Evidence for regional metamorphism in a continental rift during the Rodinia breakup
Accepted Manuscript Evidence for regional metamorphism in a continental rift during the Rodinia breakup Qiang He, Shao-Bing Zhang, Yong-Fei Zheng PII:...
Accepted Manuscript Evidence for regional metamorphism in a continental rift during the Rodinia breakup Qiang He, Shao-Bing Zhang, Yong-Fei Zheng PII: DOI: Reference:
Please cite this article as: Q. He, S-B. Zhang, Y-F. Zheng, Evidence for regional metamorphism in a continental rift during the Rodinia breakup, Precambrian Research (2018), doi: https://doi.org/10.1016/j.precamres.2018.06.009
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Evidence for regional metamorphism in a continental rift during the Rodinia breakup
Qiang He*, Shao-Bing Zhang, Yong-Fei Zheng
CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
Abstract Continental rifts are tectonic zones where substantial material and heat can be transferred from the asthenospheric mantle into the crust. Because of their high thermal gradients, they were hypothesized as a setting for regional high-temperature (HT)/low-pressure (LP) metamorphism. However, the cited example is located in an intracontinental setting, where not only the compressional regime was prominent during collisional orogeny but also the envisaged continental rift failed to run into rupture. This led to ambiguous identification of the continental rift in intraplate regions and thus rejection of this hypothesis. In the present study, we identify for the first time typical HT/LP metamorphic rocks that show the occurrence of both andalusite and sillimanite in a Neoproterozoic continental rift zone. Andalusite O isotopes confirm the continental rift setting, and comparison with zircon O isotopes supports the metamorphic origin of aluminosilicates. Pseudosection calculations indicate that the HT/LP metamorphism occurred at 1.0-3.5 kbar and 560-660°C. U-Pb dating of metamorphic titanite yields ages of ca. 750 Ma for the HT/LP metamorphism, consistent with the peak age of Rodinia breakup. Premetamorphic protoliths show bimodal lithochemistry and arc-like geochemical signatures, suggesting that they are rift magmatic rocks. These rocks would be produced during the development of continental rifting on a Grenvillian accretionary orogen that formed due to the Rodinia assembly. The continental rifting is coupled with stretching of the thinned orogenic lithosphere, accounting for the high heat flow from the asthenospheric mantle into the upper crust for the HT/LP metamorphism at shallow depths. Heat flow estimates from crustal Th, U and K contents are significantly lower than that from peak mineral assemblages, indicating that the anomalously high heat flow from the asthenospheric mantle did occur in the continental rift. Therefore, continental rifts are an important site for high thermal gradients and thus for regional HT/LP metamorphism.
1. Introduction The thermal gradients of continental crust still remain partially unresolved when concerning their relationship to tectonic settings (Liou et al., 2004; Brown, 2006). Metamorphic rocks within which peak mineral equilibrium is well preserved are the best expression of past thermal gradients. In modern plate tectonics, subduction zones are generally dominated by low thermal gradients of <10°C/km (Zheng and Chen, 2016). This leads to the occurrence of low-temperature/high-pressure blueschist-eclogite facies series in accretionary and collisional orogens (Zheng and Chen, 2017). However, it is highly controversial on which tectonic regime is responsible for high thermal gradients of >30C/km (e.g., Gibson et al., 2004; Harley, 2008; Kelsey, 2008; Clark et al., 2011; Santosh et al., 2012; Brown, 2014; Korhonen et al., 2014; Tucker et al., 2015; Zheng and Chen, 2017), which are necessary
for
the
generation
of
high-temperature
(HT)/low-pressure
(LP)
amphibolite-granulite facies series at fossil convergent plate margins (accretionary to collisional orogens). The high thermal gradients required for regional HT/LP metamorphism mostly occur in extensional settings, such as mid-ocean ridge, backarc basin and continental rift (Zheng and Chen, 2017). Seafloor spreading is associated with high thermal gradients of >60C/km, suitable for not only mid-ocean ridge magmatism and but also HT/LP metamorphism (Manning et al., 1996; Nicolas et al., 2003; Bosch et al., 2004). In continental regions, thinning of the thickened orogenic lithosphere and eventual continental rifting enhance the conductive heat transfer from the underlying asthenosphere, resulting in regional thermal anomalies in the continental crust with thermal gradients of >30°C/km (e.g., Sandiford and Powell, 1986; Olsen, 1995). For this reason, Wickham and Oxburgh (1985, 1986, 1987) proposed crustal-scale rifting, possibly in a strike-slip setting, to explain the Pyrenean HT/LP metamorphism in the Variscan orogen of Europe. However, this proposal was rejected by arguing against the presence of a rifting event during the Variscan orogeny in the Pyrenees (Matte and Mattauer, 1987; Kriegsman, 1989). Generally, HT/LP metamorphic rocks often occur in intracontinental orogens (Brown, 1993; Bucher and Grapes, 2011), where evidence for continental rifting is generally lacking. As consequence, it was uncertain to have the linkage between HT/LP metamorphic rocks and continental rifting and thus to identify continental rifts in intracontinental regions. Regional HT/LP metamorphism generally occurs at temperatures in excess of 500°C but pressures lower than the triple point of Al2SiO5 polymorphs, i.e. andalusite, sillimanite and
kyanite (Holdaway, 1971; De Yoreo et al., 1991; Bucher and Grapes, 2011). The products of such metamorphism are characterized by: (1) the formation of both andalusite and sillimanite in peraluminous rocks, and (2) the generation of petrographic textures showing the prograde metamorphic reaction from the andalusite to sillimanite stability field. In this regard, the occurrence of aluminosilicates in peraluminous rocks is a key to identification of HT/LP metamorphic rocks. The northern margin of the South China Block was splitted from supercontinent Rodinia in the middle Neoproterozoic (Li et al., 2003, 2008; Zheng et al., 2013). This is a rifted continental margin showing the occurrence of bimodal magmatism (Gao et al., 1996; Ling et al., 2003; Zheng et al., 2003; Wu et al., 2013) and the extreme
18
O depletion in resistant
minerals (Zheng et al., 2004; Zheng et al., 2007; He et al., 2016). Such a rift provides us with an excellent opportunity to clarify its relationship to regional HT/LP metamorphism. Because the peak continental rifting was developed coevally with the Rodinia breakup, it results in infiltration of continental deglacial water (Zheng et al., 2007; He et al., 2016). Thus, minerals that were related to the continental rifting can be easily recognized by their O isotope composition. More importantly, this potentially allows us to discover HT/LP metamorphic rocks in the continental rift. This paper presents a combined study of petrographic observation, whole-rock and mineral geochemistry, zircon and titanite U-Pb ages, mineral O isotopes, and pseudosection calculations for metaigneous rocks from the northern margin of the South China Block. The results are used to (1) provide the petrographic evidence for regional HT/LP metamorphism in the continental rift; (2) correlate the HT/LP metamorphism with continental rifting in a reactivated accretionary orogen, and (3) evaluate the thermal budget in continental rift zones.
2. Geological setting and samples The South China Block is tectonically composed of the Yangtze craton and the Cathaysian terrane with the Jiangnan orogen in between (Zheng et al., 2013). The Yangtze craton is bordered by the accretionary orogens of Grenvillian age in response to the Rodinia assembly (Li et al., 2002; Zheng et al., 2008), including the Jiangnan orogen in its southeast due to subduction of the Cathaysian oceanic crust beneath the Yangtze craton in the Late Mesoproterozoic to Early Neoproterozoic (Li et al., 2009; Zhang and Zheng, 2013). Although they are presently located in intracontinental settings, they were originally developed from accretionary to collisional orogenesis in the periphery of ancient cratons. The Grenvillian
orogens surrounding the Yangtze craton are mainly composed of magmatic rocks in Middle Neoproterozoic age of 830 to 740 Ma (Li et al., 2003; Zheng et al., 2008, 2009; Zhao et al., 2011; Wu et al., 2013; Zhang and Zheng, 2013). These magmatic rocks have their source rocks not only from the juvenile crust of Late Mesoproterozoic but also from the ancient crust of Middle Paleoproterozoic (Zheng et al., 2008, 2009; Zhang et al., 2014). The northern margin of the South China Block is part of the Grenvillian orogens. Although it was generated by the Grenvillian subduction of oceanic crust beneath the Yangtze craton in the Late Mesoproterozoic to Early Neoproterozoic, it was separated from Rodinia at about 750 Ma in the Middle Neoproterozoic (Li et al., 2003, 2008; Zheng et al., 2004, 2007, 2009, 2013). The present study has collected metagranite and metabasalt from the Beihuaiyang zone in the northern margin of the South China Block (Fig. 1). This lithotectonic zone is a low-grade metamorphic unit (Hacker et al., 1998; Zheng et al., 2005), in contrast to adjacent high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu orogenic belt (Zheng et al., 2003; Liou et al., 2012). This orogenic belt was produced by the subduction of South China Block beneath North China Block in the Triassic (Ernst et al., 2007; Zheng et al., 2013). While the UHP metamorphic rocks were produced by subduction of the continental crust to subarc depths of >80 km (Zheng et al., 2003; Liou et al., 2012), the low-grade metamorphic zone represents an accretionary wedge that was offscrapped from the subducting South China Block at crustal depths of <30 km (Zheng et al., 2005). In either case, their igneous protoliths are bimodal magmatic rocks of middle Neoproterozoic ages. The low-grade metamorphic zone is significant for preservation of primary petrographic textures and mineral compositions that were formed in the middle Neoproterozoic. The Beihuaiyang zone is located in the eastern edge of the low-grade metamorphic zone in the northern margin of the South China Block (Fig. 1). In the middle Neoproterozoic, the northern margin served as the rifted margin where the South China Block was ruptured from Rodinia (Li et al., 1999; Li et al., 2003; Wu et al., 2013; Zheng et al., 2013). Therefore, the Beihuaiyang zone was located in a continental rift, making it a suitable site to search for HT/LP metamorphic rocks. The continental rift setting is indicated by zircon U-Pb ages and mineral O isotopes. Magmatic zircon U-Pb dating constrains the magmatism at 720-780 Ma (Chen et al., 2003; Wu et al., 2007; Zheng et al., 2007), consistent with the timing of Rodinia breakup (Li et al., 2003; Li et al., 2008). In addition, garnet in metagranite shows negative δ18O value as low as −14.4‰ (Zheng et al., 2007), indicating involvement of the continental deglacial water. Other rock-forming minerals from the Beihuaiyang metagranite also show
negative 18O values (Zheng et al., 2007). Continental rifts are generally characterized by intensive water-rock interaction at high temperatures (Taylor, 1977; Zheng et al., 2004; Wu et al., 2007), because such sites are capable of providing high thermal gradients during lithospheric stretching (Zheng and Chen, 2017). Furthermore, the
18
O depletion was widely
developed within an area of over 20000 km2 along the northern margin of South China Block (Zheng et al., 2004, 2009), suggesting the continental rifting occurred in a regional scale. The Beihuaiyang zone in the northern edge of the South China Block is composed of Upper Neoproterozoic to Lower Paleozoic flysch sediments (the Foziling Group) and Neoproterozoic igneous rocks (the Luzhenguan complex). Samples were sampled from the Luzhenguan complex at Wozicun. Four aluminosilicates-bearing metagranite, one garnet-bearing metagranite, one titanite-bearing metagranite and one titanite-bearing metabasalt are involved in the present study. The metabasalt occurs as massive block or thick layer enclosed by the metagranite. Aluminosilicates-bearing metagranite is similar in both mineral assemblages and mineral proportions. Metagranite 14BHY02, 14BHY05 and 14BHY07 contain about 60% quartz, 10-15% plagioclase, 15-20% K-feldspar, 5% muscovite, 2% biotite, 2% andalusite and minor sillimanite. Metagranite 14BHY38 exhibits higher andalusite proportion up to ca. 25% with no sillimanite. The rest minerals include about 50% quartz, 20% K-feldspar, 3% muscovite and minor biotite. Garnet-bearing metagranite 14BHY08 contains about 60% quartz, 15-20% plagioclase, 10-15% K-feldspar, 6% biotite, 4% garnet and minor muscovite. Titanite-bearing metagranite 14BHY23 is coarse-grained and composed of about 60% quartz, 15-20% plagioclase, 20-25% K-feldspar and 1-2% titanite. In contrast, titanite-bearing metabasalt 14BHY28 is fine-grained. Quartz and plagiaclase range from 50 to 100 μm in size. Mafic minerals are mainly amphibole and biotite, which occur in a dominant amphibole-rich zone and a subordinate biotite-rich zone (Fig. 2h), respectively. Amphibole is anhedral and cracked, ranging from 200 to 300 μm in size. Biotite is subhedral to euhedral and exhibits similar size as amphibole. Both amphibole and biotite exhibit profound orientation (Fig. 2h), suggesting the metamorphic deformation.
3. Analytical methods 3.1 Raman spectroscope analysis Aluminosilicates were identified by a LabRAM HR Evolution Raman spectroscope with the laser wave length of 532 nm.
3.2 Whole-rock major and trace element analyses Whole-rock major element compositions were analyzed using a Rigaku ZSX Primus II WDXRF spectrometer. Whole-rock powder was mixed well with a pure lithium borate-fused flux, fused within a fully-automatic fusion fluxer at 1050 °C and then quenched to a flat molten glass disc for XRF analysis. Loss on ignition (LOI) was obtained by measuring the weight change after ignition in a furnace at 1050 °C for 2 hours. Trace element compositions for whole-rock samples were measured by ICPMS after acid digestion of whole-rock powder in Teflon bombs.
3.3 Electron probe microanalyser (EPMA) analysis Major element compositions of andalusite, sillimanite, spinel and titanite were measured using a Shimazu EPMA 1600 electron microprobe. The accelerating voltage was set as 15 kV, and the beam current was 20 nA. Natural silicate standards were used, and raw data were reduced using conventional ZAF correction procedures. Beam size was set as 1 μm.
3.4 Laser fluorination Minerals of 1.5–2.0 mg were heated by a 25 W CO2 laser MIR-10 and reacted with BrF5 to transform O in silicates into O2. After purified through several cold traps and molecular sieves, the released O2 was transferred to a mass spectrometer Finnigan Delta XP for oxygen isotope ratio measurement. The
18
O/16O ratio was reported in the δ18O notation relative to
VSMOW. The reproducibility is typically better than ±0.1‰ (1σ). Reference minerals and values used during the analysis are as follows: δ18O = 5.8‰ for UWG-2 garnet (Valley et al., 1995); δ18O = 3.7‰ for 04BXL07 garnet (Gong et al., 2007).
3.5 Zircon U-Pb dating and O isotope analysis Zircon was handpicked under a binocular microscope after the rocks crushed and sieved. Representative zircon grains were mounted in epoxy resin and then polished to expose their interior. Detailed morphology and internal structures of zircon were carefully observed using optical microscope and cathodoluminescence (CL) imaging. CL images were screened using a TESCAN MIRA3 scanning electron microscope. The working conditions were 10 kV and 15 nA. LA-ICPMS zircon U-Pb dating was conducted using an excimer laser ablation system (GeoLas 2005). An Agilent 7500a ICP-MS instrument was used to acquire ion-signal
intensities. The analytical protocols followed those described in (Liu et al., 2010a; Liu et al., 2010b). Off-line selection, integration of background and analytical signals, time drift correction and quantitative calibration were conducted using ICPMSDataCal (Liu et al., 2010b). SIMS zircon U-Pb dating and O isotope analysis were conducted using a CAMECA IMS-1280 HR ion microprobe at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou. For the U-Pb dating, analytical procedures, Pb/U calibration and calculation of U and Th contents were the same as those described by Li et al. (2009). Correction on the common lead was made using the measured 204Pb and the model crustal Pb isotope compositions (Stacey and Kramers, 1975). Zircon U-Pb data were handled using Isoplot/Ex_ver3.23 (Ludwig, 2003). For the O isotope analysis, the instrumentation and operating conditions were described in detail by Li et al. (2010a). The instrumental mass fractionation (IMF) was corrected using an internal zircon standard Penglai with 18OVSMOW = 5.31±0.10‰ (Li et al., 2010b). Qinghu was used as a secondary in-house standard to monitor the data quality. During the analysis session, the δ18O values for Qinghu is 5.6±0.1‰ (n=14, MSWD=2.7) consistent with the recommended value of 5.4±0.2‰ (Li et al., 2013). Correction on IMF was done following the details in (Li et al., 2010a). The corrected 18O values are reported in the standard per mil notation, along with 2 error.
3.6 Titanite U-Pb dating and trace element analysis Titanite was recognized in thin sections under an optical microscope. LA-ICPMS titanite U-Pb dating was conducted by an Agilent 7500a Q-ICPMS equipped with a 193 nm ArF excimer laser ablation system, which was applied to acquire ion-signal intensities. The spot size was 44 μm and ablation frequency was 6 Hz. Analytical procedures followed those described in (Sun et al., 2012).
207
Pb/206Pb and
206
Pb/238U ratios were calculated using the
GLITTER program (Griffin et al., 2008). Common lead correction followed the method described in (Sun et al., 2012). All of the weighted mean
206
Pb/238U ages were calculated
using Isoplot/Ex v. 3.0 (Ludwig, 2003). Titanite trace element concentrations were calibrated using 43Ca as the internal calibration and using NIST SRM 610 as the reference material.
3.7 Pseudosection calculations Phase relations for stable mineral assemblages were constructed in the MnNCKFMASH system using THERMOCALC v. 3.33 (Powell and Holland, 1988) with the internally
consistent thermodynamic data set ds55 (Holland and Powell, 1998, updated on October 2009). The bulk compositions analyzed by XRF method were used for modelling. Activity-composition (a-x) relations for solid-solution phases are as follows: garnet and biotite (White et al., 2005); cordierite (Holland and Powell, 1998); orthopyroxene (Powell and Holland,
1999);
plagioclase-K-feldspar
(Holland
and
Powell,
2003);
and
muscovite-paragonite (Coggon and Holland, 2002). Pure phases include sillimanite, andalusite, quartz and H2O. The incorporation of Mn into biotite and cordierite follows Mahar et al. (1997). The melt was modelled in the NCKFMASH system on an 8-oxygen basis following White et al. (2007).
4. Results 4.1 Petrography Andalusite and sillimanite are commonly associated with highly anhedral muscovite porphyroclasts that have their size ranging from 0.2 to 1 mm. The two Al2SiO5 polymorphs were identified by a Raman spectroscope, and their characteristic Raman shifts (Mernagh and Liu, 1991) are illustrated in Fig. 2a. Generally, the muscovite porphyroclasts show inward and irregular boundaries with anhedral andalusite and K-feldspar (Figs. 2b and 2c). The cleavage of muscovite porphyroclasts shows nearly the same direction for different parts that are crosscut by anhedral andalusite or K-feldspar. In some cases, the porphyoclastic muscovite is intimately intergrown with both anhedral andalusite and needle sillimanite, with sillimanite needles crosscutting the cleavage of muscovite (Fig. 2c). There is also sillimanite growth around andalusite with the inward-penetrating boundary (Fig. 2d). In addition, some andalusite grains are strongly zoned in color, with pink cores and colorless rims (Fig. 2e). Also observed are some dark-green spinel grains in coexistence with or included by biotite (Fig. 2f). In the same sample, the prismatic andalusite and sillimanite are intimately intergrown with relic muscovite and biotite. A sillimanite grain is included in the andalusite and connected to the matrix through a fracture (Fig. 2g).
4.2 Whole-rock major and trace elements Whole-rock major and trace element compositions are presented in supplementary Table S1. Aluminosilicates-bearing metagranite shows high contents of SiO2 (72.82-75.66 wt%) and Al2O3 (14.26-14.98 wt%). They have relatively constant K2O content of 4.06-4.91 wt% but
variable Na2O content of 1.35-3.44 wt%. Their CaO content ranges from 0.19 to 0.77 wt%, corresponding to Ca/Al ratios of 0.01-0.05. Aluminum saturation index (ASI), defined as A/CNK = 0.5Al/(Ca+0.5Na+0.5K), vary from 1.23 to 1.90. The strongly peraluminous compositions are consistent with significant crystallization of aluminosilicates. A higher ASI (1.90 for metagranite 14BHY38) corresponds to a higher proportion of andalusite. Garnet-bearing metagranite has similar SiO2 and Al2O3 contents to aluminosilicates-bearing metagranite. However, it contains a lower K2O content (3.64 wt%) but higher contents of Na2O (3.90 wt%) and CaO (0.94 wt%). These give a slightly higher Ca/Al ratio of 0.06 and a lower ASI of 1.16. Titanite-bearing metagranite contains similar SiO2, Al2O3, Na2O and CaO contents to aluminosilicate-bearing metagranite, and thus a comparable Ca/Al ratio. A higher K2O content of 6.03 wt% leads to a much lower ASI of 1.03. All the metagranite exhibit similarly low mafic components (TiO2+Fe2O3+MgO) ranging from 1.92 to 2.97 wt%. Titanite-bearing metabasalt is characterized by a low SiO2 content of 50.07 wt%，a low MgO content of 3.20 wt%, a low Mg# of 0.39 and a high Al2O3 content of 18.16 wt%, typical of low-MgO high-Al basalts (Sisson and Grove, 1993). It also contains a high mafic component of 15.6 wt% and a high CaO content of 6.66 wt%, giving a Ca/Al ratio of 0.33. In addition, it has a slightly higher Na2O content of 4.61 wt% and a lower K2O content of 2.22 wt%. Its much higher CaO content dominates the ASI calculation, resulting in a value of 0.84. Rare earth and trace element distribution patterns for the metagranite and metabasalt are shown in Fig. 3. All the metagranites show similar REE patterns with LREE enrichment (La/YbN = 9.3-26.5) and strongly to moderately negative Eu anomalies (Eu/Eu* = 0.19-0.69). They are significantly enriched in large ion lithospheric elements (LILE) such as Rb, Ba, and K, but depleted in high field strength elements (HFSE) such as Nb, Ta and Ti. Therefore, all the metagranites are characterized by arc-like trace element distribution patterns, similar to those for the upper continental crust (Rudnick and Gao, 2014). In addition, they exhibit negative Sr and P anomalies but a positive Pb anomaly. There is a significant difference in Ba concentration, which may be related to the proportions of micas. The metabasalt also shows LREE enrichment (La/YbN = 11.3), but no Eu anomaly (Eu/Eu* = 1.01). Note that the metabasalt exhibits LILE enrichment and HFSE depletion, typical of arc-like trace element composition.
4.3 Mineral chemistry Representative major element compositions of aluminosilicates and spinel in the
metagranites are listed in supplementary Table S2. The major and trace element compositions of titanite in metagranite and metabasalt are listed in supplementary Tables S3 and S4, respectively. Aluminosilicates are primarily composed of SiO2 and Al2O3 with minor and variable Fe2O3 contents (Table S2). For andalusite with strong color zoning (Fig. 2e), pink cores contain higher Fe than colorless rims (2.93-3.52 wt% Fe2O3 in pink cores versus 1.75-2.25 wt% in colorless rims), and higher Fe2O3 contents are associated with lower Al2O3. Colorless andalusite grains show similar compositions to colorless rims. Sillimanite has Fe2O3 contents of 0.88-1.34 wt%, lower than those for both pink andalusite and colorless andalusite. The compositional difference among pink andalusite, colorless andalusite and sillimanite is consistent with previous studies (Whitney and Dilek, 2000; Sepahi et al., 2004). The dark-green spinel grains in metagranite 14BHY07 (Fig. 2f) is primarily composed of Al2O3 (54.45 wt%), FeO (12.65 wt%) and ZnO (30.24 wt%) with minor MgO and MnO (Table S2). The endmember calculation gives ca. 68% gahnite, 27% hercynite and 6% spinel. This indicates the occurrence of Zn-rich spinels in the metagranite. Titanite in metagranite 14BHY23 occurs as skeletal crystals filled with quartz, plagioclase and K-feldspar. In contrast, titanite in metabasalt 14BHY28 is euhedral grains that are intimately intergrown with biotite (Fig. 4). The skeletal and euhedral titanites are also distinct in both major and trace element compositions. The skeletal titanite has higher Fe contents of 0.091-0.123 pfu and slightly lower Al contents of 0.088-0.114 pfu, in comparison with the euhedral titanite which has lower Fe contents of 0.053-0.072 pfu and higher Al contents of 0.108-0.129 pfu. The skeletal titanite is characterized by high REE concentration (23007-37764 ppm), high Y concentration (14046-40409 ppm), pronounced negative Eu anomalies (Eu/Eu* = 0.11-0.22) and flat MREE-HREE patterns with (Gd/Yb)N = 0.72-1.24 (Fig. 7d). It also contains high HFSE (Nb = 5420-10097 ppm; Ta = 134-869 ppm; Zr = 173-334 ppm; Hf = 22.7-88.8 ppm), high Th (265-675 ppm) and U (89.0-334 ppm) and high Th/U ratios varying from 1.08 to 4.54. The euhedral titanite displays relatively low REE (2235-9670 ppm), low Y concentration (235-510 ppm), no obvious Eu anomaly (Eu/Eu* = 0.84-1.03) and pronounced HREE depletion with (Sm/Yb)N = 3.76-5.48 (Fig. 7d). It has lower HFSE (Nb = 268-1901 ppm; Ta = 6.93-31.0 ppm; Hf = 6.47-46.75 ppm). Zr concentration is more variable from 146 to 849 ppm. Its Th (66.6-278 ppm) and U (14.1-310 ppm) give Th/U ratios of 0.82-5.51.
4.4 Andalusite O isotope composition Andalusite separates from metagranite 14BHY38 was analyzed for O isotope composition using the laser fluorination technique. We conducted the analysis twice to check the reproducibility, yielding δ18O values of −11.9‰ and −11.5‰, respectively. The average δ18O value is −11.7‰.
4.5 Zircon U-Pb ages and O isotopes Zircon in both metagranites and the metabasalt is typically of magmatic origin, showing clear oscillatory zoning without any overgrowth (Figs. 5 and 6). Zircon in the metagranites is euhedral with aspect ratios from 1:2 to 1:3 and length about 150-300 μm. In contrast, zircon in the metabasalt is generally euhedral to subhedral with aspect ratios from 1:1 to 1:2 and length about 100-200 μm. The results of LA-ICPMS zircon U-Pb dating for the metagranites and the metabasalt are listed in supplementary Table S5. For metagranite 14BHY05, it gives apparent 206Pb/238U ages of 700 to 789 Ma, mainly clustering at 747 to 777 Ma with a weighted average value of 758±5 Ma (Fig. 5a). The zircon has Th/U ratios of 0.53-1.43. For metagranite 14BHY08, zircon U-Pb dating yields apparent 206Pb/238U ages ranging from 733 to 801 Ma, clustering at 745 to 779 Ma with a weighted average value of 762±5 Ma (Fig. 5b). Th/U ratios are from 0.63 to 1.93. For metabasalt 14BHY28, zircon U-Pb dating gives variable ages from 750 to 937 Ma. Except for three older ages (two ~820 Ma and one ~940 Ma), the rest analyses yield apparent 206
Pb/238U ages of 750 to 790 Ma with a weighted average value of 768±6 Ma (Fig. 5c). Th/U
ratios range from 0.70 to 2.10. The ca. 768 Ma zircon grains show clear oscillatory zoning, whereas the zircon grains of older ages exhibit blurry oscillatory zoning. The results of SIMS zircon U-Pb dating and O isotope analysis for the metagranite are listed in supplementary Tables S6 and S7, respectively. For metagranite 14BHY02, zircon U-Pb dating yields apparent
206
Pb/238U ages clustering at 743 to 778 Ma with a weighted
average value of 764±5 Ma (Fig. 6a). Zircon δ18O values are from 4.9‰ to 6.2‰ with a weighted mean of 5.6±0.2‰ (Fig. 6b). Zircon apparent
206
Pb/238U ages in metagranite
14BHY05 are mainly clustering at 745 to 791 Ma except two slightly younger ages (ca. 726 Ma). They give a weighted average age of 766±5 Ma (Fig. 6c). Zircon has variable δ18O values from 4.8‰ to 6.1‰ with a weighted mean of 5.5±0.2‰ (Fig. 6d).
4.6 Titanite U-Pb dating The skeletal titanite was observed in two thin sections of metagranite 14BHY23, which are named as 14BHY23a and 14BHY23b, respectively. The euhedral titanite was also recognized in a thin section of metabasalt 14BHY28. The results of LA-ICPMS titanite U-Pb dating for both metagranite and the metabasalt are presented in supplementary Table S8. In the Tera-Wasserburg diagram (Fig. 7), the measured data are close to the lower intercept and distribute linearly, ensuring accuracy of the titanite U-Pb dating. Linear regression gives lower intercept ages of 752±30 Ma for 14BHY23a and 749±13 Ma for 14BHY23b. The weighted average values of 207Pb-corrected ages are 751±17 Ma and 748±13 Ma, respectively. One analysis for 14BHY23a yields higher MREE-HREE concentrations and an older 207
Pb-corrected age of 865 Ma. For metabasalt 14BHY28, linear regression gives a lower
intercept age of 752±17 Ma and a weighted average value of 751±11 Ma for
207
Pb-corrected
ages.
5. Discussion 5.1 Identification of regional HT/LP metamorphism The petrographic textures of muscovite, andalusite and/or sillimanite in Fig. 2 indicate the generation of metamorphic andalusite or sillimanite via the breakdown of muscovite. The classical muscovite dehydration reaction is muscovite + quartz = K-feldspar + andalusite + H2O (Spear and Kohn, 1996), which agrees well with the observed mineral assemblages and petrographic textures. Sillimanite growth around andalusite in Fig. 2d indicates the replacement of andalusite by sillimanite, suggesting elevated temperatures across the andalusite to sillimanite stability field. All of the above observations are typical for regional HT/LP metamorphism (Holdaway, 1971; De Yoreo et al., 1991). The co-existence of andalusite and sillimanite is prominent (Figs. 2c, 2d and 2g), which can be caused by the peak P-T conditions which were located near the region where the andalusite-sillimanite transition occurred. Andalusite was produced by the regional HT/LP metamorphism. This is further indicated by mineral O isotopes and P-T diagram construction. The laser fluorination for andalusite separates from the metagranite gives δ18O value of −11.7‰. It represents the O isotope composition of andalusite at the time of its formation, because the very slow O diffusion rate cannot cause significant modification subsequently (Sharp, 1995; Zheng and Fu, 1998). Such
a negative δ18O value is indicative of continental deglacial water infiltration during continental rifting (Zheng et al., 2007; Bindeman et al., 2010; Bindeman et al., 2011; He et al., 2016), suggesting the formation of andalusite was later than the continental deglaciation. The O isotope composition of muscovite in the same locality was reported by Zheng et al. (2007). Its δ18O values vary from −13.9‰ to −10.2‰. In this regard, the muscovite was hydrothermally altered at high temperatures to obtain the negative δ18O values. In addition, the andalusite is in O isotope equilibrium with the muscovite, indicating (1) the formation of the andalusite via muscovite dehydration reaction and (2) the inheritance of O isotope composition from the muscovite. In contrast, typical magmatic zircon in the metagranite has positive δ18O value of 4.8-6.2‰ with a weighted average value of 5.5±0.1‰. It indicates that zircon crystallized from a normal δ18O magma prior to the penetration of deglacial water, since partial melting of the glacial-hydrothermally altered rocks produces negative δ18O magma and thus negative δ18O zircon (Zheng et al., 2004; Tang et al., 2008; He et al., 2016). Therefore, the andalusite is a post-magmatic product. In addition, in the P-T diagram constructed by pseudosection calculations (Fig. 8), there is no overlap between the andalusite stability field and the supersolidus region. In other words, there is no the primary andalusite of magmatic origin in the compositions of metagranite 14BHY07. Collectively, the andalusite is a metamorphic product due to an increase in temperature and its negative δ18O values was produced by two-stage processes. The first step is that the muscovite obtained the
18
O-depleted signature
via the deglacial-hydrothermal alteration at a high temperature. The second step is that the muscovite underwent the dehydration reaction to produce the andalusite, which has inherited the negative δ18O values. Pseudosection calculations were made in terms of stable mineral assemblages in metagranite 14BHY07 to determine possible P-T conditions for the HT/LP metamorphism (Fig. 8). Petrographic observations show the presence of minerals such as muscovite, biotite, andalusite, sillimanite, plagioclase, K-feldspar and quartz (Fig. 2g). The occurrence of both andalusite and sillimanite indicates that the HT/LP metamorphism was conducted in the P-T region where the andalusite-sillimanite transition occurred. Therefore, the andalusite-bearing and sillimanite-bearing assemblages are the best choice for constraining the peak P-T conditions for the HT/LP metamorphism. Although most modelled aluminosilicates-bearing assemblages contain garnet and cordierite (Fig. 8), the two minerals are not observed in the thin sections of metagranite 14BHY07. This may be caused by their very low proportions as determined by the low Fe2O3 and MgO contents (Table S1). Andalusite is not stable for the
composition of metagranite 14BHY07 when partial melting occurred. In addition, under supersolidus conditions, muscovite would disappear rapidly via dehydration melting (Fig. 8), which is opposite to the petrographic observations that show survival of many muscovite relics. Therefore, we conclude that the HT/LP metamorphism took place at subsolidus conditions in the aluminosilicate stability field, i.e. 1.0-3.5 kbar and 560-660°C (Fig. 8). The estimated P-T conditions correspond to very high thermal gradients of >60°C/km at the upper crustal level. The Zn-rich spinels occurs in coexistence with or included by biotite (Fig. 2f), which is also indicative of the HT metamorphism. Biotite is a Zn-rich mineral and its Zn-saturation limit decreases with temperature (Dietvorst, 1981). During the prograde metamorphism, an increase in temperature leads biotite to accommodate the high concentrations of Zn. At declined temperatures, on the other hand, a decrease in the Zn-saturation limit of biotite results in the Zn release and thus the formation of gahnite. The formation of spinel in biotite is initially along the prominent cleavage planes (Brearley, 1987), which explains the spinel grains included in biotite.
5.2 Dating of the HT/LP metamorphism Two distinct kinds of titanite are recognized in the present study. One is the skeletal titanite in the metagranite and the other is the euhedral titanite in the metabasalt (Fig. 4). The skeletal titanite contains higher Fe, relatively lower Al and Fe/Al ratio of ~1, in contrast to lower Fe, Fe/Al ratio and relatively higher Al of the euhedral titanite (Fig. 9). The skeletal titanite has generally higher Th and U in comparison with the euhedral titanite (Fig. 9). In addition, the skeletal titanite shows higher Y, REE and HFSE concentration, pronounced negative Eu anomalies and flat MREE-HREE patterns. In contrast, the euhedral titanite displays relatively lower Y, REE and HFSE concentration, no obvious Eu anomaly and pronounced HREE depletion (Fig. 7d). The very high concentrations of Y, REE and Nb in the skeletal titanite result in the total contents of major elements as low as 90.7-94.0%. Based on the distinct major and trace element compositions of metamorphic titanite in comparison with magmatic titanite, i.e. REE and HFSE contents (Storey et al., 2007; Gao et al., 2012) and Al, Fe, Th and U contents (Rasmussen et al., 2013), the skeletal and euhedral titanites are identified as magmatic and metamorphic ones, respectively. The metabasalt has a high whole-rock Ca/Al ratio of 0.33, which is essential for crystallization of titanite and
stabilizing titanite relative to ilmenite + anorthite (Frost et al., 2001). Therefore, the euhedral titanite in the metabasalt is proposed as the product of HT/LP metamorphism. It was produced via the Ti release from biotite breakdown (Essex and Gromet, 2000; Frost et al., 2001), consistent with the co-existence of euhedral titanite and biotite. Biotite contains relatively low REE concentrations and shows a preference for LREE over HREE (Nash and Crecraft, 1985). The depletion of HREE in metamorphic titanite suggests the inheritance from biotite. In this regard, the U-Pb dating of metamorphic titanite can directly determine timing of the HT/LP metamorphism, i.e. 751±11 Ma (Fig. 7c). Although magmatic titanite is present in the metagranite, its whole-rock Ca/Al ratio of 0.05 is very low (Table S1), unsuitable for titanite crystallization. So the skeletal titanite is more likely to be a xenocryst of magmatic origin. The U-Pb dating of skeletal titanite yields very similar U-Pb ages to the euhedral titanite in the metabasalt. It gives ages of 751±17 Ma and 748±13 Ma, respectively, for the two thin sections from the same metagranite (Figs. 7a and 7b). The results suggest that the magmatic titanite suffered metasomatism possibly with resetting of its U-Pb radiometric system during the HT/LP metamorphism, consistent with its skeletal morphology. Alternatively, the U-Pb age of magmatic titanite might represent a record of magmatic activity. Similar ages of 748±3 Ma were obtained from the U-Pb dating of hydrothermally altered zircon domains from metagranite in the same locality (Zheng et al., 2007). In addition, magmatic zircon with U-Pb ages of 748±6 Ma records the low δ18O magmatism in the Beihuaiyang zone (Wu et al., 2007). Therefore, the age of ~750 Ma is the timing of not only the HT/LP metamorphism and the HT hydrothermal alteration, but also the low δ18O magmatism in the Beihuaiyang zone. Considering the contemporaneous hydrothermal alteration that would have caused widespread 18O depletion along the northern margin of the South China Block (Yui et al., 1995; Zheng et al., 1996; Zheng et al., 2003, 2004; Zheng et al., 2007; Tang et al., 2008; He et al., 2016), we propose that both HT hydrothermal alteration and HT/LP metamorphism would have occurred in a regional scale. Furthermore, this age is coevally consistent with the syn-rift phase of Rodinia breakup (Li et al., 2003; Li et al., 2008). Therefore, a typical continental rifting process associated with the breakup of Rodinia supercontinent is the most suitable for the HT hydrothermal alteration, the HT/LP metamorphism and the low δ18O magmatism. In return, the U-Pb ages of hydrothermally altered zircon, metamorphic titanite and magmatic zircon indicate that the continental rifting in the northern margin of the South China Block has its peak episode at ~750 Ma in response to the Rodinia breakup.
5.3 Protoliths of the metagranite and the metabasalt The protoliths of metagranite and the metabasalt in the present study are typical S-type granite and low-MgO high-Al basalt, respectively. The strongly peraluminous composition of metagranites suggests chemical weathering of their source rocks. The low-MgO high Al basalt is associated with a metasomatic mantle source above an oceanic subduction zone (Sisson and Grove, 1993), consistent with arc-like trace element distribution patterns for the metabasalt in the primitive mantle-normalized diagram (Fig. 3). The magmatic zircons in both metagranite and metabasalt give similar U-Pb ages of 750-780 Ma (Figs. 5 and 6), coeval with the supposed ages for the Rodinia breakup (Li et al., 2003; Li et al., 2008). Thus, the granite and basalt represent the bimodal products of continental rift magmatism during the Rodinia breakup. In this context, how did the basalt associated with the continental rifting obtain the arc-like trace element compositions? Because former orogens are vulnerable zones for supercontinental breakup (Wilson, 1966; Dewey, 1988; Vauchez et al., 1997), fossil suture zones may be reactivated for rifting orogeny (Zheng and Chen, 2017). The Grenvillian orogens were created during the Rodinia assembly in the Late Mesoproterozoic to Early Neoproterozoic. During the Grenvillian amalgamation of continental blocks into Rodinia (Li et al., 1999; Li et al., 2002; Li et al., 2008; Zheng et al., 2013), accretionary orogens were produced by oceanic subduction and marginal arc magmatism, and they were terminated by the production of arc-continent collisional orogens (Cawood et al., 2009; Zheng et al., 2013). The oceanic subduction would have generated many metasomatic mantle domains in the orogenic lithosphere (Zheng et al., 2015). Underplating of the asthenospheric mantle would transfer heat into the preexisting fertile mantle domains for partial melting, giving rise to the syn-rift basalt with arc-like trace element compositions in combined accretionary to collisional orogens. Therefore, we propose that the continental rift zone was superimposed on a Grenvillian accretionary orogen in the northern margin of South China Block. The compressional regime was prominent during the accretionary and collisional orogenesis, whereas the rifting orogeny is characterized by a stretching regime after thinning of orogenic lithosphere in response to the Rodinia breakup (Zheng and Chen, 2017). It is stretching of the thinned orogenic lithosphere that provides the high heat flow for the HT/LP metamorphism along the continental rifting zone. Generally, most regional HT/LP series metamorphic rocks (temperature dominates over pressure) are characterized by the presence of andalusite and/or sillimanite and associated with contemporaneous granitoids and HP granulites (e.g., Barton and Hanson, 1989; Brown,
1993; Zheng and Chen, 2017). The majority of HT/LP metamorphic belts show an early HP metamorphism prior to development of the final lower P mineral assemblages, which is usually ascribed to an early stage of crustal thickening in settings of accretionary to collisional orogenesis (Vissers, 1992; Amato et al., 1994; Zheng and Chen, 2017). This is consistent with the occurrence of HP granulites in thickened orogens that underwent thinning due to foundering of the lower continental crust or lithospheric mantle (Harley, 1989; Brown, 1993; Zheng and Chen, 2016). The continental rifts develop in the late stage when the orogenic lithosphere is significantly thinned (Zheng and Chen, 2017). In this regard, the HT/LP metamorphic belts are generally superimposed on preexisting HP metamorphic belts produced in compressional regimes. As such, the HT/LP metamorphic rocks were generated in the two stage processes, with the first stage of lithospheric thinning and the second stage of lithospheric stretching. If the lithospheric stretching manages to rupture the amalgamated continental blocks, HT/LP metamorphic rocks can be produced at the rifted continental margins to create a new ocean (Wilson, 1966; Burke and Dewey, 1974). If not, this kind of metamorphic rocks ended up in intracontinental orogens as proposed by Wickham and Oxburgh (1985).
6. Implications for regional HT/LP metamorphism The thermobarometric determination for aluminosilicate-bearing metagranites in the Beihuaiyang zone indicates that the regional HT/LP metamorphism took place at 1.0-3.5 kbar and 560-660°C (Fig. 8), corresponding to the very high thermal gradients of >60°C/km at the upper crustal level. Such abnormally thermal gradients cannot be provided by a normal increase in geothermal gradient for continental lithosphere. An additional heat supply is required to achieve the very high thermal gradients. In this regard, the results from the present study have important implications for heat budget in continental rifts. As listed in supplementary Table S9, heat flow values of 35-43 mW/m3 are calculated at 750 Ma based on radioactive heat-producing elements (i.e. Th, U and K) in metagranite. Following the estimation of McLaren et al. (1999), we are in a position to clarify the relationships between crustal temperatures, thermal conductivity and crustal heat flow at a depth of 10 km (Fig. 10). The observed mineral assemblages at least require a crustal heat flow of 70-120 mW/m2 since the chosen depth corresponds to the highest pressure. In addition to the heat supply by heat production elements, an additional heat supply of about 30-80 mW/m2 is necessary, which is significantly higher than the normal mantle heat flow
beneath the stable continental crust (Sclater et al., 1980). Therefore, the self-produced heat only plays a subordinate role. In other words, the existence of radioactive heat-producing elements cannot lead to the anomalously high heat flow in continental rifts. On the other hand, the very high thermal gradients can be acquired through (1) enhanced magmatic advective heat transfer from the emplacement of mantle-derived basaltic magma (Sandiford and Powell, 1986) or a large volume of granitoid plutons (Lux et al., 1986); and (2) increased conductive heat transfer from the underlying asthenosphere via shallowing of the thermal boundary layer (Houseman et al., 1981; Plat and England, 1994). In either way, continental rifts are proper sites for generating the anomalously high heat flow at crustal depths. For the origin of HT to ultrahigh-temperature (UHT) granulite-facies metamorphic rocks (Harley, 1989; Kelsey, 2008), the typical product of HT/LP metamorphic facies series (Zheng and Chen, 2017), a key issue is the temperature deficit in collisional models (Thompson and Connolly, 1995) to achieve granulite-facies metamorphism at crustal depths (Harley, 1989; Clarke, 2011). It requires additional heat sources. As argued above, there is the substantial heat supply from the asthenospheric mantle to the crust in continental rifts, making them an important candidate with the high thermal gradients for the HT to UHT metamorphism. Seeking such metamorphic rocks in rifted plate margins appears to be a promising work in the future for the origin of HT/LP metamorphic rocks. On the other hand, accretionary and collisional orogens may be reactivated by post-collisional thinning and extension (Zheng and Chen, 2017). Although such processes did not achieve continental rupture, the extensional tectonism may be responsible for regional HT/LP metamorphism in intracontinental orogens where continental rifting did not develop in rupture (Gao et al., 2017; Zheng et al., 2018). Therefore, both successful and failed rifts are preferential sites for high thermal gradients and thus for regional HT/LP metamorphism.
7. Conclusions The high-T/LP metamorphic rocks in a Neoproterozoic continental rift zone are identified by petrographic textures, such as andalusite formation via muscovite dehydration reaction and prograde transformation from the andalusite to sillimanite stability field. The negative
δ18O
value
of
andalusite
demonstrates
the
occurrence
of
continental
deglacial-hydrothermal alteration in the rift setting. Comparison with zircon O isotopes confirms that the aluminosilicates are the product of regional HT/LP metamorpohism during
the continental rifting. The HT/LP metamorphism occurred at 1.0-3.5 kbar and 560-660°C on the basis of the peak mineral assemblages. Metamorphic titanite U-Pb dating constrains the timing of HT/LP metamorphism at ca. 750 Ma, consistent with the peak age of Rodinia breakup. The premetamorphic protolith of metabasalt with the arc-like geochemical signature indicates development of the continental rift on the Grenvillian accretionary orogen. Reactivation of the accretionary orogen provides the anomalously high heat flow from the asthenospheric mantle for the HT/LP metamorphism, which is supported by the estimate of crustal heat flow based on Th, U and K contents and peak mineral assemblages. Therefore, continental rifts are a preferential site for high thermal gradients and thus for the HT/LP metamorphism, and stretching of the thinned orogenic lithosphere is the primary mechanism for transferring the high heat flow from the asthenospheric mantle into the crust.
Acknowledgments This study was supported by funds from Chinese Ministry of Science and Technology (2015CB856100) and the Natural Science Foundation of China (41590620). Thanks are due to Yi-Xiang Chen and Wei Rong for their assistance with the field trip, to Chang-Qing Yin, Long Chen and Yu-Wei Tang for discussion on petrographic textures and geochemical data, to Chun-Jing Wei for his help with the pseudosection calculations, to Ke-Qing Zong for his assistance on whole-rock trace element analysis, to Yue-Heng Yang for his assistance with titanite U-Pb dating and trace element analysis, and to Xiao-Ping Xia for his help on SIMS zircon U-Pb dating and O isotope analysis. Comments from Dr. Guochun Zhao and an anonymous reviewer are very helpful for improvement of the presentation.
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Figure captions Figure 1. Geological map showing the position of the Beihuaiyang zone at the northern margin of the South China Block. Insert is a simplified geological map for the Beihuaiyang zone. The red star labels the location of the sampling area, which is located at Wozicun in the Luzhenguan complex.
Figure 2. Representative petrographic textures for metagranite and metabasalt and Raman spectra of andalusite and sillimanite in metagranite with characteristic Raman shifts labeled. (a) Raman spectra of the anhedral andalusite and needle sillimanite. Raman analysis spots are labeled by red or white circles in the photomicrograph; (b) highly anhedral andalusite and K-feldspar associated with porphyroclastic muscovite; (c) needle sillimanite is enclosed in porphyroclastic muscovite which also coexists with anhedral andalusite and K-feldspar; (d) prismatic andalusite is partially surrounded by sillimanite with inward-penetrating boundary; (e) color zoned andalusite with a pink core and a colorless rim; (f) Zn-rich spinel coexisting with or included in biotite; (g) andalusite intimately intergrown with sillimanite, muscovite and biotite; (h) a amphibole-rich zone and a biotite-rich zone of the metabasalt showing a clear boundary. The euhedral titanite occurs in the biotite-rich zone.
Figure 3. Chondrite normalized REE patterns and primitive mantle normalized trace element spidergram for metagranite and metabasalt. Chondrite values are from (Sun and McDonough, 1989) and primitive mantle values are from (McDonough and Sun, 1995). Trace element concentrations of continental arc and oceanic arc basalts (CAB and OAB) are from (Kelemen et al., 2003). Also shown are trace element compositions of the upper continental crust (UCC, Rudnick and Gao, 2014) and GLOSS-II (Plank, 2014).
Figure 4. The skeletal titanite in the metagranite and the euhedral titanite in the metabasalt.
Figure 5. LA-ICPMS zircon U-Pb dating results for the metagranite and metabasalt. (a-c) Concordia diagrams of U-Pb isotope data. Also shown are representative zircon CL images; (d) histogram of apparent 206Pb/238U ages.
Figure 6. SIMS zircon U-Pb dating and O isotope analysis results for the metagranite. (a, c) Concordia diagrams of U-Pb isotope data with representative zircon CL images displayed; (b, d) histograms of zircon δ18O values.
Figure 7. Titanite U-Pb dating results and REE patterns for metagranite and metabasalt. (a-c) Tera-Wasserburg diagrams of U-Pb isotope data with weighted average value of 207
Figure 8. Pseudosection calculations of P-T conditions for metagranite 14BHY07 in the MnNCKFMASH system. The red line labels the subsolidus aluminosilicates-bearing assemblages, i.e. the estimated P-T conditions for the peak stage of HT/LP metamorphism. The blue area highlights the andalusite stability field. The depth of shading reflects increased variance, and the darkest fields are quinivariant. Abbreviations: g, garnet; mu, muscovite; bi, biotite; opx, orthopyroxene; cd, cordierite; pl, plagioclase; ksp, K-feldspar; q, quartz; and, andalusite; sill, sillimanite; ky, kyanite.
Figure 9. Fe, Al, Th and U contents of the skeletal titanite in the metagranite and the euhedral titanite in the metabasalt.
Figure 10. The possible relationships between crustal temperatures, thermal conductivity and crustal heat flow at a depth of 10 km. Calculation of the isothermal lines are based on the approximated temperature at depth related to crustal heat flow, thermal conductivity and heat production of the overlying sequence (assuming to be 2 mW/m2 according to Mclaren et al. 1999). The thermal conductivity of 1.5 to 2.0 is adopted based on a range of granite at temperatures higher than 300 °C (Jaupart et al., 2016). The blue and red areas show the heat flow ranges estimated from U, Th and K contents and mineral assemblages, respectively.
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41
42
Highlights
Both metamorphic andalusite and sillimanite occur in metagranitic rocks from a Neoproterozoic continental rift.
Metamorphic titanite U-Pb dating yields consistent ages with the peak time of Rodinia breakup.
High-T/low-P metamorphism is evident during the continental rifting in the middle Neoproterozoic.
The continental rift was superimposed on a Grenvillian orogen developed during the Rodinia assembly.
Continental rifting transferred high heat flow from the asthenospheric mantle into the crust.
