Transition from oceanic subduction to continental collision recorded in the Bangong-Nujiang suture zone: Insights from Early Cretaceous magmatic rocks in the north-central Tibet

Gondwana Research 78 (2020) 77e91

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Transition from oceanic subduction to continental collision recorded in the Bangong-Nujiang suture zone: Insights from Early Cretaceous magmatic rocks in the north-central Tibet Wei Wang a, b, Ming Wang b, *, Qing-Guo Zhai a, **, Chao-Ming Xie b, Pei-Yuan Hu a, Cai Li b, Jin-Heng Liu b, c, An-Bo Luo b a

Institute of Geology, Chinese Academy of Geological Science, Beijing, 100037, China College of Earth Sciences, Jilin University, Changchun, 130061, China c Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 June 2019 Received in revised form 5 September 2019 Accepted 7 September 2019 Available online 8 October 2019

The transition from oceanic subduction to continental collision is a key stage in the evolution of ancient orogens. We present new data for Early Cretaceous diorite and granite porphyry from northecentral Tibet to constrain the evolution of the BangongeNujiang Tethyan Ocean (BNTO). The diorites have moderate SiO2 and high MgO contents, similar to high-Mg andesites. Zircon grains yield UePb ages of 128e124 Ma and positive εHf(t) values between þ13.2 and þ 16.3, corresponding to Hf depleted-mantle model ages (TDM) of 281e131 Ma. The high-Mg diorite was probably formed by partial melting of hydrous mantle wedge fluxed by slab-derived fluids in an oceanic subduction setting. The granite porphyries yield zircon UePb ages of 117e115 Ma and zircon εHf(t) values ranging from þ0.1 to þ4.5. Most samples have high SiO2 and Fe2OT3 contents, variable FeOT/MgO and Ga/Al ratios, and are depleted in Ba, Sr, P, and Ti, similar to I- and A-type granites. The granite porphyries were most likely derived from partial melting of juvenile dioritic or granodioritic crust due to break-off of the BNTO lithosphere following collision between the Lhasa and Qiangtang blocks. The Early Cretaceous high-Mg diorite and Atype granite porphyry thus record the Early Cretaceous transition from oceanic subduction to continental collision along the BangongeNujiang suture zone (BNSZ). © 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Keywords: Tibet High-Mg diorite A-type granite Bangong-Nujiang suture zone Tethys

1. Introduction The transition from oceanic subduction to continental collision is key to understanding the formation and evolution of ancient orogens. However, it is commonly difficult to identify and reconstruct this transitional history in orogenic belts. The Tibetan Plateau is one of the youngest and largest orogens on Earth, and was created by the amalgamation of crustal terranes derived from Gondwana and Asia following the closure of the Tethyan oceans (e.g., Yin and Harrison, 2000; Pan et al., 2012; Metcalfe, 2013). The BangongeNujiang suture zone is the remnant of the Meso-Tethyan Ocean (or northern branch of the Neo-Tethyan Ocean), which

* Corresponding author. College of Earth Sciences, Jilin University, Changchun, 130061, 2199 Jianshe Street, China. ** Corresponding author. Institute of Geology, Chinese Academy of Geological Science, Beijing, 100037, No. 26 Baiwanzhuang Road, China. E-mail addresses: [email protected] (M. Wang), [email protected] (Q.-G. Zhai).

opened when the Cimmerian continent separated from Gondwana during the late Paleozoic, then closed again during the late Mesozoic (Fig. 1; S¸engor and Natlin, 1996; Pan et al., 2012; Metcalfe, 2013). Thus, the suture preserves information on the transition from oceanic subduction to continental collision from the Jurassic to Early Cretaceous. In recent decades, numerous studies with a focus on structural geology, paleontology, sedimentology, and igneous petrology have explored the evolution of the BangongeNujiang Tethyan Ocean (BNTO; Wang and Dong, 1984; Yin et al., 1988; Yin and Harrison, 2000; Zhu et al., 2006b; Bao et al., 2007; Kapp et al., 2007; Fan et al., 2014, 2015, 2017a; Zhu et al., 2016, 2019; Chen et al., 2017a, 2017b; Liu et al., 2017a, 2018a, 2018b; Ma et al., 2017). However, many of the details, in particular relating to the history of subduction and closure, are debated. Two main hypotheses exist to explain the closure of the ocean. The first suggests that closure occurred in the Jurassic to early Early Cretaceous, based mainly on: (i) the existence of an angular unconformity between the Early Cretaceous Dongqiao Formation and underlying ophiolites (Wang

https://doi.org/10.1016/j.gr.2019.09.008 1342-937X/© 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

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Fig. 1. (a) Tectonic framework of the Lhasa Terrane, Tibet (Zhu et al., 2009), showing the localities of the Early Cretaceous igneous rocks. Literature data are from Kang et al. (2008, 2009, 2010), Yuan (2009), Zhu et al. (2009, 2011), Yu (2010), Zhang et al. (2010a, 2010b, 2011, 2012), Peng et al. (2011), Huang et al. (2012), Qu et al. (2012), Ma (2013), Wu et al. (2013, 2014a, 2014b, 2015), Sui et al. (2013), Chen et al. (2014). LSSZ ¼ Longmu CoeShuanghu Suture Zone; BNSZ ¼ Bangong CoeNujiang Suture Zone; SNMZ ¼ Shiquan RivereNam
lange Zone; LMF ¼ LuobaduieMilashan Fault; IYZSZ ¼ InduseYarlung Zangbo Suture Zone. Tso Me

and Dong, 1984; Yin et al., 1988); (ii) Jurassic conglomerates (ca. 166 Ma) that overlie marine limestones unconformably (Ma et al., 2017), and; (iii) the presence of collisional granites in Amdo (140e120 Ma; Xu et al., 1985; Dewey et al., 1988). The second hypothesis posits that closure occurred later, due to the presence of Early Cretaceous ocean island basalts (Zhu et al., 2006b; Fan et al., 2014), marine sedimentary strata (Fan et al., 2015), and arc-type magmatic rocks in the Lhasa and Qiangtang terranes as well as within the BangongeNujiang suture zone (Cao et al., 2016; Fan et al., 2017a, 2017b; Liu et al., 2014; Hao et al., 2016, 2019; Li et al., 2016a; Wang et al., 2017). The formation of Mesozoic magmatic rocks, which are widespread along the BangongeNujiang suture zone (Fig. 1), relates to subduction and/or closure of the BNTO, and subsequent continental collision (Zhu et al., 2009, 2011; 2016; Kang et al., 2009, 2010; Qu et al., 2012; Sui et al., 2013; Wu et al., 2014b, 2015; Chen et al., 2014, 2017b; Liu et al., 2014, 2018a; Fan et al., 2017b). Here, we conducted a detailed petrological, whole-rock geochemical, and zircon UePb and LueHf isotopic analysis of newly-discovered Early Cretaceous high-Mg diorite and A-type granite in the Asa area, northecentral Tibet (Fig. 1). These data are used to constrain the processes associated with the transition from oceanic subduction to continental collision in the BangongeNujiang suture zone. 2. Geological background and petrography The Tibetan Plateau lies in the eastern part of the AlpineeHimalayan tectonic domain, and consists of several terranes separated by suture zones (S¸engor and Natlin, 1996; Yin and Harrison, 2000; Pan et al., 2012; Metcalfe, 2013). The BangongeNujiang suture zone, in the northecentral Tibetan Plateau, is bordered by the Qiangtang terrane to the north, and the Lhasa terrane to the south (Fig. 1). The suture is characterized by a series of ophiolite suites and sedimentary flysch. Previous studies have shown that the ophiolites range in age from Early Jurassic to Early Cretaceous (Girardeau et al., 1984; Liu et al., 2002, 2016; Shi et al., 2004, 2008; 2012; Shi, 2007; Qiu et al., 2004; Bao et al., 2007; Wang et al., 2008b, 2016; Xu et al., 2015). The Mugagangri Group consists of bathyal-to-abyssal flysch that was deposited in

the BNTO between the Late Triassic and Early Cretaceous (Fan et al., 2015, 2017a; Zeng et al., 2015; Huang et al., 2015c). Jurassic to Early Cretaceous magmatic rocks are widespread within, and on both sides of, the BangongeNujiang suture zone (Qu et al., 2012; Sui et al., 2013; Chen et al., 2014; Li et al., 2014a, 2014b; 2016b; Liu et al., 2014, 2017a; Fan et al., 2016; Hao et al., 2016; Zeng et al., 2016; Zhu et al., 2016; Hu et al., 2017; Wang et al., 2017). The Lhasa terrane is subdivided by the Shiquan RivereNam Tso
lange Zone and the LuobaduieMilashan Fault into northern, Me central, and southern subterranes (Fig. 1; Zhu et al., 2013). Neoproterozoic to Cambrian rocks in the Amdo and Nyainqentanglha areas represent the ancient crystalline basement of the Lhasa terrane (Dewey et al., 1988; Guynn et al., 2006; Zhu et al., 2013; Hu et al., 2018). Paleozoic rocks are mainly limestone, shale, sandstone, and conglomerate (Kapp et al., 2005; Zhu et al., 2009, 2011, 2013; Dong et al., 2011a, 2011b). Mesozoic volcanic and sedimentary strata trend broadly eastewest in the central and northern parts of the Lhasa terrane, and include the Qushenla and Duoni formations, and the Zenong Group, most of which were deposited in the Early Cretaceous (Zhu et al., 2006a, 2011; Kang et al., 2008, 2009; 2010; Sui et al., 2013; Wu et al., 2013, 2014b; Chen et al., 2014; Hu et al., 2017). Early Cretaceous granitic rocks (132e103 Ma) are also widely distributed in the central and northern Lhasa subterranes (Zhang et al., 2010b, 2011; 2012; Ma, 2013; Yu, 2010; Zhu et al., 2016). The Qiangtang terrane is subdivided into north and south subterranes by the Longmu CoeShuanghu suture zone (Li et al., 1995; Zhai et al., 2011a, 2011b). The LowereMiddle Jurassic deposits, together with Early JurassiceEarly Cretaceous magmatic rocks that are widely distributed in the south of the southern Qiangtang subterrane, are considered to have formed through northward subduction of the BNTO (Zeng et al., 2006; Li et al., 2011, 2014a; 2016a, 2016b; Liu et al., 2012, 2017a; 2017b; Hao et al., 2016; Huang et al., 2017; Ma et al., 2017; Wang et al., 2017). The Asa area is located ~120 km west of Nyima County, northecentral Tibetan Plateau (Fig. 1). Little work has been undertaken in this area because of its high elevation and rudimentary road network. Our recent detailed mapping has revealed numerous intermediate to felsic intrusions (diorite and granite porphyry) in the area (Fig. 2). The diorite and granite porphyry occur mainly as

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Fig. 2. Simplified geologic map of the Asa area, north-central Tibet.

irregular intrusions with diameters ranging from hundreds to thousands of meters, and which intrude JurassiceCretaceous flysch deposits (Fig. 3a and b; see Supplementary Table 1 for sample locations). The diorite comprises plagioclase, clinopyroxene, and hornblende. Grains of plagioclase are euhedral and elongate, and show polysynthetic twinning (Fig. 3c). Pyroxene and hornblende are mostly anhedral and exhibit variable chloritization. Phenocrysts in granite porphyry are mainly quartz and feldspar, together comprising around 25% by volume (Fig. 3d). Feldspar phenocrysts comprise euhedral to subhedral plagioclase and orthoclase. Quartz phenocrysts are anhedral. Biotite grains have irregular, ragged margins and are 0.05e0.25 mm across (Fig. 3d). The matrix is composed of microlitic quartz, plagioclase, and K-feldspar. Alteration of these rocks involved both carbonation and sericitization.

3. Analytical methods Zircon grains were separated at the Special Laboratory of the Geological Team of Hebei Province, Langfang, China. Cathodoluminescence (CL) imaging was undertaken at the Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China. UePb isotopic analyses were performed using laser ablationeinductively coupled plasmaemass spectrometry (LAeICPeMS) at the Geological Laboratory Centre, China University of Geoscience, Beijing, China. Detailed analytical methods are similar to those of Yuan et al. (2004). Common Pb was corrected for following the methods of Andersen (2002). Age uncertainties are quoted at the 1s level for individual analyses, and at the 95% confidence level for weighted-mean age calculations. Isoplot/Ex (version 3.0) was used to calculate weighted-mean ages and to plot

Fig. 3. Field photographs and photomicrographs of diorite and granite porphyry in the Asa area, north-central Tibet. Abbreviations: J-K ¼ Jurraic- Cretaceous flysch deposit, Amp ¼ Amphibole, Bi¼ Biotite, Or ¼ Orthoclase, Pl ¼ Plagioclase, Cpx ¼ Clinopyroxene, Q ¼ Quartz.

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concordia diagrams (Ludwig, 2003). In situ zircon LueHf isotopic analyses were conducted either at the same location as UePb analyses, or within the same textural domain based on CL images. Analyses were undertaken using a Neptune Plus multi-collectoreICPeMS (MCeICPeMS) coupled to a NWR 213 nm laser-ablation system at the Beijing Createch Test Technology, China. Analyses used a 40 mm diameter spot size, 10 Hz laser repetition rate, and a laser energy of 10e11 J cm2. The ablated material was transported into the MCeICPeMS with a high-purity He carrier gas. Detailed analytical procedures and instrumental operating conditions are similar to those described by Hou et al. 176 (2007). The Hf/177Hf ratio was normalized to 179 Hf/177Hf ¼ 0.7325, and Hf isotopic data were age-corrected using a176Lu decay constant of 1.867  1011 a1 (Soderlund et al., 2004). Zircon εHf(t) values and Hf model ages were calculated using the methods of Bouvier et al. (2008) and Griffin et al. (2000), respectively. Whole-rock geochemical analyses were conducted at the China University of Geosciences, Beijing, China. Major-element concentrations were determined by X-ray fluorescence spectrometry (XRF; PS-950), for which the analytical accuracy for all major elements was better than 1%. Trace-element compositions were determined using an Agilent-7500a ICPeMS, for which analytical accuracy was better than 5%. Detailed analytical procedures follow Zhai et al. (2013).

0.283081e0.283172, corresponding to positive εHf(t) values of þ13.2 to þ16.3, and young TDM ages of 131e281 Ma (Fig. 11). Forty-four LueHf isotopic analyses from three granite porphyry samples (N16T12, N16T27, and N16T54) yielded lower 176Hf/177Hf values (0.282707e0.282834) and εHf(t) values (þ0.1 to þ4.5; Fig. 11), but older TDM ages (782e620 Ma) than the diorite.

4.3. Geochemistry 4.3.1. Alteration effects Whole-rock geochemical data are presented in Supplementary Table 3. Petrographic observations and high loss-on-ignition (LOI) values (Fig. 3; LOI ¼ 0.87e7.30 wt%) indicate that the rocks have undergone variable degrees of alteration. Consequently, to constrain the petrogenesis of the rocks, we used immobile element concentrations, including high-field-strength elements (HFSEs; e.g., Nb and Ta), rare-earth elements (REEs; e.g., La, Ce, and Sm), and Th, which are less affected by alteration. In addition, we used major element (e.g., MgO, Fe2OT3, and K2O), transition element (e.g., Cr and Ni), Ba, and Sr concentrations, as all show no clear trends with increasing LOI value, suggesting they were also not significantly modified by alteration. The major-element oxide concentrations have been normalized to 100 wt% on an anhydrous basis.

4. Results 4.1. Geochronology Seven samples were selected for zircon UePb dating, including two of diorite (N16T10 and N16T52) and five of granite porphyry (N16T11, N16T12, N16T25, N16T27, and N16T54). The results are given in Supplementary Table 1 and illustrated in Fig. 4. Zircon grains from the two diorite samples are dominantly euhedral and prismatic and exhibit oscillatory zoning in CL images. They are 50e200 mm in length and have aspect ratios of 2:1 (Fig. 4a and b). The analyzed grains have Th contents of 786e8346 ppm and U contents of 85e1362 ppm, with Th/U ratios of 1.63e9.49, typical of magmatic zircon (Hoskin and Schaltegger, 2003). Twelve analyses of sample N16T10 yielded 206Pb/238U ages of 131e125 Ma, and a weighted-mean age of 128.0 ± 1.1 Ma (MSWD ¼ 0.73). Eighteen zircon grains from sample N16T52 gave 206Pb/238U ages of 128e121 Ma, and a weighted-mean age of 123.67 ± 0.92 Ma (MSWD ¼ 2.1; Fig. 4a and b). Both ages are interpreted to indicate the timing of crystallization of the diorites. Five samples of granite porphyry (N16T11, N16T12, N16T25, N16T27, and N16T54) were selected for LAeICPeMS zircon UePb geochronology. Zircon grains are predominantly euhedral, elongate, prismatic, 50e200 mm in length, have aspect ratios of 2:1 to 3:1, and exhibit clear oscillatory zoning in CL images (Fig. 4ceg). Th and U concentrations are 25e535 ppm and 30e358 ppm, respectively, and Th/U ratios 0.51e1.5, consistent with a magmatic origin. The five samples yielded weighted-mean ages of 116.07 ± 0.93 Ma (N16T11; n ¼ 17; MSWD ¼ 0.46), 115.4 ± 1.0 Ma (N16T12; n ¼ 16; MSWD ¼ 0.54), 116.8 ± 1.3 Ma (N16T25; n ¼ 11; MSWD ¼ 0.18), 114.66 ± 0.93 Ma (N16T27; n ¼ 17; MSWD ¼ 0.9), and 115.85 ± 0.96 Ma (N16T54; n ¼ 19; MSWD ¼ 0.14). These ages are interpreted as recording the timing of emplacement of the granite porphyries. 4.2. Zircon LueHf isotopic compositions Fifty-five zircon grains analyzed for UePb geochronology were also selected for LueHf isotopic analyses (Supplementary Table 2). One diorite sample (N16T52) has 176Hf/177Hf values of

4.3.2. Diorite Diorite samples contain low to moderate SiO2 (51.4e61.8 wt%), high Fe2OT3 (6.31e9.93 wt%), Al2O3 (12.58e20.42 wt%), CaO (6.02e10.23 wt%), and TiO2 (0.88e1.92 wt%) contents (Fig. 5aed, Fig. 6). They have variable Mg# (¼ 100  atomic Mg/(Mg þ Fe2þ)) values (58e73) and much higher MgO contents (4.91e7.37 wt%) than Early Cretaceous intermediate rocks from the northern Lhasa terrane (Fig. 5e). Primitive-mantle-normalized trace-element diagrams (Fig. 7a) show that all samples are strongly enriched in Th and Pb, and depleted in Nb, Ta, and Ti. Moreover, they show flat to slightly enriched chondrite-normalized REE patterns (Fig. 7b) with (La/Yb)N values of 0.68e2.50.

4.3.3. Granite porphyry The granite porphyry samples are characterized by high SiO2 (74.9e78.5 wt%), moderate Al2O3 (11.62e13.78 wt%) and CaO (0.28e2.83 wt%), and low TiO2 (0.29e0.38 wt%) and K2O (1.25e3.60 wt%) contents (Figs. 5 and 6). In the SiO2 versus AR diagram (AR ¼ (Al2O3 þ CaO þ Na2O þ K2O)/(Al2O3 þ CaO e Na2O e K2O)), they display calc-alkaline to alkaline features (Fig. 10a), and are predominantly peraluminous, with variable Al2O3/ (CaO þ Na2O þ K2O) values (A/CNK ¼ 0.97e1.43; Fig. 10b). Harker diagrams (Fig. 5) reveal that the rocks display strong negative correlations between SiO2 and TiO2, Al2O3, Fe2OT3, MgO, MnO, and P2O5. The granite porphyry samples can be subdivided into two groups based on their geochemical compositions. Group I (i.e., samples N16T11, N16T25, and N16T27) has higher Zr concentrations (259.8e337.4 ppm) than Group II (124.4e177.6 ppm; i.e., sample N16T54). In primitive-mantle-normalized trace-element diagrams (Fig. 9a), Group I samples are more enriched in Rb, Th, U, Zr, K, Pb, Nd, and Hf relative to Group II samples. Both groups are strongly depleted in Ba, Sr, Nb, Ta, and Ti. As shown in Fig. 9b, the rocks have seagull-shaped chondrite-normalized REE patterns with distinct total REE contents (Group I: 229e274 ppm; Group II: 116e132 ppm), an enrichment in light over heavy REE (LaN/ YbN ¼ 4.54e5.75), and pronounced negative Eu anomalies (Eu/ Eu* ¼ 0.23e0.29; Fig. 9b).

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Fig. 4. Zircon U-Pb Concordia diagrams for the Asa diorite and granite porphyry, with CL images of analyzed zircon grains. The solid and dashed circles indicate the locations of LAICP-MS U-Pb dating and Hf analyses, respectively.

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Fig. 5. SiO2 versus selected major element oxides (wt.%) for the studied samples. Data for comparison: Regional Early Cretaceous magmatic rocks (Kang et al., 2008, 2009; 2010; Yuan, 2009; Zhu et al., 2009, 2011; Yu, 2010; Zhang et al., 2010a, 2010b; 2011, 2012; Peng et al., 2011; Huang et al., 2012; Ma, 2013; Wu et al., 2013, 2014b; Sui et al., 2013); A-type granite from the Xainza area, Tibet (Qu et al., 2012; Chen et al., 2014); Aluminous A-type Granites from the Lachlan Fold Belt, Southeastern Australia (King et al., 1997); Guguan lavas at the Mariana arc (Elliott et al., 1997).

5. Discussion 5.1. Petrogenesis 5.1.1. Diorite High-magnesian andesites (HMA) are characterized by moderate SiO2 contents (52e63 wt%) and high Mg# (>55) (Kelemen, 1995; McCarron and Smellie, 1998; Zhao et al., 2007). The Asa diorite samples have high MgO (4.91e7.37 wt%) and Mg# values (58e73), and are compositionally similar to HMA (Fig. 8a). Several models have been proposed to explain the origin of HMA. These include: (1) interaction of subducted sediment-derived

melts with mantle peridotite (e.g., Shimoda et al., 1998; Tatsumi, 2001; Tatsumi and Yoshiyuki, 2006; Hanyu et al., 2002; Wang et al., 2008a; Zeng et al., 2016); (2) partial melting of a subducted slab followed by interaction with the overlying mantle (e.g., Kelemen et al., 2003, 2007; Bryant et al., 2011); (3) interaction of delaminated lower continental crust with inflowing mantle peridotite (e.g., Xu et al., 2002, 2006; Gao et al., 2004; Huang et al., 2008); (4) mixing between crust and mantle-derived materials (e.g., Qian and Joerg, 2010; Streck et al., 2007); and (5) direct melting of depleted hydrous mantle (e.g., Kushiro, 1969, 1974; Mysen and Boettcher, 1975; Kuroda et al., 1978; Tatsumi and Ishizaka, 1981; Elliott et al., 1997; Grove et al., 2002, 2003;

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Fig. 6. SiO2 versus Zr/TiO2 *0.0001 diagram (Winchester and Floyd, 1977) for the Asa diorite and granite porphyry, north-central Tibet.

emplacement of the Asa diorite (Lei et al., 2015; Yi et al., 2018; Liu et al., 2019). The Asa diorite is also unlikely to have formed by assimilation, as no coeval mafic and felsic members required for such assimilation have been reported (Streck et al., 2007). Similarly, the relatively uniform εHf(t) compositions and absence of any peridotite enclaves are inconsistent with an assimilation model for the Asa diorite (Fig. 11). We argue that the Asa diorite formed by direct melting of depleted and hydrated mantle wedge peridotite. The diorite has a composition similar to the Guguan lavas in the Mariana arc, which are attributed to melting fluxed by slab-derived aqueous fluid (Elliott et al., 1997, Fig. 7). The pronounced positive zircon εHf(t) values (þ14.7 to þ15.6; Fig. 11) are consistent with a depletedmantle source. Nb/Ta (9.0e16.9) ratios, which are lower than primitive-mantle values (17.5 ± 2.0; Sun and McDonough, 1989), as well as high Ba/Th (31.3e280.6), and low Th/Yb (0.09e0.49) ratios, are also consistent with fluid-fluxed melting in an arc environment (Fig. 8e; Turner et al., 1996; Woodhead et al., 2001; Nebel et al., 2007). Ratios of Nb/Yb that are similar to N-MORB, and Th/Yb ratios that are higher than N-MORB, also indicate a mantle source metasomatized by slab fluids (Fig. 8f; Pearce, 2014; Hao et al., 2019).

Mitchell and Grove, 2015). The Asa diorite has high Mg# values (58e73), and high concentrations of Cr and Ni (up to 218 and 145 ppm, respectively). In addition, zircon grains from the samples have low U/Yb ratios (0.02e0.69), similar to those in oceanic crust (Fig. 8b), indicating a mantle signature (Sun and McDonough, 1989; Grimes et al., 2007). In the first model, subducted sediments can be from the oceanic lithosphere or continental crust (e.g. Shimoda et al., 1998; Wang et al., 2008a). However, with extremely low Th concentrations (0.34e2.14 ppm) and Th/La ratios (0.09e0.27), the studied samples have no clear affinity with marine sediments (Zeng et al., 2016, Fig. 8c). In addition, their low-K tholeiitic characteristics are distinct from those of high-K calc-alkaline magmatism that results from continental subduction (Fig. 8d; Wang et al., 2008a). Partial melting of a subducting slab will result in melts with low heavy REE concentrations (Martin, 1999; Katz et al., 2004), inconsistent with the compositions of the Asa diorite (Fig. 7b). Hence, the studied samples cannot have formed through models 1 and 2 presented above. Model (3) above is also not applicable, as delamination of lower crust beneath the northern and central Lhasa subterranes probably occurred in the Late Cretaceous, significantly later than

5.1.2. Granite porphyry In general, A-type granites are characterized by high SiO2 contents, high FeOT/MgO and Ga/Al ratios, enrichment in Ce, Y, and HFSE (e.g., Zr and Nb), strong depletion in Ba, Sr, P, and Ti, seagulltype REE patterns, and negative Eu anomalies (Whalen et al., 1987; Eby, 1990; King et al., 1997; Frost et al., 2001; Bonin, 2007; Zhang, 2013). As described above, samples of Group I granite porphyry have high Ga/Al ratios, and high Zr and Y concentrations, suggesting an A-type affinity (Fig. 10d and e; Whalen et al., 1987). Furthermore, the samples have high zircon saturation temperatures of 840 C-885  C (Supplementary Table 3; Watson and Harrison, 1983), consistent with A-type granites (King et al., 1997, 2001; Bonin, 2007). By contrast, Group II samples have lower concentrations of many trace elements (e.g., Ba, Sr, and REE) than Group I samples (Fig. 9a and b), with Zr concentrations (<200 ppm) that are similar to those of highly-fractionated granites (Fig. 10d and e; Whalen et al., 1987). Moreover, the samples have higher K2O contents (1.45e1.83 wt%) than M-type granites (<1 wt%; White, 1979; White and Chappell, 1983). The negative correlation between SiO2 and P2O5 contents indicates that Group II samples are highly fractionated I-type granites rather than S-type granites (Fig. 5h; Chappell, 1999; Wu et al., 2003).

Fig. 7. Primitive-mantle-normalized trace element and chondrite-normalized REE patterns for the Asa diorite, north-central Tibet. Primitive-mantle and chondrite normalizing values are from Sun and McDonough (1989).

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Fig. 8. Plots of SiO2 versus MgO (a; McCarron and Smellie, 1998), U/Yb versus Y (b; Grimes et al., 2007), Th/La versus Th (c; Zeng et al., 2016), K2O versus SiO2 (d; Le Maitre, 2002), Th/Yb versus Ba/La (e; Yin et al., 2015), Th/Yb versus Nb/Yb (e; Pearce, 2014) for the Asa diorite. Data for comparison: Regional Early Cretaceous intermediate rocks (Wu et al., 2013; Kang et al., 2009, 2010；Zhu et al., 2009; Huang et al., 2012; Zhang et al., 2010b, 2011).

In a Zr/Hf versus Zr diagram, the granite porphyry samples define a negative trend (Fig. 10c), suggesting that both Group I and II samples underwent fractional crystallization (Wu et al., 2017). Negative Ba, Sr, and Eu anomalies indicate fractional crystallization of feldspar (Fig. 9a and b; Eby, 1990), in which the observed trends in BaeSr and BaeEu/Eu* plots indicate a dominance of K-feldspar fractionation (Figs. 5b and 10f, g), consistent with the low K2O concentrations (1.25e3.60 wt%) of the rocks (Fig. 5g; Eby, 1990; Singh et al., 2006). This is quite different from other Early Cretaceous silicic igneous rocks along the BangongeNujiang suture zone (including Xainza A-type granites (XZAG)) and aluminous A-type granites from the Lachlan Fold Belt, Australia (ALAG). Fractional crystallization of accessory minerals such as apatite may be responsible for the observed trends in the (La/Yb)N versus La

diagram (Fig. 5h), the strong depletion in P, and the negative correlation between P2O5 and SiO2 contents (Fig. 10h; Wu et al., 2003). A-type granites may form from varied sources and through different mechanisms, including: (1) extensive fractional crystallization of mantle-derived basaltic magmas (Turner et al., 1992; Shellnutt and Zhou, 2007); (2) mixing between mantle- and crustderived magmas (Yang et al., 2006; Pankhurst et al., 2013), and; (3) ~o partial melting of crustal rocks (Skjerlie and Johnston, 1992; Patin Douce, 1997; Wu et al., 2002; Shellnutt et al., 2011; Frost and Frost, 2011). The Asa A-type granite porphyries are peraluminous and have high silica contents, inconsistent with the first model, which produces metaluminous low-silica magmas (Frost and Frost, 2011; Whitaker et al., 2008). The second model is also unlikely due to absence of basaltic or dioritic enclaves, and the relatively uniform

Fig. 9. Primitive-mantle-normalized trace element and chondrite-normalized REE patterns for the Asa granite porphyry. Primitive-mantle and chondrite normalizing values are from Sun and McDonough (1989).

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Fig. 10. Plots of SiO2 versus AR (a; Wright, 1969; AR ¼ (Al2O3 þCaOþNa2OþK2O)/(Al2 O3þCaO-Na2O-K2O)), A/NK versus A/CNK (b; Le Maitre, 1989), Zr/Hf versus Zr (c), Zr versus 10000*Ga/Al (d; Whalen et al., 1987), FeOT/MgO versus ZrþNbþCeþY (e; Whalen et al., 1987), Ba versus Eu/Eu* (f; Eby, 1990), Ba versus Sr (g; Singh et al., 2006), (La/Yb)N versus La (h; Wu et al., 2003), TiO2/MgO versus (K2OþNa2O)/Al2O3 (i) for the Asa granite porphyry, north-central Tibet. FG: Fractionated felsic granites; OGT: unfractionated M  , I-, and Stype granites; AF: Alkaline feldspar; Ap: Apatite; Allan: Allanite; Bi: Biotite; Kf: K-feldspar; Mon: monazite; Pl: Plagioclase; Sph: sphene; Zr: zircon.

Hf isotopic compositions (εHf(t) ¼ þ0.1 to þ4.5; Fig. 11; Bolhar et al., 2008). Thus, partial melting of crustal rocks most likely accounts for formation of the Asa A-type granite porphyry. As discussed above, the Asa A-type granite porphyries underwent fractional crystallization dominated by K-feldspar. All analyzed samples have similar ratios of (Na2O þ K2O)/Al2O3 (0.37e0.62) and TiO2/MgO (0.67e1.67) to samples from the SZAG (0.50e0.61 and 0.67e2.22), which are interpreted as having been derived from melting of dioritic or granodioritic crust (Fig. 10i; ~ o Douce, 1997; Chen et al., 2014). Moreover, as these samples Patin display positive εHf(t) values (þ0.1 to þ4.5), we suggest that the Asa A-type granite porphyries probably originated from anatexis of juvenile dioritic or granodioritic crust. I-type granites can be generated by fractional crystallization of mafic magmas or by intracrustal partial melting (Chappell and White., 1974; Chappell, 1999). The Asa I-type granite porphyry has similar geochemical features and zircon Hf isotopic compositions to those of the A-type granite porphyry (Figs. 5, 9e11), suggesting a common source. We suggest that variable degrees of fractional crystallization may explain the geochemical differences between the I- and A-type

granite porphyries. 5.2. Transition from oceanic subduction to continental collision 5.2.1. Early Cretaceous High-Mg dioritic and A-type granitic magmatism in the Lhasa terrane Early Cretaceous magmatic rocks are widely distributed along the BangongeNujiang suture zone, some of which are related either to southward subduction of the BNTO lithosphere or collision between the Lhasa and Qiangtang blocks (Fig. 1; Zhu et al., 2009, 2011, 2016; Sui et al., 2013; Wu et al., 2013, 2014a, 2014b, 2015; Zhang et al., 2010a, 2010b, 2011; Huang et al., 2012; Kang et al., 2009, 2010; Qu et al., 2012; Chen et al., 2014; Hao et al., 2016; Hu et al., 2017; Chen et al., 2017b; Li et al., 2018). These rocks include andesitic rocks with ages of 117e110 Ma (Kang et al., 2009; Zhang et al., 2010b; Huang et al., 2012; Wu et al., 2013; Chen et al., 2014; Hu et al., 2017). Notably, Early Cretaceous HMA or diorite is rare along the BangongeNujiang suture zone. Jurassic HMAs (164e162 Ma) are the only such rocks that have been reported from the area, which are interpreted as recording the initial stages of

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Therefore, we argue that the BNTO had closed, and the Lhasa terrane had collided with the Qiangtang terrane, at or before 117e115 Ma (Fig. 13). In addition, coeval bimodal volcanic rocks (112e109 Ma) and A2-type granitoids (116e112 Ma) have been reported from the Yanhu and Xainza areas, respectively (Sui et al., 2013; Chen et al., 2014). These rocks suggest that a hightemperature extensional setting probably existed along the northern margin of the Lhasa terrane during the Early Cretaceous. These magmas were probably produced by break-off of the southward subducting slab after collision between the Lhasa and Qiangtang terranes (Fig. 13; Zhu et al., 2016).

Fig. 11. Zircon εHf(t) values versus ages for the Early Cretaceous igneous rocks in the central and northern Lhasa subterranes. Literature data: Zhu et al. (2009), 2011; Ma (2013); Yu (2010); Sui et al. (2013); Chen et al. (2014); Zhang et al. (2010b), 2012; Huang et al. (2012); Peng et al. (2011).

subduction of this Tethyan Ocean (Zeng et al., 2016; Liu et al., 2018b). In this study, we report the discovery of Early Cretaceous high-Mg diorite (128e124 Ma) from the Asa area, which provides important clues to constrain the evolution of the BNTO during this period. The record of Early Cretaceous felsic magmatism is diverse along the BangongeNujiang suture zone, including the occurrence of I-, S-, and A-type granitic rocks (Zhu et al., 2016). However, A-type granitic rocks with zircon UePb ages of 116e110 Ma have previously been reported from only the Xainza area in the central part of this suture zone (Qu et al., 2012; Chen et al., 2014). We report the occurrence of coeval A-type granite porphyries (117e115 Ma) from the Asa area, in the central western part of the BangongeNujiang suture zone. Thus, Early Cretaceous A-type granitic rocks may have been widely distributed along the BangongeNujiang suture zone. 5.2.2. Tectonic setting The presence of HMA is usually considered to be an indicator of oceanic subduction (Kay, 1978; Cameron et al., 1979; Tatsumi et al., 2003; Rogers et al., 1985; Crawford et al., 1989; Shimoda et al., 1998; Kelemen, 1995; Elliott et al., 1997; Tatsumi, 2005; Smithies and Champion, 2000; Tatsumi and Hanyu, 2003). For example, the Mariana arc is considered to have formed due to westward subduction of the Pacific Plate beneath the Philippine Sea Plate (Elliott et al., 1997). The high Al2O3, and low TiO2, Zr, Y, Nb, and Ta contents of the Asa high-Mg diorite, and the absence of pronounced negative Eu anomalies (Fig. 7b), are similar to arc-related andesites in orogenic belts, suggesting formation in a subduction setting (Bailey, 1981). Furthermore, the rocks are enriched in Th and Pb, and depleted in Nb, Ta, and Ti, similar to the Guguan lavas in the Mariana arc (Fig. 7). These features provide evidence that subduction of the BNTO was ongoing and the ocean basin couldn’t completely close at 128e124 Ma (Fig. 13). A-type granite commonly forms in extensional tectonic settings (Eby, 1990, 1992). In this study, the Asa A-type granite porphyry samples have similar Rb (56.2e132.9 ppm) and Y þ Nb (66.3e81.2 ppm) contents to post-collisional granites (Pearce et al., 1984). Furthermore, they show geochemical affinities with A2-type granites, suggesting that they were derived from crustal melting after orogeny or continental collision (Fig. 12a and b; Eby, 1992).

5.2.3. Transition from oceanic subduction to continental collision The BNTO was recognized as a Meso-Tethys ocean and probably opened from the early Permian to Early Jurassic (S¸engor and Natlin, 1996; Yin and Harrison, 2000; Zhu et al., 2011, 2013; 2016; Pan et al., 2012; Metcalfe, 2013; Li et al., 2019). However, its closure time and the transition processes from oceanic subduction to continental collision are still unclear. The subduction of this oceanic lithosphere had taken place in the Early Jurassic, as inferred from the oldest SSZ (supra-subduction zone) type ophiolites (190e184 Ma) in the Dongqiao and Amdo areas in the middle of the suture zone (Xia et al., 2008; Huang et al., 2015a, 2015b; Liu et al., 2016; Wang et al., 2016), as well as Jurassic arc-related magmatic rocks (Li et al., 2014a, 2014b; 2016b; Liu et al., 2014, 2018b; Cao et al., 2016; Fan et al., 2016; Hao et al., 2016; Zeng et al., 2016). In this study, we discovered Early Cretaceous high-Mg diorite in the Asa area, providing further evidence that this oceanic subduction continued until ca. 128e124 Ma. The BangongeNujiang suture zone is defined by a >2000 km long ophiolite belt within the Tibetan plateau, and records a complex evolution. The closure of the BNTO basin was most likely diachronous (Yin and Harrison, 2000). The Asa area lies in the central part of the BangongeNujiang suture zone (Fig. 1), where variably deformed Mesozoic sedimentary and magmatic rocks are well preserved. The angular unconformities between the ophiolites and rocks of the overlying Lower Cretaceous Dongqiao Formation suggest that the transition from oceanic subduction to continental collision between the Lhasa and Qiangtang terranes occurred in the Early Cretaceous (Wang and Dong, 1984; Yin et al., 1988). This is supported by sedimentary rocks from the Nyima basin, where sedimentary environments changed from marine to non-marine during the Early Cretaceous (Kapp et al., 2007). Furthermore, Early Cretaceous magmatic rocks are widely distributed along the BangongeNujiang suture zone (Fig. 1). The geochemical compositions of S-type granitoids (125e110 Ma; Tang et al., 2015; Sun et al., 2015; Zhu et al., 2016) and volcanic rocks (118e110 Ma; Zhang et al., 2010b; Chen et al., 2014, Hu et al., 2017) in the Nagqu and Baingoin areas indicate formation in a syn- or post-collision setting related to the collision of the Lhasa and Qiangtang terranes in the late Early Cretaceous. In this study, the Asa A2-type granitic rocks (117e115 Ma) provide new constraints on this collision. These rocks could be equivalent to the Xainza A-type granitic rocks (116e112 Ma; Qu et al., 2012; Chen et al., 2014), and record breakoff of the BangongeNujiang Tethyan oceanic lithosphere. Our data are able to constrain the transition from oceanic subduction to continental collision within the BangongeNujiang suture zone (Fig. 13). The BNTO lithosphere, between the Lhasa and Qiangtang terranes, probably began to subduct (with either single or double-sided subduction) at least in the Early Jurassic. This led to the generation of SSZ-type ophiolites in the Dongqiao and Amdo areas, and arc-related magmatic rocks along the BangongeNujiang suture zone (Xia et al., 2008; Huang et al., 2015a, 2015b; Liu et al., 2014, 2016; Wang et al., 2016; Li et al., 2014a, 2014b; 2016a, 2016b, 2017a; 2017b, 2018b; Fan et al., 2016; Hao et al., 2016; Zeng et al., 2016). This oceanic subduction continued until ca. 128e124 Ma.

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Fig. 12. Plots of Y-Nb-Ce (a) and Rb/Nb versus Y/Nb (b; Eby, 1992) for the Asa granite porphyry, north-central Tibet.

The ocean closed when the Lhasa terrane collided with the Qiangtang terrane during the late Early Cretaceous (ca. 117e115 Ma). The transition from oceanic subduction to continental collision of the BNTO thus occurred in the period ca. 124 to 117 Ma. 6. Conclusions (1) We report the discovery of Early Cretaceous high-Mg diorite (128e124 Ma) and A2-type granite porphyry (117e115 Ma) in the Asa area, northecentral Tibet.

(2) The high-Mg diorite was formed by partial melting of hydrous mantle wedge peridotite associated with oceanic subduction of the BNTO. The A2-type granite porphyry was derived from melting of juvenile dioritic or granodioritic crust triggered by slab break-off following closure of the ocean. (3) The Asa high-Mg diorite and A-type granite porphyry record the transition from oceanic subduction to continental collision within the BangongeNujiang suture zone, and constrain the timing of closure between 124 and 117 Ma.

Fig. 13. A tentative geodynamic model for tectonic evolution of the Bangong-Nujiang suture zone in the Early Cretaceous.

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Acknowledgments We thank Fan Jianjun, Li Xingkui, Yu Yunpeng, and Dong Yuchao for their help in the field. This research was supported by the National Science Foundation of China (Grant Nos. 41602230, 91755103, and 41872240), the China Geological Survey (Grant Nos. DD20190370, DD20190060, DD20160026 and DD20160015), the Ministry of Science and Technology of China (Grant No. 2016YFC0600304), and the Institute of Geology, Chinese Academy of Geological Sciences (Grant Nos. J1705 and YYWF201704). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.gr.2019.09.008. References Andersen, T., 2002. Correction of common lead in UePb analyses that do not report 204Pb. Chem. Geol. 192, 59e79. Bailey, J.C., 1981. Geochemical criteria for a refined tectonic discrimination of orogenic andesites. Chem. Geol. 32 (1), 139e154. Bao, P.S., Xiao, X.C., Su, L., Wang, J., 2007. 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