4). deviates.
N= number of analyses and MSWD= mean square of weighted
Source data are listed in Appendix 3 (slide is equal to polished section and
sand is equal to heavy concentrate in the text).
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BSE images of discrete monazite grains of some representative samples on
which the EMP ages, with ThO2 and Y2O3 contents (wt% in the parenthesis), and Z: zircon in monazite.
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NanoSIMS U-Pb and Pb-Pb ages are compared.
BSE images of monazite alteration zonations including 1: unaltered core
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Fig. 8.
(mnz 1), 2: intermediate monazite (mnz 2), and 3: the outer zone (mnz 3).
The mnz
3 is characterized by the presence of abundant huttonite (the Th- and U-rich silicate The mnz 2 may be present (a-b) or absent (c-d) when the
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with a bright reflection).
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pristine monazite (mnz 1) was subjected to alteration probably involving fluid attacks.
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Numbers in the parenthesis are EMP ages (Ma) with ThO2 and Y2O3 contents (wt%).
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Fig. 9-1. Monazite inclusions with the EMP age results (Ma) and ThO2 contents (wt% in the parenthesis) in K-feldspar and plagioclase from a biotite gneiss (sample
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11GD04).
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Fig. 9-2. Monazite inclusions with the EMP age results (Ma) and ThO2 contents (wt% in the parenthesis) in biotite and muscovite from a gneissic granite (sample 11GD01).
Fig. 10.
BSE images with results of spot analysis of EMP apparent ages (Ma) and
ThO2 and Y2O3 contents (wt% in the parenthesis), and CeLα, ThMα, UMα and YLα X-ray compositional maps of two representative monazites from sample 23GX11 (S denotes samples from heavy concentrates). (a-e) The presence of ~1750, ~540, and ~440 Ma domains (BSE) in grain M03; (f-j) ~450, ~260 and ~230 Ma domains in grain M24, in which the ~450 Ma domain is relatively low in Ce and Th.
The color
bar in (e) is equally applied for all X-ray maps. 52
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Fig. 11.
Cumulative age probability curves of studied rocks from the Yunkai massif.
Resemblance of two curves with
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zircon ages with histograms of four groups of rock.
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(a) EMP monazite ages of a meta-sandstone (sample 23GX11), and (b) total U-Pb
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peaks at the Neoproterozoic and Early Paleozoic indicates the common provenance for monazite and zircon.
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Fig. 12. Schematic sketch showing the now-exposed Early Paleozoic metamorphic
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terrains (dark) and Triassic reactivated belts (hatched) in the South China and Indochina Blocks in a position relative to the distribution of Permo-Triassic Emeishan An inferred boundary of E-LIP
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large igneous province (E-LIP; Shellnutt, 2014).
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influence in the deep crust (the dashed line in red) is constrained by the Triassic high-temperature granulite xenoliths and enclaves in Daoxian (DX) basalts (Li et al.,
respectively.
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2016) and Darongshan (DRS) granitoids (Chen et al., 2011; Zhao et al., 2012), Metamorphism triggered by the same heat source form E-LIP and
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collision of these two Blocks along the Ailaoshan-Song Ma suture (in green) might have affected the Triassic rework of Song Chay Massif (SCM) and Yunkai massif (YKM), and build-up of the Xuefengshan Belt (XFSB) in South China Block, and Kontum massif (KTM) and Truong Son Belt (TSB) in Indochina Block at various extents.
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List of Tables
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Table 1. Description of studied samples from the Yunkai massif
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Table 2. Percentage of constituent minerals and information of monazite inclusions of the studied rocks Table 3. Comparison of EMP and NanoSIMS ages on same BSE domain in monazites
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Table 4. Record of monazite and zircon ages on rocks from the Yunkai massif.
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List of Appendices (designated to be included in the data repository)
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Appendix 1. LA-ICPMS zircon U-Pb dating results for samples from Yunkai massif.
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Appendix 2. Statistics of U-Pb zircon ages of dated grains from the studied samples.
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Appendix 3. EMP monazite ages of rock samples from Yunkai Massif.
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Appendix 4. Original EMP monazite ages used for comparison with NanoSIMS ages
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Fig. 1.
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Fig. 1. (a) Relevant tectonic units in the Yangtze and Cathaysia sub-blocks of the South China Block, including Yunkai massif (YKM), Song Chay massif (SCM), Darongshan (DRS), Xuefengshan Belt (XFSB), Hainan Island (HN), Wuyishan (WYS)-Biyunshan (BYS) massif in the South China, and Kontum massif (KTM) and Truong Son Belt (TSB) in the Indochina Block. The formerly proposed Early Paleozoic units are marked by blue and Triassic units, by pink colors. Ailaoshan (ALS)-Song Ma (SM) zone is the collision boundary between the South China and Indochina (+Simao) Blocks. (b) The simplified geological map of the Yunkai massif (modified from Zhou, 1995) and sample localities. (c) Relative position of major tectonic blocks in East Asia. Names of fault are 1: the Wuzhou-Bobai Fault, 2: the Luoding-Yuocheng Fault (one of the major shear zones), 3: the Wuchuang-Sihui Fault, 4: Chenzhou-Linwu Fault, and 5: Poko Zone (Ferrari et al., 2008).
55
ACCEPTED MANUSCRIPT (b)
(a)
Pl
Bt Pl
Ms
Ch Qt z
500 µm
Qt
Kfs
z
Ap
SC R
(c)
T
l
z
IP
Qt
Bt
500 µm
(d)
Bt Pl
Qtz Pl
Ms
Qt
NU
Ms
z
500 µm
500 µm
MA
(e) Kfs
(f) Qt
Kfs
Ms
z
Qtz
D
Chl
Gr
Pl
CE P
TE
Ms
500 µm
(h)
l Grt
(g)
Kfs
Bt Kfs
z
AC
500 µm
Ch
Qt
Pl
Ms
t
Bt
Pl 500 µm
Pl Rt
Qt z
500 µm
Fig. 2.
Fig. 2. Back-scattered electron (BSE) images showing some special petrographic features of four sample groups. The gneissic granites in Group 1 show (a) granular texture (sample 11GDGZA1) and (b) relatively high extent of deformation (sample 11GD01). Biotite gneiss in Group 2 (sample 23GX05-2) is a gneissoid enclave consisting mainly of (c) granular quartz, plagioclase and biotite, and a two-mica gneiss in Group 2 (sample 23GX15) shows (d) relict plagioclase grain. Biotite gneiss in Group 3 (sample 11GD05) contains (e) K-feldspar and plagioclase porphyroblasts, and garnet-tourmaline schist in Group 3 (sample 23GX14) generally shows (f) garnet poikiloblast with abundant quartz inclusions surrounded by foliated muscovite and chlorite. Group 4 undeformed granites show (g) quartz, plagioclase and biotite grains surrounded by interstitial K-feldspar (samples 28GX20), and (h) minor amounts of garnet and rutile (sample 28GX22). Abbreviation of mineral names follows those described by Whitney and Evans (2010).
56
ACCEPTED MANUSCRIPT
(a)
Ms Chl
(b)
Bt
Qtz Zrn
Chl
T
Ms Mnz
Mnz
IP
Ms
SC R
Fe-Mn-Ti oxide
Chl
(c)
Mnz
Qtz
Bt
(d)
Qtz
NU
Bt
Ms
Bt Mnz
Zrn
MA
Ms
Ms
Qtz
D
Qtz
CE P
Bt
TE
Ms
Mnz
AC
Qtz
Chl
Chl
Ms Qtz
Ms
(e)
(f) Chl
Ilm
Ms
Mnz
Ilm Sercite Chl
Ms Chl
Ilm
Fig. 3.
Fig. 3. BSE images showing mineral phases that enclose monazite grain (Mnz) are consistent with biotite, muscovite, chlorite, quartz and ilmenite based on energy dispersive X-ray spectra. Monazites in muscovite (a-b), in biotite contacting with quartz (c), in biotite contacting with muscovite (d), in biotite penetrated by muscovite veinlets (e), and in muscovite associated with ilmenite (f) are generally parallel to the foliation of the NE-trending Group 2 (a-d) and the NW-trending Group 3 (e-f) rocks.
57
AC
CE P
TE
D
206
Pb/238U
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
207
Pb/235U Fig. 4.
58
AC
CE P
TE
D
206
Pb/238U
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
207 235 207Pb/235U
Pb/
U
Fig. 4 (Cont.)
Fig. 4. Concordia diagrams and weighted mean 206Pb/238U age (2σ) for zircons from Yunkai samples in four groups: (a-d), (e-i), (j-n) and (o-p). Note that high population of Neoproterozoic zircons is a common feature in Groups 2 and 3 samples. N= number of analyses and MSWD= mean square of weighted deviates. Source data are listed in Appendix 1.
59
ACCEPTED MANUSCRIPT
(d) 23GX04R M09 (e) 23GX04R M15 (a)11GD01S M10 (c) 11GDGZA1S (b)11GDGZA1R 233 (5.48) 228 (7.78) 231 (7.67) 231 (6.77) 248 (8.27) 231 (4.44) 246 M09 232 (6.38) M08 236 M (5.84) (7.23) 247 1 (7.15) 237 (4.39)
259 (11.91) 265 (12.05)
243 (7.14) 233 (6.99) 240 (5.80) 435 (7.29) 427 (7.13) 437 (4.47) 441 (7.28)
D
TE
232 (5.59) 234 (5.87)
246 (1.99)
232 (2.24)
235 (8.32) 433 (6.80) 229 (8.22) 443 (6.87) (r) 28GX20R M03 (q) 11GD05R M06 237 Ap (7.95) 249 (7.29)
431 (5.40) 438 (5.42)
239 (6.85) 228 (6.02) 238 (6.74) 234 (6.10) 236 (6.13)
238 (5.76) 241 (6.42) 249 (6.60)
247 (s) 28GX20R (6.46) 224 M07 (5.26)
230 (5.71) 221 (5.67)
Allanite
Fig. 5.
AC
CE P
228 (5.81)
253 224 (5.23) (6.08) 234 (5.28)
(p) 11GD04R (o) 11GD04R 242(2.23) Kfs M03 M40
245 (1.80) 240 (1.78) 234 (1.90)
Huttonite
(l) 11GD06S M11 (m) 11GD07R M07 (n) 11GD03R M01 Huttonite 250 (5.55) 235 (9.06) 441 (7.37)
(k) 11GD06R M04 236 (7.67)
234 (6.05) 238 (6.49)
Ap
244 (4.61)
226 (5.82)
MA
(j) 23GX15R 240 (5.59) M54
SC R
445 (8.11) 440 (10.61) 439 (4.38) 437 (4.24) 445 (5.93)
250 (7.94)
230 (5.39) (f) 23GX04S M31 (g) 23GX04S M15 (i) 23GX15R M25 (h) 23GX05-1R M08 233 (4.45) 239 (10.43) 439 (21.45)
NU
235 (8.25)
IP
225 (5.90)
T
0
Fig. 5. BSE images of monazite in the polished section (R) and heavy concentrate (S) of the representative samples, with the EMP age results (Ma) and ThO2 contents (wt% in the parenthesis). Although different BSE domain and zonation, morphological feature, and degree of alteration are shown, they invariably present Triassic ages in all samples, including the overgrowing rim of two grains (d and o) that exhibit an Early Paleozoic core.
60
CE P
Fig. 6. EMP monazite isochron and the weighted mean age results for four groups of rocks in the Yunkai massif (a-d for Group 1, e-i for Group 2, j-n for Group 3 and o-p for Group 4). N= number of analyses and MSWD= mean square of weighted deviates. Source data are listed in Appendix 3 (slide is equal to polished section and sand is equal to heavy concentrate in the text).
AC
ig. 6.
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
61
ACCEPTED MANUSCRIPT
(a) 11GD05 NM19
(b) 11GD06 NM03 232 Ma (5.53, 1.68) 246 Ma (5.45, 1.67)
T
427 Ma (5.31, 2.23) 417 Ma (4.63, 2.67) 413 Ma (4.61, 2.72) 421 Ma (5.95, 2.42)
Z
IP
409 Ma (Pb-Pb)
(c) 23GX11 NM08
(d) 23GX04 NM06
MA
379 Ma (U-Pb) 379 (U-Pb) 435 Ma Ma (6.29, 1.18) 424 Ma (6.31, 1.16)
TE
CE P
553 Ma (14.41, 1.17) 563 Ma (13.11, 1.31)
D
545 Ma (11.00, 1.30)
503 Ma (U-Pb) 562 Ma (Pb-Pb)
(e) 11GD01 NM14
AC
429 Ma (6.67, 1.06) 443 Ma (6.92, 1.28)
NU
234 Ma (7.17, 1.11) 236 Ma (7.18, 1.14)
SC R
393 Ma (U-Pb) 425 Ma (Pb-Pb)
Z
Z
236 Ma (6.12, 1.58) 237 Ma (6.14, 1.57)
238 Ma (U-Pb) (f) 11GD01 NM15
222 Ma (4.97, 2.37) 209 Ma (4.91, 2.41) 210 Ma (4.85, 2.09) 226 Ma (5.00, 2.45)
229 Ma (4.04, 2.16)
224 Ma (U-Pb)
219 Ma (U-Pb)
229 Ma (5.51, 2.48)
218 Ma (U-Pb)
228 Ma (5.77, 1.85)
214 Ma (U-Pb) Fig. 7.
Fig. 7. BSE images of discrete monazite grains of some representative samples on which the EMP ages, with ThO2 and Y2O3 contents (wt% in the parenthesis), and NanoSIMS U-Pb and Pb-Pb ages are compared. Z: zircon in monazite.
62
ACCEPTED MANUSCRIPT (a) 23GX04S M15
237 Ma (6.75, 3.80) 231 Ma (6.74, 3.96) 246 Ma (6.83, 3.70)
(c) 23GX04R M22 232 Ma (6.31, 4.59) (Th,U)O4 227 Ma (6.30, 4.67) 230 Ma (5.53, 3.48) 224 Ma (5.48, 3.54)
2
T
1
IP
1
244 Ma (4.61, 3.42)
233 Ma (4.45, 3.73)
SC R
3
(Th,U)SiO4
3
(d) 11GD07R M01
(b) 11GD04R M30
1
MA
2
NU
Huttonite
231Ma (2.09, 3.68)
D
3
1 242 Ma (7.69, 1.90) 239 Ma (7.44, 1.85) 239 Ma (7.05, 1.43)
Fig. 8.
CE P
TE
221 Ma Ma (1.99, (1.93, 3.24) 3.22) 223 223 Ma (1.99, 3.24)
3
AC
Fig. 8. BSE images of monazite alteration zonations including 1: unaltered core (mnz 1), 2: intermediate monazite (mnz 2), and 3: the outer zone (mnz 3). The mnz 3 is characterized by the presence of abundant huttonite (the Th- and U-rich silicate with a bright reflection). The mnz 2 may be present (a-b) or absent (c-d) when the pristine monazite (mnz 1) was subjected to alteration probably involving fluid attacks. Numbers in the parenthesis are EMP ages (Ma) with ThO2 and Y2O3 contents (wt%).
63
ACCEPTED MANUSCRIPT Ab
(a) 11GD04
Bt Chl
IP
M10
T
Synchysite
Bt
SC R
M12
Kfs
Mag
Ab Kfs
Ilm (c)
M11 Ab Ms
(d) M12
D
(b) M10
MA
Ms
NU
Bt
Zr
246Ma (2.20)
230Ma (1.95) 227Ma (1.88)
CE P
227Ma (1.93) 242Ma (1.50)
TE
M11
229Ma (2.45) 238Ma (2.19) 243Ma (1.92)
Fig. 9-1.
AC
Fig. 9-1. Monazite inclusions with the EMP age results (Ma) and ThO2 contents (wt% in the parenthesis) in K-feldspar and plagioclase from a biotite gneiss (sample 11GD04).
64
ACCEPTED MANUSCRIPT Zr Pl
(a) 11GD01
Ilm
ap
Zr
Ms
Bt
T
Qtz
Ms
Ilm
M05
M08
Pl
M03
SC R
M04
M07 M06
Qtz
MA
NU
Bt
CE P
(d) M07
AC 239 Ma (6.03)
Pl
224 Ma (5.68)
223 Ma (5.22)
TE
225 Ma (5.78)
Kfs
212 Ma (6.80)
D
(b) M03
250 Ma (5.81)
Pl
IP
Zr
(e) M06
(c) M04
224 Ma (6.45) 235 Ma (5.13)
(f) M05
249 Ma (5.88)
241 Ma (4.80)
224 Ma (7.02) 228 Ma (6.08)
Fig. 9-2. Fig. 9-2. Monazite inclusions with the EMP age results (Ma) and ThO2 contents (wt% in the parenthesis) in biotite and muscovite from a gneissic granite (sample 11GD01).
65
(f) 23GX11S M24
(c)
Ce (d)
IP
(b)
SC R
1763Ma (5.68, 1.36) 1746Ma (5.63, 1.00)
538Ma (4.71, 3.54) 540Ma (4.67, 3.57) 541Ma (4.87, 3.89) 541Ma (4.85, 2.72) 536Ma (4.78, 3.67) 533Ma (6.46, 2.60) 543Ma (5.23, 3.49) 1755Ma (6.59, 0.85) 1728Ma (6.73, 0.92) 1748Ma (6.24, 0.79) 443Ma (5.00, 3.69) 446Ma (5.40, 1.39) 436Ma (5.15, 4.29) 438Ma (6.28, 2.58) 1743Ma (5.82, 0.76) 1769Ma (5.73, 0.94)
U
Th (e)
Y
NU
(a) 23GX11S M03
T
ACCEPTED MANUSCRIPT
(g)
(h)
Ce
Th
(i)
(j)
U
Y
MA
220Ma (7.29, 1.22) 232Ma (7.31, 1.18) 235Ma (8.36, 1.84)
TE
D
458Ma (6.44, 2.09) 458Ma (6.08, 2.96) 459Ma (6.44, 2.30)
CE P
257Ma (13.81, 1.10) 251Ma (14.32, 1.19)
Fig. 10.
AC
Fig. 10. BSE images with results of spot analysis of EMP apparent ages (Ma) and ThO2 and Y2O3 contents (wt% in the parenthesis), and CeLα, ThMα, UMα and YLα X-ray compositional maps of two representative monazites from sample 23GX11 (S denotes samples from heavy concentrates). (a-e) The presence of ~1750, ~540, and ~440 Ma domains (BSE) in grain M03; (f-j) ~450, ~260 and ~230 Ma domains in grain M24, in which the ~450 Ma domain is relatively low in Ce and Th. The color bar in (e) is equally applied for all X-ray maps.
66
Fig. 11.
AC
CE P
TE
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
Fig. 11. Cumulative age probability curves of studied rocks from the Yunkai massif. (a) EMP monazite ages of a meta-sandstone (sample 23GX11), and (b) total U-Pb zircon ages with histograms of four groups of rock. Resemblance of two curves with peaks at the Neoproterozoic and Early Paleozoic indicates the common provenance for monazite and zircon.
67
Fig. 12
D
MA
NU
SC R
IP
T
ACCEPTED MANUSCRIPT
AC
CE P
TE
Fig. 12. Schematic sketch showing the now-exposed Early Paleozoic metamorphic terrains (dark) and Triassic reactivated belts (hatched) in the South China and Indochina Blocks in a position relative to the distribution of Permo-Triassic Emeishan large igneous province (E-LIP; Shellnutt, 2014). An inferred boundary of E-LIP influence in the deep crust (the dashed line in red) is constrained by the Triassic high-temperature granulite xenoliths and enclaves in Daoxian (DX) basalts (Li et al., 2016) and Darongshan (DRS) granitoids (Chen et al., 2011; Zhao et al., 2012), respectively. Metamorphism triggered by the same heat source form E-LIP and collision of these two Blocks along the Ailaoshan-Song Ma suture (in green) might have affected the Triassic rework of Song Chay Massif (SCM) and Yunkai massif (YKM), and build-up of the Xuefengshan Belt (XFSB) in South China Block, and Kontum massif (KTM) and Truong Son Belt (TSB) in Indochina Block at various extents.
68
ACCEPTED MANUSCRIPT Table 1. Description of studied samples from the Yunkai massif Sample No. Latitude
Longitude
Locality
Lithology
Remarks
22°25’44.6” 110°31’08.1” Longsun-Gaozhou highway
Biotite gneiss
D*= N18E
23GX05-2
22°25’44.6” 110°31’08.1” Longsun-Gaozhou highway
Biotite gneiss
D*= N18E
23GX11
220031.3” 1110350.4” Near the Gaozhou reservoir
23GX13-1
222754.0” 1105652.1” Highway 207 at 3355.8 km
Meta-sandstone Garnet-tourmaline schist
23GX13-2
222754.0” 1105652.1” Highway 207 at 3355.8 km
23GX14
223257.4” 1105646.5” Highway 207 at 3344.9 km
23GX15 28GX20
IP
D*= N32W No monazite D*= N32W
221635.0” 1101043.7” Near the Lutou reservoir
Two-mica gneiss
D*= N10E
2316’02.0” 11106’06.1” Road 187 near the 10 km mark 2254’38.1” 11047’10.5” Dahe village in Yunrong Town (south to Xindi)
Undeformed granite Yin-nan batholith
SC R
Biotite gneiss Garnet-tourmaline schist
NU
28GX22
T
23GX04
Undeformed granite Beiliu batholith
11GDGZA1 22°00’36.2” 111°04’10.2” Longshou village in Yunlu town Gneissic granite 22°02’06.4” 111°03’04.0” Road between Shilong-Changpo Gneissic granite
11GD02
22°12’40.0” 111°07’18.7” Shuikou village
11GD03
22°32’56.8” 110°56’45.8” Highway 207 at 3344 km
Gneissic granite Garnet-tourmaline schist
11GD04
22°29’57.2” 110°56’13.2” Highway 207 at 3350 km
Biotite gneiss
11GD05
22°27’41.5” 110°57’08.3” Highway 207 at 3355-56 km
Biotite gneiss
11GD06
22°13’19.1” 110°35’44.9” Baoxu-Liujin
Two-mica gneiss
11GD07
22°18’26.4” 110°33’07.5” Liujin village
Gneissic granite
No monazite
CE P
TE
D
MA
11GD01
AC
D*: orientation of deformation in the field.
69
ACCEPTED MANUSCRIPT Table 2. Percentage of constituent minerals and information of monazite inclusions of the studied rocks.
Sample No.
Qtz
Pl
Kfs
Bt
Chl
11GD01
35
25
10
20
3
Ap
Grt
Ep
Tur
11GD02
40
30
10
11GDGZA1
38
30
12
(1)
(2)
(1)
23GX11
60
(3)
(6)
11GD07
40
12
(1)
(4)
(1)
35
15
20
25
(10)
(3)
(3)
(13)
35
25
31
(6)
(11)
(15)
23GX15
40
10
(4)
(2)
11GD06
35
Group 2 Gneissic granite
Bt gneiss
23GX04
TE
(6)
23GX05-2
CE P
(6)
AC
Two-mica gneiss
Grt tur schist
30
(2)
Bt gneiss
6
<3
<1
9
<3
<2
10
<1
<1
<2
15
<1
15
5
NU 30
(10)
2
3 (3)
<3
<3
<3
23
2
20
5
(32)
(1)
(18)
8
20
5
15
<2
(1)
(2)
<2
<2
25
25
<1
8
<2
<1
5
4
<2
12
<2
(1)
(4)
23GX13-1
30
25
5
20
5
23GX14
22
8
<2
12
15
(1)
(1)
11GD04
<3
30
45
12
(16)
(1)
(8)
(10)
(6)
11GD05
35
20
15
6
23GX13-2
<3
(1)
(2)
(9)
2
(7)
25
(4)
11GD03
16
MA
Meta-sandstone
5
(20)
D
(1)
IP
Gneissic granite
SC R
Group 1
Group 3
Ms
T
Lithology
(3)
25 (8)
5
4
10
15
<2
18
3
<1
<1
<3
(2)
40
30
<2
(3)
70
ACCEPTED MANUSCRIPT Group 4 15
18
25
(7)
(3)
(4)
(5)
(4)
28GX22
40
10
30
14
(1)
(2)
(1)
(1)
(4)
<3
<2
<2
<1
<3
<1
<1
T
34
IP
Undeformed granite 28GX20
SC R
1. Abbreviation of mineral names follows those described by Whitney and Evans (2010).
AC
CE P
TE
D
MA
NU
2. Number in the parenthesis (italic) indicates grains of monazite inclusions enclosed by this mineral or in the matrix of this sample.
71
ACCEPTED MANUSCRIPT Table 3. Comparison of EMP and NanoSIMS ages on same BSE domain in monazites#. EMP
NanoSims
UO2 Y2O3 PbO ThO2* (wt %) (wt %) (wt %) (wt %)
Age
U-Pb Error (2s)
206* 238
/
Age
IP
(Ma)
Error (2s)
207 206*
/
Error (2S)
0.0549
0.0005
409
20
(Ma)
0.84 0.95 0.71 0.87
2.51 1.13 2.30 0.59
0.140 0.102 0.159 0.092
7.867 10.236 8.569 9.258
419.4 234.8 437.3 235.3
30.8 17.3 30.6 22.7
0.0569 0.0011
356.8
6.7
0.0579 0.0011 0.0325 0.0006
362.8 206.1
6.8 3.9
6.33 5.34 6.80 5.49
1.06 0.82 0.53 0.68
0.65 2.22 1.17 1.68
0.100 0.083 0.158 0.078
9.765 7.998 8.538 7.693
242.4 245.7 436.0 238.6
13.9 23.7 31.6 20.3
0.0473 0.0319 0.0629 0.0621
0.0010 0.0006 0.0015 0.0013
298.2 202.6 393.1 388.2
6.2 4.0 9.1 8.2
0.0575
0.0006
513
24
0.0553
0.0005
425
20
13.76 11.00 6.30 6.21 6.86
1.00 0.69 0.22 0.18 0.14
1.24 1.30 1.17 1.39 1.65
0.410 0.308 0.128 0.127 0.133
17.063 13.288 7.023 6.781 7.320
557.9 544.7 429.5 441.3 428.3
31.5 40.0 31.6 32.4 31.5
0.0812 0.0019
503.1
11.5
0.0589
0.0005
562
19
0.0606 0.0012 0.0589 0.0012 0.0598 0.0012
379.1 369.1 374.4
7.6 7.2 7.3
2.80 5.44 6.13 NM06 11GDGZA1 4.61 NM09-1 8.57 NM09-2 5.37 NM11
0.49 1.19 0.91
1.54 1.63 1.58
0.044 0.093 0.091
4.387 9.295 9.062
234.8 237.3 236.5
22.7 22.9 21.5
0.0277 0.0007 0.0268 0.0006 0.0376 0.0008
175.8 170.4 238.2
4.6 4.0 5.0
0.0516
0.0010
267
42
0.26 0.43 0.29 0.16 0.45 0.66
2.25 1.77 1.82 1.60 1.49 2.29
0.053 0.105 0.062 0.044 0.132 0.126
5.431 9.964 6.309 4.678 13.208 7.083
231.2 249.4 230.9 222.8 236.6 418.5
21.0 18.3 22.5 21.5 17.4 31.8
0.0309 0.0006 0.0314 0.0006 0.0300 0.0007
195.9 199.1 190.8
4.0 4.0 4.2
0.0336 0.0007 0.0597 0.0013
212.8 373.7
4.3 8.2
0.0548
0.0006
403
24
0.54 0.57 0.51 0.56 0.61 0.61
0.061 0.071 0.055 0.073 0.069 0.074
6.687 7.278 5.691 7.583 7.164 7.684
216.9 228.6 228.9 228.0 228.1 228.1
20.9 21.9 22.1 22.0 22.0 22.0
0.0346 0.0344 0.0353 0.0337 0.0330 0.0327
219.2 217.8 223.6 213.4 209.4 207.6
11.5 11.1 11.4 4.9 4.4 4.9
NM19-1 NM19-2 NM21-1 NM21-2
NM02-1 NM02-2 NM03-1 NM03-2
NM08-2 NM08-3 NM16-1 NM16-2
D
NM08-1
NM04-2
4.16
11.75 4.93
NM18-3
11GD01 NM14-1 NM14-2 NM15-1 NM15-2 NM20-1 NM20-2 #
AC
NM18-1 NM18-2
4.93 5.51 4.04 5.77 5.19 5.71
2.33 2.48 2.16 1.85 2.23 2.22
TE
23GX04 NM04-1
MA
23GX11
NU
11GD06
SC R
5.43 7.18 6.25 6.44
CE P
11GD05
Pb-PbA ge Error (2s) (Ma)
Error (2s)
T
Smpl. no. ThO2
0.0018 0.0018 0.0018 0.0008 0.0007 0.0008
Examples showing two age groups and BSE domains in one single grain see Fig. 6.
EMP compositions listed here are the average of 1-4 spot analysis of the original data (Appendix 4). 206*/238=
206
Pb*/238U ratio and 207/206*= 207Pb/206Pb* ratio, in which 206Pb*= radiogenic 206Pb.
Pb-Pb ages with error >20% are not included.
72
ACCEPTED MANUSCRIPT Table 4. Record of monazite and zircon age clusters on rocks from the Yunkai massif. Sample no.
Adopted Age (Ma)
EMP Monazite Age-1 (Ma)
MS WD
EMP Monazite Age-2 (Ma)
MS WD
LA-ICPMS Zircon Age (Ma)
MS WD
N
11GD01
232.84.3
23313 (n=58)
0.025
23115 (n=38)
0.029
437.45.5 (n=22)
1.70
27
11GDGZA1
232.85.2
23018 (n=30)
0.038
23416 (n=38)
440.64.1 (n=23)
0.41
27
11GD02
n.a..
0.54
28
23814
0.030
(No mnz) ~1760, ~860, ~540, ~440, ~250
439.64.5 (n=18)
23GX11
(No mnz) 23814 (n=10) ~437, ~370
443.58.9 (n=5)
0.10
41
22910 (n=7)
5.3
30
98927 (n=16)
5.1
22
95038 (n=9)
5.1
22
435.05.1 (n=12)
0.79
31
22910
235.55.5 (n=67)
0.026
23GX05-2
234.1±5.4
234.1±5.4 (n=58)
0.16
23GX15
235.54.1 239.74.1 435.05.1
235.54.1 (n=102)
23311
23311 (n=14)
23GX13-1
n.a.
23GX13-2
23815
(No mnz) 23815 (n=9) ~433, ~362
23GX14
235.98.2
11GD03 11GD04
11GD07
23411 (n=17)
0.073
n.d.
n.d. 240.84.4 (n=66) 44128 (n=5)
0.17 0.021
n.d.
0.12
n.d.
235.98.2 (n=27)
0.097
n.d.
90432 (n=10)
24411
24411 (n=14)
0.035
(No mnz)
No age cluster
245.15.4 239.25.7 435.05.1
236.04.6 (n=105) 236.17.2 (n=21) 43616 (n=15)
0.14 0.24 0.047
(No mnz) 244.59.3 (n=18) 438.18.8 (n=51)
245.15.4 (n=5)
0.26
42
0.22 0.069
431.35.0 (n=10)
0.66
30
28GX20
232.14.4
233.16.2 (n=50)
0.27
23016 (n=34)
0.032
250.93.3 (n=16)
1.6
25
28GX22
229.28.4
229.28.4 (n=21)
0.069
n.d.
247.72.3 (n=18)
0.43
23
TE
CE P
Group 4
AC
11GD05
D
Group 3
0.091
0.065 0.065
MA
11GD06
0.21
0.026
IP
SC R
236.88.7 (n=53) 440.710.0 (n=12)
23GX04
NU
Group 2
T
Group 1
No age cluster
(No mnz)
100420 (n=20)
Monazite age-2: obtained from monazite in heavy concentrates. Zircon age: obtained from zircon in heavy concentrates (N= total number of zircon spot analysis). MSWD= mean square of weighted deviates.
9.1
30
4.6
33
n.d.
Monazite age-1: obtained from interstitial monazite and inclusions in major phases from petrographic slide.
35
n.a.: not available; n.d.: not determined.
73
30