The meta-gabbroic complex of Fushui in north Qinling orogen: A case of syn-subduction mafic magmatism

    The meta-gabbroic complex of Fushui in north Qinling orogen: a case of syn-subduction mafic magmatism Hong-Fu Zhang, Hong Yu, Ding-Wu Zhou, Juan Zhang, Yu-Peng Dong, Guo-Wei Zhang PII: DOI: Reference:

S1342-937X(14)00175-0 doi: 10.1016/j.gr.2014.04.010 GR 1259

To appear in:

Gondwana Research

Received date: Revised date: Accepted date:

18 January 2014 21 April 2014 30 April 2014

Please cite this article as: Zhang, Hong-Fu, Yu, Hong, Zhou, Ding-Wu, Zhang, Juan, Dong, Yu-Peng, Zhang, Guo-Wei, The meta-gabbroic complex of Fushui in north Qinling orogen: a case of syn-subduction mafic magmatism, Gondwana Research (2014), doi: 10.1016/j.gr.2014.04.010

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ACCEPTED MANUSCRIPT The meta-gabbroic complex of Fushui in north Qinling orogen:

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a case of syn-subduction mafic magmatism

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Hong-Fu Zhang1,2, Hong Yu2, Ding-Wu Zhou1, Juan Zhang1, Yu-Peng Dong1, Guo-Wei Zhang1

State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an

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710069, China

State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese

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Academy of Sciences, Beijing 100029, China

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Abstract:

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SIMS zircon geochronology and oxygen isotopes, as well as LA-ICPMS zircon Lu-

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Hf isotopic analyses, were carried out on a suite of rocks from the Fushui complex to probe its origin and tectonic significance for the evolution of the Qinling Group in the north Qinling orogenic belt. The Fushui complex comprises several phases of meta-gabbros: the dominant rock type is light-colored meta-gabbro, followed by mica-bearing and darkcolored meta-gabbros, with abundant dark gabbroic enclaves with different sizes and shapes and occasional occurrence of mica-bearing gabbroic pegmatite dikes. Age determination shows that the magmatic intrusion began at about 500 Ma and surged at 490480 Ma with the latest zircon crystallized at about 476 Ma. This age spectrum is remarkably coeval with the peak metamorphism (500-480 Ma) of the surrounding Qinling Group metamorphic rocks dated by zircons from eclogites, retrograded eclogites, HP mafic granulites and UHP felsic gneisses, representing syn-subduction mafic magmatism. These

ACCEPTED MANUSCRIPT late Cambrian to early Ordovician zircons display a slightly enriched Hf isotopic feature with a limited range in Hf(t) (-5~-2) and yield uniform TDM age (1.15~1.38 Ga). They also

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show a relatively small variation in δ18O (6.69±0.44‰ (2) ~ 8.75±0.34‰ (2)), much

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higher than the normal mantle value. The isotopic feature suggests that the Fushui complex

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was originally derived from enriched lithospheric mantle, which was metasomatized by melts from ancient continental sediments and altered MORB basalts. The Carboniferous

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zircons, dominantly occurring as rims of early Ordovician cores, display very similar Hf

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TDM ages (1.19-1.27 Ga) and oxygen isotopes as their counterparts, corresponding to their recrystallization at 335 Ma during amphibolite facies metamorphism when exhumed. Our

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study on the Fushui complex, together with the recently reported zircon U-Pb age data on

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the surrounding HP-UHP metamorphic rocks demonstrate that the Qinling Group preserves a complete cycle of tectonic evolution in an orogenic belt from an oceanic basin spreading,

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and mini-continent formation to deep subduction of mini-continent and multiple stage exhumations.

Keywords: Zircon geochronology; Hf and oxygen isotopes; Syn-subduction magmatism; Qinling Orogenic Belt; Fushui gabbroic complex.

1. Introduction The Qinling Mountains, lying in the middle part of China, is not only a critical geographic and hydrologic demarcation line, but also an important historical and cultural boundary. The mountains resulted from the Paleozoic Qinling orogeny, was modified by

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corresponding author: [email protected].

ACCEPTED MANUSCRIPT Mesozoic tectonic deformation and finally affected by Cenozoic intra-plate uplift (Zhang et al., 1995; 2001; Ratschbacher et al., 2003; Dong et al., 2011a&b; Bader et al., 2013; Wu

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and Zheng, 2013). The Qinling orogen is an important part of the Central Orogenic Belt of

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China, located between the North China Block (NCB) and South China Block (SCB) (Fig. 1a), and linked with the Tongbai-Dabie-Sulu orogens to the east and the Qilian-Kunlun

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orogens to the west. Since the 1980s, many geologists have investigated the tectonic

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evolution of the Qinling orogen, and numerous results have been obtained from the integration of geology, petrology, geophysics and geochemistry (e.g. Zhang et al., 1995;

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2001; 2013; Meng and Zhang, 2000; Ratschbacher et al., 2003; Dong et al., 2008; 2011a&b; Yuan et al., 2008; Zhu et al., 2011; Bader et al., 2013; Liu et al., 2013a&b; Wang et al.,

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2013a&b; Bader et al., 2013; Wu and Zheng, 2013; Li et al., 2014). The Qinling orogen is

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regarded as a composite orogen that experienced multiple stages of tectonic development.

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The final collision between the NCB and SCB occurred in the early Mesozoic along the Mianlue suture zone (Zhang et al., 1995; 2001; Meng and Zhang, 2000; Dong et al., 2011a; Wu and Zheng, 2013; Li et al., 2014), defining the present tectonic framework. The Qinling orogen comprises three major tectonic blocks: the North Qinling Belt (NQB) which includes the southern margin of the NCB; the South Qinling Belt (SQB); and the northern margin of the SCB in a sequence from the north to the south (Fig. 1b). These blocks were separated by the Shangzhou-Danfeng Suture Zone (SDSZ) and Mianlue Suture Zone (MLSZ), respectively (Fig. 1b). However, the early tectonic evolution of the Qinling orogen is poorly constrained, such as the timing of oceanic crust spreading and subduction and subsequent collision resulting in the closure of Paleotethys. The Qinling Group is a key to solve some of the key problems related to the evolution of the Qinling orogen (Fig. 1b). The Qinling Group is bound in its south by the

ACCEPTED MANUSCRIPT SDSZ, which is considered as the major suture zone in the Qinling orogen. The presence of the Neoproterozoic ophiolite melange in the Qinling Group was taken as the evidence for

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the existence of an oceanic crust subduction and related volcanism (Dong et al., 2011a&b;

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Wu and Zheng, 2013). In its northern and southern margins and middle part of the Qinling Group, several HP-UHP metamorphic rocks including eclogites, retrograde eclogites, HP

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mafic granulites and UHP felsic gneisses have been reported (Fig. 1b). Recent studies on

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these rocks have demonstrated that the early Paleozoic HP-UHP metamorphism was the consequence of northward continental deep-subduction along the SDSZ (Hu et al., 1994;

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Liu et al., 1995; 1996; 2013; Yang et al., 2002; 2003; Chen et al., 2004; Chen and Liu 2011; Cheng et al., 2011; 2012; Zhang et al., 2011a; Wang et al., 2011). These new results

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provide insights into the process and age of subduction of the oceanic crust as well as the

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amalgamation among continental blocks during the early Paleozoic in the Qinling orogen.

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The oceanic/continental subduction and continental collision could result in HP-UHP metamorphism (Liu et al., 2013 and reference therein). Mafic and felsic magmatism during the subduction, collision and post-collisional processes would occur due to dehydration and change in the thermal regime of subducted and exhumed slabs in progressive and retrogressive metamorphism. The tectonic setting and origin of meta-gabbros and granitoids in the Qinling orogen can be used to constrain the above processes and the timing of closure of the oceanic basins, continental collision and exhumation. Zircons from Paleozoic granitoids in NQB show three episodes of magmatism with peaks at ~500, ~452 and ~420 Ma (Zhang et al., 2013). These magmatic pulses are generally correlated with HP-UHP metamorphism at ca. 500 Ma, retrograde granulite-facies metamorphism at ca. 450 Ma and amphibolite-facies metamorphism at ca. 420 Ma, respectively (Zhang et al., 2013). In this study, we carried out a detailed investigation on the meta-gabbroic complex

ACCEPTED MANUSCRIPT of Fushui with a view to understanding the petrogenesis of this complex, and to reveal the

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tectonic significance.

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2. Geological background

The North Qinling Belt (NQB) is bound on the north by the Luonan-Luanchuan fault

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and on the south by the SDSZ (Fig. 1b) and extends for more than one thousand kilometers

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from east to west. The belt consists of four tectonic units, i.e. the Kuanping, Erlangping, Qinling and Danfeng groups from north to south. The Kuanping Group displays an

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association of greenschists, mica-quartz schists and quartz-rich marbles, and preserves young concordant detrital zircons with ages of 550-450 Ma from meta-sandstones,

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confirming that this group was formed in a period from the late Neoproterozoic to the early

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Paleozoic (Zhu et al., 2011; Shi et al., 2013). To the south of the Kuanping group, the

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Erlangping Group is dominated by greenschist- to amphibolite-facies backarc basin volcanic rocks associated with some sedimentary rocks. The Erlangping Group was intruded by 490-480 Ma granitoids (Zhang et al., 2013), suggesting its formation before 490 Ma.

The Qinling Group occurs as scattered lenticular bodies in the middle of the NQB (Fig. 1b), and consists mainly of biotite-plagioclase and garnet-sillimanite gneisses, mica-quartz schists, graphite marbles and amphibolites or garnet amphibolites, with some eclogites. Detrital zircons from the Qinling Group yield a range of ages from 1.5 to1.9 Ga (Lu et al., 2006; Shi et al., 2013) and zircon from gneissic granitoids show an age of 950 Ma (Wang et al., 2005; Liu et al., 2013 a&b). These observations suggest that the protoliths of the Qinling Group were formed in the Neoproterozoic. The Danfeng Group is exposed in the southern NQB (Fig. 1b). It is composed of arc volcanic-sedimentary rocks that underwent

ACCEPTED MANUSCRIPT greenschist to lower amphibolite facies metamorphism and was intruded by 430-517 Ma gabbroic rocks (Dong et al., 2011b). The youngest concordant detrital zircon age is 827 Ma,

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implying that the Danfeng Group was deposited after 827 Ma (Shi et al., 2013)).

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Another striking feature is the existence of many early Paleozoic granitoid plutons (Wang et al., 2009; Zhang et al., 2013) and anatectic veins in the NQB. A number of HP-

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UHP metamorphic rocks and ophiolites have been identified in the boundary areas between

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the Shaanxi and Henan provinces, with the peak metamorphism being about 500 Ma (Liu et al., 2013). All these data suggest that the NQB is an important tectonic belt that formed

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during the Neoproterozoic to early Paleozoic. Detrital zircon investigation on crustal evolution further reveals that the North Qinling terrain can be regarded as a separate micro-

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continent with a pre-Neoproterozoic evolution different from that of the North China block

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(Zhu et al., 2011; Liu et al., 2013a&b). The final assembly of the North Qinling terrain to

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the North China block took place after about 640 Ma (Zhu et al., 2011).

3. Fushui meta-gabbro complex The Fushui complex is a typical and largest meta-gabbroic complex in the North Qinling metamorphic terrane with an outcrop are of about 52.5 km2, and distributed along the region from the Fushui Town in the Shangnan County, Shaanxi Province to the Xiping Town in the Xixia County, Henan Province. It is tectonically situated at the northern side of the SDSZ, to the south of the Songshugou peridotite massif (Fig. 2). The complex extends in NWW direction parallel to regional tectonic line. Highly affected by the shearing during the formation of the SDSZ, the complex has experienced deformation and metamorphism and developed ductile shear structure with the widespread occurrence of mylonitization in its southern portion (Fig. 2, such as sample FS13-10). Gneissic structure is strongly

ACCEPTED MANUSCRIPT developed in the margin of the complex and the intensity of the gneissosity gradually becomes weak from the margin to the interior. The complex contains abundant peridotite

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fragments as enclaves with sharp contacts with the host (Fig. 2).

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The Fushui complex comprises a variety of phases and rock types: the dominant rock type is light-colored meta-gabbro as represented by sample FS13-05, followed by

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mica-bearing meta-gabbro (FS13-07), with a subordinate dark-colored meta-gabbro (FS13-

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09) (Fig. 2). These gabbros are medium to coarse grained and composed of primary minerals such as plagioclase, clinopyroxene, mica (Fig. 3). Orthopyroxene occurs only in

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few less metamorphosed rocks. These rocks were more or less altered to a certain degree with the clinopyroxene partially or completely transformed to amphibole or the zoisite

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together with albitization of plagioclase to form aggregate of albite, epidote, sphene, and

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apatite (Fig. 3). Some large grains of plagioclase remain relatively fresh with a weak

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sericitization (Fig. 3, FS13-09). These features together with the occurrence of inclusionfree rims of the large plagioclase crystals suggest that these rocks were subjected to fluidassisted amphibolite facies metamorphism. The ubiquitous occurrence of magnesium mica in these rocks suggests that the initial intrusion occurred at deeper levels and the alkali-rich feature of the primary magma, consistent with the high potassium in the whole rock analyses (Dong et al., 1997; Pei et al., 1997). The Fushui complex contains abundant dark gabbroic enclaves with size varying from centimeter to several meters and shapes of irregular breccia, crumb or tadpole set in the light-colored meta-gabbros. A representative sample FS13-06 has a similar mineralogy to its host (Fig. 2). The porphyritic structure is common in the enclave with large fresh pyroxene and/or plagioclase occurring as phenocrysts (Fig. 3).

ACCEPTED MANUSCRIPT The mica gabbroic pegmatite dike of 2-3 meters is occasionally found in the Fushui complex (Fig. 2). The sample collected in this study (FS13-04) is analogous in mineralogy

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to the host, but shows large mineral grains with the mica and pyroxene up to 2-3

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centimeters. It contains feldspar and is also altered with pyroxene uralitization (Fig. 3). All these rock types have similar major oxide compositions with SiO2 contents of 45-52 wt.%

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and the MgO contents of 4-10 wt.% for the majority (Dong et al., 1997; Pei et al., 1997).

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Broadly, they show basaltic composition with pyroxene fractionation. Moreover, the thin veins of monzonitic, syenitic and granitic composition of

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apparently later stage also occur in the complex, which were excluded in this study.

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4. Analytical methods

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Zircons were separated from samples using conventional heavy liquid and magnetic

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separator and further purified by handpicking under a binocular stereoscope. Subsequently, the grains were mounted in transparent epoxy together with a variety of reference zircon crystals, and polished sufficiently to expose any potentially older cores, and coated with ca. 30 nm of gold for ion-probe analysis. Before U-Pb dating, oxygen and Hf isotopic analyses, cathodoluminescence (CL) images were obtained using a microprobe at State Key Laboratory of Continental Dynamics, Department of Geology, the Northwest University, in order to identify zircon internal textures and choose potential target sites. The zircons were first analyzed for U-Pb isotopic dating, and then oxygen isotopes, and finally the same site was used to obtain the Hf isotope composition.

ACCEPTED MANUSCRIPT 4.1 Geochronology U-Pb dating of zircons was performed using a Cameca IMS-1280 ion microprobe at

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the SIMS Lab, Institute of Geology and Geophysics (IGG), Chinese Academy of Sciences. Analytical procedures were similar to those described by Whitehouse et al. (1997) and Li et

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al. (2009). A primary beam of O2- was accelerated at -13 kV, with an intensity of ca. 8-10

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nA. The aperture illumination mode was used with a 200 m primary beam mass filter

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aperture to produce even sputtering over the analyzed area (ellipsoidal spot size ca. 20 x 30 m). In the secondary ion beam optics, a 30 eV energy window was used, together with a

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mass resolution of approximately 5400, in order to separate Pb+ peaks from molecular

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interferences. Rectangular lenses were activated in the secondary ion optics to increase the

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transmission at high mass resolution. A single electron multiplier was used in ion-counting mode to measure secondary ion beam intensities.

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U-Th-Pb abundances and their isotopic ratios were determined relative to the reference zircon 91500 (Wiedenbeck et al., 1995), analyses of which were interspersed with those of unknown grains, using operating and data processing procedures described by Li et al. (2009). Measured compositions were corrected for common Pb using

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Pb, and a

present-day crustal composition (Stacey and Kramers, 1975) assuming that the common Pb is largely surface contamination introduced during sample preparation. Data reduction was carried out using the Isoplot/Ex v. 2.49 program (Ludwig, 2001).

4.2 Oxygen isotope Zircon oxygen isotopes were measured using a Cameca IMS-1280 at the SIMS Lab of the IGG, with analytical procedures similar to those reported by Li et al. (2010a). The

ACCEPTED MANUSCRIPT Cs+ primary ion beam was accelerated at 10 kV, with an intensity of ca. 2 nA (Gaussian mode with a primary beam aperture of 200 m to reduce aberrations) and rastered over a 10

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m area. The spot size was about 20 m in diameter. The normal incidence electron flood

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gun was used to compensate for sample charging. Negative secondary ions were extracted

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with a -10 kV potential. Oxygen isotopes were measured using multi-collection mode. The mass resolution used to measure oxygen isotopes was ca. 2500. The nuclear magnetic

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O⁄16O ratios were normalized using Vienna Standard Mean 18

O⁄16O = 0.0020052), and then corrected for the

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16 hr on mass 17. Measured

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resonance probe was used for magnetic field control with stability better than 3 ppm over

Ocean Water compositions (VSMOW;

instrumental mass fractionation factor (IMF) as follows:

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(18O)M = ((18O/16O)M  0.0020052 -1)  1000(‰) IMF = (18O)M(reference) - (18O)VSMOW,

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(18O)sample =(18O)M - IMF

In this way, the results are reported in the conventional 18O notation with reference to VSMOW in per mil. Two working zircon reference materials "Qinghu" and "Penglai" were used to monitor the machine stability. With highly stable relative gain between two Faraday cup (FC) amplifiers, the reproducibility of the "Penglai" and "Qinghu" zircon reference samples was better than 0.26‰ (1s), and the internal precision of a single analysis was generally between 0.1‰ and 0.3‰ (2 SE) (Li et al., 2010b).

4.3 Lu-Hf isotope In-situ zircon Hf isotopic analyses were carried out at the IGG using a Neptune MCICPMS with an ArF excimer laser ablation system. During analyses, spot sizes of 32 or 63

ACCEPTED MANUSCRIPT m and a laser repetition rate of 10 Hz with 100 mJ were used. Details of the technique are described by Xu et al. (2004). Reference zircon (91500) gave

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Lu/177Hf ratio of 0.00031, similar to the commonly

Hf/177Hf ratio of 0.28230806 (2) measured using the solution method

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accepted

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Hf/177Hf ratio of

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0.28229116 (2, n=35) and

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(Blichert-Toft, 2008). The notations of Hf, fLu/Hf and THf are after Zhang et al. (2011b).

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5. Analytical results

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The CL images of representative zircons taken after U-Pb age determination and oxygen isotope analyses are presented in Fig. 4. A summary of the U-Pb dating, oxygen

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and Hf isotopes of zircons from the Fushui meta-gabbroic complex is given in Table 1 with

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the whole data set provided as electronic Supplementary Table. The CL images reveal a variety of internal structures from irregular patchy and ribbon texture to relatively fine-

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scale oscillatory zoning typical of magmatic crystallization. Apart from the large variation in grain morphology, zircons from this complex also show a marked range in their U and Th contents and Th/U ratios (Table 1 and Fig. 5). However, these zircons gave a limited range in U-Pb ages (Table 1 and Figs. 6 and 7), Hf isotope ratios (Table 1 and Fig. 7), and oxygen isotopes (Table 1 and Figs. 8 and 9). The salient features are briefly summarized in the following sections.

5.1 Crystal morphology Based on morphology, the zircon crystals in our samples can be divided into three populations. The grains from the light-colored meta-gabbro (FS13-05) and the micabearing meta-gabbro (FS13-07) are 80-350 m in sizes and have granular or stumpy

ACCEPTED MANUSCRIPT euhedral morphology with the length:width ratios about 1:1-1:2 (Fig. 4). They show a variety of internal structures in CL images from irregular patchy texture to relatively fine-

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scale oscillatory zoning typical of magmatic crystallization (Fig. 4). These zircons show a

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relatively narrow range in U and Th contents and Th/U ratios (Table 1 and Fig. 5). Zircons from the dark-colored meta-gabbro (FS13-09), the mica gabbroic pegmatite

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dike (FS13-04), and the dark gabbroic enclave (FS13-06) are large crystals ranging from

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100 m to over 600 m and show stumpy euhedral shapes in small grains or irregular fragments during the zircon separation with the rocks broken to 60 mesh. They display a

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variety of dark internal structures in CL images from irregular patchy or ribbon texture to wide-scale oscillatory zoning typical of mafic magmatic crystallization (Fig. 4). Some

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crystals have an envelope shape such as FS13-09 G15 (Fig. 4) or taxitic textures with the black and white ribbon (FS13-06, Fig. 4). These zircons have an extremely large variation

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in U (134~2770 ppm) and Th (93-6765 ppm) contents and Th/U (0.28~2.44) ratios (Table 1 and Fig. 5), especially for samples FS13-06 and FS13-09. The third population of a few zircons occurs as single crystals (Fig. 4, FS13-06 G17 and G34) or dominantly as rims of other two population crystals (Fig. 4; FS13-07 G1 and G6; FS13-09 G14). Interestingly, whilst it is difficult to identify this population from other two in CL images (Fig. 4), this population of zircons have extremely low Th/U ratios (0.09~0.14) due to the low Th contents (129~180 ppm) and high U contents (1043~1472 ppm) (Table 1 and Fig. 5), a typical metamorphic origin.

5.2 Zircon geochronology

ACCEPTED MANUSCRIPT 45 analyses on 44 zircon crystals from sample FS13-05 yield an excellent U-Pb concordant age of 490.21 Ma (MSWD = 0.14) (Table 1 and Fig. 6). The data also produce

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a similar weighted mean age 49010 Ma (MSWD = 0.051) (Table 1 and Fig. 6). This result

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suggests that these zircons were formed at 490 Ma and did not suffer any Pb loss during

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later amphibolite facies metamorphism. 38 analyses on 31 zircon crystals from sample FS13-07 of another dominant type of meta-gabbros in this complex yield no U-Pb

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concordant age, but a weighted mean age 47811 Ma (MSWD = 0.053) (Table 1 and Fig. 6). This age is generally consistent within errors with the zircon TIMS U-Pb age of

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480.03.4 Ma reported in meta-gabbros from this complex (Li et al., 2006). These data

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suggests at least two stages of magma pulses.

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Zircons from the dark gabbroic enclave (FS13-06) have two groups: the first group yield a good U-Pb concordant age of 497.24.4 Ma (MSWD = 0.063) with the weighted

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mean age of 49718 Ma (n=17, MSWD = 0.037) (Table 1 and Fig. 6). This age is slightly older than the age of the above host rock, but consistent with the baddeleyite TIMS U-Pb age of 501.41.2 Ma obtained from the same complex (Li et al., 2006). Thus, about 500 Ma may represent the first pulse of magma intrusion in this complex. Zircons of the second group yield no concordant age, but a weighted mean age of 47318 Ma (n=15, MSWD = 0.007) (Table 1 and Fig. 6), showing a prolonged or two stages of crystallization. Zircons from samples FS13-04 and FS13-09 yield excellent U-Pb concordant ages of 477.11.2 Ma (MSWD = 6.2) and 482.80.96 Ma (MSWD = 0.00) and weighted mean ages of 47611 Ma (n=33, MSWD = 0.066) and 482.69.2 Ma (n=52, MSWD = 0.055) (Table 1 and Fig. 6), respectively. The youngest age of 476 Ma obtained in FS13-04 is

ACCEPTED MANUSCRIPT consistent with its occurrence of gabbroic pegmatite dike, representing the latest stage of magma pulse.

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A few zircons occur as single crystals or as rims of other crystals from different

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samples (Fig. 4) yield a good U-Pb concordant age of 335.21.4 Ma (MSWD = 2.3) and a

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weighted mean age of 335.69.2 Ma (n=13, MSWD = 0.084) (Table 1 and Fig. 6). This age

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is interpreted as the metamorphic age at the amphibolite facies during the later uplifting.

5.3 Zircon Hf isotope

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The population of Late Cambrian to early Ordovician zircons in our samples shows a limited range in Hf isotope compositions (Table 1 and Fig. 7) with the majority in the range

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of -5 ~ -2 in Hf(t) and -15 ~ -12.5 in Hf(0). These zircons also yield a limited range in TDM ages (1.15-1.38 Ga) with a mode around 1.18-1.30 Ga (Table 1 and Fig. 7).

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The Carboniferous zircons from this meta-gabbroic complex display markedly uniform Hf isotopic compositions with the Hf(t) between -7.9 and -5.8 (Table 1 and Fig. 7). The Hf(0) (-15 ~ -12.6) values are remarkably consistent with the Hf(0) values of the Late Cambrian to early Ordovician zircons (Table 1 and Fig. 7). The Carboniferous zircons also produce a same narrow range in TDM ages (1.19-1.27 Ga) as those of the Late Cambrian to early Ordovician zircons (Table 1 and Fig. 7).

5.4 Zircon oxygen isotope Late Cambrian to early Ordovician zircons in our samples have a relatively small variation in δ18O (6.69±0.44‰ (2) – 8.75±0.34‰ (2); Table 1 and Fig. 8), in response to the limited variation in Hf isotopes. Two populations have been observed: zircons from the

ACCEPTED MANUSCRIPT samples FS13-05 and FS13-07 show slightly higher δ18O with a peak around 8 and the highest up to the 8.75±0.34‰ (2), i.e. to the δ18O-highly enriched supracrustal value. In

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contrast, zircons from other samples FS13-09, FS13-06 and FS13-04 indeed display a

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slightly lower δ18O with the peak at 7.4, still much higher than the normal mantle value (Fig. 9).

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Carboniferous zircons have very similar δ18O values to the Late Cambrian to early

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Ordovician zircons in the specific samples (Fig. 8).

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6. Discussion

6.1 Episodic magma intrusion into the deep lower crust

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Late Cambrian to early Ordovician multiple pulses of magma: About two hundred analyses on 187 zircons from the Fushui meta-gabbroic complex clearly show late

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Cambrian to early Ordovician multiple pulses of mafic magma (Table 1 and Figs. 4 and 6). The extremely large variations in U and Th contents and their high Th/U ratios generally over 0.5 (Table 1 and Fig. 5) as well as the feature of CL images demonstrate that these late Cambrian to early Ordovician zircons were of magmatic origin. Thus the excellent U-Pb concordant ages and/or weighted mean ages obtained in this study from these rocks should reflect their crystallization ages. Therefore, the concordant age of 497 Ma (Fig. 6) obtained in zircons from the dark gabbroic enclave are taken to represent the first pulse of magmatism. This result is consistent with the pioneering work of TIMS U-Pb dating on the baddeleyite (501.41.2 Ma) from the Fushui complex (Li et al., 2006). Thus, the magma intrusion began at about 500 Ma. An alternative interpretation is that zircons of 500 Ma crystallized in the mantle rather than during the emplacement since some zircons have a

ACCEPTED MANUSCRIPT prolonged mantle residence time as indicated by xenocrystic zircons from the western Eger rift (Siebel et al., 2009). Whatever which explanation, the initial magmatism undoubtedly

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began at about 500 Ma.

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Following the initial magmatism was the massive intrusion of magma that produced the light-colored meta-gabbros at about 490 Ma as indicated by the excellent concordant

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age given by zircons from sample FS13-05 (Fig. 6). The third pulse of magma represents

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the dark-colored and mica-bearing meta-gabbros (samples FS13-09 and FS13-07; Fig. 6) with ages of 482-478 Ma. This period of magmatism was also identified in zircons by the

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TIMS method which gave a 480.03.4 Ma age (Li et al., 2006). Thus, the age of ca. 480 Ma reflects the third magma pulse. The magma intrusion was completed at about 476 Ma

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when the mica gabbroic pegmatite dike formed (Fig. 6). Our results thus identify an episodic late Cambrian to early Ordovician magma intrusion of the Fushui meta-gabbroic

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complex at the peaks of about 500 Ma, 490 Ma, 480 Ma, and 476 Ma. Lower crust depth of magma intrusion: Petrological and mineralogical observation such as the presence of typical retrograde metamorphic amphibole, epidote group minerals and albite (Fig. 3) demonstrates that this complex must have experienced amphibolite facies retrograde metamorphism which will be discussed in the next section. This suggests that the initial magma must have intruded at domains deeper than the amphibolite facies, perhaps the garnet granulite facies of lower crust. No thermal contact metamorphic aureoles were observed between the meta-gabbros and the wall rocks (Dong et al., 1997; Pei et al., 1997; this study) which also illustrate that the complex intruded to the considerable depth of deep crust, with the similar temperature and pressure conditions between the magma and the ambient wall rock. Sharp contact of the gabbro with entrained peridotite further

ACCEPTED MANUSCRIPT suggests that the complex initially formed at the depth of the lower crust, since the

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Songshugou peridotite is believed to represent the fragment of the oceanic lithosphere.

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6.2 Carboniferous retro-metamorphism and exhumation

Minor zircons rims surrounding magmatic grains were observed in three different

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samples (Fig. 4; FS13-07 G1 and G6; FS13-09 G14) which yield much younger U-Pb

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concordant and weighted mean ages of around 335 Ma than the intrusion ages (Table 1 and Fig. 6). Their extremely low Th/U ratios (0.09~0.14) (Table 1 and Fig. 5) demonstrate that

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these zircons are of a typical metamorphic origin. Thus, the 335 Ma age corresponds to retrograde metamorphism when the complex was uplifted to the depth of amphibolite facies

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from the granulite facies. The similarity in compositions such as the Hf(0) and TDM ages

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(Fig. 7) and δ18O values (Fig. 8) between Early Carboniferous and late Cambrian to early 18

O-enriched or

18

O-depleted components with

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Ordovician zircons suggests that no

different Lu/Hf ratios were involved during the amphibolite facies retrograde metamorphism although aqueous fluid must have been involved to produce water-bearing minerals.

The similar reheating events were also recorded in the felsic garnet gneiss and garnet amphibolite of the Qinling complex (Bader et al., 2013). The matrix monazites from felsic garnet gneiss yielded a continuous series of progressively younger ages, suggesting early Ordovician metamorphism overprinted by early Carboniferous reheating (~343 Ma, Bader et al., 2013). A garnet amphibolite contains relic rutile and ilmenite in titanite and the titanite formed at 324 ± 12 Ma (Bader et al., 2013), within error overlapping the monazite ages of felsic garnet gneiss and the zircon ages obtained in this study. These are the youngest ages of the retro-metamorphism obtained so far.

ACCEPTED MANUSCRIPT The mechanism of exhumation of the complex during the period from early Ordovician to Early Carboniferous is difficult to be constrained. A clue can be drawn from

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the deformation structure of the complex. The following deformation characteristics have

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been recorded: 1) meta-gabbroic body distributed as a concentric zoning; 2) the gneissic structure only developed at the margin of the intrusive, parallel with the contact and

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regional structure (Fig. 2); 3) the planar fabric pattern and three-dimensional morphology

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of deformation of the inclusions reflect a flattening deformation (Pei et al., 1993). According to these characteristics, Pei et al. (1993) proposed that the emplacement

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mechanism of the complex was dyke propagation in the early stage followed by oblique diapirism of a balloon-like expansion, and experienced a transition from passive into active

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emplacement. In other words, the emplacement mechanism was controlled by the

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lithosphere extension, subsequent large-scale sinistral crustal shearing and its shortening

complex.

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effect on the local level. In general, the Fushui complex is a syntectonic plutonic intrusive

The fact that the gabbro gravels have not been found in the upper Cretaceous continental red basin deposition, but frequently found in Quaternary sediments on the southern side of the Fushui complex reveals that the complex was exposed after the Late Cretaceous, but prior to the Quaternary (Pei et al., 1997). Thus, a schematic figure can be drawn to show the process of uplift of the Fushui complex (Fig. 10) with 500-480 Ma magma intrusion into lower crust to produce the gabbros, followed by exhumation of the gabbros to the middle crust during early Cretaceous where they were metamorphosed with the replacement of pyroxene and plagioclase by minerals of amphibolite facies. Finally the pluton was uplifted as an oblique diaper to form the Fushui meta-gabbroic complex presently seen.

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6.3 Magma sources and lithospheric mantle enrichment

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To characterize the magma source, the crustal assimilation factor must be discussed.

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As described above, the initial magma intruded at a greater depth, perhaps of the lower crust. Thus, the assimilation of upper and middle crustal materials is impossible, consistent

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with no inherited zircon or old zircon core in these samples (Fig. 4). The low SiO2 and

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relatively high MgO contents (Dong et al., 1997; Pei et al., 1997) show that the primary magma was derived from the mantle and the assimilation of lower crustal materials is

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insignificant. Though magma fractionation indeed occurred in this complex, it did not cause any significant change in isotopic composition. Thus, the hafnium and oxygen

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isotopic compositions can be used to reflect the magma source character of these gabbros.

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The negative Hf(t) and the high δ18O values given by the zircons of the gabbros

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(Table 1 and Figs. 7 and 8) suggest their derivation from slightly enriched lithospheric mantle rather than the normal asthenosphere since the latter generally has a depleted Hf isotope feature (Hf(t) >5) and normal mantle oxygen isotope (δ18O = 5.2‰ ~ 5.8‰ with a predominant variation in 5.5±0.5‰) obtained by the Mid-Ocean-Ridge Basalt (MORB) glasses from the Pacific Ocean Islands and mid-Atlantic ridge (Harmon and Hoefs, 1995; Cooper et al., 2004; Bindeman et al., 2012). The generally constant Hf and oxygen isotopes with the predominance in a narrow range (Hf(t)= -5~-2 and δ18O = 7-8‰; Figs. 7 and 8) observed in these zircons from diverse phases and rock types imply that the magma source, i.e. the lithosphere mantle, was relatively homogenous in terms of Hf and oxygen isotopes. Slightly higher oxygen isotopes in the high silica light-colored gabbros as compared to the more mafic dark-colored counterparts (Fig. 8) may reflect a lower degree of partial melting

ACCEPTED MANUSCRIPT rather than a heterogeneous source, i.e. more felsic component involved in the melting, which previously incorporated into the mantle during the mantle metasomatism.

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We now evaluate the process that resulted in the enrichment of the relatively young

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arc-like lithosphere mantle. The coupled Hf and oxygen isotopes in zircons are extremely important in deciphering such an enriched process. Kemp et al. (2006, 2007) first used the

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zircon δ18O versus Hf relation and found it to be useful for identifying the mixing of the

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different components such as the mantle and supracrustal materials. As shown in Fig. 9, all zircons from this complex have much high δ18O ratios than the normal mantle value

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(Valley et al., 1998), suggesting considerable input of melts with very high δ18O ratios such as supracrustal materials.

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Now we address the supracrustal materials, whether subducted altered oceanic basalts or ancient continental sediments or both. The mafic character of these gabbros and much

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higher δ18O than the normal mantle value requires that the lithospheric mantle source was metasomatized by small volume melt with an extremely high δ18O ratio, thus ancient continental sediments must be involved. This is also shown by the zircon Hf TDM ages. Available studies show that the protoliths of the MORB basalts for the eclogites, highpressure granulites and retrograde eclogites were formed during the period of about 850650 Ma (Table 2; Liu et al., 2009; Chen and Liu, 2011; Cheng et al., 2011; Li et al., 2012; Wang et al., 2011; Liu et al., 2013). The corresponding lithospheric mantle should also be formed during that period. The much older and constant TDM ages (1.18-1.30 Ga) obtained from these zircons suggest that the lithospheric mantle was metasomatized by melts from ancient continental sediments. The relatively low Pb isotopes (206Pb/204Pb)i =16.2 ~ 17.2) of the whole rock analyses of the Fushui complex (Dong et al., 1997) are also consistent with the above suggestion.

ACCEPTED MANUSCRIPT Whole rock analyses on the Fushui complex show that the magma source had a relatively low (143Nd/144Nd)i (Nd(t) = -3.5 ~ +0.2), but an exceedingly high (87Sr/86Sr)i

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(0.70942 ~ 0.71328) composition (Dong et al., 1997), displaying a clear oceanic water

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alteration feature. Thus, melts or fluids from the altered oceanic basalts must have been involved in the formation of gabbroic magma source. In general, both the ancient

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continental sediments and altered oceanic basalts were involved in the generation of the

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magma sources.

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6.4. Syn-subducted magmatism and the evolution of the Qinling Group Syn-subducted magmatism: Several recent studies on a variety of HP-UHP

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metamorphic rocks in the Qinling Group show that these metamorphic rocks underwent peak metamorphism during 500-480 Ma, followed by retrograde metamorphism at about

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460-420 Ma, as shown by the zircon U-Pb ages (Table 2; Yang et al., 2002; Chen et al., 2004; Su et al., 2004; Liu and Sun, 2005; Li et al., 2009, 2012; Liu et al., 2009; Chen and Liu, 2011; Cheng et al., 2011, 2012; Wang et al., 2011; Zhang et al., 2011a; Liu et al., 2013). This peak metamorphism was coeval with the period of the Fushui magma intrusion. Thus, the Fushui complex is a typical syn-subducted mafic magmatism. The enriched geochemical feature of these gabbros with a low Nd and high oxygen isotopes is also consistent with the syn-subducted melting of the previously metasomatized lithospheric mantle of the Neoproterozoic newly-formed mini-continent (Fig. 11a). Subduction of the mini-continent: As shown on Fig. 1 and Table 2, HP-UHP metamorphic rocks were found in many places such as Guanpo, Shuanghuaishu, Qingyouhe, Songshugou, Zhaigen, Xixia. Almost all the rocks of the Qinling Group underwent HP-UHP metamorphism to produce eclogite, retrograde eclogite (garnet

ACCEPTED MANUSCRIPT amphibolite and plagioclase amphibolite), garnet pyroxenite, HP mafic granulite, and UHP felsic gneiss, as well as the Songshugou peridotite massif. This means that the mini-

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continent as represented by the present block of the Qinling Group was subducted to the

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depth of eclogite facies on a whole (Fig. 11a).

Scenarios of the Qinling Group or North Qinling Unit: Several stages of the

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evolution of the North Qinling Unit can be identified as follows with the schematic

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cartoons showing these evolutionary stages given in Fig. 11a. (1) 850-650 Ma: Zircon U-Pb dating shows that the protoliths of these HP-UHP metamorphic rocks dominantly formed at

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the period of 850-650 Ma (Table 2). Petrological studies including the whole rock trace element patterns show that the protoliths of the eclogites, retrograde eclogites, and HP

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mafic granulites had N-MORB and/or E-MORB characteristics (Chen and Liu et al., 2011;

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Dong et al., 2008). Thus, this period was the birth of the North Qinling unit through the

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MORB-like volcanism in a wide oceanic basin or continental margin, creating a minicontinent or a microblock. The lithosphere profile beneath the mini-continent was the peridotite, sheeted dikes (Fig. 11c), pillow lava, continental sediments, and altered basalts from bottom up, i.e. the typical ophiolite suite were produced. (2) Prior to 500 Ma, melts from the subducted altered basalts and continental sediments of the Paleotethyan oceanic lithosphere infiltrated the arc-like mantle to yield metasomatized enriched lithospheric mantle, which had an enriched character with high oxygen isotope and low Nd isotope compositions. (3) 500-480 Ma: the lithosphere of the mini-continent of the North Qinling unit was as a whole subducted to the depth of the eclogite facies. The altered basalts and sheeted dikes were metamorphosed to eclogites (Chen and Liu, 2011; Cheng et al., 2012) or HP mafic granulites (Chen et al., 2004; Zhang et al., 2011a; Wang et al., 2011; Liu et al., 2013), and the ancient continental sediments to the UHP felsic gneisses (Yang et al., 2002;

ACCEPTED MANUSCRIPT Liu et al., 2009; 2013; Zhang et al., 2011a) with some carbonates to marbles. The lithospheric mantle was highly deformed with extensive mylonitization and re-

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crystallization of dunites and the sheeted dikes transformed to eclogites (Fig. 11c). During

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this period, the mafic melt from the partial melting of the previously metasomatized lithospheric mantle intruded into the lower crust to produce the Fushui gabbroic complex,

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which included the sample of the transition zone of the lower crust-lithospheric mantle. The

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anatexis of the felsic gneiss yielded 500~480 Ma granitoid dikes widespread in the Qinling Group (Zhang et al., 2013 and references therein). (4) 460-420 Ma: the subducted and

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metamorphosed lithospheric slab was uplifted to granulite facies and amphibolite facies due to the break-up of the subducted oceanic slab and the opening of the Erlangping back-arc

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basin. The eclogites and UHP felsic gneiss were subsequently overprinted by medium

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pressure granulite facies metamorphism at 450 Ma and then amphibolite facies retrograde

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metamorphism at 420 Ma to produce the ubiquitous presence of the garnet amphibolites and plagioclase amphibolites in the Qinling Group (Su et al., 2004; Liu and Sun, 2005; Li et al., 2009, 2012; Liu et al., 2009, 2013; Chen and Liu, 2011; Cheng et al., 2011). The eclogite were also retrograded to garnet amphibolite with fresh garnet and fine-grained plagioclase amphibolites with no garnet (Fig. 11c). No zircons formed at this period in the Fushui gabbros. Paleozoic felsic magmatism as constrained from several zircon U-Pb studies on granites from the Qinling Group also shows three peaks at 500 Ma, 452 Ma and 420 Ma (Zhang et al., 2013), respectively, consistent with the above metamorphic and retrograde metamorphic peaks. Therefore, the first episode of granitic magmatism is inferred to have resulted from continental subduction, i.e. the syn-subducted melting of sedimentary rocks. The second and third episodes of magmatism were attributed to subducted slab uplifting and decompressional melting of the UHP felsic gneiss and

ACCEPTED MANUSCRIPT retrograde amphibolites to produce the widespread occurrence of early Paleozoic granites and migmatites in the North Qinling unit. Partial melting of the enriched lithspheric mantle

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and its crust due to the mantle upwelling beneath the Erlangping unit yielded 440 Ma

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Erlangping gabbros (Wang et al., 2013b) and about 460 Ma mafic volcanic and 430 Ma intermediate-felsic granitoids (Bader et al., 2013). (5) 335 Ma: the Fushui gabbros exhumed

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to amphibolite facies and underwent retrograde metamorphism to crystallize the zircon rims.

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(6) Tertiary-present: the subducted continental slab exhumed to form the presently observed the North Qinling unit and Fushui meta-gabbroic complex (Fig. 11b). In summary,

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rocks of the Qinling Group record a complete cycle of tectonic evolution of an orogenic belt from an oceanic basin spreading, through mini-continent formation to deep continental

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7. Conclusions

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subduction and multiple stage exhumations.

The following salient conclusions can be drawn from this study: (1) Fushui gabbroic complex resulted from episodic magma intrusion, which is consistent with the peak metamorphism (500-480 Ma) of the surrounding UHP metamorphic rocks. Thus the Fushui complex is a typical syn-subduction product of mafic magmatism. (2) Limited range in zircon Hf(t) and a high and small variation in δ18O demonstrate that the Fushui meta-gabbroic complex was originally derived from the lithospheric mantle, which was metasomatized by the melts from the ancient continental sediments and altered MORB-like basalts. (3) When the gabbros exhumed to amphibolite facies at 335 Ma, they underwent retrograde metamorphism to produce the meta-gabbroic complex presently exposed.

ACCEPTED MANUSCRIPT (4) Integration with recent zircon U-Pb dating work on the surrounding UHP metamorphic rocks, we conclude that the Qinling Group record a complete cycle of tectonic

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evolution in an orogenic belt from an oceanic basin spreading, mini-continent formation

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to deep subduction of the mini-continent and multiple stage exhumations.

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ACKNOWLEDGEMENTS

The authors would like to thank Tang G.Q. and Lin X.X. for their assistance with zircon age

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determination and oxygen isotope analyses and Yang J.H. for his assistance with zircon Lu-Hf isotope

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analyses. Professor M. Santosh is thanked for his careful revision on the earlier version of the manuscript. Two anonymous reviewers are thanked for their constructive comments improved the quality a lot. This research was financially supported by the Innovation Team Project from the Ministry

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of Education and the Natural Science Foundation of China (Grant 91014007 and 91214203).

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Siebel W., Schmitt A.K., Danišík M., Chen F.K., Meier S., Weiß S., Eroglu S., 2009. Prolonged mantle residence of zircon xenocrysts from the western Eger rift. Nature Geoscience 2, 886-890. Su L., Song S.G., Song B., Zhou D.W., Hao J.R., 2004. The SHRIMP zircon U-Pb ages of the garnet pyroxenite and Fushui complex from Songshugou area and its constrain on the tectonic evolution of Qinling orogenic belt. Chinese Science Bulletin 49(12), 1209-1211 (in Chinese). Stacey J.S., Kramers J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters 26, 207-221. Valley J.W., Kinny P.D., Schulze D.J., Spicuzza M.J., 1998. Zircon megacrysts from kimberlite: oxygen isotope variability among mantle melts. Contributions to Mineralogy and Petrology 133, 1-11. Wang H., Wu Y.B., Gao S., Liu X.C., Gong H.J., Li Q.L., Li X.H., Yuan H.L, 2011. Eclogite origin and timings in the North Qinling terrane and their bearing on the amalgamation of the South and North China blocks. Journal of Metamorphic Geology 29(9), 1019-1031.

ACCEPTED MANUSCRIPT Wang H., Wu Y.B., Gao S., Liu X.C., Liu Q., Qin Z.W., Xie S.W., Zhou L., Yang S.H., 2013a. Continental origin of eclogites in the North Qinling terrane and its tectonic implications.

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Precambrian Research 230, 13-30. Wang H., Wu Y.B., Qin Z.W., Zhu L.Q., Liu Q., Liu X.M., Gao S., Wijbrans J.R., Zhou L., Gong H.J.,

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Yuan H.L., 2013b. Age and geochemistry of Silurian gabbroic rocks in the Tongbai orogen,

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central China: Implications for the geodynamic evolution of the North Qinling arc-back-arc system. Lithos 179, 1-15.

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Wang T., Zhang Z.Q., Wang X.X., Wang Y.B., Zhang C.L., 2005. Neoproterozoic collisional deformationg in the core of the Qinling orogen and its age: constrained by zircon SHRIMP dating

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of strongly deformed syn-collisional granites and weakly deformed granitic veins. Acta Geologica Sinica 79, 220-231 (in Chinese with English abstract).

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Wang T., Wang X.X., Tian W., Zhang C.L., Li W.P., Li S., 2009. North Qinling Paleozoic granite

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associations and their variation in space and time: implications for orogenic processes in the orogens of Central China. Science in China(D) 52, 1359-1384.

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Whitehouse M.J., Claesson S., Sunde T., Vestin J., 1997. Ion microprobe U-Pb zircon geochronology and correlation of Archaean gneisses from the Lewisian Complex of Gruinard Bay, northwestern Scotland. Geochimica et Cosmochimica Acta 61, 4429-4438. Wiedenbeck M., Alle P., Corfu F., Griffin W.L., Meier M., Oberli F., Vonquadt A., Roddick J.C., Speigel W., 1995. Three natural zircon standards for U-Th-Pb, Lu-Hf, trace-element and REE analyses. Geostand Newsletter 19, 1-23. Wu Y.B., Zheng Y.F., 2013. Tectonic evolution of a composite collision orogen: An overview on the Qinling-Tongbai-Hong'an-Dabie-Sulu orogenic belt in central China. Gondwana Research 23, 1402-1428. Xu P., Wu F.Y., Xie L.W., Yang Y.H., 2004b. Hf isotopic compositions of the standard zircons for U-Pb dating. Chinese Science Bulletin 49, 1642-1648. Yang J.S., Xu Z.Q., Pei X.Z., Shi R.D., Wu C.L., Zhang J.X., Li H.B., Meng F.C., Rong H., 2002. Discovery of diamond in North Qinling: evidence for a giant UHPM belt across Central China and

ACCEPTED MANUSCRIPT recognition of Paleozoic and Mesozoic dual deep dubduction between North China and Yangtze Plates. Acta Geologica Sinica 76, 484-495 (in Chinese with English abstract).

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Yang J.S., Liu F.L., Wu C.L., Wan Y.S., Zhang J.X., Shi R.D., Chen S.Y., 2003. Two ultrahigh pressure metamorphic events recognized in the Central Orogenic Belt of China: Evidence from the U-Pb

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dating of coesite-bearing zircons. Acta Geologica Sinica 77, 463-477 (in Chinese with English

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abstract).

Yuan X.C., Li S.F., Hua J.R., 2008. Lithospheric structure of the Qinling intracontinental orogen.

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Geology in China 35(1), 1-17 (in Chinese with English abstract). Zhang C.L., Liu L., Wang T., Wang X.X., Li L., Gong Q.F., Li X.F., 2013. Granitic magmatism related

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to early Paleozoic continental collision in North Qinling. Chinese Science Bulletin 58, 1-7. Zhang G.W., Meng Q.R., Lai S.C., 1995. Tectonics and structure of the Qinling Orogenic belt. Science

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in China(D) 38, 1379-1394.

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Zhang G.W., Zhang B.R., Yuan X.C., Xiao Q.H., 2000. The Qinling orogenic belt and continental Dynamics. Science Press, Beijing, pp. 885 (in Chinese).

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Zhang H.F., Ying J.F., Tang Y.J., Li X.H., Feng C., Santosh M., 2011b. Phanerozoic reactivation of the Archean North China Craton through episodic magmatism: Evidence from zircon U-Pb geochronology and Hf isotopes from the Liadong Peninsula. Gondwana Research 19, 446-459. Zhang J.X., Yu S.Y., Meng F.C., 2011a. Ployphase Early Paleozoic metamorphism in the northern Qinling orogenic belt. Acta Petrologica Sinica 27, 1179-1190 (in Chinese with English abstract). Zhu X.Y., Chen F.K., Li S.Q., Yang Y.Z., Nie H., Siebel W., Zhai M.G., 2011. Crustal evolution of the North Qinling terrain of the Qinling Orogen, China: evidence from detrital zircon U-Pb ages and Hf isotopic composition. Gondwana Research, 20: 194-204.

Figure Captions Fig. 1. Geological sketch of the Eastern North Qinling Terrane (b) with the inset map (a) showing the Qinling-Tongbai-Dabie-Sulu Orogenic Belts (modified after Dong et al. (2011a&b) and Wang et al. (2013)). The star shows the locality of zircon U-Pb dating on HP-UHP metamorphic rocks in North

ACCEPTED MANUSCRIPT Qinling Terrane. Line A to B shows the crustal cross-section of the eastern Qinling orogenic belt and

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related North and South China Blocks in figure 11b.

Fig. 2. Simplified geological map showing the Fushui meta-gabbroic complex as well as the Songshugou

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peridotite massif and its related garnet-bearing plagioclase amphibolites (modified after Pei et al., 1997;

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Dong et al., 2008). Also shown are sample locations and photographs of some samples. The black dot

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shows the sample reported by Li et al. (2006).

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Fig. 3. Photomicrographs of samples from the Fushui meta-gabbroic complex.

Fig. 4. Cathodoluminescence (CL) images of representative zircons before Cameca ims-1280 U-Pb age determination and oxygen isotope analysis. The G numbers correspond to the analysis numbers listed in

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the Table supplementary. Solid circles show locations of ablation spots for the U-Pb age determination

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and oxygen isotope analysis, and dashed circles indicate locations of ablation spots for Hf isotopic analyses and numbers below the U-Pb (206Pb/238U) ages refer to oxygen isotopes followed by Hf(t)

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values.

Fig. 5. Th/U versus U (ppm) and Th (ppm) for zircons from the Fushui meta-gabbroic complex

Fig. 6. Zircon U-Pb concordia diagrams for samples from the Fushui meta-gabbroic complex.

Fig. 7. Histograms showing 206Pb/238U ages, TDM Hf model ages, εHf(0) and εHf(t) values of the zircons from the Fushui meta-grabbroic complex. The εHf(t) value was calculated for the respective age from the individual zircons.

Fig. 8. Oxygen isotope histograms of the zircons from the Fushui meta-grabbroic complex.

Fig. 9. Plot of εHf versus O isotope of zircons from the Fushui meta-grabbroic complex. The dotted line denotes the mixing trend between the mantle- and supracrust-derived components. Hfm/Hfc in the curve

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= 5.3‰ and the supracrustal zircon εHf = -13 and δ18O = 10‰. δ18O (5.3‰±0.6‰ (2)) for the normal

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mantle-derived magma is after Valley et al. (1998).

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Fig. 10. Schematic drawing showing the exumation process of the Fushui complex.

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Fig. 11. (a): Schematic cartoons showing the evolution of the eastern Qinling orogenic belt with an emphasis on the formation of the North Qinling Unit from it birth through continental subduction to its

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exhumation (modified after Wu and Zheng, 2013 and Wang et al., 2013b; referred from Dong et al., 2011a and Bader et al., 2013). The explanation is given in text. The star shows the position of the rock

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sample in (c) at the different stages. (b): Schematic diagram showing the crustal cross-section of the

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eastern Qinling orogenic belt and related North and South China Blocks though the line A to B in figure 1b. The legend is same as in figure 1; XF, SD, and LL represents the Xiangfan Fault, Shangxian-

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Danfeng Suture Zone, and Luonan-Luanchuan Fault, respectively. The Moho is taken after Yuan et al. (2008). (c): Photograph of the Songshugou peridotite inclusion collected near the meta-grabbro sample FS13-05 in Fig. 2. The sample represents a transition zone of lower crust-lithosphere mantle and initially an oceanic lithosphere composed of the peridotite and sheeted mafic dikes. When subducted to the eclogite facies at 500-480 Ma the peridotite was mylonitized and the mafic dike metamorphosed to eclogite. When exhumed to the amphibolite facies (460-420 Ma), the coarse-grained eclogite was retrograded to garnet amphibolite and the fine-grained one to the amphibolite with all the garnets transformed to plagioclases.

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Grain (spot)

U (ppm)

Th (ppm)

Pb (ppm)

206

Pb/238U

Th/U

 O(‰) 18

Hf(0)

Hf(t)

TDM (Ga)

-18.0~12.9

-7.2~-4.7

1.17~1.38

.282326~.282403

-15.5~12.3

-4.8~-2.2

1.20~1.30

.282338~.282418

-15.1~12.4

-4.9`-2.2

1.16~1.28

.282315~.282411

-16.1~12.6

-5.5~-2.2

1.17~1.30

.282318~.282417

-15.9~12.3

-5.5~-2.0

1.16~1.30

-15.5~12.0

-5.0~-1.6

1.15~1.28

-15.0~12.6

-7.9~-5.8

1.19~1.27

(176Hf/177Hf)i

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Sample

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Table 1：The summery of the U-Pb dating, Oxygen and Hf isotopes of zircons from the Fushui meta-gabbroic complex

FS13-05

44(45)

225~761

123~862

22~85

0.55~1.17

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Concordia age = 490.2±1 Ma (MSWD=0.14); 206Pb/238U weighted mean age = 490±10 Ma (n=45;MSWD=0.051) 475~508

7.14~8.24

.282257~.282404

17(17)

278~2770

234~6765

30~406

0.66~2.44

485~511

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FS13-06

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Concordia age = 497.2±4.2 Ma (MSWD=0.063); 206Pb/238U weighted mean age = 497±18 Ma (n=17; MSWD=0.0037) 6.69~7.64

No excellent concordia age; 206Pb/238U weighted mean age = 473±18 Ma (n=15;MSWD=0.007) 15(15)

134~1097

93~2281

13~141

0.67~2.08

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FS13-06

467~477

6.76~7.81

Concordia age = 477.1±1.2 Ma (MSWD=6.2); 206Pb/238U weighted mean age = 476±11 Ma (n=33; MSWD=0.066) FS13-04

31(33)

159~2048

124~2648

16~183

0.28~1.89

463~492

6.94~7.82

FS13-07

31(38)

209~585

115~533

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No excellent concordia age; 206Pb/238U weighted mean age = 478±11 Ma (n=38; MSWD=0.053) 19~59

0.55~1.09

463~495

7.47~8.75

FS13-09

49(52)

225~2236

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Concordia age = 482.8±0.96 Ma (MSWD=0.00); 206Pb/238U weighted mean age = 482.6±9.2 Ma (n=52; MSWD=0.055) 171~2850

23~248

0.76~1.53

464~504

6.70~8.00

.282329~.282427

Concordia age = 335.2±1.4 Ma (MSWD=2.3); 206Pb/238U weighted mean age = 335.6±9.2 Ma (n=13; MSWD=0.084) FS13-06

4(4)

FS13-07

6(6)

FS13-09

3(3)

1043~1472

129~180

60~84

0.09~0.14

330~348

6.79~8.35

.282344~.282401

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Peak metamorphic Age (Ma)

Guanpo

657±18

502±11 490±4 507±9

791±6

787±16 843±7

503-487 480±6 490±6 501±10 518±19 506±7 514±9 500±10 484±4 503±5

573±40

500±6 495±2

796±13 814±45

485±3 499±2 504±7 486±4 489±6

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Retrograde age (Ma) Eclogite

Method

Data source

LA-ICPMS SIMS SIMS

Chen and Liu 2011 Cheng et al. 2012 Cheng et al. 2012

LA-ICPMS LA-ICPMS LA-ICPMS SHRIMP LA-ICPMS LA-ICPMS SHRIMP LA-ICPMS LA-ICPMS LA-ICPMS

Chen and Liu 2011 Cheng et al. 2011 Liu et al. 2013 Su et al. 2004 Liu and Sun 2005 Li et al. 2009 Liu et al. 2009 Liu et al. 2009 Li et al. 2012 Liu et al. 2013

LA-ICPMS LA-ICPMS

Liu et al. 2009 Liu et al. 2013

LA-ICPMS LA-ICPMS SHRIMP SIMS LA-ICPMS

Chen et al. 2004 Liu et al. 2013 Zhang et al. 2011a Wang et al. 2011 Wang et al. 2011

SHRIMP LA-ICPMS SHRIMP LA-ICPMS

Yang et al. 2002 Liu et al. 2009 Zhang et al. 2011a Liu et al. 2013

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Locality

Zhaigen

Songshugou

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418±5 452±5 Garnet pyroxenite

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450±3, 426±1 HP mafic granulite

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>750

453±9

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655±9 Songshugou

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Retrograde eclogite Guanpo Qingyouhe

>1200

507±37 486-511 506±3 497±8

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<832±25

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UHP felsic gneiss

Guanpo Songshugou

448±4, 421±2

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Graphical abstract

ACCEPTED MANUSCRIPT Research Highlights  Fushui complex is a typical syn-subduction product of mafic magmatism

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 Fushui complex was originally derived from the continental sediments and altered basalts metasomatized lithospheric mantle

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 The Qinling Group recorded a wholesale deep subduction of mini-continent and

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multiple stage exhumations