Late Triassic syn-exhumation magmatism in central Qiangtang, Tibet: Evidence from the Sangehu adakitic rocks

Accepted Manuscript Full length Article Late Triassic syn-exhumation magmatism in central Qiangtang, Tibet: Evidence from the Sangehu adakitic rocks Han Liu, Bao-di Wang, Long Ma, Rui Gao, Li Chen, Xiao-bo Li, Li-quan Wang PII: DOI: Reference:

S1367-9120(16)30310-8 http://dx.doi.org/10.1016/j.jseaes.2016.10.009 JAES 2831

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

17 April 2016 8 October 2016 14 October 2016

Please cite this article as: Liu, H., Wang, B-d., Ma, L., Gao, R., Chen, L., Li, X-b., Wang, L-q., Late Triassic synexhumation magmatism in central Qiangtang, Tibet: Evidence from the Sangehu adakitic rocks, Journal of Asian Earth Sciences (2016), doi: http://dx.doi.org/10.1016/j.jseaes.2016.10.009

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Late Triassic syn-exhumation magmatism in central Qiangtang, Tibet: Evidence from the Sangehu adakitic rocks

Han Liua,b, Bao-di Wanga,b,*, Long Maa, Rui Gaoc, Li Chena,b, Xiao-bo Lia,b, Li-quan Wanga,b

a

Chengdu Institute of Geology and Mineral Resources, Chengdu, 610081, China

b

Research Center for Tibetan Plateau Geology, China Geological Survey, Chengdu,

610081, China c

Chengdu University of Technology, Chengdu, 610059, China

Abstract: The geodynamic setting of Late Triassic magmatic activity along the Longmu Co–Shuanghu suture zone (LSSZ) in central Qiangtang, Tibet is a matter of debate. This paper presents zircon LA–ICP–MS U–Pb ages, zircon Hf isotopic compositions, and whole-rock geochemical data for the Sangehu (SGH) granitic intrusion in central Qiangtang, and addresses the petrogenesis of Late Triassic magmatism, and the history of collision between the northern and southern Qiangtang terranes. The SGH pluton consists mainly of biotite adamellite with mafic microgranular enclaves (MMEs), and small amounts of K-feldspar granite. The biotite adamellite,

MMEs,

and

K-feldspar

granite

give ages of 207.8 ± 3.0 Ma,

212.4 ± 31 Ma, and 211.6 ± 3.8 Ma, respectively. The MMEs show magmatic textures and acicular apatite, and are coeval with the host biotite adamellite, suggesting they were produced by magma mixing. All samples from the SGH pluton show high Sr and low Y contents, and positive Eu anomalies, similar to adakitic rocks. The high K2O contents and low Mg#, Cr, and Ni contents, and enriched Hf isotopic characteristics of the zircons indicate that these magmas were derived from the partial melting of thickened crust. However, the whole-rock geochemical data and zircon Hf isotopic compositions also reveal heterogeneity at the source. The combined magmatic and metamorphic records suggest that Triassic magmatic activity in central Qiangtang was closely related to the collision of the northern and southern Qiangtang terranes. The

large-scale Late Triassic (225–200 Ma) magmatic event in central Qiangtang may have resulted from the breakoff of the Longmu Co–Shuanghu Tethys Ocean lithospheric slab in the early Late Triassic (236–230 Ma). The Late Triassic magmatic rocks, including adakitic rocks, are coeval with retrograde high-pressure (HP) to ultrahigh-pressure(UHP) metamorphic rocks in central Qiangtang, and show characteristics of syn-exhumation magmatism. The early adakitic rocks (>220 Ma) were generated by melting of the edge of the slab, which was heated by upwelling asthenospheric mantle after subduction slab breakoff, and the late adakitic rocks (<220Ma) were generated by partial melting of the thickened crust. Keywords: Tibetan Plateau, adakitic rock, syn-exhumation magmatism, slab breakoff, Tethys

1 Introduction The Longmu Co–Shuanghu suture zone (LSSZ) is the consequence of the closure of the Longmu Co–Shuanghu Tethys Ocean (LSTO). Recent studies have shown that this ocean existed from the Ordovician to the Permian (Zhu et al., 2006; Li et al., 2007a, 2008a; Wang et al., 2008; Wu et al., 2010; Zhai et al., 2013a). Subduction likely started in the Late Devonian or earlier, as Late Devonian–Early Permian island-arc-type magmatic rocks are distributed along the LSSZ (Zhang et al., 2014a; Jiang et al., 2015; Liu et al., 2015; Wang et al., 2015). However, the timing of LSTO closure is disputed (Zhang et al., 2006; Li et al., 2007b; Liu et al., 2011; Zhai et al., 2011a, 2011b; Liang et al., 2012); resolving the timing of closure will improve our understanding of the processes involved in the collision of the southern and northern Qiangtang terranes. Some researchers have used the timing of peak high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism to constrain the closure time of the LSTO (Zhang et al., 2006; Pullen et al., 2008; Dong et al., 2010), and have suggested that the LSTO closed in the Early–Middle Triassic as a result of collision between the southern and northern Qiangtang terranes (Wang et al., 2006; Peng et al., 2015). However, these HP–UHP metamorphic rocks may have formed by deep subduction of

the LSTO (Liu et al., 2011; Zhai et al., 2011a, 2011b). Late Triassic (225–205 Ma) arc-type volcanic rocks are widely distributed across the southern margin of the northern Qiangtang terrane and within the LSSZ, and although their origin is debated, these rocks are consistent with southward subduction of the Jinshajiang oceanic lithosphere (Wang et al., 2011a) or northward subduction of the Longmu Co–Shuanghu seafloor (Zhai and Li, 2007; Zhai et al., 2013b) beneath the northern Qiangtang terrane. On the other hand, some of these volcanic rocks are considered to have been formed in a continental rift setting, marking the opening of the Mesozoic Qiangtang Basin (Wang et al., 2008; Fu et al., 2010), which implies that the LSTO had closed before the Late Triassic. Collisional granitoidal magmatism preserves information about crust–mantle interactions, providing a petrogenetic ‘window’ into the evolution of deep crustal sources and geodynamic processes, and is important in understanding the tectonic evolution of an orogenic belt. In comparison with the widespread granitic magmatism on the southern margin of the southern Lhasa terrane, which is related to closure of the Yarlung Zangbo Neo-Tethys Ocean, the granitic magmatism along the LSSZ is small in scale, but also forms a granitic belt that stretches more than 1000 km from west to east (Hu et al., 2014a; Tao et al., 2014; Zhang et al., 2014b; Peng et al., 2015). In this paper, we present geochronological and geochemical data for the Late Triassic Sangehu (SGH) granitic pluton in the western section of the LSSZ, and we use these to interpret the geodynamic evolution of the Qiangtang terrane. By combining our data with regional results, we provide insight into the geodynamic mechanisms of Late Triassic magmatism and the processes involved in the collision of the southern and northern Qiangtang terranes.

2 Geologic setting The Tibetan plateau is composed, from north to south, of the Songpan−Ganzi, Qiangtang, Lhasa, and Himalaya terranes (Fig. 1a; Chung et al., 2009; Zhu et al., 2012). The Qiangtang terrane is separated from the Songpan−Ganzi terrane by the Jinshajiang suture zone (JSSZ) to the north, and from the Lhasa terrane by the

Bangong Co–Nujiang suture zone (BNSZ) to the south (Zhang et al., 2011a). The Qiangtang terrane itself is divided into the southern and northern terranes by the LSSZ (Fig. 1b; Li et al., 1987; Kapp et al., 2003; Pan et al., 2012). The LSSZ is one of several major suture zones in Tibet, and it trends east–west across the Qiangtang terrane, projecting eastward into the Changning–Menglian suture zone (CMSZ), and extending into the Qingmai region in Thailand (Pan et al., 2012; Wang et al., 2013a, 2013b; Zhai et al., 2013a). The presence of HP–UHP metamorphic rocks (eclogite and blueschist) within the LSSZ suggests that the southern Qiangtang continental crust was subducted northward during continental collision between the southern and northern Qiangtang terranes (Zhang et al., 2006; Zhang and Tang, 2009). The Triassic intrusive rocks are usually interpreted to have formed in a collisional setting (Hu et al., 2010a; Wang et al., 2011b; Hu et al., 2014; Peng et al., 2015), but Triassic bimodal volcanic rocks (Fu et al., 2010; Zhang et al., 2011a) and arc-type volcanic rocks (Zhai and Li, 2007; Zhai et al., 2013b) are also reported. Some of the Triassic volcanic and intrusive rocks are geochemically similar to adakitic rocks (Zhai et al., 2013b; Zhang et al., 2014b). Therefore, the geodynamic setting of Triassic magmatic activity along the LSSZ is debated, and several different models have been proposed for the magmatism (Fu et al., 2010; Zhang et al., 2011a; Zhai et al., 2013b; Hu et al., 2014a; Peng et al., 2015). The study area is located in the western segment of the LSSZ, which in this area has been divided into the Hongjishan ophiolite mélange, and the Taoxinghu ophiolite mélange due to structural deformation (Fig. 2a). Both belts contain complete ophiolite mélange successions, including metamorphic peridotite, pyroxenite, gabbro, basalt, plagiogranite, and chert (Zhai et al., 2013a; Lou et al., 2014), accompanied by HP–UHP metamorphic rocks (Zhai et al., 2011a, 2011b; Zhang et al., 2014c). The Chaduogangri accretionary complex is located between the two ophiolite mélange belts (Pan et al., 2012), and consists of Carboniferous–Permian strata, mainly clastic rocks with minor volcanic rocks, carbonate rocks, and cherts, similar to the southern Qiangtang strata. Triassic magmatic activity occurred frequently in the Chaduogangri accretionary

complex. The volcanic rocks are primarily intermediate–acidic volcanic (or volcaniclastic) rocks, interlayered with small amounts of basic volcanic rocks (Wang et al., 2007; Zhai and Li, 2007), and they overlie the early Carboniferous Chameng Formation, the late Carboniferous–early Permian Zhanjin Formation, and the middle Permian Longge Formation (Fig. 2a). The SGH granitic pluton is located in the eastern part of the Chaduogangri accretionary complex, and is one of several large granitic plutons along the LSSZ (Zhang et al., 2014c).

3 Field relationships and petrography The SGH granitic pluton lies ~10 km southeast of Sangehu Lake in central Qiangtang, has an outcrop area of ~30 km2, and an elliptical shape with a NE-trending long axis (Fig. 2b). The Late Carboniferous–Early Permian Zhanjin Formation, which is the wall rock to the SGH pluton (Fig. 2b), comprises clastic rocks with minor mafic volcanic rocks and tillite (pebbly slate; Jiang et al., 2015), as well as hornfels near the contact with the pluton. A northeast-striking valley cuts the SGH pluton and likely marks the presence of a fault. The SGH pluton is composed of biotite adamellite and K-feldspar granite, with minor enclaves within the biotite adamellite. The biotite adamellite is medium- to coarse-grained, and is composed mainly of plagioclase (25%–30%), K-feldspar (20%–25%), quartz (20%–25%), and biotite (~10%) with minor (<5%) apatite, zircon, and opaque minerals. The K-feldspar granite is medium- to coarse-grained, and consists of K-feldspar (35%–40%), quartz (25%–30%), plagioclase (15%–25%), and mica (~5%), with minor zircon, apatite, titanite, and iron oxides. The enclaves are 5–50 cm in size and are randomly distributed in the host biotite adamellite. Each enclave has a sharp contact with its host, and is almost perfectly round (Fig. 3a). The enclaves are dioritic in composition, fine-grained, and contain plagioclase (55%–65%), biotite (25%–30%), quartz (7%–12%), and minor K-feldspar (5%), with accessory (<3%) of iron oxides, zircon, and acicular apatite (Fig. 3b).

4 Analytical methods

4.1 Whole-rock major and trace element analyses Major oxide contents were determined at the Chengdu Center, China Geological Survey (CDCGS), Chengdu, by X-ray fluorescence spectrometry (XRF) on glass disks using a Rigaku ZSX100e spectrometer. Analytical errors were usually <5%. Trace element analyses were performed at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS), Guangzhou, by inductively coupled plasma–mass spectrometry (ICP–MS). The detailed analytical protocol was described by Chen et al. (2010). The analytical precision for rare earth elements (REE) and high field-strength elements (HFSE) was >5%. 4.2 Zircon U–Pb dating Zircons for LA–MC–ICP–MS U–Pb dating were separated from the rock samples by using conventional heavy liquid and magnetic separation techniques. Samples 11SGH-2, 11SGH-6, and 11SGH-9 are from the biotite adamellite, enclave, and K-feldspar granite, respectively. The internal structure of the zircons was assessed using cathodoluminescence (CL) images obtained with an EMPA-JXA-8100 scanning electron microscope at GIGCAS; these CL images are shown in Fig. 4. The U–Pb isotopic compositions of the zircons were analyzed by LA–MC–ICP–MS at GIGCAS. Zircon 91500 was used as an external standard for U–Pb dating, and the preferred U–Th–Pb isotopic ratios are from Wiedenbeck et al. (1995). Off-line selection and integration of the background and analyte signals, and time-drift correction and quantitative calibration for the trace element analyses and U–Pb dating were performed using ICP–MS–DataCal software (Liu et al., 2010a). All ages were plotted using the Isoplot 3.0 program (Ludwig, 2003), with 1σ errors presented for all results. The zircon U–Pb isotopic data are listed in Table 2 and shown in Fig. 5. 4.3 Zircon Hf isotope analyses In situ Hf isotope analyses of zircon were conducted at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan, by LA–ICP–MS (a GeoLas 2005 laser-ablation system attached to an Agilent 7500a ICP–MS) with a beam size of 44 μm, and a laser pulse frequency of 8 Hz. Initial 176Hf/177Hf and εHf(t) values were calculated with the measured U–Pb

ages, and with reference to the chondritic reservoir (CHUR) at the time of zircon growth from the magmas, at ratios of 176Hf/177Hf = 0.282772 and 176Lu/177Hf = 0.0332 (Blichert-Toft and Albarede, 1997). Single-stage Hf model ages (TDM(Hf)) were calculated with reference to the depleted mantle at a present-day 0.28325 and

176

176

Hf/177Hf ratio of

Lu/177Hf of 0.0384 (Griffin et al., 2000). Crustal model ages

(TCDM(Hf)) were calculated assuming

176

Lu/177Hf = 0.015 for the average continental

crust (Griffin et al., 2002). The off-line selection and integration were also performed using ICP–MS–DataCal software (Liu et al., 2010a). The zircon Hf isotopic data are listed in Table 3.

5 Results 5.1 Zircon U–Pb data The zircons in the three samples are predominantly euhedral or subhedral, have short to long prismatic forms (60–200 μm long), length/width ratios of 1:4–1:1, and exhibit clear oscillatory zoning in CL images (Fig. 4). Twenty-four zircon grains from each sample were analyzed by LA–ICP–MS. All of the Th/U ratios for samples 11SGH-6 (0.14–0.60) and 11SGH-9 (0.13–1.46), and most of the Th/U ratios for sample 11SGH-2 (0.06–2.85), are greater than 0.1. These ratios, together with the crystal forms and growth zoning (Fig. 4), suggest that the zircons are of magmatic origin (Hoskin and Schaltegger, 2003). Excluding the discordant zircons, 7 analyses for sample 11SGH-2 plot on or close to the concordia, have an 206Pb/238U age range of 203 to 214 Ma, and yield a weighted mean

206

Pb/238U age of 207.8 ± 3.0 Ma (MSWD = 1.14; Fig. 5a). The 16 spots for

sample 11SGH-6 give a weighted mean

206

Pb/238U age of 212.4 ± 3.1 Ma (MSWD =

2.5; Fig. 5b). For sample 11SGH-9, 17 spots yield Ma, with a weighted mean

206

206

Pb/238U ages from 201 to 227

Pb/238U age of 211.6 ± 3.8 Ma (MSWD = 3.3; Fig. 5c).

These ages are interpreted as crystallization ages, and suggest that the SGH pluton formed in the Late Triassic. Notably, samples 11SGH-2 and 11SGH-9 contain inherited zircons (Table 2) that have ages older than the crystallization age. These inherited zircons are likely

xenocrysts that were trapped by the granitic magma during ascent, and show distinctive bright CL images, in contrast to the relatively dark CL images of the Late Triassic magmatic zircons. 5.2 Whole-rock geochemical data Petrographic observations and the range in loss on ignition (LOI = 0.43–1.17 wt.%; Table 1) values indicate that the SGH samples experienced low degrees of alteration. The seven samples from the SGH granitoids (biotite adamellite and K-feldspar granite) are siliceous, with 68.58–75.26 wt.% SiO2, 0.12–0.27 wt.% TiO2, 13.71–16.82 wt.% Al2O3, 1.38–2.56 wt.% Fe2O3T, 0.32–0.58 wt.% MgO (Mg# = 32–37), and 6.42–7.44 wt.% total alkalis (K2O + Na2O). The biotite adamellite shows slightly lower SiO2 contents and higher TiO2, Al2 O3, MgO, CaO, Na2O, and Fe2O3T contents than the K-feldspar granite. The three samples of enclaves hosted in the biotite adamellite have dioritic compositions with 60.96–66.95 wt.% SiO2, and show lower total alkalis (K2O + Na2O = 5.36–6.28 wt.%) and higher TiO2 (0.51–0.71 wt.%), Al2O3 (16.16–18.18 wt.%), MgO (1.59–2.47 wt.%; Mg# = 38–42), and Fe2O3T (5.23–6.97 wt.%) contents than the K-feldspar granite and host biotite adamellite. The high K2O contents (1.93–4.39 wt.%) for all the samples show that they belong to the high-K calc-alkaline series (Fig. 6a), although the granitoids have higher K2O/Na2O values (1.03–1.60) than the enclaves (0.56–0.75). All the samples have A/CNK values greater than 1 (molar Al2O3/(CaO + K2O + Na2O) = 1.10–1.47), indicating that they are peraluminous rocks (Fig. 6b), and they have low Na2O contents (2.74–3.74 wt.%). In addition, they show negative correlations between P2O5 and SiO2, suggesting an S-type affinity (Chappell and White, 2001). Figure 7a shows both the biotite adamellites and their enclaves are relatively depleted in Nb and Ti, and enriched in Rb, Th, and U. They have similar chondrite-normalized REE patterns (Fig. 7b). The host biotite adamellite shows LREE enrichment ([La/Yb]N = 8.09–13.86), with moderate positive Eu anomalies (Eu/Eu* = 1.15–1.83), and the enclaves show LREE enrichment ([La/Yb] N = 11.15–18.36), and small negative or positive Eu anomalies (Eu/Eu* = 0.89–1.10). The K-feldspar granites show similar trace and rare earth element characteristics to

the biotite adamellites and their enclaves (Fig. 7), but have higher Th contents and a greater depletion in Nb and Ti. 5.3 Zircon Lu–Hf isotopic data The initial 176Hf/177Hf ratios and εHf(t) values are calculated at t = 208 Ma, 212 Ma, and 211 Ma for samples 11SGH-2, 11SGH-6, and 11SGH-9, respectively. One spot in 11SGH-2 and one spot in 11SGH-6 yield strongly negative εHf(t) values (−17.3 and −29.1, respectively), and were positioned close to the zircon cores. These cannot represent Hf isotopic features of magmatic zircons, but instead indicate that the enclaves and host biotite adamellite contain inherited zircon. Ten zircons from sample 11SGH-2 have variable εHf(t) values ranging from −7.0 to −2.6 and give TCDM ages of 1258–1503 Ma (Table 3). Eleven analyses of zircons from sample 11SGH-6 give negative εHf(t) values of −3.8 to −0.6, yielding Hf crustal model ages (TCDM) of 1149–1328 Ma. Twelve spot analyses were obtained for zircons from sample 11SGH-9, yielding high εHf(t) values from −5.7 to −0.9, and TCDM of 1163–1431 Ma.

6 Discussion 6.1 Types of mafic micro-granular enclaves and their petrogenesis There are several different hypotheses for the petrogenesis of enclaves in felsic plutons, including an origin as wall rock xenoliths, as residues after partial melting, or as blobs of coeval mafic magma, termed mafic micro-granular enclaves (MMEs; Didier and Barbarin, 1991); the latter is the most frequently described. The enclaves in the biotite adamellite of the SGH pluton are rounded, suggesting that the magma from which they crystallized existed as liquid globules within the host magma. They also show igneous micro-textures, sharp contacts, and an age coeval with the host rocks, consistent with MMEs. Several studies have suggested that MMEs in granitic rocks are produced by magma mixing (Didier and Barbarin, 1991; Andersen et al., 2007; Zhao et al., 2012), but alternative models have been proposed, including an origin as cumulates formed by early crystallization (Clemens and Wall, 1988; Dodge and Kistler, 1990), or by

magma–liquid immiscibility (Watson, 1976; Zhu, 1995). The SGH MMEs have similar chondrite-normalized patterns and total rare earth element concentrations (ΣREE) as the host rocks (Fig. 7b), thereby precluding an origin as cumulates formed by early crystallization. Moreover, if the enclaves represent cumulates, both the MMEs and host rocks should have the same mineral granularity, and their sharp contacts with the host do not support crystal accumulation processes (Kumar and Rino, 2006). Although the MMEs have different major element contents to the host rocks, they have similar trace element compositions (excluding Zr, Nb, P, and Y contents; Fig. 7a), which is inconsistent the liquid immiscibility hypothesis (Watson, 1976). The SGH MMEs contain acicular apatites, which provide evidence of rapid cooling during crystallization, similar to the characteristics of MMEs produced by the mixing of small volumes of hot basic magma with cooler granitic melt (Sparks and Marshall, 1986; Xu et al., 2004). As mentioned above, the magmatic textures, sharp contacts, and zircon U–Pb ages coeval with the host rocks also support a magma-mixing origin for the MMEs. The whole-rock major and trace element chemistry may also be used to assess the possibility of mixing and the nature of the mixing relationship. Linear trends in Figure 8 strongly suggest a mixing relationship between the MMEs and host rocks (Langmuir et al., 1978; Kaygusuz and Aydincakir, 2009). The compositions of the major element oxides suggest significant differences between the MMEs and host rocks; however, their trace element compositions are similar (Fig. 7b), which can be interpreted as resulting from non-synchronous diffusion between major elements and trace elements during magma mixing (Lesher, 1994; Zhao et al., 2012). The similarity in trace element compositions between the MMEs and host rocks suggests that the diffusion had approached equilibrium. 6.2 Adakite-like geochemical affinities All the samples in the SGH pluton have high SiO 2 (60.96–75.26 wt.%) and Al2 O3 (13.71–16.82 wt.%), and low Y (7.37–11.87 ppm) and Yb (0.67–1.31 ppm) contents, along with high Sr contents (269.3–479.3 ppm) and Sr/Y ratios (27.0–42.3), and they plot in the adakite field in the Sr/Y–Y diagram (Fig. 9). In addition, they are

characterized by small negative to positive Eu anomalies (Eu/Eu* = 0.89–1.83), as is typical of adakitic rocks (Defant and Drummond, 1990; Martin, 1999; Xu et al., 2002). Low contents of MgO (0.32–0.80 wt.%; Mg# = 35.3–41.2), and compatible trace elements (e.g., Ni, Cr) indicate that these samples belong to the low-Mg group of adakitic rocks. In addition to the SGH rocks, adakitic rocks have been documented in the Baohu and Juhuashan areas in central Qiangtang (Zhai et al., 2013b). Compared with the SGH adakitic rocks, the Baohu and Juhuashan rocks have higher MgO contents, with Mg# = 51.1–57.9. Previous studies have proposed five possible origins for adakitic rocks: partial melting of a subducted slab (Defant and Drummond, 1990), partial melting of thickened mafic lower crust (Atherton and Petford, 1993), partial melting of delaminated lower crust (Xu et al., 2002; Gao et al., 2004), differentiation of a basaltic magma under high pressure (Castillo et al., 1999; Macpherson et al., 2006), or mixing between basaltic and rhyolitic magma (Guo et al., 2007; Qin et al., 2010). The SGH adakitic rocks have high K2O contents (1.93–4.39 wt.%), and their zircons yield negative εHf(t) (Fig. 10), and old Hf crustal model ages (Table 3), thereby precluding an origin by melting of young subducted slab. However, rocks generated by the melting of subducted oceanic crust and sediments can also display these geochemical characteristics. Furthermore, the Hf isotope composition of zircons in this study suggests that any addition of subducted sediment melts to the magma must have been negligible. For example, Late Cretaceous (90 Ma) adakitic intrusive rocks in the Kelu area (Jiang et al., 2012), which were formed by the partial melting of 90% basaltic oceanic crust and 10% oceanic sediments, have high, positive εHf(t) values. Given that all SGH pluton samples yield negative εHf(t) values, their origin cannot be attributed to the melting of oceanic crust containing an old sedimentary component. A linear trend between Th/Yb and Th/Sm is also not observed (Elburg et al., 2002); this is inconsistent with melting of oceanic sediment, and suggests that the model for the generation of adakitic rocks in the SGH pluton require revision. The Late Triassic magmatic rocks in central Qiangtang are dominated by acidic rocks, with only a few intermediate and basic rocks, and are located relatively far

from the SGH pluton, which they predate (Zhai and Li, 2007; Fu et al., 2010; Zhai et al., 2013b; Li et al., 2014); thus, it is difficult to explain the origin of the SGH adakitic rocks by fractional crystallization of a basaltic magma (Castillo et al., 1999). Modeling of the partial melting of delaminated lower crust yields adakitic rocks with relatively high MgO contents and Mg#, because of interaction of crustal material with mantle rocks (Xu et al., 2002), whereas the adakitic samples from the SGH pluton have low MgO (0.32–0.80 wt.%; Fig. 11a) and Mg# (35.3–41.2), with low Ni and Cr contents (Fig. 11b, c). The low Mg# and Cr and Ni contents of the host rocks suggest that they did not undergo mixing with a mantle-derived mafic magma (Qin et al., 2010). This view is consistent with the fact that the volume of host rock can exceed 99% in outcrop; therefore, mixing between felsic adakitic magma and mafic magma is not a suitable explanation for the origin of the SGH adakitic rocks. Thus, we conclude that the SGH adakitic melts were derived from the partial melting of thickened lower crust (Fig. 11). 6.3 Magma source Adakitic rocks in the SGH pluton are characterized by low Mg# and Cr and Ni contents, negative εHf(t) (Fig. 11), and old Hf crustal model ages (Table 3), suggesting that they were derived from the partial melting of thickened lower crust. The inherited zircons from the MMEs and their host rocks have strongly negative εHf(t) values (−17.3 and −29.1), indicating that their source must have contained an older crustal component. Moreover, all the samples yield low Nb/Ta values (6.48–12.48, with a mean of 9.94), similar to the ratio (8.33) of lower crust (Rudnick and Gao, 2003). In addition, they contain high Sr, and low HREE and Y contents, indicating that the MMEs, biotite adamellite, and K-feldspar granite were derived from a magma source containing garnet and pyroxene. The MMEs and K-feldspar granite are characterized by weakly-developed negative Eu anomalies (Eu/Eu* = 0.89–1.10 and 0.89–1.11, respectively), in contrast to the positive Eu anomalies of the biotite adamellite (1.15–1.83), which suggests that plagioclase was a minor residual phase at the source of the MMEs and K-feldspar granite. Phase equilibrium and partial melting experiments on basalt show that the presence of rutile as a residual phase can produce

the characteristic depletions in Nb and Ta of adakitic melts (Xiong et al., 2006; Nair and Chacko, 2008). However, the SGH samples only show negative anomalies in Nb, suggesting that the pressure in the magma source did not reach that required for rutile stability (1.5 GPa; Xiong et al., 2006). The SGH samples also show flat HREE patterns, low Ti contents (Fig. 6), and Y/Yb = 8.68–11.00 (i.e, ≈10), indicating that amphibole played an important role in the generation of the adakitic melts (Sisson, 1994; Zhu et al., 2008). This mineral phase is known to significantly fractionate Nb and Ta strongly under some conditions (Liang et al., 2009) and could account for the absence of negative Ta anomalies in the SGH samples. The MMEs have incompatible trace elements abundances higher than, or similar to the host rocks, suggesting that the host biotite adamellites were not derived from MME melts by fractional crystallization, but originated from a different source. The comparable trace element abundances of the K-feldspar granite and biotite adamellite, however, suggest that they were derived from similar sources (Fig. 7a). In addition, the εHf(t) values of the K-feldspar granite (–5.7 to –0.9) and biotite adamellite (−7.0 to −2.6) also show only slight differences (Table 3). Thus, we conclude that the biotite adamellite and K-feldspar granite were derived from similar sources that differed from the source of the MMEs. Moreover, if the biotite adamellite was derived from the fractional crystallization of a basaltic magma, or shared a source with the MMEs or Triassic basalts in central Qiangtang, then such rocks should be more abundant. This contrasts with the dominance of felsic rocks in the Late Triassic magmatic suite in central Qiangtang, which contains only minor intermediate and mafic rocks. In the absence of a fluid phase, the dehydration-melting of H2O-bearing minerals becomes important, and the concentrations of K2O and SiO2 in the melt depend on those in the protolith (Xiong et al., 2011). The high SiO2 (68.58–75.26 wt.%) contents of the biotite adamellite and K-feldspar granite show that they were derived from melting of moderately acidic rocks; the low K2O (1.97–2.54 wt.%) and SiO2 (60.96–66.95 wt.%) contents of the MMEs reflect their meta-basaltic source. The former yield high Al2O3/(MgO + FeOT) values (2.76–5.33), similar to those of the partial melts of metasedimentary rocks, and the latter have low Al2O3/(MgO + FeOT)

values (1.17–1.51), consistent with those for melts derived from metabasalts (Wang et al., 2003). 6.4 Implications for dynamic evolution 6.4.1 Late Triassic magmatism and collision between the southern and northern Qiangtang terranes There is much debate over the geodynamic setting of the Late Triassic magmatism in central Qiangtang. Several scenarios have been proposed, including continental rift (Wang et al., 2008b; Fu et al., 2010), subduction (Liang et al., 2012), collisional settings (Wang et al., 2006; Zhang et al., 2006; Pullen et al., 2008), and the transition from oceanic crust subduction to continent–continent collision (Zhai et al., 2013b). A continental rift setting for the Late Triassic magmatism can be excluded based on an absence of observable field relationships between bimodal volcanics in the area. Bimodal volcanic rocks are characteristic of continental rifting (Davies and Macdonald, 1987; Wilson, 1989), and although Late Triassic basic and acidic volcanic rocks occur in central Qiangtang (Fu et al., 2010; Wang et al., 2010; Zhang et al., 2011a; Zhai et al., 2013b), they do not occur in the same outcrop or section, therefore do not constitute a bimodal volcanic suite as they were likely derived from unrelated sources. Furthermore, A-type granite magmatism is typical of continental rift settings (Eby, 1990; Li et al., 2007), and almost no Late Triassic A-type granite occurs along the LSSZ (Hu et al., 2010a; Zhai et al., 2013b; Zhang et al., 2014b; Chen et al., 2014; Hu et al., 2014; Peng et al., 2015). Zircon saturation temperatures for the Late Triassic granites are typically less than 800 °C, lower than the crystallization temperature of A-type granite (Skjerlie and Johnston, 1992; King et al., 1997). Thus, mantle heat sources had little influence on the genesis of the Late Triassic granites, which, additionally, do not display the alkaline or peralkaline features typical of within-plate granites (Fig. 5a). Although a few Late Triassic magmatic rocks show geochemical characteristics similar to those of island-arc magmas (Zhai et al., 2013b), geochemical characteristics alone are insufficient to prove their formation in a subduction environment, magmas may also be generated by subduction of the oceanic slab during collision (Liegeois,

1998; Sylvester, 1989). Assuming the LSTO had not closed in the Late Triassic, and the collision between the northern and southern Qiangtang terranes started in the early Jurassic (Liang et al., 2012), collision-related early Jurassic magmatic and metamorphic events should be preserved in the geological record (Zheng et al., 2009). However, all the reported ages for metamorphic events along the LSSZ are Triassic (Kapp et al., 2000, 2003; Pullen et al., 2008; Dong et al., 2010; Zhai et al., 2011a), and most of the magmatic events do not extend into the early Jurassic (Zhai et al., 2013b, Peng et al., 2015). Thus, whether or not the collision between the northern and southern Qiangtang terranes occurred during the Early Jurassic remains inconclusive. Consequently, the change from subduction of oceanic crust to continent –continent collision at 216 Ma remains unproven. The Triassic granites along the LSSZ are high-K calc-alkaline (Fig. 5a), showing peraluminous characteristics (Fig. 5b), similar to those of collisional granite (Fig. 13; Liegeois, 1998). Additionally, the Triassic magmatic events were concomitant with metamorphic events in central Qiangtang (Fig. 12), consistent with the coupling of magmatism and metamorphism during collision. Moreover, stress usually causes the alignment of minerals in collisional granites, therefore, the presence of gneissic granitoids along the LSSZ in central Qiangtang spanning the whole Triassic (Wang et al., 2011b; Li et al., 2014; Hu et al., 2014) is indicative of collision throughout the Triassic (Fig. 13). Thus, we propose that the Late Triassic magmatic events were closely related to the collision between the northern and southern Qiangtang terranes. 6.4.2 Slab breakoff In addition to Triassic magmatism, the HP metamorphic rocks in central Qiangtang are also considered to be related to collision between the southern and northern Qiangtang terranes (Zhang et al., 2006; Pullen et al., 2008; Dong et al., 2010). The presence of both low-temperature and high-pressure metamorphism, which provide evidence of oceanic deep subduction (Zhai et al., 2011a, 2011b; Liu et al., 2011), has been used to refute Triassic collision in the past. Numerical simulations suggest, however, that HP–UHP metamorphic rocks exhumed after slab breakoff, may also undergo low-temperature metamorphism (Duretz et al., 2012). Therefore, we argue

that a more reasonable explanation for the Late Triassic magmatic and metamorphic records may be provided by the slab breakoff model. The Late Triassic basalts in central Qiangtang postdate peak HP–UHP metamorphism, and show ocean island basalt (OIB) features, as well as evidence of crustal contamination (Ma et al., 2016), unlike typical ocean intra-plate basalts (Zhai et al., 2011b). Moreover, their geochemical characteristics, such as high Zr/Y values and negative Nb–Ta anomalies, are similar to those of ~110 Ma mafic enclaves in the BNSZ, and ~50 Ma mafic dikes from the Gaoligong Mountains, both of which are proposed to be related to slab breakoff (Xu et al., 2008; Zhang et al., 2011b). In addition, the presence of retrograde metamorphic minerals (e.g., glaucophane, phengite, and amphibole) indicates that the eclogite in central Qiangtang underwent retrograde

metamorphism

from

eclogite

facies

to

blueschist

facies,

and

epidote–amphibolite facies (Dong et al., 2010; Zhai et al., 2011a; Liu et al., 2011). Retrograde metamorphic minerals are younger than those formed during peak metamorphism (e.g., garnet and zircon in eclogite facies; Fig. 12), consistent with rapid exhumation of the eclogite along an isothermal decompression P–T path (Guillot et al., 2009). Slab breakoff is the most effective exhumation mechanism for HP–UHP metamorphic rocks (Von Blanckenburg and Davies, 1995; Duretz et al., 2012; Zheng, 2012). Furthermore, as the byproduct of exhumation of HP–UHP metamorphic rocks after slab breakoff, the Late Triassic magmatic activity was accompanied by the retrograde metamorphism of eclogite along the LSSZ in central Qiangtang (Fig. 12). This represents syn-exhumation magmatism (Davies and von Blanckenburg, 1995; Zheng et al., 2009) similar to that in other orogenic belts, such as the Alps (von Blanckenburg and Davies, 1995; Marchant and Stampfli, 1997), South Altyn Tagn (Yang et al., 2012), and the Qinling–Dabie orogenic belt (Sun et al., 2002). Following slab breakoff, asthenospheric mantle upwelling generates partial melting that produces basaltic magmas, and underplating of these magmas induces crustal melting (Yang et al., 2012), causing the post-peak metamorphism transition from basic to acidic magmatism in the Late Triassic (Fig. 12b). The Hf isotopic compositions of zircons indicate a change occurred in the Late

Triassic adakitic rocks, from an early depletion (>220 Ma) to a late enrichment (<220 Ma; Fig. 10), and the abundances of continental crustal components (e.g., Th) increased gradually over the same period (Fig. 14). These compositional changes may reflect the change from subducted slab melting to continental crust melting during rejuvenation (Zhai et al., 2013b; Zhang et al., 2014b). We suggest that the early adakitic rocks were derived from slab melting, when upwelling asthenospheric mantle heated the margin of the slab window following slab breakoff, and the late adakitic rocks were produced by partial melting of thickened lower crust, as a result of underplating by mantle-derived basaltic melts at the base of the crust. Thus, the magmatic and metamorphic records, and Hf isotopic compositions of zircons preserve a slab breakoff event during the collision between the southern and northern Qiangtang terranes. However, there remains some debate over the timing of slab breakoff (Zhai et al., 2011b; Zhang et al., 2011a; Hu et al., 2014). Numerical modeling shows that subducted slabs attain peak pressure conditions during slab breakoff (Duretz et al., 2012), which causes asthenospheric upwelling and induced exhumation of the HP–UHP metamorphic rocks and syn-exhumation magmatism (Davies and von Blanckenburg, 1995; Zheng et al., 2009). Given that slab breakoff is a gravity effect, it cannot instantaneously alter the horizontal direction of movement of the subducting slab. Therefore, it is likely that the residual southern Qiangtang lithosphere continued to be subducted beneath the northern Qiangtang lithosphere during the slab breakoff stage, and eclogite-facies metamorphism continued along the leading edge of the subducting slab. The last eclogite-facies metamorphic event, and the initial eruption of mantle-derived basaltic magma in central Qiangtang occurred during the Triassic at 236–230 Ma, and subsequent retrograde metamorphism and large-scale exhumation-related magmatism occurred after 230 Ma (Fig. 12). Therefore, we conclude that slab breakoff in central Qiangtang occurred at or before 236–230 Ma. 6.4.3 Try “An integrated geodynamic model” On the basis of our analysis of Triassic magmatism and metamorphism along the LSSZ in central Qiangtang, we suggest that the LSTO closed during the Triassic by

continent–continent collision between the southern and northern Qiangtang terranes, as illustrated in a three-stage geodynamic model (Fig. 15). Stage A (250–236 Ma): The subduction of LSTO lithosphere beneath the northern Qiangtang terrane may have begun as early as the Late Devonian (Wang et al., 2015), and arc-related magmatic activity continued until the Permian (Zhang et al., 2014a; Liu et al., 2015;). Ongoing subduction ultimately led to the closure of the LSTO in the Early Triassic (Yang et al., 2011; Wang et al., 2011b), causing the welding together of the northern and southern Qiangtang terranes by continent–continent collision. Coeval syn-collisional magmatism and deformation occurred along the collision zone (Fig. 15). This stage is represented by HP–UHP metamorphic rocks in the LSSZ. Peak eclogite-facies metamorphism, as recorded by zircon and garnet in the eclogite, occurred at 245–230 Ma (Fig. 12a), suggesting that the southern Qiangtang continental crust had subducted beneath the northern Qiangtang terrane during continental collision (Zhang et al., 2006; Zhang and Tang, 2009). Magmatism at the time of peak eclogite-facies metamorphism was weak (Fig. 12b), due to strong N–S extrusional stresses along the LSSZ, which accounts for the occurrence of Early–Middle Triassic gneissic granites in central Qiangtang (Wang et al., 2011; Hu et al., 2014). Stage B (236–230 Ma): The LSTO slab was destroyed as continued slab pull and mineral phase changes at depth produced excess negative buoyancy (Niu, 2014), eventually resulting in slab breakoff at 236–230 Ma (Fig. 15b). Contemporaneous magmatism is represented by mafic lavas in central Qiangtang (Wang et al., 2006; Zhu et al., 2006b; Ma et al., 2016), which were produced by partial melting induced by upwelling of hot asthenosphere in the slab window. This mantle-derived basaltic volcanism lasted for only a few millions of years, because the progressive convergence of the southern and northern Qiangtang terranes obstructed the upwelling channel, and favoured underplating at the base of thickened continental lithosphere (Fig. 15b). At the same time, adakitic melts derived from the partial melting of the subducted slab accumulated in the slab window. Stage C (230–200 Ma): Horizontal compressive stress between the northern and

southern Qiangtang terranes still played a major role in the early stages of Stage C; therefore, only minor exhumation of the HP–UHP metamorphic rocks took place (Fig. 12b), along with small-scale magmatic activity (Fig. 12b). With time, the vertical stress resulting from underplating by mantle-derived basaltic melts increased, and after 225 Ma this was the dominant tectonic process, causing exhumation of the HP–UHP metamorphic rocks and retrograde metamorphism from eclogite-facies to blueschist-facies

by

near-isothermal decompression

(Fig.

15c).

Large-scale

syn-exhumation magmatism took the form of ascending adakitic and mafic melts (Fu et al., 2010; Zhai et al., 2013b), which subsequently induced anatexis of the crust (Davies and von Blanckenburg, 1995; Atherton and Ghani, 2002), and generated the widespread 225–200 Ma acidic magmatism in central Qiangtang (Fig. 12b, Fig. 15c). The late Triassic adakitic SGH pluton was one of the products of this syn-exhumation magmatism.

7. Conclusions (1) U–Pb dating of zircons from the biotite adamellite, K-feldspar granite, and an MME yield ages of 207.8 ± 3.0 Ma, 211.6 ± 3.3 Ma, and 212.4 ± 3.1 Ma, respectively, indicating that the SGH pluton in central Qiangtang was emplaced in the Late Triassic. (2) The composition of the SGH pluton has an adakitic affinity, with high K2O, and low MgO and compatible trace element (e.g., Ni, Cr) concentrations, and negative zircon εHf(t) values, suggesting melting of thickened lower crust. (3) The Triassic magmatism and metamorphism along the LSSZ in central Qiangtang was related to the collision between the southern and northern Qiangtang terranes. Slab breakoff likely took place between 236 Ma and 230 Ma. (4) The Late Triassic adakitic rocks show evidence of a change in petrogenesis, from melting of the subducted slab, to melting of thickened continental crust, related to rejuvenation following slab breakoff, which resulted in the exhumation of HP–UHP metamorphic rocks and syn-exhumation magmatism.

Acknowledgments We are very grateful for constructive comments and suggestions by Editor-in Chief, Mei-Fu Zhou, the handling editor, Prof. Wyman, and two anonymous reviewers, which greatly improved the manuscript. We also thank Ning Chen and Guozheng Mao for assistance with field work, Zhaochu Hu for Hf isotopic analyses, and Xianglin Tu for LA–ICP–MS U–Pb dating. This research was supported by the Natural Science Foundation of China (41303028), the Science and Technology Foundation of Sichuan Province (2014JQ0025), the Major State Basic Research Program of the People’s Republic of China (2015CB452601), and the Program of the China Geological Survey (DD20160015 / 121201010000150014).

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Figure captions Fig. 1. (a) Tectonic framework of the Tibetan Plateau. JSSZ = Jinshajiang suture zone; LSSZ = Longmu Co–Shuanghu suture zone; BNSZ = Bangong Co–Nujiang suture zone; YZSZ = Yarlung Zangbo Suture Zone. (b) Longmu Co–Shuanghu suture zone, showing zircon U–Pb ages for Triassic magmatic rocks from this study. Fig. 2. (a) Simplified geological map of the study area in central Qiangtang (modified after L. Q. Wang et al., 2013), showing zircon U–Pb ages for Triassic magmatic and metamorphic rocks. Age data sources: (1) this study, (2) Chen et al. (2014), (3) Hu et al. (2014b), (4) Wang et al. (2007), (5) Zhai et al. (2013b), (6) Hu et al. (2010a), (7) Li et al. (2014), (8) Hu et al. (2010b), (9) Fu et al. (2008), (10) Li et al. (2007), (11) Zhai and Li (2007), (12) Zhai et al. (2011a), (14) Zhai et al. (2014a). (b) Geological map of the SGH area. Fig. 3. (a) Enclave hosted in biotite adamellite from the SGH pluton. (b) Photomicrograph of a typical enclave (cross-polarized light). Ap = Apatite; Bi = Biotite; Pl = Plagioclase. Fig. 4. Representative cathodoluminescence (CL) images of zircons used for

206

Pb/238U

dating of samples 11SGH-2, 11SGH-6, and 11SGH-9 from the SGH pluton. Solid circles indicate U–Pb analysis spots and dashed circles indicate Hf isotope analysis spots. Fig. 5. U–Pb concordia diagrams for zircons from samples 11SGH-2, 11SGH-6, and 11SGH-9 from the SGH pluton.

Fig. 6. (a) K2O versus SiO2 (Peccerillo and Taylor, 1976) and (b) A/NK versus A/CNK (Rickwood, 1989) diagrams for the Triassic granites of the study area. Data are from this study and Hu et al. (2010a, 2014a), Wang et al. (2011b), Tao et al. (2014), Zhang et al. (2014c), Chen et al., (2014), and Peng et al. (2015). Fig. 7. (a) Primitive-mantle-normalized spidergram, and (b) chondrite-normalized REE patterns for the Late Triassic adakitic rocks. Chondrite and primitive mantle values are from Sun and McDonough (1989); fields for adakitic rocks from Baohu and Juhuashan are from Zhai et al. (2013b). Fig. 8. Na2O/CaO versus Al2O3/CaO diagram (after Barth et al., 2000) for the MMEs and host biotite adamellite from the SGH pluton. Fig. 9. Sr/Y versus Y diagram (after Defant and Drummond, 1990) for samples from the SGH pluton. Data sources for the Baohu and Juhuashan adakitic rocks are as in Fig. 7. Fig. 10. Hf isotopic compositions of Late Triassic adakitic rocks from central Qiangtang. Data for Baohu adakitic rocks are from Zhai et al. (2013b). DM= depleted mantle and CHUR= chondritic uniform reservoir Fig. 11. (a) MgO versus SiO2 (after Karsli et al., 2010), (b) Ni versus SiO2, and (c) Cr versus SiO2 (after Huang et al., 2009) diagrams for Late Triassic adakitic rocks from central Qiangtang. Data sources for adakitic rocks are as in Fig. 7. Fig. 12. (a) Histograms of Triassic metamorphic ages in central Qiangtang. Data sources: Lu–Hf ages of garnet, Pullen et al. (2008); U–Pb ages of zircon, Zhai et al. (2011); 40

Ar/39Ar ages of glaucophane, Li et al. (1997) and Zhai et al. (2009); phengite

abundances, Kapp et al. (2003), Li et al. (2006, 2008b), Zhai et al. (2009, 2011a), and Liang et al. (2012); amphibole abundances, Kapp et al. (2003) and Pullen et al. (2011);

muscovite and biotite abundances, Kapp et al. (2000, 2003), Dong et al. (2009), Zhang et al. (2010), and Pullen et al. (2011); actinolite abundances, Wang et al. (2006) and Li et al. (2007b). (b) Histograms of Triassic igneous ages. Data sources: Kapp et al. (2000); Zhu et al. (2006); Li et al. (2007b); Wang et al. (2007, 2008, 2010); Zhai and Li (2007); Fu et al. (2008, 2010); Hu et al. (2010a, 2010b, 2014a, 2014b); Wang et al. (2011); Yang et al. (2011); Zhang et al. (2011); Liu et al. (2013); Zhai et al. (2013b); Chen et al. (2014); Li et al. (2014); Tao et al. (2014); Zhang et al. (2014); Peng et al. (2015); Ma et al. (2016). Fig. 13. Rb versus Y + Nb discrimination diagram (after Förster, 1997) for Triassic granites from central Qiangtang. Additional data are from Hu et al. (2010a, 2014a), Wang et al. (2011b), Tao et al. (2014), Zhang et al. (2014c), Chen et al. (2014), and Peng et al. (2015). Fig. 14. Th versus SiO2 diagram for Late Triassic adakitic rocks from central Qiangtang. The arrow shows the increase of Th contents the Late Triassic adakitic rocks after 220 Ma, which indicates the change from subducted slab melting to continental crust melting during rejuvenation. Data sources for adakitic rocks (at 219 Ma and 223 Ma) are as in Fig. 7. Fig. 15. Schematic illustration of continent–continent collision between the southern and northern Qiangtang terranes. (a) Northward subduction of the southern Qiangtang terrane beneath the northern Qiangtang terrane led to ultra-high pressure (UHP) metamorphism and syn-collisional magmatism. (b) Breakoff of the Longmu Co–Shuanghu Tethys Ocean slab resulted in eruption of OIB -type mafic lavas (OIB = ocean island basalt). (c) Exhumation of the UHP metamorphic rocks and syn-exhumation magmatism.

Table captions Table 1 Major (wt.%) and trace element (ppm) compositions of samples from the SGH pluton in central Qiangtang. Table 2 LA–ICP–MS U–Pb data of zircons from samples from the SGH pluton in central Qiangtang. Table 3 Lu–Hf isotopic data of samples from the SGH pluton in central Qiangtang.

Table 1 Major (wt.%) and trace element (ppm) compositions of samples from the SGH pluton in central Qiangtang. Sample

11SGH-1 11SGH-2

Rock type SiO2

11SGH-3

11SGH-4 11SGH-5 11SGH-6 11SGH-7 11SGH-8 11SGH-10

Biotite damellite

diorite

71.84 70.4 adamellite 14.93 15.64 adamellite 1.98 2.85

71.11

70.12

68.58

15.5

15.71

16.82

2.08

2.41

2.64

K2O

2.64 adamellite 2.62 0.58 0.8 adamellite 3.25 3.44

Na2O

3.17

TiO2

11SGH-11

BHVO-2 AGV-1

GSR-1

K-feldspar granite

60.96

61.26

75.26

73.86

72.82

18.18

17.39

13.71

14.58

13.78

2.56

66.95 diorite 16.16 diorite 5.23

6.97

6.74

1.38

1.97

1.95

2.64

2.71

3.39

3.4

3.65

1.86

0.36

1.46

0.62

0.69

0.7

1.59

2.3

2.47

0.32

0.53

0.52

3.5

3.44

3.96

1.93

2.54

2.48

3.77

4.39

4.85

3.26

3.22

3.32

3.48

3.43

3.74

3.31

2.75

2.74

2.94

0.19

0.26

0.2

0.22

0.27

0.51

0.71

0.7

0.12

0.23

0.29

P2O5

0.04

0.05

0.04

0.04

0.05

0.08

0.13

0.12

0.02

0.03

0.09

MnO

0.43

0.54

0.6

0.55

0.69

0.62

0.96

1

0.67

1.17

0.05

LOI

0.43

0.54

0.6

0.55

0.69

0.62

0.96

1

0.67

1.17

Mg#

40.81

39.78

41.23

40.26

39.16

41.71

43.71

46.31

35.31

38.77

A/CNK

1.10

1.13

1.12

1.12

1.13

1.16

1.20

1.18

1.14

1.47

Sc

4.36

5.57

4.55

5.01

5.03

9.21

12.95

14.35

3.12

5.31

Ti

1043

1416

1101

1198

1487

2811

3987

3949

641

1249

V

24.24

28.64

21.25

26.72

25.29

71.17

95.68

118.90

12.48

Cr

40.68

39.02

25.38

39.52

27.74

45.32

40.83

36.70

22.78

Co

2.73

3.74

2.82

3.03

3.14

6.59

10.33

11.34

Ni

11.49

11.90

9.24

10.54

8.87

11.45

14.24

Cu

3.81

5.32

3.86

4.68

4.34

6.49

Zn

25.92

31.87

28.75

28.85

38.66

Ga

14.65

16.68

15.79

16.24

Rb

121.90

146.90

141.20

Sr

370.60

347.60

Y

8.77

Zr

Al2O3 Fe2O3T CaO MgO

32.1

12.3

6.1

26.60

318

118

23.2

31.93

290

10.7

4.68

1.36

1.33

44.8

15.6

3.21

12.76

8.53

9.66

119

16.2

2.07

11.41

8.75

4.47

6.28

135

56.2

3.41

75.29

145.60

97.57

17.47

25.73

108

85.9

27.8

18.28

21.07

24.99

20.74

14.16

17.08

21.3

20.5

19.4

136.60

165.50

191.70

280.00

229.30

132.00

219.50

10

69.2

464

382.20

378.70

479.30

401.50

359.40

332.00

392.50

269.30

398

659

102

10.76

11.38

11.56

11.87

9.39

11.86

7.59

7.37

9.97

27.6

19.2

62.3

87.83

106.70

107.90

104.50

130.40

111.80

117.50

104.00

100.70

116.90

177

227

169

Nb

6.12

8.83

7.11

7.63

9.36

16.04

21.95

14.07

4.63

8.34

19.7

15.5

39.8

Cs

4.26

4.39

6.39

4.00

6.45

8.91

15.35

8.40

3.74

6.84

0.11

1.18

38.6

Ba

598.10

584.50

693.50

651.30

918.10

255.40

277.60

294.60

873.40

778.40

137

1236

343

La

10.31

17.16

16.28

15.91

23.69

24.37

19.11

16.82

19.84

19.12

15.9

38.7

54.5

Ce

19.38

33.49

31.24

29.07

43.62

45.61

36.78

32.50

37.32

37.43

38.5

67.8

107

Pr

2.21

3.78

3.51

3.40

5.03

5.22

4.28

3.77

4.33

4.13

5.52

7.76

12.9

Nd

8.00

13.43

12.63

12.32

17.73

18.58

15.52

13.42

15.30

14.38

25.7

33.3

46.9

Sm

1.51

2.39

2.34

2.29

2.95

3.22

2.99

2.44

2.56

2.55

6.13

5.76

9.74

Eu

0.89

0.91

0.91

0.90

1.05

0.91

0.82

0.82

0.87

0.71

2.03

1.68

0.82

Gd

1.41

2.02

2.04

1.96

2.51

2.50

2.54

2.01

2.10

2.21

6.11

4.99

9.31

Tb

0.22

0.33

0.31

0.34

0.37

0.35

0.39

0.28

0.28

0.32

0.98

0.69

1.63

Dy

1.32

1.78

1.85

1.85

1.98

1.75

2.04

1.36

1.39

1.63

5.22

3.6

10.1

Ho

0.30

0.39

0.38

0.40

0.39

0.34

0.40

0.27

0.25

0.36

1

0.68

2.1

Er

0.88

1.07

1.22

1.15

1.16

0.93

1.17

0.77

0.75

0.99

2.5

1.78

6.3

Tm

0.15

0.17

0.19

0.18

0.19

0.13

0.18

0.12

0.12

0.15

0.34

0.26

1.07

Yb

0.91

1.21

1.31

1.25

1.23

0.95

1.23

0.81

0.67

0.91

1.99

1.68

7.36

Lu

0.15

0.17

0.19

0.19

0.18

0.14

0.18

0.13

0.12

0.14

0.29

0.27

1.13

Hf

2.68

3.01

3.13

3.17

3.77

3.42

4.01

3.21

2.99

3.27

4.29

5.16

6.39

Ta

0.92

1.36

0.97

1.09

0.98

1.30

2.11

1.13

0.54

0.98

1.23

0.89

7.16

Pb

28.45

29.41

32.58

30.18

30.90

16.39

36.19

17.11

27.23

14.63

2.25

33.1

31.9

Th

7.93

12.48

10.59

11.68

11.14

9.39

9.40

9.45

11.72

13.48

1.18

6.38

54.2

U

1.56

1.90

2.10

1.74

2.02

2.53

3.00

1.96

1.09

1.36

0.4

1.88

18.8

P

100

157

87

170

162

122

109

65

157

231

Rb/Ba

0.20

0.25

0.20

0.21

0.18

0.75

1.01

0.78

0.15

0.28

Rb/Sr

0.33

0.42

0.37

0.36

0.35

0.48

0.78

0.69

0.34

0.82

Sr/Y

42.28

32.30

33.59

32.76

40.38

42.76

30.30

43.76

53.25

27.00

Eu/Eu*

1.83

1.23

1.24

1.26

1.15

0.94

0.89

1.10

1.11

0.89

A/CNK = mol Al2O3T/(CaO + K2O + Na2O); Mg# = 100 × mol MgO/(MgO + FeOT); FeOT = 0.8998 × Fe2O3T.

Table 2 LA–ICP–MS U–Pb data of zircons from samples from the SGH pluton in central Qiangtang. Spot

Element content (ppm)

Isotope ratio

Th/U 207

Pb/206Pb

(1σ)

207

Pb/235U

(1σ)

Age (Ma) 206

Pb/238U

(1σ)

207

Pb/235U (1σ)

206

Pb/238U

(1σ)

Pb

Th

U

1

80.95

203

2305

0.09

0.0480

0.0021

0.2228

0.0104

0.0330

0.0006

204

9

209

2

105.69

241

3021

0.08

0.0509

0.0021

0.2295

0.0096

0.0320

0.0004

210

8

203

3

3

61.86

144

598

0.24

0.0649

0.0027

0.8752

0.0415

0.0957

0.0027

638

22

589

16

4

148.9

343

2574

0.13

0.1593

0.0075

0.8439

0.0442

0.0373

0.0007

621

24

236

4

5

110.2

327

2369

0.14

0.1065

0.0049

0.5439

0.0262

0.0358

0.0005

441

17

227

3

6

105.6

546

2796

0.20

0.0644

0.0034

0.2932

0.0156

0.0327

0.0009

261

12

207

5

7

69.44

121

2032

0.06

0.0490

0.0025

0.2247

0.0116

0.0327

0.0007

206

10

207

4

8

109.6

1430

3043

0.47

0.0734

0.0033

0.3065

0.0141

0.0295

0.0007

271

11

188

4

9

116.6

1023

3059

0.33

0.0630

0.0027

0.2952

0.0152

0.0327

0.0009

263

12

208

6

10

261

2295

3126

0.73

0.2734

0.0103

1.5871

0.0670

0.0409

0.0010

965

26

258

6

11

180

10570

3711

2.85

0.1781

0.0075

0.6633

0.0322

0.0261

0.0007

517

20

166

4

12

125.3

1728

3063

0.56

0.0771

0.0041

0.3408

0.0194

0.0315

0.0008

298

15

200

5

13

75.74

123

2206

0.06

0.0423

0.0019

0.1954

0.0089

0.0327

0.0007

181

8

208

4

14

128.2

3000

3028

0.99

0.1204

0.0052

0.4970

0.0222

0.0295

0.0007

410

15

187

4

15

76.0

719

1585

0.45

0.1077

0.0075

0.5897

0.0479

0.0364

0.0009

471

31

231

6

16

362

5428

2395

2.27

0.4372

0.0154

3.3078

0.1389

0.0534

0.0014

1483

33

335

8

17

128.8

928

3168

0.29

0.0752

0.0037

0.3386

0.0176

0.0320

0.0008

296

13

203

5

18

233

1220

2742

0.44

0.2516

0.0106

1.5829

0.0696

0.0448

0.0011

964

27

283

7

19

172.0

1002

2422

0.41

0.1476

0.0077

0.9436

0.0491

0.0468

0.0015

675

26

295

9

20

91.4

592

2310

0.26

0.0607

0.0028

0.2874

0.0130

0.0337

0.0008

257

10

213

5

21

317

2773

3361

0.82

0.2582

0.0123

1.5877

0.0854

0.0422

0.0011

965

34

266

7

22

128.5

2683

3151

0.85

0.0753

0.0037

0.3406

0.0150

0.0318

0.0008

298

11

201

5

23

82.48

169

2241

0.08

0.0442

0.0024

0.2132

0.0096

0.0337

0.0007

196

8

214

4

24

302

2527

3125

0.81

0.2678

0.0158

1.6905

0.0831

0.0432

0.0010

1005

31

273

6

1

51.78

228

1368

0.17

0.0447

0.0025

0.2177

0.0124

0.0345

0.0006

200

10

218

4

2

51.81

194

1317

0.15

0.0560

0.0032

0.2700

0.0156

0.0347

0.0007

243

12

220

4

3

49.88

281

1361

0.21

0.0457

0.0020

0.2130

0.0100

0.0331

0.0005

196

8

210

3

4

57.7

502

1473

0.34

0.0466

0.0022

0.2213

0.0108

0.0340

0.0005

203

9

216

3

5

75.4

444

1735

0.26

0.0908

0.0049

0.4363

0.0247

0.0340

0.0007

368

17

215

4

6

89.8

643

2405

0.27

0.0931

0.0042

0.3637

0.0168

0.0281

0.0006

315

12

179

4

7

17.82

265

440

0.60

0.0480

0.0039

0.2223

0.0199

0.0335

0.0007

204

17

212

5

8

29.65

115

844

0.14

0.0492

0.0023

0.2187

0.0104

0.0321

0.0006

201

9

204

3

9

43.31

220

1161

0.19

0.0472

0.0022

0.2221

0.0110

0.0340

0.0007

204

9

215

4

10

50.20

238

1338

0.18

0.0494

0.0025

0.2332

0.0147

0.0335

0.0011

213

12

213

7

11

27.49

135

686

0.20

0.0776

0.0052

0.3736

0.0274

0.0338

0.0006

322

20

215

4

12

70.5

373

1735

0.21

0.0616

0.0027

0.2950

0.0136

0.0344

0.0007

263

11

218

4

13

39.62

213

1027

0.21

0.0431

0.0020

0.2095

0.0103

0.0349

0.0006

193

9

221

4

14

46.17

287

1243

0.23

0.0451

0.0019

0.2073

0.0090

0.0328

0.0005

191

8

208

3

15

50.93

273

1294

0.21

0.0530

0.0024

0.2549

0.0117

0.0344

0.0005

231

9

218

3

16

28.66

133

804

0.16

0.0471

0.0026

0.2111

0.0117

0.0322

0.0006

194

10

204

4

17

73.0

460

1878

0.24

0.0669

0.0033

0.2979

0.0149

0.0319

0.0006

265

12

202

4

18

38.94

216

1037

0.21

0.0473

0.0026

0.2198

0.0123

0.0334

0.0006

202

10

212

4

19

122.7

429

2656

0.16

0.1563

0.0069

0.6607

0.0292

0.0302

0.0007

515

18

192

4

20

41.95

302

1138

0.27

0.0542

0.0027

0.2425

0.0114

0.0323

0.0005

220

9

205

3

21

52.4

374

1296

0.29

0.0700

0.0041

0.3330

0.0235

0.0332

0.0007

292

18

210

4

22

47.33

246

1199

0.21

0.0619

0.0033

0.2943

0.0146

0.0342

0.0006

262

11

217

4

23

63.5

242

1519

0.16

0.0904

0.0039

0.4188

0.0171

0.0331

0.0005

355

12

210

3

24

57.96

347

1543

0.22

0.0538

0.0022

0.2544

0.0107

0.0336

0.0006

230

9

213

4

1

155.1

1019

2285

0.45

0.2144

0.0101

1.1772

0.0569

0.0398

0.0007

790

27

252

5

2

68.13

331

1862

0.18

0.0473

0.0022

0.2186

0.0103

0.0334

0.0006

201

9

212

4

3

34.46

404

944

0.43

0.0471

0.0025

0.2070

0.0109

0.0316

0.0006

191

9

201

4

4

53.66

410

1441

0.28

0.0425

0.0020

0.1993

0.0093

0.0334

0.0006

185

8

212

4

5

98.9

2151

1491

1.44

0.1668

0.0079

0.9095

0.0438

0.0385

0.0007

657

23

244

4

11SGH-2 4

11SGH-6

11SGH-9

6

87.5

1851

2188

0.85

0.0728

0.0038

0.3262

0.0163

0.0317

0.0006

287

12

201

4

7

159.7

332

874

0.38

0.0598

0.0029

1.3305

0.0623

0.1560

0.0029

859

27

935

16

8

56.9

420

1463

0.29

0.0619

0.0037

0.2998

0.0202

0.0333

0.0007

266

16

211

4

9

20.50

102

575

0.18

0.0443

0.0028

0.1979

0.0117

0.0325

0.0007

183

10

206

4

10

41.76

211

1141

0.18

0.0447

0.0023

0.2157

0.0120

0.0341

0.0006

198

10

216

4

11

46.89

363

1201

0.30

0.0658

0.0032

0.3008

0.0139

0.0330

0.0006

267

11

210

4

12

53.9

599

1347

0.44

0.0662

0.0046

0.3210

0.0274

0.0338

0.0009

283

21

214

5

13

25.59

527

617

0.85

0.0460

0.0030

0.2002

0.0122

0.0320

0.0006

185

10

203

4

14

23.87

98.5

688

0.14

0.0485

0.0025

0.2168

0.0112

0.0322

0.0005

199

9

204

3

15

35.0

542

872

0.62

0.0458

0.0026

0.2089

0.0119

0.0329

0.0006

193

10

209

4

16

37.53

130

1022

0.13

0.0448

0.0022

0.2099

0.0110

0.0338

0.0007

193

9

214

4

17

70.59

380

1878

0.20

0.0483

0.0021

0.2306

0.0102

0.0345

0.0007

211

8

219

4

18

83.8

527

1666

0.32

0.1180

0.0058

0.5980

0.0320

0.0364

0.0007

476

20

230

5

19

86.38

202

932

0.22

0.0526

0.0022

0.6242

0.0284

0.0851

0.0018

492

18

527

11

20

26.5

801

550

1.46

0.0458

0.0027

0.2082

0.0128

0.0327

0.0007

192

11

208

4

21

44.28

230

1111

0.21

0.0472

0.0023

0.2366

0.0118

0.0359

0.0007

216

10

227

4

22

56.48

298

1460

0.20

0.0449

0.0020

0.2231

0.0106

0.0356

0.0008

204

9

226

5

23

30.97

180

836

0.22

0.0479

0.0031

0.2345

0.0162

0.0347

0.0008

214

13

220

5

24

93.8

304

635

0.48

0.0582

0.0028

1.0448

0.0527

0.1270

0.0027

726

26

771

15

Table 3 Lu–Hf isotopic data of samples from the SGH pluton in central Qiangtang. Spot

Age (Ma)

176

Hf/177Hf

1σ

176

Lu/177Hf

1σ

176

Yb/177Hf

1σ

εHf(0) 1σ εHf(t) 1σ TDM

TCDM

fLu/Hf

11SGH-2: 207.8 ± 3.0 Ma, εHf(t) = −7.0 to −2.6 (11 analyses) 1

208

0.282497

0.000020

0.001944

0.000016

0.060593

0.000486

-9.7

2

208

0.282450

0.000017

0.001396

0.000013

0.039214

0.000469

-11.4 0.8 -7.0 0.8 1146 1503 -0.96

0.9 -5.4 0.9 1096 1415 -0.94

6

208

0.282576

0.000020

0.001818

0.000057

0.055146

0.001921

-6.9

7

208

0.282482

0.000016

0.001159

0.000020

0.031930

0.000733

-10.2 0.8 -5.8 0.8 1093 1437 -0.97

9

208

0.282538

0.000020

0.002186

0.000042

0.067583

0.001491

-8.3

0.9 -4.0 0.9 1043 1336 -0.93

12

208

0.282574

0.000025

0.004323

0.000064

0.150363

0.002427

-7.0

1.0 -3.0 1.0 1052 1281 -0.87

13

208

0.282517

0.000021

0.002084

0.000115

0.062219

0.003598

-9.0

0.9 -4.7 0.9 1070 1375 -0.94

17

208

0.282163

0.000031

0.002053

0.000021

0.059781

0.000636

-21.5 1.2 -17.3 1.2 1579 2066 -0.94

20

208

0.282530

0.000024

0.002118

0.000021

0.068110

0.000816

-8.5

1.0 -4.3 1.0 1052 1350 -0.94

22

208

0.282509

0.000017

0.001819

0.000008

0.060714

0.000279

-9.3

0.8 -5.0 0.8 1075 1391 -0.95

23

208

0.282489

0.000021

0.001959

0.000130

0.059727

0.004316

-10.0 0.9 -5.7 0.9 1106 1429 -0.94

0.9 -2.6 0.9 978

1258 -0.95

11SGH-6: 212.4 ± 3.1 Ma, εHf(t) = −3.8 to −0.6 (12 analyses) 3

212

0.282537

0.000017

0.001049

0.000009

0.027716

0.000338

-8.3

0.8 -3.8 0.8 1013 1328 -0.97

4

212

0.282584

0.000021

0.001256

0.000029

0.036256

0.000822

-6.7

0.9 -2.2 0.9 952

7

212

0.282561

0.000020

0.001974

0.000005

0.053105

0.000520

-7.5

0.9 -3.1 0.9 1004 1288 -0.94

8

212

0.282585

0.000014

0.000555

0.000007

0.015172

0.000228

-6.6

0.7 -2.1 0.7 934

1231 -0.98

9

212

0.282630

0.000019

0.001413

0.000007

0.041742

0.000237

-5.0

0.8 -0.6 0.9 891

1149 -0.96

10

212

0.281822

0.000018

0.001100

0.000011

0.033124

0.000304

-33.6 0.8 -29.1 0.8 2012 2717 -0.97

12

212

0.282620

0.000019

0.001614

0.000032

0.049573

0.001277

-5.4

0.8 -0.9 0.9 909

13

212

0.282536

0.000016

0.000765

0.000011

0.021428

0.000296

-8.4

0.8 -3.8 0.8 1007 1328 -0.98

14

212

0.282595

0.000020

0.001421

0.000031

0.040709

0.001045

-6.3

0.9 -1.8 0.9 941

1217 -0.96

15

212

0.282585

0.000017

0.001767

0.000040

0.051524

0.000908

-6.6

0.8 -2.2 0.8 964

1239 -0.95

18

212

0.282576

0.000018

0.001597

0.000027

0.045285

0.000800

-6.9

0.8 -2.5 0.8 972

1255 -0.95

24

212

0.282594

0.000019

0.001659

0.000081

0.049877

0.002155

-6.3

0.8 -1.9 0.9 948

1220 -0.95

1237 -0.96

1169 -0.95

11SGH-9: 211.5 ± 4.5 Ma, εHf(t) = −5.7 to −0.9 (12 analyses) 2

211

0.282616

0.000017

0.001403

0.000015

0.040600

0.000659

-5.5

0.8 -1.1 0.8 909

1175 -0.96

3

211

0.282592

0.000024

0.003112

0.000101

0.087105

0.002586

-6.4

1.0 -2.2 1.0 989

1236 -0.91

4

211

0.282582

0.000024

0.001268

0.000007

0.036942

0.000225

-6.7

1.0 -2.2 1.0 954

1241 -0.96

6

211

0.282604

0.000025

0.001977

0.000015

0.064081

0.000574

-5.9

1.0 -1.6 1.0 941

1203 -0.94

8

211

0.282559

0.000019

0.001575

0.000047

0.048155

0.001455

-7.5

0.8 -3.1 0.9 995

1288 -0.95

9

211

0.282616

0.000020

0.001085

0.000051

0.030584

0.001414

-5.5

0.9 -1.0 0.9 903

1174 -0.97

10

211

0.282579

0.000017

0.000996

0.000027

0.029726

0.000956

-6.8

0.8 -2.3 0.8 952

1244 -0.97

11

211

0.282585

0.000017

0.001207

0.000030

0.035513

0.000835

-6.6

0.8 -2.1 0.8 949

1234 -0.96

12

211

0.282623

0.000019

0.001502

0.000017

0.044538

0.000557

-5.3

0.8 -0.9 0.8 903

1163 -0.95

15

211

0.282484

0.000017

0.000998

0.000023

0.029595

0.000821

-10.2 0.8 -5.7 0.8 1086 1431 -0.97

16

211

0.282621

0.000019

0.001211

0.000037

0.034932

0.000946

-5.4

0.9 -0.9 0.9 899

1165 -0.96

20

211

0.282592

0.000020

0.001668

0.000070

0.045445

0.002388

-6.4

0.9 -2.0 0.9 952

1226 -0.95

Spot numbers are the same as in Table 2. εHf(t) = 10,000 × {[(176Hf/177Hf)S − (176Lu/177Hf)S × (eλt − 1)]/[(176Hf/177Hf)CHUR,0 − (176Lu/177Hf)CHUR × (eλt − 1)] − 1}. TDM = 1/λ × ln{1 + [(176Hf/177Hf)S − (176Hf/177Hf)DM]/[(176Lu/177Hf)S − (176Lu/177Hf)DM]}. TCDM = TDM − (TDM − t) × [(fcc − fs)/(fcc − fDM)]. fLu/Hf = (176Lu/177Hf)s/(176Lu/177Hf)CHUR − 1. (176Hf/177Hf)CHUR,0 = 0.282772 and (176Lu/177Hf)CHUR = 0.0332 (Blichert-Toft and Albarède, 1997); (176Hf/177Hf)DM = 0.28325 and (176Lu/177Hf)DM = 0.0384 (Griffin et al., 2000); (176Lu/177Hf)mean crust = 0.015 (Griffin et al., 2002); λ = 1.867 × 10−11 a−1 (Soderlund et al., 2004); fcc = [(176Lu/177Hf)mean crust/(176Lu/177Hf)CHUR] − 1; fs = fLu/Hf; fDM = [(176Lu/177Hf)DM/(176Lu/177Hf)CHUR] – 1.

Graphical abstract

Highlights 1. The Sangehu pluton was emplaced in the Late Triassic, and the enclaves in the biotite adamellite of the Sangehu pluton were produced by the magma mixing. 2. The Sangehu pluton has the composition of adakitic affinity, and was derived from melting of thickened lower crust. 3. The relationship between magmatism and metamorphism in the Triassic in central Qiangtang. 4. The Late Triassic magmatic rocks show characteristics of syn-exhumation magmatism following slab breakoff. 5. Three-stage geodynamic model of collision of the southern and northern Qiangtang terranes.