Geochronology, petrogenesis, and tectonic implications

Tectonophysics 666 (2016) 90–102

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Middle Triassic volcanic rocks in the Northern Qiangtang (Central Tibet): Geochronology, petrogenesis, and tectonic implications Sheng-Sheng Chen a,b,⁎, Ren-Deng Shi a,c, Guo-Ding Yi a,b, Hai-Bo Zou d a

Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China Chinese Academy of Sciences Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China d Department of Geosciences, Auburn University, Auburn, AL 36849, USA b c

a r t i c l e

i n f o

Article history: Received 16 April 2015 Received in revised form 31 August 2015 Accepted 25 October 2015 Available online 10 November 2015 Keywords: Magmatism Triassic Crust growth Qiangtang Tibetan Plateau

a b s t r a c t Although voluminous magmatism occurred during the Triassic in the Qiangtang terrane, the petrogenesis of the volcanic rocks and the associated tectonic scenarios remain mysterious. This study focuses on new identified primitive volcanic rocks from the Yanshiping area, Northern Qiangtang subterrane. The whole-rock majortrace elemental, Sr–Nd isotopic data, and zircon U–Pb age of volcanic rocks are reported in this paper in order to understand their petrogenesis and tectonic setting. The studied volcanic rocks can be grouped into two types, i.e., Nb-enriched basalts and basaltic andesites (NEBs) in the low sequence and arc basalts in the upper sequence. Zircon U–Pb dating using LA–ICP-MS techniques yields the concordant age with a weighted mean 206 Pb/238U age of ca. 242–241 Ma for the NEBs and ca. 240 Ma for the arc basalts. The distinct geochemical and isotopic characteristics of whole-rock and varying Th–U–REE components of zircon grains from the volcanic rocks suggest that the NEBs were derived from partial melting of relatively enriched mantle wedge that have been metasomatised by slab-related melts and that the arc basalts originated from partial melting of mixing of mantle wedge and depleted asthenospheric mantle in response to the slab breakoff. Our new geochemical and geochronological results, in combination with regional studies, imply that the Middle Triassic magmatism was generated in the tectonic setting of northward subduction of the Paleo-Tethys or Bangong-Nujiang Ocean. Subduction-related processes that include melting of the slab and the slab breakoff during the Permian-Middle Triassic made an important contribution to crustal growth in the Qiangtang terrane, whereas vertical crustal growth associated with collision-related setting during the Late Triassic is nonsignificant. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The Qiangtang terrane is a large part of the Tibetan Plateau and bounded to the north by Jinsha suture zone (JSSZ) and to the south by the Bangong-Nujiang suture zone (BNSZ) (Fig. 1a). It's generally characterized by the presence of the N500-km-long and up to 100-km-wide E.G. Longmu Tso-Shuanghu suture zone (LSSZ) in the central Qiangtang terrane and extensive Triassic magmatic rocks in the Qiangtang terrane. This configuration implies that the Triassic is a critical period in the tectonic evolution of the Qiangtang terrane. Therefore, many studies have been carried out on the correlation between the magmatism and metamorphism in order to decipher the tectonic evolution of the Triassic Qiangtang terrane (Fu et al., 2010; P. Hu et al., 2014; P.Y. Hu et al., 2014; Kapp et al., 2000, 2003; Peng et al., 2014; Pullen et al.,

⁎ Corresponding author at: Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China. Tel.: +86 10 84097062; fax: +86 10 84097079. E-mail address: [email protected] (S.-S. Chen).

http://dx.doi.org/10.1016/j.tecto.2015.10.017 0040-1951/© 2015 Elsevier B.V. All rights reserved.

2008; Wang et al., 2008; Yang et al., 2011; Yin and Harrison, 2000; Zhai et al., 2011a,b, 2013a,b; Zhang et al., 2006a,b; Zhao et al., 2014). A general consensus exists that the Qiangtang terrane can be divided into two parts, i.e., the Northern and Southern Qiangtang subterranes as a consequence of the presence of Carboniferous ophiolites and Triassic high-pressure Qiangtang metamorphic belt (QMB) in the central Qiangtang terrane (Li et al., 2006; Zhai et al., 2011a,b). The recent researches on the QMB have proposed some competing tectonic models. For example, the QMB is considered to be composed of Songpan-Ganzi flysch which was deposited in the Paleo-Tethys, and tectonically eroded fragments of the Qiangtang continental margin that were underthrust southward beneath the Qiangtang terrane along the JSSZ during the Triassic (Kapp et al., 2000, 2003; Pullen et al., 2008). Alternatively, the QMB can mark a Triassic in site suture zone that separates the Southern Qiangtang subterrane from the Northern Qiangtang subterrane (e.g., P. Hu et al., 2014; P.Y. Hu et al., 2014; Li and Zheng, 1993; Peng et al., 2014; Yang et al., 2011; Zhai et al., 2011a,b, 2013a,b; Zhang et al., 2006a,b; Zhao et al., 2014). Recently, Zeng et al. (2015) propose that Late Triassic initial subduction of the Bangong-Nujiang Ocean beneath the Qiangtang terrane can explain the exhumation of Triassic

S.-S. Chen et al. / Tectonophysics 666 (2016) 90–102

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Fig. 1. Schematic map showing (a) tectonic outline of the Tibetan Plateau and Mesozoic magmatic rocks in the Qiangtang terrane and (b) the location of Triassic volcanic rocks in the Yanshiping area. JSSZ = Jinsha Suture Zone; LSSZ = Longmu Tso-Shuanghu Suture Zone; BNSZ = Bangong-Nujiang Suture Zone; SNMZ = Shiquan River-Nam Tso Mélange Zone; LMF = Luobadui-Milashan Fault; IYZSZ = Indus-Yarlung Zangbo Suture Zone.

Qiangtang metamorphic event. If this is the case, the interpretation means that the formation of Triassic metamorphism in the central Qiangtang terrane was associated with the northward subduction of the Bangong-Nujiang Ocean. In addition, understanding the origin of the Triassic volcanic rocks is also a key to constrain the geodynamic evolution of the Triassic Qiangtang terrane. Nonetheless, the tectonic setting of the Triassic tectono-magmatism in the Qiangtang terrane is still a matter of debate (Fu et al., 2010; Li et al., 2015; Peng et al., 2014; Wang et al., 2008; Yang et al., 2011; Zhai et al., 2011a,b, 2013a; Zhao et al., 2014). In general, the Triassic volcanic rocks in the Qiangtang terrane have been interpreted as arc magmatism in relation to subduction or collision processes (P. Hu et al., 2014; P.Y. Hu et al., 2014; Li et al., 2015; Peng et al., 2014; Wang et al., 2008; Yang et al., 2011; Zhai et al., 2013a,b; Zhao et al., 2014), as the product of a continental rift setting (Fu et al., 2010), or as the result of back-arc basin extension (Zeng et al., 2015). Although there have been lots of Paleozoic ophiolites along the QMB within the Qiangtang area (Fig. 1a; Wu et al., 2013; P.Y. Hu et al., 2014; Zhai et al., 2013c), this paper focuses mainly on a study of the Triassic volcanic rocks along the north of the Amdo-Basu area in the Northern

Qiangtang subterrane. We firstly report here a suite of Triassic primitive volcanic rocks that include niobium-enriched basalts and basaltic andesites (NEBs) and arc basalt in the Yanshiping area, Northern Qiangtang subterrane (Fig. 1a). In this paper, the results of whole-rock major-trace elemental, Sr–Nd isotopic compositions, and in situ U–Pb age and rare earth elements analyses for zircon grains from the volcanic rocks are presented. In combination with regional studies, these new data can offer constraints on primitive magma genesis and a new interpretation to account for tectonic implications of the Triassic volcanic rocks. If the petrogenesis of Yanshiping Triassic NEBs are associated with the adakite metasomatic process (Sajona et al., 1993, 1996), partial melting of subducted oceanic crust is an important additional continental crustal growth mechanism for the Qiangtang terrane during the Triassic. 2. Geological setting It is generally argued that the Tibetan Plateau, from north to south, consists of four main continental terranes: the Songpan-Ganze, the Qiangtang, the Lhasa, and the Himalaya. These terranes are separated

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by the Jinsha suture zone (JSSZ), the Bangong-Nujiang suture zone (BNSZ), and the Indus-Yarlung Zangbo suture zone (IYZSZ), respectively (Fig. 1a; Yin and Harrison, 2000). The Qiangtang terrane is a major W–E trending tectonic belts in the Tibetan Plateau. The LSLSZ was first proposed by Li (1987), and various studies have showed that the LSLSZ crosses the middle Tibetan Plateau eastwards for 2000 km from Longmuco through Shanghu, Baqing, and eastern Tibet, and it separates the Southern Qiangtang terrane from the Northern Qiangtang terrane (Li, 1987; Li et al., 2008; Wu et al., 2013; Fan et al., 2014a). The LSSZ is composed dominantly of a tectonic complex of ophiolite mélange, gneisses, blueschist, eclogite, greenschist, glaucophane-bearing marble, quartzite, metabasites, and minor cherts (e.g., Kapp et al., 2000, 2003; Li and Zheng, 1993; Li et al., 2006; Pullen et al., 2008, 2011; Zhai et al., 2011a,b, 2013a,b; Zhang et al., 2006a,b; Zhao et al., 2014). This suture zone is considered to result from closure of the Paleo-Tethys which was located between the Northern and Southern Qiangtang subterranes during the Mesozoic. Zircon U–Pb age data suggested that the ophiolites formed in the Ordovician, Silurian, Carboniferous, and Permian (P. Hu et al., 2014, and reference therein). The QMB extends from Longmu Co area in the northwest to Shuanghu area in the east over a distance of ~ 500 km (Fig. 1a). However, the way in which it extends eastwards remains disputed (e.g., Zhang and Tang, 2009; Yang et al., 2011; P. Hu et al., 2014; P.Y. Hu et al., 2014; Zhang et al., 2014), because of the absence of ophiolitic rocks, blueschists or eclogites in central of the Northern Qiangtang subterrane (Fig. 1a). In contrast, there are predominant Late TriassicLate Jurassic marine siliciclastic rocks and limestones in the eastern region (Fig. 1a; e.g., Li and Zheng, 1993; Zhang et al., 2002; Li et al., 2006; Pan et al., 2004). The stratum of the Yanshiping area in the Northern Qiangtang subterrane consists primarily of Jurassic clastic rocks and carbonates including Quemocuo Formation, Buqu Formation, Xiali Formation, Suowa Formation and Xueshan Formation and Triassic carbonates, and coal-bearing clastics that include Bolila Formation and Bagong Formation (Fig. 1b). Overall, the area between Yanshiping and Amdo displays identical sedimentary processes associated with alternative marine and continental environment (Chen et al., 2002; BGTAR, 2005). An important regional characteristic is the presence of NWW to NW-trending thrust faults and the formation of fold system, with thrusting direction from north to south (Fig. 1b; BGTAR, 2005). This feature is probably related to Late Jurassic-early Cretaceous Lhasa–Qiangtang collision (Chen et al., 2015a; Coward et al., 1988). The volcanic-sedimentary sequence (about 500 m long) in this study is exposed in the Yanshiping area, roughly 120 km north of the Amdo County. It is in systematic tectonic contact with the Jurassic surrounding sedimentary rocks. The volcanic-sedimentary sequence is dominated by volcanic rocks, interbedded limestones and sandstones. The volcanic rocks are characterized by a variety of rock types that include

volcanic breccias, volcanic agglomerates, and pillow basalts (Fig. 2). It's important to note that the occurrences of voluminous volcanic breccias and agglomerates with thickness of more than 100 m are indicative of a strong volcanic explosion (Fig. 3a). According to contrasting eruptive environments field occurrence, the volcanic-sedimentary sequence can be divided into two groups: the lower sequence and the upper sequence. The volcanic rocks from upper sequence are characterized by the presence of spheroidal weathering (Fig. 3b), columnar jointing (Fig. 3c) and interbeded red sandstone, indicative of a continental environment. Whereas the pillow lavas (Fig. 3d) occur in the lower of the sequence, implying that they were generated in a marine environment. In addition, there is an interbedded limestone between the upper and lower volcanic rocks (Fig. 2). Thus, the sequence from the lower to the upper sequence records the gradual shift of eruptive environments from marine to continental facies. Accordingly, the volcanic rocks in this study can be divided into the upper volcanic rocks (UVR) and the lower volcanic rocks (LVR). In order to make a systematically research of the magmatic evolution in the Yanshiping area, we sampled the volcanic rocks from the upper sequence to the lower sequence (Fig. 2). Although the studies volcanic rocks have been subjected to variable alteration, they preserve original textures. The UVR shows porphyritic texture and is composed of augite phenocryst (about 8 to 10 vol.%). The matrix consists dominantly of altered needle-like plagioclase, augite, magnetite, and cryptocrystalline-glassy material (Fig. 3e). Parts of the plagioclases are altered into fine sericite. Magnetite fills among fine plagioclases in the matrix. Minor secondary carbonates occur in pores. Plagioclase is ubiquitous in the LVR and as shown in Fig. 3f, there are two different sizes of plagioclase. The larger plagioclase crystals occur as lath shaped and the smaller plagioclase is needle-like. Thus, the former was formed in the early stage and the latter was generated in the late stage of magmatic evolution. The compositions of groundmasses are similar to the UVR with the exception of magnetite, which is more in the UVR. 3. Analytical methods 3.1. LA–ICP-MS zircon U–Pb dating U–Pb dating and trace element analyses of zircons were accomplished simultaneously by LA–ICP-MS at the Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences. To accurately analyze the zircons' internal structure and determine the dating domains, cathodoluminescence (CL) images were obtained using the electron microprobe at the Institute of Mineral Resources, Chinese Academy of Geological Sciences. Laser sampling was performed using a New Wave and ATL 193 nm ArF excimer laser ablation system (UP193FX), which is characterized by a

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Fig. 2. Structural cross section showing spatial relationships of the Yanshiping volcanic rocks.

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Fig. 3. Field photographs (a–d) and photomicrographs (e–f) of Triassic volcanic rocks in the Yanshiping area. (a) volcanic breccias and agglomerates, (b) spheroidal weathering, (c) columnar jointing in the upper part of sequence; (d) pillow lavas in the low part of sequence; (e) photomicrographs of sample AD1314 in the upper part of sequence; and (f) photomicrographs of sample AD1315VIII in the low part of sequence.

short pulse duration (b4 ns) and variable spot sizes (10, 15, 20, 25, 35, 50, 75, 100, or 125 μm). The Plesovice and SL natural zircon references were used as external standards for the matrix-matched calibration in the U–Pb dating process. The NIST SRM 612 reference glasses were analyzed as external standards for the trace element content calibration. A Plesovice, a Qinghu and a NIST SRM 612 zircon crystal were analyzed every five to ten sample analyses. Off-line isotope ratios and trace element concentrations were calculated by GLITTER_Ver4.0 (Achterbergh et al., Macquarie University). The common Pb correction and ages of the samples were calibrated and calculated using ComPbCorr # 3.17 (Anderson, 2002). The U–Pb concordia diagrams, weighted mean calculations and probability density plots of the U–Pb ages were made using Isoplot/Ex_ver 3 (Ludwig, 2003). 3.2. Whole-rock geochemical analysis Rock samples were carefully selected and sawed into slabs. The central parts were crushed and ground to finer than 200 mesh (~80 μm) for the bulk-rock analyses. Major elements were determined using Axiosmax X-ray fluorescence spectrometry (XRF), and trace elements, including rare earth and refractory elements, were analyzed using an XSeries II ICP mass spectrometry (Thermo Fisher Corp., USA) at the Hebei Institute of Regional Geological and Mineral Resources Survey. Loss on ignition (LOI) values were obtained by using an electronic analytical balance after heating the samples to 1000 °C. The analytical precisions for all major element abundances are better than 2–3 wt.%. Duplicate analysis of samples and rock standards (GSR-1

and GSR-2) yielded analytical errors of less than 5 wt.% for trace elements with concentrations of N10 ppm and less than 10 wt.% for those with concentrations of b 10 ppm. The Rb–Sr and Sm–Nd isotopic analyses followed procedures similar to those described by Li et al. (2012) and Yang et al. (2010). Whole-rock powders for Sr and Nd isotopic analyses were dissolved in Savillex Teflon screw-top capsules after being spiked with mixed 87 Rb– 84 Sr and 149 Sm– 150 Nd tracers prior to HF + HNO 3 + HClO 4 dissolution. Rb, Sr, Sm and Nd were separated using the classical two-step ion exchange chromatographic method and measured using a Thermo Fisher Scientific Triton Plus multi-collector thermal ionization mass spectrometer at IGGCAS. The whole procedure blank was less than 300 pg for Rb–Sr and 100 pg for Sm–Nd. The Rb–Sr and Sm–Nd isotopic ratios were corrected for mass fractionation by normalizing to 88 Sr/ 86 Sr = 8.375209 and 146Nd/ 144 Nd = 0.7219, respectively. The international standard samples, NBS-987 and JNdi-1, were used to evaluate instrument stability during the period of data collection. The measured values for the NBS-987 Sr standard and the JNdi-1 Nd standard were 87Sr/86Sr = 0.710277 ± 0.000009 (n = 9, 2 SD) and 143Nd/ 144 Nd = 0.512126 ± 0.000011 (n = 9, 2 SD), respectively. The USGS reference material BCR-2 was measured to monitor the accuracy of the analytical procedures, with the following results: 87Sr/86Sr = 0.705012 ± 0.000011 and 143 Nd/144Nd = 0.512623 ± 0.000015. The 87Sr/86Sr and 143Nd/144Nd data of BCR-2 show good agreement with previously published data based on the TIMS and MC–ICP-MS techniques (Li et al., 2012; Yang et al., 2010).

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4. Analytical results 4.1. Geochronology and zircon trace elements The results of geochronology and zircon trace elements for representative volcanic samples in the Yanshiping area are listed in Appendixes A–B. Two samples from the LVR and UVR, respectively, are analyzed for LA–ICP-MS zircon U–Pb dating. U–Pb concordia diagrams and corresponding chondrite-normalized rare earth element (REE) patterns of zircon grains are shown in Figs. 4 and 5a, respectively. Zircons collected from the LVR (samples AD1315-VII4 and AD1315-VIII5) are mostly 60 to 100 μm in size, euhedral, and short to long prismatic, with aspect ratios of 1:2 to 1:3 (Fig. 4). Most zircon grains are characterized by complete dark cathodoluminescence (CL) images and very weak zonation patterns. These features are likely associated with exceptionally higher U, REE and Th contents in the zircons (Fig. 5a–b; Wu and Zheng, 2004). The uranium concentrations range from 539 to 3637 ppm, and thorium contents vary from 4869 to 47,411 ppm. Their Th/U ratios ranging from 5.0 to 15.7 are inconsistent with metamorphic zircons (0.001 to 0.1; Rubatto, 2002). These Th/U ratios are even higher than those of typical magmatic zircon (0.2–1.5; e.g., Vavra et al., 1999; Hartmann et al., 2000), suggesting that they were formed in a thorium-enriched environment (Tucker et al., 2013; Zou et al., 2010). They are featured by fractionated REE patterns of heavy rare earth element (HREE) enrichment, pronounced positive Ce anomalies (Ce/Ce* of 15 to 75 (Ce/Ce* = CeN/SQRT(LaN*PrN))), and insignificant negative Eu anomalies (Fig. 5a). These features are consistent with magma-derived zircons

0.048

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(Hoskin and Schaltegger, 2003). Zircon grains in the lower sequence yield concordant 206Pb/238 U ages, with a mean of ca. 241.0 ± 1.6 Ma (MSWD = 0.15) for AD1315-VII4 and ca. 242.3 ± 1.3 Ma (MSWD = 0.57) for AD1315-VIII5 (Fig. 4). Zircon grains from the UVR (samples AD1315-V and AD1314) are typically 70–150 μm in length, euhedral, short to long prismatic, with aspect ratios of 1:2 to 1:4 (Fig. 4). Oscillatory zoning is a common characteristic in these zircon grains. Uranium concentrations range from 74 to 1347 ppm, and thorium contents range from 25 to 1118 ppm. Their Th/U ratios vary from 0.26 to 1.99, which are suggestive of magmatic zircon (Hoskin and Schaltegger, 2003). Considering that the trace element abundance in zircons generally decreases from granitic through mafic to ultramafic rocks, the trace element contents of zircons can be an indicator of source rock type (Belousova et al., 2002). For example, the average concentration of a rare earth element (REE) is several hundred to 1000 ppm and rarely over 2000 ppm in mafic rocks (Belousova et al., 2002 and references therein). In this paper, most of the zircons in samples AD1314 and AD1315-V (Fig. 6c–d) in the upper part of sequence display REE abundances less than 2000 ppm (Appendix B). Although these zircons show relatively narrow magmatic oscillatory zoning, we tend to suggest that they are derived from mafic rocks. Zircon grains collected in the UVR define the weighted mean 206Pb/238U age of 240.6 ± 1.1 Ma (MSWD = 0.48) for AD1314 and 240.4 ± 1.1 Ma (MSWD = 0.71) for AD1315-V (Fig. 4). During our LA–ICP-MS analyses, all the spot analyses were located on rims, thus, we suggest that these ages (ca. 242–240 Ma) represent the time of eruption of the volcanic rocks. The above-reported

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Fig. 5. Plot of (a) chondrite-normalized REE patterns and (b) Th–Th/U of zircon grains from Yanshiping volcanic rocks. The normalizing values are from Sun and McDonough, 1989.

geochronology data reveal that the UVR is slightly younger than the LVR, consistent with our field investigations, since the LVR is overlain by the UVR. 4.2. Major, trace elemental and Sr–Nd isotopic geochemistry The results of major, trace elemental and Sr–Nd isotopic data for representative volcanic samples in the Yanshiping area are listed in Appendixes C–D. Majority of the LVR and UPR have high MgO contents (N7 wt.%) and Mg# values (molar 100 ∗ MgO/(MgO + FeOT) N 64), which are close to compositions of primitive arc magma (e.g., Leat et al., 2002; Tatsumi and Eggins, 1995). Considering the presence of relatively high loss on ignition (1.28 to 2.66 wt.%), the SiO2 vs. Zr/TiO2*0.0001 diagram (Winchester and Floyd, 1977) can be used to classify volcanic rocks. Accordingly, Fig. 6a shows that the UVR and LVR are basalts and basalts–andesites, respectively. Positive Mg# vs. Al2O3 linear relationship (Fig. 6b) in the UVR indicates the presence of obvious fractionation of plagioclase. All of the analyzed samples are strongly enriched in light rare earth element (LREE) relative to heavy rare earth element (HREE) (Fig. 7a, b). The LVR displays obvious Yb negative anomalies when compared with the UVR, and this implies that garnet was a major component in the source mantle of the LVR. In primitive mantle-normalized trace element spidergrams (Fig. 7c, d), both the LVR and UVR display negative Nb and Ta anomalies, indicative of typical characteristics of arc magma. The LVR displays obviously positive Ba and Sr anomalies, with Ba = 194.1 to 2065.0 ppm, Sr = 602.7 to 1166.0 ppm. Whereas the UVR shows negative Ba and Sr anomalies, with Ba = 46.8 to 194.2 ppm,

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Sr = 107.4 to 260.8 ppm. In addition, the LVR is remarkably distinguished from the UVR by (i) obvious high field-strength elements (HFSEs) contents, with Nb ranging from 11.0 to 15.8 ppm, Zr ranging from 150 to 197 ppm and Th ranging from 2.3 to 4.2; (ii) high (Nb/La)N (0.5 to 1.0), (Nb/U)N (0.3 to 0.6) and Sr/Y ratios (27.5 to 63.1); and (iii) low (Rb/Ba)PM (0.3 to 1.3) and (Rb/Sr) PM ratios (0.4 to 2.9). On the other hand, the LVR is different from OIB, N-MORB and E-MORB (Sun and McDonough, 1989) in that it displays obviously negative Nb and Ta anomalies and relatively lower Nb/U (9.9 to 18.6) and Ce/Pb (1.5 to 7.9) ratios. The contents of TiO2 are relatively high and range from 1.29 wt.% to 1.66 wt.% with average value of 1.49 wt.%. It's important to note that these significant characteristics of the LVR are almost identical to Nb-enriched basalts and basaltic andesites (NEBs) in the Leyte and Mindanao (Philippines) and Vizcaino Peninsula, Mexico (e.g., Sajona et al., 1994, 1996; Aguillón-Robles et al., 2001). Sr–Nd isotopic compositions of the Yanshiping volcanic rocks, combined with previously published data, are illustrated in Fig. 8. The volcanic rocks in this study have relatively tightly clustered initial 87Sr/86Sr (0.70745 to 0.70862) and positive to negative initial εNd (− 1.11 to + 1.99) values, which have been calculated by their zircon LA–ICPMS U–Pb ages (Appendix A). Overall, their isotopic compositions are different from the Late Triassic felsic volcanic rocks along the QMB (Zhai et al., 2013a,b), Triassic adakites from the Songpan-Ganzi terrane (Wang et al., 2011), and the Late Triassic granite in the Qiangtang (Li et al., 2015; Peng et al., 2014). In addition, compared with the UVR (εNd(t) values of + 1.71 to + 1.99), the LVR exhibits negative initial εNd(t) (− 1.11 to − 0.40). This significant characteristic suggests that the LVR and UVR were derived from varying mantle sources.

21

(a)

(b)

Rhyolite

20

Com/Pan Rhyodacite Dacite

Al2O3 (wt. %)

SiO2 (wt. %)

70

Trachyte

60

Andesite

TrAn Phonolite

50

Sub-Ab Ab

Bas-Trach Neph

pl

19

acc

um

ula

tio

n

18 17

n

atio

tion

16

rac pl f

Lower volcanic rocks Upper volcanic rocks

40 0.001

0.01

0.1

Zr/TiO2*0.0001

1

10

15 53

58

63

68

73

78

Mg#

Fig. 6. Plot of (a) Zr/TiO2*0.0001-SiO2 (Winchester and Floyd, 1977), (b) Mg#-Al2O3 displaying rocks classification for Yanshiping volcanic rocks.

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1000

(a)

100.0

10.0

1.0

La

Ce

(c)

Upper volcanic sequence

Rock/Primitive Mantle

Rock/Chondrite

1000.0

Pr

Nd Sm Eu

Gd

Tb

Dy Ho

Er

Tm Yb

100

10

1

Lu

Rb

Th Ba

1000

(b)

Lower volcanic sequence

Rock/Primitive Mantle

Rock/Chondrite

1000.0

100.0

10.0

1.0

Ce

Pr

Nd Sm Eu

Gd

Tb

Dy Ho

Er

Tm Yb

Lu

Nb U

La Ta

Pb Ce

Sr Pr

Zr Nd

(d)

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Sm Gd Dy Ho Tm Lu Eu Tb Y Er Yb

Lower volcanic sequence

100

10

1 La

Upper volcanic sequence

Rb

Ba

Th

U

Nb

Ta

La

Ce

Pb

Pr

Sr

Nd

Zr

Hf

Sm Gd Dy Ho Tm Lu Eu Tb Y Er Yb

Fig. 7. Primitive mantle-normalized trace element spidergrams for the volcanic rocks from the Yanshiping area, central Tibet. The chondrite and primitive mantle values are from Sun and McDonough (1989).

5. Discussion 5.1. Alteration effects Because the analyzed volcanic rocks were formed in the Triassic and one sample from this study has high loss-on-ignition (LOI) values (up to 2.66 wt.%) (Appendix C). Therefore, the evaluation of effects of alteration is important before interpreting the data. Previous studies have suggested that the concentrations of the REEs, HFSEs (such as Zr, Nb), and transition elements Sc, and Th are not influenced by a wide range

of metamorphic conditions (Bienvenu et al., 1990). This is consistent with our results that the REE and HFSE contents of the Yanshiping volcanic rocks display the feature of coherent and subparallel concentration patterns (Fig. 7). In the case of the Sr–Nd isotopic composition, Sr isotopic ratios of oceanic rocks are easily increased by seawater alteration; however, both alteration and metamorphism have not significantly changed Nd isotopic compositions (e.g., White, 1993). Therefore, the following discussion will be based primarily on REEs, HFSEs, Sc, Th and Nd isotopic, which are the least affected by alteration or metamorphism. 5.2. Petrogenesis of the Yanshiping volcanic rocks

4 2 0 -2 Triassic lavas along the QMB

εNd(t)

-4 -6 -8 -10

Triassic adakites from the Songpan-Ganzi terrane

Triassic granite in the Qiangtang

-12 Lower volcanic rocks

-14

Upper volcanic rocks

0.702

0.707

0.712 87

0.717

0.722

0.727

86

Sr/ Sr(i)

Fig. 8. Plots of 87Sr/86Sr(i)-Nd isotopes for Yanshiping and surrounding volcanic rocks. The Triassic adakites in Songpan-Ganzi terrane are from Wang et al. (2011). The Late Triassic lavas along the QMB are and Zhai et al. (2013a,b). The Late Triassic granite in the Qiangtang are from Peng et al. (2014) and Li et al. (2015).

5.2.1. Lower volcanic rocks The petrogenesis of the NEBs remains a persistent question, and two major hypotheses have been proposed that include (i) melting of a geochemically enriched component in the mantle wedge (e.g., Castillo et al., 2002) and (ii) partial melting of the mantle wedge that has been metasomatized by adakitic melts (e.g., Defant et al., 1992, 2002; Sajona et al., 1993, 1994, 1996; Aguillón–Robles et al., 2001; Defant and Kepezhinskas, 2001; Smithies et al., 2005; Wang et al., 2007, 2008). It seems likely that the LVR originated from a relatively enriched mantle source due to large part to moderate enrichment of LREE (i.e., (La/Yb)N = 3.3–8.0) and relatively enriched Nd isotopic compositions (εNd (t): − 1.11 to − 0.40). Nevertheless, they differ from plume-related basalts (e.g., OIB and E-MORB-types of Sun and McDonough, 1989) in the presence of high field strength elements (HFSEs, e.g., Nb–Ta) negative anomalies (Fig. 7), which are suggestive of the characteristic of subduction-related magmatic suite. In addition, variable contributions of slab-derived component (e.g., sediment or fluid) would also result in relatively low or negative εNd(t) value. Therefore, it's possible that adakitic metasomatism of a mantle wedge played an important role in the petrogenesis of the Yanshiping NEBs. The diagnostic geochemical characteristics of trace element abundances in igneous zircons can be used as a geochemical tracer, because they are sensitive to crystallization environment (e.g., Belousova et al.,

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2002; Heaman et al., 1990). In this paper, the zircon grains from the LVR are significantly different from those in the UVR by exceptionally high thorium (Th) contents, i.e., 4869 - 47,411 ppm in the former and 25–1118 ppm in the latter (Fig. 5b). Generally, thorium is a moderate mobile element and derived primarily from subduction-related sediments overlying the subducted oceanic crust in subduction environments (Plank, 2005; Polat and Hofmann, 2003). Thus, high Th contents suggest that abundant subduction-related sediments have been involved in the mantle generation of the LVR beneath the Yanshiping area. This is further consistent with relatively high Th contents and Th/Yb ratios in the whole-rock of the LVR than in the UVR (Appendix C). Additionally, the Nb/Ta ratios are higher in the LVR (14.7 to 19.7) than in the UVR (9.6–13.7, one sample 16.3), and this is agreement with observations that weak negative Ta anomalies relative to significantly negative Nb anomalies occur in primitive mantle-normalized spidergrams (Fig. 7). These features may be associated with modification of the subarc mantle source by silicic melts derived from the subducting slab (e.g., Stolz et al., 1996). On the other hand, the LVR occurs as a subhorizontal trend in the plot of (La/Yb)N versus Sc/Yb (Fig. 9b). Given the higher partition coefficient Kd (Sc) for garnet relative to Yb and La, this sub-horizontal trend or limited Sc/Yb ratios variation is indicative of the garnet stability field where related magmatic melts were generated (Hole et al., 1995). Other processes such as pyroxene fractional crystallization or relative shallow partial melting exhibit sub-vertical or sloping trends in the

1.20

97

Fig. 9b, respectively (Hole et al., 1995). Thus, a subhorizontal trend defined by the Yanshiping NEBs is consistent with the interpretation that their parental magmas were generated in the garnet stability field (e.g., Hole et al., 1995; Sajona et al., 1996; Wyman et al., 2000). This is further supported by obviously negative Yb negative anomalies in chondrite-normalized REE patterns (Fig. 7b). The relative enrichment of HFSEs (Fig. 7d) and low Zr/Nb ratios (Fig. 9c) in the LVR are associated with the presence of Ti-bearing phases in the magma source (Sajona et al., 1996). It's suggested that low degree of partial melting will result in higher Gd abundances and lower Ti/Gd ratios, as a result of Ti retention in residual amphibole (Hollings, 2002). Thus, the melts produced in the presence of amphibole display the expectedly negative trends in the Gd–Ti/Gd plot (not shown). It's important to note that the LVR follows this negative trend, confirming that the amphibole was presented in the magma source. In addition, Sajona et al. (1996) originally proposed a mantlenormalized Nb/Yb versus Nb/La plot to assess the petrogenesis of the Mindanao NEBs and distinguish them from other arc basalts and within-plate basalts. This plot was further adopted by some authors (e.g., Wang et al., 2008; Wyman et al., 2000). The LVR displays remarkably negative correlation between (Nb/Yb)N and (Nb/La)N (Fig. 9d). This trend is analogous to other NEBs and suggestive of bulk partial melting with a Ti-phase (Sajona et al., 1996; Wyman et al., 2000). Hence, we suggest that Ti-phase played an important role in the petrogenesis of the LVR.

25.00

(a)

1.00

(b)

20.00

NEBs Sc/Yb

(Nb/La)N

0.80 0.60

15.00

Partial melting above the garnet stability field

10.00

“garnet stability”

0.40 5.00

0.20

cpx crystallization

Lower volcanic rocks Upper volcanic rocks

0.00 4.00

6.00

0.00 0.0

8.00 10.00 12.00 14.00 16.00 18.00

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5.0

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15.0

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(La/Yb)N 2.00

(c)

(d)

25.0

Mindanao NEB

1.50

15.0

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10.0

(Nb/La)N

Zr/Nb

20.0 Marquesas Island (OIB)

NEBs 1.00

Mindanao CA 1

0.50 5.0 5

0.0 4.00

6.00

8.00 10.00 12.00 14.00 16.00 18.00

Nb

0.00 0.00

5.00

10.00

4 3

2

15.00

(Nb/Yb)N

Fig. 9. Plots of (a) Nb–(Nb/La)N, (b) (La/Yb)N–Sc/Yb (modified from Wyman et al., 2000), (c) Nb–Zr/Nb, and (d) (Nb/Yb)N–(Nb/La)N (Sajona et al., 1996) for the Yanshiping volcanic rocks. In (d), the fields of Mindanao CA, Mindanao NEB, and Marquesas Island (OIB) are from Wyman et al. (2000), and numbered vectors are the effiects of (1) bulk partial melting without a Ti-phase, (2) bulk partial melting with a Ti-phase, (3) slab fluid introduction of La or closed system fractionation of an Nb-bearing phase, (4) open system fractionation where mass crystallized smass assimilated, (5) open system fractionation where mass crystallized = mass assimilated (Sajona et al., 1996; Wyman et al., 2000). The ENBAs from Wang et al. (2007); Wyman et al. (2000); Sajona et al. (1993, 1996); Aguillón-Robles et al. (2001).

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Based on the above discussion, therefore, it is reasonable to infer that the following two processes account for the petrogenesis of the LVR. (i) Low slab-derived melt/peridotite ratios commonly result in consuming slab-derived melts during their ascent and then metasomatic mantle source is generated above the subducting plate (Martin et al., 2005), leading to the precipitation of amphibole (e.g., Prouteau et al., 2001; Hastie et al., 2011). (ii) Subsequent melting of metasomatical mantle, accompanied by the breakdown of amphibole and releasing HFSE into the melts can form Nb-enriched magma (e.g., Kepezhinskas et al., 1996; Defant and Kepezhinskas, 2001). 5.2.2. Upper volcanic rocks Fractional crystallization is commonly responsible for the existence of petrologic and geochemical variations among the different volcanic rocks in the orogenic zone (e.g., Pearce et al., 1990). With regard to the Yanshiping volcanic rocks; however, both the LVR and UVR trend to have similar MgO contents and Mg# values (Appendix C), which is obviously inconsistent with trend of fractional crystallization from the LVR to the UVR. In addition, the lack of correlation between εNd(t) and SiO2 values (Fig. 10a) for the UVR and LVR are not consistent with extensive crustal contamination. By comparison, partial melting plays a key role in the petrogenesis inferred from the occurrence of positive correlation in the plot of La vs. La/Sm or La/Yb (Fig. 10b). The absence of overlapped or continuous variation of isotopic composition (εNd(t) values of + 1.71 to + 1.99 for the UVR and − 1.11 to − 0.40 for the LVR; Fig. 8) and correlation between 147Sm/144Nd and 143 Nd/144Nd suggest that they are likely ascribed to partial melting of heterogeneous source regions. Contrasting Th, U contents and Th/U ratios in the zircons (Fig. 5b) and different characters of associated Cathodoluminescence (CL) images (Fig. 4) are further consistent with the presence of heterogeneous source. Compared to the LVR, the UVR has relatively high Sc/Yb ratios (Fig. 9b), confirming that partial melting occurred mainly above the garnet stability field. This process is analogous to the trend of modern MORB (Wyman et al., 2000). Thus, we consider that possible mechanism to explain the variation of the LVR and the UVR involves partial melting of heterogeneous source regions. The most significant feature of the UVR is large negative Nb anomalies in primitive mantle-normalized diagram (Fig. 7b), exhibiting features of arc components (Perfit et al., 1980; Sun and McDonough, 1989). In this paper, the trace elemental geochemical features clearly suggest that the UVR was derived from an amphibole-free magma source (Fig. 9d), which has less subducted sediments contribution due to lower Th contents (Fig. 5b). This is also supported by relatively less SiO2 contents in the UVR than in the LVR, because addition of a slab melt to mantle peridotite would result in elevated SiO2 contents in the

5

magma (Sobolev et al., 2007). Commonly, this type of arc basalt originates mainly from partial melting of mantle wedge which has been metasomatized by aqueous fluids rather than sediments released by the downgoing oceanic slab (e.g., Woodhead and Johnson, 1993). Importantly, the UVR displays relatively depleted isotopic composition (εNd(t) of +1.71 to +1.99) when compared with the LVR (Appendix D), indicating addition of depleted asthenospheric component to the magma source. Evidence for asthenospheric components in the magma source is further indicated by relatively low Ba/Nb ratios (Appendix C; Jahn et al., 1999). Commonly, two possible mechanisms may account for the presence of asthenospheric component in subduction zone (i) slab-window and (ii) asthenospheric corner flow. The close association of subduction-related volcanism and within-plate alkalic volcanism is generally ascribed to the slab-window mechanism (Hole et al., 1991). It's important to note that Middle Triassic OIB-like alkaline diabase in the Tuohepingco area, central Qiangtang is considered to be a result of breakoff of the subducted slab (Zhang et al., 2011). In addition, the new identified Middle Triassic (ca. 235 Ma) rhyolites with negative εHf(t) values ranging from −14.75 to −5.91 in the Tanggula Pass area adjacent to the Yanshiping area may be mainly derived from ancient continental crust and associated with geodynamic background of the slab breakoff (Chen et al., unpublished data). Therefore, a reasonable explanation of the magma genesis would favor that the break-off induced the hot asthenosphere to underplate the Yanshiping area and subsequently mixed mantle wedge, generating the magmatic activities. 5.3. Implications for Phanerozoic crustal growth in Qiangtang terrane There are two competing ways of crust growth, i.e., subduction and collision, in the orogenic belt (e.g., Mo et al., 2008; Sengör et al., 1993); however, continental crustal growth mechanism for the Qiangtang terrane remains unclear. The most remarkable feature of Triassic magmatic rocks in the Qiangtang terrane is the fact that they display positive or negative εHf(t) or εNd(t) values, indicative of the presence of varying magma sources, e.g., asthenoshperic mantle, lithospheric mantle, continental lower crust. Yang et al. (2011) reported the positive εHf(t) values of the zircons (+ 9.7 to + 16.7) that were collected in the Permo-Triassic intermediate-acidic arc-related magmatic rocks in the Northern Qiangtang terrane, indicating derivation of magma from a relatively uncontaminated and depleted mantle. Triassic adakitic melts have also been identified in the Tuotuohe area in the Northern Qiangtang subterrane and in the Baohu and Juhuashan areas in the QMB (Wang et al., 2008; Zhai et al., 2013a), suggestive of the presence of melting of the subducted slab. Most of the Triassic adakitic rocks (219–

6

(a)

4

(b)

5

3

1

4

Cr

La/Sm

εNd(t)

2

us

tal

as

sim

ila

0

tio

3

g

ltin

e fm

eo

gre

2

n

e gd

sin

-1

rea

c De

1

-2

Lower volcanic rocks

Fractional crystallization

Upper volcanic rocks

-3 45.0

0 47.0

49.0

51.0

SiO2(wt.%)

53.0

55.0

57.0

0

10

20

La (ppm)

Fig. 10. Plots of (a) SiO2–Nd isotopes and (b) La–La/Sm for the Yanshiping volcanic rocks.

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236 Ma) in the Tuotuohe area close to the Yanshiping area reported by Wang et al. (2008) were dated by K–Ar ages. In this paper, zircon U–Pb dating using LA–ICP-MS techniques yields older concordant ages (242–240 Ma). It's important to note that older NEBs and related younger adakites were also identified in some ancient orogenic zone such as Altaids, China (Mao et al., 2012). We tend to suggest that the presence of younger adakites may be associated with persistent slab melting. Similarly, we propose that the slab-derived adakitic melts and fluids metasomatized mantle wedge peridotites and induced partial melting of the mantle wedge to lead to the generation of the NEBs magma in the Yanshiping area based on the geochemical characteristics. In addition, Middle Triassic slab breakoff played an important role in the growth of the Qiangtang terrane crust, as evidenced by the presence of OIB-like mafic volcanic rocks (ca. 234 ± 4 Ma) with positive εNd(t) values (+ 1.7 to + 3.2) in the central Qiangtang (Zhang et al., 2011). This is because the slab breakoff provided a channel for the upwelling and underplating of asthenospheric mantle to the relatively shallow level. The LSSZ crosses the middle Tibetan Plateau eastwards for 2000 km from Longmuco through Shanghu, Baqing, and eastern Tibet (Fig. 1a). In particular, the distance between the Yanshiping and Tuohepingco is more than 500 km. Thus such large regional extension makes us tend to suggest that the UVR in the Yanshiping (240 Ma) and the Tuohepingco OIB-like rocks (234 Ma) may be in response to the same tectonic event, although there is the presence of minor age discrepancy (about 6 Ma). Accordingly, the UVR in this paper displays significantly positive εNd(t) values (+1.71 to +1.99) and arc-like geochemical characteristics (e.g., negative Nb–Ta anomalies), indicative of addition of asthenospheric mantle relative to early stage of magma source of the NEBs in response to break-off of the subducting slab. Thus, Permian-Middle Triassic continental crust in the Qiangtang terrane was made up of young mantle-derived material. In contrast, all the Late Triassic igneous rocks probably generated in a post-collisional tectonic setting have highly negative εHf(t) (− 8.3 to −15.3) and εNd(t) (−7.9 to −11.1) values (Li et al., 2015; Peng et al., 2014; Zhai et al., 2013a,b). The most probable explanation is that these magmatic rocks have a dominantly crustal source heated by upwelling of asthenospheric mantle but insignificant mantle-derived magma contribution. Collectively, we suggest that partial melting of subducted oceanic crust and the upwelling of asthenospheric mantle resulting from the slab breakoff during the Permo-Middle Triassic played an important role in the growth of Qiangtang terrane crust and that Late Triassic collision-related and vertical crustal growth was nonsignificant.

Hf/3

Continental arc

y

rra

a tle

B OR

ite lei

PB alin Alk B WP

an

M

Oceanic island tholeiite

Th/Yb

N-M

tho

(b)

1.00

E th -MO ol ei RB iti cW

arc and Isl lt asa cB Ar tal nen nti

The Yanshiping volcanic rocks are significantly characterized by relative depletion of Nb–Ta and enrichment in LILE (Fig. 7), a diagnostic feature of subduction-related volcanic rocks. Fig. 11 demonstrates variable tectonic settings through the most widely used tectono-magmatic discrimination diagrams in terms of distinct features of relatively immobile elements. The Hf/3-Th-Ta diagram (Fig. 11a; Wood, 1980) illustrates clearly that the Yanshiping volcanic rocks plot within the field of continental arc basalt. Moreover, in the plot of Ta/Yb vs. Th/Yb (Fig. 11b; Condie, 2005), all the studied samples have higher Th/Yb ratios than E-MORB and are also within the continental arc field. In particular, the presence of the NEBs strongly suggests the contributions of slab-derived melts into the magma sources, and similar process has been recognized in many modern subduction settings such as the Leyte and Mindanao (Philippines) and Vizcaino Peninsula, Mexico (e.g., Hastie et al., 2011; Martin et al., 2005; Sajona et al., 1993, 1994). The features of zircon chemistry provide a further means of distinguishing tectono-magmatic affiliation of the basalts, since the diagnostic geochemical characteristics of trace elements in igneous zircons are sensitive to crystallization environment and therefore can be used as a geochemical tracer (e.g., Belousova et al., 2002; Heaman et al., 1990). It's generally believed that zircon from the igneous rocks does not display a Ce anomaly (Hidaka et al., 2002). Nevertheless, most of the analyzed igneous zircons in this study display variably positive Ce anomalies in chondrite-normalized REE patterns (Fig. 5a), indicating that the zircons formed in a special environment. Considering that the variation of the oxidation state from Ce3 + to Ce4 + occurs under oxidizing conditions, Ce can be fractionated from other REE, resulting in the occurrence of positive Ce anomaly. Such characteristic is also in good agreement with an arc-related environment, since oxygen fugacity is high in arc magma source as a consequence of relatively high water contents (e.g., Perfit et al., 1980). Thus, we propose that the genesis of the Yanshiping magmatic activities is associated with subduction-related process. However, which subduction event that resulted in the generation of the Yanshiping volcanic rocks remains unclear. The identification of Carboniferous ophiolites in the QMB indicates the occurrence of the Paleo-Tethys Ocean between the Northern and Southern Qiangtang subterranes during the Late Paleozoic (Li and Zheng, 1993; Li et al., 2006; Zhai et al., 2013a,b). As a consequent, closure of the Paleo-Tethys Ocean resulted in the generation of subduction- and collision-related magmatic rocks and high-pressure metamorphic rocks in the Qiangtang terrane during the Mesozoic

SHO CA

CA TH

E-MORB

0.10

e

N-MORB Lower volcanic rocks Upper volcanic rocks

Co

Th

5.4. Tectonic implications

10.00

(a)

99

0.01 0.01

Ta

0.10

1.00

10.00

Ta/Yb

Fig. 11. Discrimination diagrams of tectonic setting of (a) Hf/3–Th–Ta (Wood, 1980) and (b) Ta/Yb–Th/Y (Condie, 2005) for the Yanshiping volcanic rocks.

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(e.g., Li et al., 2006; Zhai et al., 2013a,b). In recent years, more and more precise chronological studies have been conducted on magmatic zircons (U–Pb dating) from the igneous rocks and metamorphic minerals (Ar– Ar and Lu–Hf dating) from metamorphic rock in the Qiangtang terrane. For example, glaucophane in the QMB shows Permian 40Ar/39Ar plateau ages (ca. 272–285 Ma; Deng et al., 2000) and Late Triassic Lu–Hf ages (223.4 ± 4.5 Ma; Pullen et al., 2008). In the Gemo area, the eclogite may be dated back to ca. 244 ± 11 Ma based on Lu–Hf geochronology (Pullen et al., 2008), ca. 230 ± 3 Ma and ca. 237 ± 4 based on zircon U–Pb ages (Zhai et al., 2011a,b). The 40Ar/39Ar ages for sodic amphiboles from the Qiangtang blueschists are ca. 223 ± 4 Ma and ca. 227 ± 4 Ma (Zhai et al., 2009). Other metamorphic thermal records include the phengite 40Ar/39Ar ages of ca. 223 to ca. 211 Ma for the eclogite and garnet–phengite schist (Liang et al., 2012; Zhai et al., 2011a,b), and the 40Ar/39Ar ages of ca. 203–222 Ma for white mica from metapelite (Kapp et al., 2000, 2003; Zhai et al., 2009). Details of the exhumation mechanism are beyond the scope of this paper, but these metamorphic events are considered to be associated with subduction or collision environments (Kapp et al., 2000, 2003; Pullen et al., 2008; Zhai et al., 2013a,b). In the case of Triassic magmatism, a large volume of Triassic magmatic rocks have also been verified along the QMB or in the Northern Qiangtang subterrane (Fu et al., 2010; Peng et al., 2014; Yang et al., 2011; Zhai et al., 2013a,b; Zhang et al., 2011). For example, the Permo-Early Triassic arc magmatism (ca. 275 Ma–248 Ma) in the north of the Shuanghu suture, Northern Qiangtang subterrane indicates the presence of northward subduction of the Paleo-Tethyan beneath the Northern Qiangtang subterrane during the Permo-Early Triassic (Yang et al., 2011). The Middle Triassic alkaline diabase (ca. 234 Ma) in the Tuohepingco area, central Qiangtang is thought to be a result of melting of upwelling asthenosphere which was attributed to breakoff of the subducted slab (Zhang et al., 2011). The Late Triassic felsic volcanic rocks and granites (205 Ma–223 Ma) in the QMB and the Northern Qiangtang subterrane (Fu et al., 2010; Peng et al., 2014; Zhai et al., 2013a,b; Zhao et al., 2014) were likely related to continent-continent collision between the Southern and Northern Qiangtang subterranes. Particularly, the Late Triassic Nadi Kangri Formation consist dominantly of the basal conglomerates, volcanic rocks (ca. 205–225 Ma) and minor eclogite gravels and blueschist fragments, suggesting that these HP metamorphic rocks had been rolled back or exhumed back to the surface during the Late Triassic (Peng et al., 2014). In these scenarios, therefore, the Yanshiping magmatism with age of ca. 242–240 Ma likely occurred in the process of the Paleo-Tethys oceanic subduction beneath the Northern Qiangtang subterrane. Recently, Zeng et al. (2015) proposed that the Late Triassic exhumation of the Qiangtang metamorphic belt and associated magmatism were formed in the tectonic context of back-arc rifting. According to their model, the QMB resulted from the initial (ca. 220–210 Ma) northward subduction of the Bangong-Nujiang Ocean (Zeng et al., 2015); however, this model is controversial. Because this model is in terms of the time of the so-called Triassic strata, and the time of the socalled Triassic strata is only based on the youngest detrital zircons, whereas various studies have suggested that the so-called Triassic strata may be formed from Jurassic to Early Cretaceous based on the fossils and zircon U–Pb dating of the volcanic interlayers of these strata (Bureau of Geology and Mineral Exploration of Tibet Province, 1993; Fan et al., 2015a,b; Xie et al., 2009). On the other hand, with respect to the central Qiangtang metamorphic belt, Shuanghu is the eastern end of its outcrop, but the way in which it extends eastwards remains highly debated (Zhang and Tang, 2009; Yang et al., 2011; P. Hu et al., 2014). Recently, the LSSZ is thought to connect the North Lancangjiang suture zone (NLSZ) and Changning Menglian suture zone (CMSZ), marking a Paleo-Tethys Ocean basin in Northern Qiangtang subterrane based on the limited data currently available for the geology of the Northern Qiangtang subterrane (P. Hu et al., 2014). Alternatively, the QMB extends southeastward and is

linked with Amdo-Basu high-pressure metamorphic belt (AMB) within the BNSZ (Fig. 1a; e.g., Zhang and Tang, 2009; Yang et al., 2011). As shown in Fig. 1a, not all volcanic rocks in the Qiangtang Terrane have a distribution which is strictly associated with the LSSZ. By contrast, some Triassic volcanics in the Yanshiping, Tuotuohe, and Dingqing areas occur along the north of the AMB within the BNSZ (Fig. 1a). This characteristic suggests that the tectonic evolution of the Paleo-Tethys along the QMB extended southeastward to the Amdo–Basu area (Yang et al., 2011). Other evidence such as similar protoliths of eclogite, P–T estimations, time for the high pressure metamorphism further supports for the linkage between the QMB and AMB (Zhang and Tang, 2009; Yang et al., 2011). Although there is no evidence showed that the Bangong-Nujiang Ocean has subducted during the Middle Triassic only based on the studies of Zhonggang and Nadong ocean island in the western portion of the BNSZ (Fan et al., 2014b,c), it's agreed that the Bangong-Nujiang Ocean was characterized by diachronism of the tectonic evolution from east to west. Recently, our group has identified the Late-Middle Triassic subduction-related volcanic rocks in the Amdo area (220–226 Ma; Chen et al., 2015) and the Tanggula Pass area (235 Ma; Chen et al., unpublished data), indicating that the middle of the Bangong-Nujiang Ocean may have subducted in the middle Triassic or even earlier. In addition, previous studies have shown that both the Paleo-Tethys and the Bangong-Nujiang Ocean are subducted northwards (Chen et al., 2015b; P. Hu et al., 2014; Wu et al., 2013; Yang et al., 2011). Thus the magmatism in the north of the Amdo and the areas eastern is likely related to the tectonic evolution of Bangong-Nujiang Ocean. 6. Conclusions The new data and interpretations presented in this paper have several important implications, not only for the understanding of the Triassic tectono-magmatic evolution of the Qiangtang terrane, but also for the understanding of subduction-related magmatism. Our major conclusions can be summarized as follows: (1) Primitive volcanic rocks in the Yanshiping area are divided into two parts, i.e., the lower volcanic rocks and upper volcanic rocks. Zircon U–Pb dating using LA–ICP-MS techniques reveals that the age of the lower volcanic rocks and upper volcanic rocks are ca. 242–241 Ma and ca. 240 Ma, respectively. (2) Petrogenetically, the lower volcanic rocks were derived from partial melting of mantle source regions that have been variably metasomatised by slab-related melts, whereas the upper volcanic rocks originated from partial melting of mixing of mantle wedge and depleted asthenospheric mantle in response to the slab breakoff. (3) Our new geochemical and geochronological results, in combination with regional studies, suggest that the Middle Triassic magmatism was formed in the geodynamic background of northward subduction of the Paleo-Tethys or the BangongNujiang Ocean. (4) Crustal growth in the Qiangtang terrane was dominated by subduction-related processes from Permian to Middle Triassic, whereas vertical crustal growth in the Late Triassic collisionrelated setting was nonsignificant. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.tecto.2015.10.017. Acknowledgments We thank Editor Rob Govers, Kuo-Lung Wang and an anonymous reviewer for their constructive reviews that have significantly improved the quality of this paper. This study was jointly funded by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB03010203), the National Program on Key Basic Research

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Project of China (973 Program) (Grant no. 2011CB403101) financed by the Ministry of Science and Technology, and the National Natural Science Foundation of China (Grant no. 41372063, 41172059). We thank Qi-Shuai Huang, De-Liang Liu, and Xiao-Han Gong for their assistance in fieldwork, and Ya-Hui Yue for their expert assistance in zircon LA\\ICP-MS U\\Pb dating. Discussion with Pei-Ping Song is helpful.

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