Post-Collisional Potassic Magmatism in the Eastern Lhasa Terrane, South Tibet: Products of Partial Melting of Mélanges in a Continental Subduction Channel

Gondwana Research 41 (2017) 9–28

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Post-collisional potassic magmatism in the eastern Lhasa terrane, South Tibet: Products of partial melting of mélanges in a continental subduction channel

Lihong Zhang a,b, Zhengfu Guo a,⁎,MaoliangZhanga,b, Zhihui Cheng a,b, Yutao Sun a,b a Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China b University of Chinese Academy of Sciences, Beijing 100049, China article info abstract

Article history: Post-collisional, potassic magmatic rocks widely distributed in the eastern Lhasa terrane provide significant Received 29 May 2015 information for comprehensive understanding of geodynamic processes of northward subduction of the Indian Received in revised form 18 October 2015 lithosphere and uplift of the Tibetan Plateau. A combined dataset of whole-rock major and trace elements, Accepted 3 November 2015 Sr–Nd–Pb isotopes, and in situ zircon U–Pb dating and Hf–O isotopic analyses are presented for the Yangying Available online 23 December 2015 potassic volcanic rocks (YPVR) in the eastern part of the Lhasa terrane, South Tibet. These volcanic rocks consist of trachytes, which are characterized by high K O(5.46–9.30 wt.%), SiO (61.34–68.62 wt.%) and Al O (15.06– Keywords: 2 2 2 3 – – Post-collisional potassic magmatism 17.36 wt.%), and relatively low MgO (0.47 2.80 wt.%) and FeOt (1.70 4.90 wt.%). Chondrite-normalized rare Zircon U–Pb dating earth elements (REE) patterns display clearly negative Eu anomalies. Primitive mantle-normalized incompatible Assimilation and fractional crystallization (AFC) trace elements diagrams exhibit strong enrichment in large ion lithophile elements (LILE) relative to high field Indian mélanges strength elements (HFSE) and display significantly negative Nb–Ta–Ti anomalies. Initial isotopic compositions South Tibet 87 86 143 144 indicate relatively radiogenic Sr [( Sr/ Sr)i = 0.711978–0.712090)] and unradiogenic Nd [( Nd/ Nd)i = 206 204 0.512121–0.512148]. Combined with their Pb isotopic compositions [( Pb/ Pb)i = 18.615–18.774, 207 204 208 204 ( Pb/ Pb)i = 15.708–15.793, ( Pb/ Pb)i =39.274–39.355)], these data are consistent with the involvement of component from subducted continental crustal sediment in their source region. The whole-rock Sr– Nd–Pb isotopic compositions exhibit linear trends between enriched mantle-derived mafic ultrapotassic magmas and relatively depleted crustal contaminants from the Lhasa terrane. The enrichment of the upper mantle below South Tibet is considered to result from the addition of components derived from subducted Indian continental crust to depleted MORB-source mantle during northward underthrusting of the Indian continental lithosphere beneath the Lhasa terrane since India–Asia collision at ~55 Ma. Secondary Ion Mass Spectrometry (SIMS) U–Pb zircon analyses yield the eruptive ages of 10.61 ± 0.10 Ma and 10.70 ± 0.18 Ma (weighted mean ages). Zircon

Hf isotope compositions [ƐHf(t) = −4.79 to −0.17], combined with zircon O isotope ratios (5.51–7.22‰), imply an addition of crustal material in their petrogenesis. Clinopyroxene-liquid thermobarometer reveals pressure (2.5–4.1 kbar) and temperature (1029.4–1082.9 °C) of clinopyroxene crystallization, suggesting that depth of the magma chamber was 11.6–16.4 km. Energy-constrained assimilation and fractional crystallization (EC– AFC) model calculation indicates depth of assimilation and fractional crystallization in the region of 14.40– 18.75 km underneath the Lhasa terrane, which is in consistent with depth of the magma chamber as suggested by clinopyroxene-liquid thermobarometer. Based on the whole-rock major and trace elements and Sr–Nd–Pb isotope compositions, combined with EC–AFC modeling simulations and zircon Hf–O isotope data, we propose that the YPVR resulted from assimilation and fractional crystallization (AFC) process of the K-rich maﬁcprimitive magmas, which were caused by partial melting of the Indian continental subduction-induced mélange rocks. © 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction 2006b; Zhao et al., 2006, 2009; Gao et al., 2007a, 2007b, 2009, 2010; Guo et al., 2007, 2013, 2015; Wang et al., 2008, 2014, 2015; J.L. Chen During India–Asia collision and subsequent northward subduction et al., 2010; Chen et al., 2011, 2012; Guo and Wilson, 2012; Hou et al., of the Indian continental lithosphere, multi-stage magmatism and 2013; Zhu et al., 2013; Jiang et al., 2014; Ma et al., 2014; Liuetal., crust–mantle interaction took place in the Lhasa terrane, South Tibet 2015). The magmatism in continental collision settings records (e.g., Miller et al., 1999; Ding et al., 2003, 2006; Mo et al., 2003, 2006a, recycling of subducted crustal components at continental subduction zone and uplift process of the Tibetan Plateau (e.g., Chung et al., 2003, 2005, 2009; Ding et al., 2003; Hou et al., 2006, 2013; Guo et al., 2007, ⁎ Corresponding author at: No. 19, Beitucheng Western Road, Chaoyang District, Beijing 100029, China. Tel.: +86 10 82998393; fax: +86 10 62010846. 2013, 2015; Zhao et al., 2009; Guo and Wilson, 2012). Post-collisional, E-mail address: [email protected] (Z. Guo). K-rich (including ultrapotassic and potassic) magmatic rocks (25–

http://dx.doi.org/10.1016/j.gr.2015.11.007 1342-937X/© 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. 10 L. Zhang et al. / Gondwana Research 41 (2017) 9–28

8 Ma) in the Lhasa terrane are thought to link to partial melting events Miller et al., 1999; Ding et al., 2003; Nomade et al., 2004; Zhao et al., associated with deep geodynamic processes in the Himalaya–Tibet 2009; J.L. Chen et al., 2010; Chen et al., 2012; Zhou et al., 2010; C.Z. Liu continent orogenic zone, such as northward subduction of the Indian et al., 2011, 2014; D. Liu et al., 2011, 2014; Liu et al., 2015; Wang et al., continental lithosphere (Ding et al., 2003; Hou et al., 2006; Guo et al., 2014). Because these previous studies simply presented whole- 2013, 2015) and convective thinning of over-thickened Tibetan conti- rock major element, trace element and Sr–Nd–Pb isotopic data of the nental lithosphere (e.g., Turner et al., 1996; Miller et al., 1999; Chung potassic rocks in South Tibet, lack of detailed zircon Hf–O isotopic data et al., 2003, 2005, 2009; Sun et al., 2007; Zhao et al., 2009; C.Z. Liu and comprehensive mineralogical and petrological data has precluded et al., 2011, 2014; D. Liu et al., 2014; Liu et al., 2015; Tian et al., 2012; further understanding of the origin and evolution of these potassic Wang et al., 2014) and break-off of a northward subducted slab of the magmas. Indian continental lithosphere (e.g., Mahéo et al., 2002; Replumaz We focus on the Yangying potassic volcanic rocks (YPVR) (Fig. 1), et al., 2010, 2013, 2014). Intense controversies still remain in spite of which have been thought to exhibit many typical outcrops of many previous studies, which proposed that ultrapotassic and potassic the post-collisional potassium-rich magmatic rocks in South Tibet (Li magmas are derived from: (1) asthenospheric mantle enriched by et al., 1992). In this study, we report new systematic dataset of whole- materials from subducted Indian continental lithosphere (e.g., Arnaud rock major, trace elements and Sr–Nd–Pb isotopes, in situ zircon U–Pb et al., 1992; Guo et al., 2013, 2015); and (2) enriched metasomatized age and Hf–O isotopes of the YPVR, South Tibet. These data, combined lithospheric mantle and/or maﬁc lower crust (e.g., Turner et al., 1996; with previously published geochemical and geophysical data, allow us

Fig. 1. (a) Simplified map showing distribution of the Cenozoic magmatic rocks in the Lhasa terrane, South Tibet (modified from Guo et al., 2015). (b) Simplified geological map of the Yangying potassic volcanic field (modified from Li et al., 1992; Zhou et al., 2010). L. Zhang et al. / Gondwana Research 41 (2017) 9–28 11 to develop a more robust petrogenetic model for the YPVR in South 3. Petrography Tibet. Fourteen samples were collected for analyses from the Yangying 2. Geological setting volcanic field (Fig. 1b). The analyzed samples have porphyritic textures and most phenocrysts with size ranging from 1 to 5 mm. Phenocryst Tibetan Plateau is composed of the Kunlun-Qaidam, Songpan-Ganzi, minerals consist of clinopyroxene, alkali feldspar, plagioclase and phlog- Qiangtang and Lhasa terranes from north to south, which were integrated opite scattered in the groundmass composed of feldspar, clinopyroxene, together during closure of the Tethys Oceans since the Paleozoic times apatite and Fe–Ti oxides (Table 1 and Fig. 2). The clinopyroxene crystals (Yin and Harrison, 2000). The Lhasa and Qiangtang terranes are are mostly euhedral (Fig. 2a). Almost all the plagioclase phenocrysts separated by the Jurassic-Cretaceous Bangong-Nujiang suture (BNS), have reaction rims of alkali feldspars (Fig. 2). Phlogopite phenocrysts whereas the Indus-Tsangpo suture (ITS) marks the boundary between are characterized by altered and oxidized dark rims of Fe–Ti oxides the Lhasa terrane and the Himalayas (Fig. 1a; Kapp et al., 2003, 2007). (Fig. 2e and f). Precambrian basement has been only discovered in the central and northern Lhasa terrane, as represented by metamorphic rocks to the 4. Analytical methods west of the Nam Lake (~750 Ma; Hu et al., 2005) and the Amdo gneiss (852 Ma; Guynn et al., 2006). Paleozoic to Mesozoic sedimentary strata 4.1. Whole-rock elemental and Sr–Nd–Pb isotopic analyses and Jurassic to Cretaceous volcanic rocks comprise sedimentary cover of the Lhasa terrane (Zhu et al., 2008). All samples from Yangying potassic volcanic field (Fig. 1b) were The onset of Indian and Asian continent collision was still a contro- analyzed for their major and trace elements and Sr–Nd–Pb isotopic versial issue; estimates vary from as early as the late Cretaceous (70– compositions at Beijing Research Institute of Uranium Geology (BRIUG).

65 Ma; Gnos et al., 1997; Yin and Harrison, 2000; Mo et al., 2007; Xia FeO concentrations were obtained by KMnO4 titration. Sample powders et al., 2011) to the Paleocene (65–55 Ma, e.g., Ding et al., 2005; Hu for trace elements analyses were digested with mixed HNO3 + HF acid et al., 2012; X. Hu et al., 2015; Chung et al., 2009; J. Chen et al., 2010; in steel-bomb coated Teflon beakers to assure the complete dissolution Yi et al., 2011; Jiang et al., 2014; F.Y. Wu et al., 2014)orevenlater of refractory minerals. The trace elements were measured using a (55–50 Ma, e.g., Patriat and Achache, 1984; Tapponnier et al., 2001; Finnigan Element II ICP–MS at the BRIUG following the procedures Royden et al., 2008; Najman et al., 2010; Aitchison et al., 2011; described by Li (1997). The analytical precisions for trace elements Shellnutt et al., 2014). The India–Asia continent collision has led to exten- were between 5% and 10% depending on the concentration level of sive magmatism in the Lhasa terrane, generating widespread Cenozoic ig- aspecific element. The major and trace element analytical data are neous rocks (Fig. 1a) which are mainly classified as Gangdese batholith presented in Table S1 in the Supplementary data. (e.g., Jiang et al., 1999; Dong et al., 2005, 2008; Mo et al., 2005; Wen Sr–Nd–Pb isotopes were determined by an Isoprobe-T thermal ioni- et al., 2008; Zhu et al., 2008, 2009, 2011, 2013; Ji et al., 2009a, 2009b, zation mass spectrometry (TIMS) at the BRIUG following the procedures 2012, 2014), Linzizong volcanic rocks (e.g., Mo et al., 2005, 2006a, of GB/T17672-1999. Powder samples were mixed for isotope dilution

2006b; Lee et al., 2007, 2009, 2012), adakitic rocks (e.g., Chung et al., and dissolved using HF + HNO3 +HClO4 in sealed Teflon capsules on 2003, 2009; Hou et al., 2004, 2013; Gao et al., 2007a, 2010; Guo et al., a hot plate for 24 h. After the separation of the Rb, Sr and light REE in 2007; King et al., 2007; Xu et al., 2010; Chen et al., 2011; Guan et al., a cation-exchange column, the Sm and Nd were further purified using 2012; Hébert et al., 2014; Jiang et al., 2014; Ma et al., 2014; L.Y. Zhang a cation-exchange column, conditioned and eluted with dilute HCl. et al., 2014; Zeng et al., 2017; S. Wu et al., 2014; Y. Hu et al., 2015)and The isotopic mass fractions were normalized to 143Nd/144Nd = 0.7219 K-rich magmatic rocks (e.g., Miller et al., 1999; Williams et al., 2001, and 87Sr/86Sr = 0.1194, respectively. Repeated analyses of the 2004; Ding et al., 2003, 2006; Nomade et al., 2004; Zhao et al., 2009; J.L. 86Sr/88Sr ratio of the standard NBS987 and 146Nd/144Nd ratio of the Chen et al., 2010; Chen et al., 2012; Zhou et al., 2010; Guo et al., 2013, standard SHINESTU gave 0.710250 ± 0.000007 (2σ) and 0.512118 ± 2015; C.Z. Liu et al., 2011, 2014; D. Liu et al., 2011, 2014; Liu et al., 2013, 0.000003 (2σ), respectively. Total chemical blanks were b200 pg 2015; Wang et al., 2008, 2014; Huang et al., 2015). Ultrapotassic and po- for strontium and b50 pg for neodymium (X. Zhao et al., 2014). The tassic magmatic rocks in South Tibet are found as lavas, plugs and dikes analytical precision for Rb/Sr and Sm/Nd ratios were below 1%. For Pb with small volumes, which mainly crop out in near NS trending rifts to isotope measurements, Pb was separated from the silicate matrix and the west of longitude 87°E while potassic lavas have only been discov- purified using AG1 × 8 anionic ion-exchange columns with dilute HBr ered in two volcanic fields (Majiang and Yangying) to the east of longi- as eluant. During the period of analyses repeat analyses of the interna- tude 87°E (Fig. 1a). tional standard NBS981 yielded 204Pb/206Pb = 0.0591107 ± 0.000002; Yangying volcanic field is located at central segment of the NE–SW trending Yadong-Gulu rift, about 80 km west of Lhasa (Fig. 1b). YPVR mainly consist of trachytes which cover an area less than 10 km2. Table 1 These lavas are controlled by NS trending high-angle normal faults Phenocryst and groundmass mineral assemblages of the Yangying potassic volcanic rocks. and crop out as the lava flow and dome which are cut by Pujiemu Valley, Sample no. Rock type Phenocrysts Groundmass

Nangzeng Valley and Qialagai Valley from south to north (Fig. 1b; Li YY-01 Trachyte Cpx + Sani + Pl + Phl Cpx + Sani + Pl + Ap + Fe–Ti et al., 1992, 1994). High-temperature hydrothermal activities (e.g., hot YY-02 Trachyte Cpx + Sani + Pl + Phl Cpx + Sani + Pl + Ap + Fe–Ti springs, soil microseepage) are pervasive around the Yangying volcanic YY-03 Trachyte Cpx + Sani + Pl + Phl Cpx + Sani + Pl + Ap + Fe–Ti field (Li et al., 1994). Volatiles and volcanic gases from high- YY-04 Trachyte Cpx + Sani + Pl + Phl Cpx + Sani + Pl + Ap + Fe–Ti YY-05 Trachyte Cpx + Sani + Pl + Phl Cpx + Sani + Pl + Ap + Fe–Ti temperature hydrothermal systems in the Yangying volcanic field YY-06 Trachyte Cpx + Sani + Pl + Phl Cpx + Sani + Pl + Ap + Fe–Ti might be related with shallow magmatic heat source, similar to those YY-07 Trachyte Cpx + Sani + Pl + Phl Cpx + Sani + Pl + Ap + Fe–Ti from Yangbajing geothermal field ~30 km north of Yangying (Zhao YY-08 Trachyte Sani + Pl + Phl Sani + Pl + Ap + Fe–Ti et al., 1993; Brown et al., 1996; Nelson et al., 1996; Liao and Zhao, YY-09 Trachyte Sani + Phl Sani + Pl + Ap – 1999; Wei et al., 2001). Recent studies show that total soil microseepage YY-10 Trachyte Cpx + Sani + Pl + Phl Cpx + Sani + Pl + Ap + Fe Ti – fl 4 −1 YY-11 Trachyte Cpx + Sani + Pl + Phl Cpx + Sani + Pl + Ap + Fe Ti CO2 ux (~4.43 × 10 ta ; Guo et al., 2014b) of the Yangying volcanic YY-12 Trachyte Cpx + Sani + Pl + Phl Cpx + Sani + Pl + Ap + Fe–Ti 4 −1 field is comparable to that (~8.59 × 10 ta ; L.H. Zhang et al., 2014)of YY-13 Trachyte Cpx + Sani + Pl + Phl Cpx + Sani + Pl + Ap + Fe–Ti the Yangbajing geothermal field with well-developed geothermal ener- YY-14 Trachyte Cpx + Sani + Pl + Phl Cpx + Sani + Pl + Ap + Fe–Ti gy, suggesting considerable potentiality for geothermal power genera- Abbreviations are as follows: Ap, apatite; Cpx, clinopyroxene; Fe–Ti, Fe–Ti oxides; Phl, tion in the Yangying volcanic field, South Tibet. phlogopite; Pl, plagioclase; Sani, sanidine. 12 L. Zhang et al. / Gondwana Research 41 (2017) 9–28

Fig. 2. Representative microphotographs of the YPVR (cross-polarized). (a) Euhedral clinopyroxene phenocrysts. (b) Euhedral plagioclase and sanidine phenocrysts. (c) Sanidine showing thin reaction rim of alkali feldspar. (d) Plagioclase showing thin rim of alkali feldspar. (e) Altered phlogopite phenocrysts. (f) Corroded sanidine and phlogopite phenocrysts. Abbreviations are as follows: Ap, apatite; Cpx, clinopyroxene; Phl, phlogopite; Pl, plagioclase; Sani, sanidine.

207Pb/206Pb = 0.914338 ± 0.00007; 208Pb/206Pb = 2.164940 ± U and Pb isotopes were measured using Cameca IMS-1280 SIMS. 0.000015. Pb isotope fractionations were corrected using correction The O2− primary ion beam was accelerated at ~13 kV, with an intensity factors from the certiﬁed values of the international standard NBS981. of ca. 10 nA and the ellipsoidal spot is about 20 μm×30μm. Positive The analytical precision for Pb isotopic ratios was below 0.05%. The Sr, secondary ions use a 60 eV energy window and a mass resolution of Nd and Pb isotopic data are presented in Table S2 in the Supplementary ~5400. Each measurement consists of 40 cycles, and the total analytical data. time is 12 min. Analytical procedures are described in details (Li et al., 2009, 2012; Balintoni et al., 2011; Dan et al., 2012, 2016; Eyal et al., 4.2. Zircon U–Pb dating 2014; Wang et al., 2015). Zircon U–Pb isotopic data are presented in Table S3 in the Supplementary data. Zircon U–Pb dating, together with pre-analytical prepared work, was achieved at the Institute of Geology and Geophysics, Chinese 4.3. Zircon Hf–Oisotopicanalyses Academy of Sciences (IGGCAS), Beijing. The pre-analytical preparations contain the following two parts: (1) zircon grains were separated using Zircon oxygen isotopes were measured using Cameca IMS-1280 at heavy-liquid and magnetic techniques and then they were mounted in the IGGCAS. The Cs+ primary ion beam was accelerated at 10 kV with epoxy resin and polished for analyses; (2) transmitted, reﬂected light intensity of ca. 2 nA and rastered over a 10 μm area with a spot diameter images and cathodoluminescence (CL) images of the zircon were of 20 μm. Oxygen isotopes were measured in multi-collector mode obtained. Cathodoluminescence images were used to check the internal using two off axis Faraday cups. The internal precision of single analyses structures of individual zircon grains and to select positions for analyses was generally better than 0.2 ‰ for 18O/16O ratio. The instrumental (Fig. 6a and b). mass fractionation factor (IMF) was corrected using 91,500 zircon L. Zhang et al. / Gondwana Research 41 (2017) 9–28 13

18 standard with (δ O)VSMOW = 9.9‰ (Wiedenbeck et al., 2004). Mea- Dan et al., 2012, 2016; Wang et al., 2015). Zircon oxygen isotopic data sured 18O/16O was normalized by using VSMOW (Vienna Standard are listed in Table 2. Mean Ocean Water) compositions, and then corrected for the instru- Lu–Hf isotopes were measured using laser-ablation multi-collector mental mass fractionation factor. The working conditions and analytical inductively coupled plasma mass spectrometry (LA–ICP–MS) at the procedures have been described in details (Li et al., 2009; Su et al., 2011; IGGCAS. Lu–Hf isotopic analyses were obtained on the same zircon grains

Table 2 Zircon Hf–O isotope data of the Yangying potassic volcanic rocks.

176 177 176 177 176 177 C 18 Sample t (Ma) Yb/ Hf Lu/ Hf Hf/ Hf ± 2σ ƐHf(t) ƐHf(0) TDM(Ma) TDM(Ma) fSample δ O(‰) YY-08-01 10.0 0.005357 0.000203 0.282705 0.000015 −2.59 −2.81 758 1231 −0.99 6.47 YY-08-02 62.9 0.040641 0.001667 0.282900 0.000016 5.41 4.08 507 761 −0.95 6.35 YY-08-03 10.6 0.007239 0.000272 0.282730 0.000017 −1.72 −1.95 726 1176 −0.99 6.13 YY-08-04 10.9 0.005083 0.000194 0.282699 0.000016 −2.79 −3.03 767 1245 −0.99 6.19 YY-08-05 10.9 0.007185 0.000263 0.282729 0.000022 −1.76 −1.99 727 1179 −0.99 6.69 YY-08-06 63.5 0.072463 0.002993 0.282896 0.000026 5.21 3.92 533 774 −0.91 5.65 YY-08-07 10.3 6.86 YY-08-08 10.4 0.009920 0.000394 0.282681 0.000014 −3.45 −3.68 796 1287 −0.99 7.16 YY-08-09 10.5 0.014747 0.000526 0.282711 0.000020 −2.39 −2.62 757 1219 −0.98 6.75 YY-08-10 10.3 6.67 YY-08-11 10.3 0.028974 0.001027 0.282681 0.000018 −3.45 −3.67 809 1286 −0.97 6.51 YY-08-12 10.6 0.025624 0.000929 0.282704 0.000017 −2.63 −2.85 774 1234 −0.97 6.63 YY-08-13 10.5 0.032422 0.001144 0.282695 0.000019 −2.95 −3.17 792 1254 −0.97 6.41 YY-08-14 59.2 0.048940 0.001882 0.282922 0.000021 6.07 4.83 480 715 −0.94 5.60 YY-08-15 10.9 6.21 YY-08-16 10.5 0.011925 0.000489 0.282698 0.000014 −2.85 −3.08 775 1248 −0.99 5.51 YY-08-17 10.9 0.008937 0.000325 0.282707 0.000015 −2.54 −2.78 759 1228 −0.99 6.61 YY-08-18 10.3 0.016233 0.000594 0.282702 0.000018 −2.73 −2.95 771 1240 −0.98 6.62 YY-08-19 65.4 5.66 YY-08-20 62.9 0.066980 0.002635 0.282871 0.000017 4.31 3.03 565 831 −0.92 5.62 YY-08-21 10.1 0.008673 0.000324 0.282754 0.000025 −0.88 −1.10 693 1122 −0.99 6.50 YY-08-22 9.9 0.013494 0.000490 0.282687 0.000018 −3.27 −3.48 790 1274 −0.99 6.54 YY-08-23 10.9 0.019257 0.000676 0.282647 0.000015 −4.64 −4.87 849 1362 −0.98 6.66 YY-08-24 11.0 0.006092 0.000226 0.282686 0.000018 −3.26 −3.51 786 1275 −0.99 7.22 YY-08-25 67.0 0.040666 0.001570 0.282974 0.000304 8.11 6.69 399 590 −0.95 5.82 YY-08-26 10.2 0.011021 0.000405 0.282752 0.000021 −0.94 −1.16 697 1126 −0.99 6.32 YY-08-27 10.8 0.015357 0.000569 0.282735 0.000019 −1.52 −1.76 724 1163 −0.98 6.19 YY-08-28 10.2 0.013133 0.000483 0.282704 0.000017 −2.65 −2.87 766 1235 −0.99 6.27 YY-08-29 10.5 0.009313 0.000351 0.282702 0.000015 −2.71 −2.94 766 1239 −0.99 6.18 YY-12-01 10.9 0.017196 0.000618 0.282694 0.000019 −3.00 −3.23 783 1258 −0.98 6.02 YY-12-02 10.8 0.004635 0.000177 0.282710 0.000016 −2.42 −2.65 751 1221 −0.99 6.13 YY-12-03 61.1 0.076558 0.003048 0.282944 0.000018 6.86 5.62 462 666 −0.91 5.71 YY-12-04 10.6 0.049378 0.001679 0.282704 0.000017 −2.65 −2.87 791 1235 −0.95 5.96 YY-12-05 10.5 6.44 YY-12-06 37.5 0.059471 0.002473 0.282951 0.000016 6.65 5.87 444 661 −0.93 5.53 YY-12-07 63.2 0.026749 0.001078 0.282850 0.000014 3.65 2.29 571 873 −0.97 6.27 YY-12-08 10.5 0.008267 0.000305 0.282692 0.000015 −3.05 −3.28 779 1261 −0.99 6.59 YY-12-09 63.3 0.035398 0.001460 0.282905 0.000013 5.59 4.24 498 749 −0.96 5.76 YY-12-10 9.6 0.016490 0.000595 0.282698 0.000018 −2.85 −3.06 776 1248 −0.98 6.53 YY-12-11 10.7 0.051714 0.001778 0.282680 0.000017 −3.47 −3.70 827 1288 −0.95 6.34 YY-12-12 10.5 0.037633 0.001318 0.282774 0.000022 −0.17 −0.39 683 1077 −0.96 6.60 YY-12-13 10.7 0.019032 0.000681 0.282673 0.000022 −3.73 −3.96 813 1304 −0.98 6.15 YY-12-14 58.7 0.034657 0.001429 0.282933 0.000015 6.47 5.23 458 689 −0.96 5.77 YY-12-15 10.2 0.009032 0.000328 0.282700 0.000022 −2.77 −2.99 768 1243 −0.99 6.41 YY-12-16 10.2 0.011190 0.000417 0.282643 0.000016 −4.79 −5.01 849 1371 −0.99 6.46

176 177 176 177 4 Initial Lu/ Hf and Hf/ Hf ratios and εHf(t) (the parts in 10 deviation of initial Hf isotope ratios between the zircon sample and the chondritic reservoir) values were calculated with the reference to the chondritic uniform reservoir (CHUR) at the time of zircon growth from magmas. The 176Lu decay constant was 1.867 × 10−11 year−1 (Söderlund et al., 2004). 176 177 176 177 176 177 The ratios of ( Lu/ Hf)CHUR = 0.0336 and ( Hf/ Hf)CHUR = 0.282785 (Bouvier et al., 2008), ( Lu/ Hf)DM =0.0384(Grifﬁn et al., 2000) were also shown. And then the depleted 176 177 mantle model ages (TDM) were calculated with reference to depleted mantle at a present-day Hf/ Hf ratio of 0.28325 similar to that of the average MORB (Nowell et al., 1998). The C hafnium isotopic “crustal” model ages (TDM ) were also calculated for each zircon grain by assuming its parental magma to have been derived from an average continental crust (MC), with 176Lu/177Hf = 0.015, that originated from the depleted mantle source (Grifﬁnetal.,2002). The calculation formulas used in Table 2 are presented below. 23 2 3 . . . 176 176 176 λ 6 Hf HfÞ 7 6 Hf HfÞ − Lu HfÞ e t−1 7 6 177 7 6 177 177 7 ε ðÞ¼6 Sample − 7 4; ε ðÞ¼6 Sample Sample − 7 4; Hf 0 4 . 15 10 Hf t 4 . 15 10 176 176 176 λt Hf HfÞ Hf HfÞ − Lu 177 HfÞ ðÞe −1 177 177 CHUR 8 CHUR. . 9 CHUR > 176 176 > <> Hf HfÞ − Lu HfÞ => 177 177 f −f ¼ 1 þ Sample DM ; C ¼ −ðÞ− MC Sample ; TDM λ ln> 1 . > TDM TDM TDM t − > 176 176 > f MC f DM : Hf HfÞ − Lu 177 HfÞ ; 177 DM . Sample. . 176 Lu HfÞ 176 Lu HfÞ 176 Lu HfÞ 177 177 177 f ¼ Sample −1; f ¼ MC −1; f ¼ DM −1 Sample 176 MC 176 DM 176 Lu 177 HfÞ Lu 177 HfÞ Lu 177 HfÞ CHUR CHUR CHUR t, crystallization time of zircon; 206Pb/238U ages were used for zircons younger than 1000 Ma. 14 L. Zhang et al. / Gondwana Research 41 (2017) 9–28 that were previously analyzed for U–Pb and O isotopes, with ablation pits of 40–80 μm in diameter, ablation time of 26 s and repetition rate of 8 Hz. The detailed analytical procedures are described in Li et al. (2010) and Wang et al. (2015). Zircon Hf isotopic data are presented in Table 2.

4.4. Mineral compositional analyses

The compositions of phenocryst minerals of the YPVR were determined using a JEOL JXA-8100 electron microprobe at the IGGCAS. Analytical conditions include an accelerating voltage of 15 kV, beam current of 2 × 10−8 A, a spot diameter of 5 μm for clinopyroxene, phlogopite and feldspar. The analytical procedures are described in details (Cheng and Kusky, 2007; Tam et al., 2011; H. Xu et al., 2015). The bulk compositions of phenocryst minerals are presented in Tables S4, S5 and S6 in the Supplementary data.

5. Results

5.1. Whole-rock major, trace elements and Sr–Nd–Pb isotopes

Samples of the YPVR are all plotted in the field of trachyte (Fig. 3a), which have relatively high SiO2 (61.34–68.62 wt.%), K2O(5.46– 9.30 wt.%), Al2O3 (15.06–17.36 wt.%), low MgO (0.47–2.80 wt.%) and FeOt (1.70–4.90 wt.%) contents (Table S1 in the Supplementary data). These trachytes are potassic (K2O/Na2O N 1) according to the criteria proposed by Nelson (1992) and mainly lie in the shoshonitic series (Fig. 3b) with the exception of sample YY-09 which might be altered in the hydrothermal field (Li et al., 1992). MgO, FeOt, CaO decrease with increasing SiO2 in Harker diagrams (not shown), indicating fractional crystallization of clinopyroxene and plagioclase. Negative correlation between SiO2 and TiO2 (or P2O5) might be interpreted as the fractional crystallization of Fe–Ti oxides and apatite. Al2O3 firstly increases and then decreases with increasing SiO2, implying removal of feldspar from magma during low-pressure fractional crystallization. The chondrite-normalized rare earth elements (REE) patterns of the YPVR are characterized by enrichment of LREE and distinctive negative Eu anomalies (δEu = 0.55–0.68; Table S1 in the Supplementary data and Fig. 4a), which can be explained by fractional crystallization of plagioclase. Primitive mantle-normalized incompatible trace elements diagrams of the studied rocks show positive anomalies for large ion lithosphile element (LILE, e.g., K, Ba and Rb; Fig. 4b) and significantly negative anomalies for high field strength elements (HFSE, e.g., Nb, Ta and Ti; Fig. 4b). The studied YPVR have relatively radiogenic initial Sr isotopic com- 87 86 position [( Sr/ Sr)i =(0.711978–0.712090)] and unradiogenic initial Fig. 3. (a) K2O+Na2O (wt.%) vs. SiO2 (wt.%) for the YPVR and other K-rich volcanic rocks 143 144 of the Lhasa terrane. All data plotted have been recalculated to 100 wt.% on a volatile-free Nd isotopic composition [( Nd/ Nd)i = (0.512121–0.512148)] fi (Table S2 in the Supplementary data), which differs from Sr–Nd isotopic basis. Classi cation boundaries are from Le Bas et al. (1986) and Le Maitre et al. (1989). Filled and open symbols represent, respectively, data from this study and the published compositions of the Gangdese batholith, adakites and Linzizong volcanic literatures of Miller et al. (1999); Williams et al. (2001, 2004); Ding et al. (2003, 2006); rocks (Fig. 5a). The YPVR are characterized by fairly radiogenic Pb isotopic Jiang (2003); Jiang et al. (2003); Gao et al. (2007a, 2009); Sun et al. (2007); Wang et al. 206 204 207 204 signatures [( Pb/ Pb)i = (18.615–18.774), ( Pb/ Pb)i = (15.708– (2008, 2014); Zhao et al. (2009); Chen et al. (2012); J.L. Chen et al. (2010); Tian et al. 208 204 (2012); Guo et al. (2013); D. Liu et al. (2011, 2014) and reference therein. Rock types 15.793), ( Pb/ Pb)i = (39.274–39.355)](Table S2 in the Supple- shown by letters are as follows: S2, basaltic trachyandesite; S3, trachyandesite; T, mentary data), which are plotted above the North Hemisphere trachyte; R, rhyolite; U3, tephriphonolite; Ph, phonolite; O1, basaltic andesite; O2, – Reference Line (NHRL; Hart, 1984) as shown in Fig. 5b e, suggesting andesite; O3, dacite. (b) K2O (wt.%) vs. SiO2 (wt.%) diagram for same samples as plotted enriched source for the Yangying potassic magma. in (a). The dividing lines show the classification boundaries from Rickwood (1989).

5.2. Zircon U–Pb geochronology represents zircon xenocrysts or inherited core with lower Th/U ratios As shown in cathodoluminescence (CL) images (Fig. 6a and b), (0.23–0.83) that were captured during evolution of the Yangying zircons for U–Pb dating from samples YY-08 and YY-12 are mostly potassic magma, suggesting potential crustal materials involved in the euhedral and display long to short prismatic shapes with crystal lengths Yangying magma generation. In situ zircon U–Pb age of the Group 1 of about 50–250 μm, which could be divided into two groups: (1) Group 1 zircons could be interpreted as crystallization age of the YPVR (Wu is composed of magmatic zircons exhibiting weak oscillatory zoning and et al., 2007; Chiu et al., 2009). uniform internal texture (Corfu et al., 2003; Hoskin and Schaltegger, Magmatic zircons from sample YY-08 are coeval and yield a lower 2003). Most of these zircons have Th/U ratios higher than 1 (1.1–4.3; intercept age of 10.62 ± 0.10 Ma (MSWD = 0.88) and a weighted Table S3 in the Supplementary data), in consistent with those of mag- mean 206Pb/238U age of 10.61 ± 0.10 Ma (MSWD = 0.88) in the Tera- matic zircons (N0.5; Hoskin and Schaltegger, 2003); and (2) Group 2 Wasserburg U–Pb concordia diagram (Fig. 6c). Five zircon grains with L. Zhang et al. / Gondwana Research 41 (2017) 9–28 15

slightly higher than those of mantle zircons (δ18O = 5.3 ± 0.3‰; Valley, 2003).

5.4. Mineral chemistry

Clinopyroxene is ubiquitous and the most abundant phase occurred both as phenocryst and groundmass in the YPVR (Fig. 2a), and plotted in the ﬁelds of augite and endiopside (Fig. 7a). These clinopyroxene phenocrysts are characterized by relatively low AlVI/AlIV (0.5–4), Ti/Al (0.091–0.25) ratios, and low Al (b0.2 apfu) and Na (b0.05 apfu) contents (Table S6 in the Supplementary data), suggesting the crystallization of the clinopyroxene occurring at low pressure situation (b10 × 103 bar; Dobosi and Fodor, 1992; Haase et al., 1996; Seyler and Bonatti, 1994; McCarthy and PatiÑO Douce, 1998). Meanwhile, the AlVI/AlIV ratios have relatively broad ranges, suggesting the AlVI of clinopyroxene crystallizing much deeper than that of AlVI beneath the Lhasa terrane crust and the changing depth of clinopyroxene crystallization. Alkali feldspar and plagioclase are the most common minerals in the

YPVR (Fig. 2). The plagioclases (An30–39) are plotted within the ﬁelds of andesine and K-andesine in An–Ab–Or diagram (Fig. 7b), while all the

alkali feldspars have relatively uniform end-member values (Or38–54 Ab45–59 An3–10)(Fig. 7b), including euhedral alkali feldspar phenocrysts (Fig. 2b and f), cores and rims of overgrowth alkali feldspars (Fig. 2a, c–e). Phlogopite phenocrysts are very common in the South Tibetan potassic and ultrapotassic magmatic rocks. All analyzed phlogopites in the YPVR are plotted within the ﬁeld of ultrapotassic magmatic rocks in the Lhasa terrane (Fig. 7c). Moreover, the analyzed phlogopite

phenocrysts are characterized by high-Ti (TiO2 = 5.55–7.93 wt.%), low MgO (15.46–18.60 wt.%) and low K2O(7.52–8.44 wt.%) (Table S4 in the Supplementary data), which resemble cores of phlogopite in ultrapotassic rocks in the Lhasa terrane (Miller et al., 1999; Gao et al., 2007b), suggesting a genetic relation between the YPVR and ultrapotassic magmas in South Tibet. Fig. 4. (a) Chondrite-normalized rare earth element diagram of the YPVR. (b) Primitive mantle-normalized incompatible trace element patterns of the YPVR. Normalization factors are from Sun and McDonough (1989). Data for the average composition of 6. Discussion subduction channel mélange rocks are from Marschall and Schumacher (2012) and references therein. Data sources of the potassic and ultrapotassic rocks in the Lhasa 6.1. Origin of YPVR and the relationships between the ultrapotassic and terrane are as in Fig. 3. potassic magmas in the Lhasa terrane

U–Pb ages ranging from 63 to 67.4 Ma are interpreted as xenocrystic The YPVR have different geochemical characteristics (i.e., whole- crystals which represent contaminants added to the Yangying magmas. rock major elements, trace elements and Sr–Nd–Pb isotope ratios) Eleven analyses of magmatic zircons from sample YY-12 yields a lower from those of Miocene mantle-derived ultrapotassic magmatic rocks intercept age of 10.72 ± 0.14 Ma (MSWD = 1.3) and a weighted mean in the Lhasa terrane (e.g., Miller et al., 1999; Zhao et al., 2009; Guo 206Pb/238U age of 10.70 ± 0.18 Ma (MSWD = 1.3) (Fig. 6d), in consis- et al., 2013, 2015; D. Liu et al., 2014; Liu et al., 2015) and of Miocene tent with crystallization age of sample YY-08. U–Pb ages of zircon adakitic rocks derived by partial melting of thickened lower crust in xenocrysts range from 58.7 to 67 Ma (Table S3 in the Supplementary South Tibet (e.g., Chung et al., 2003, 2009; Gao et al., 2007a; Guo et al., data). In situ zircon U–Pb ages for the YPVR in this study are consistent 2007; Chen et al., 2011; Li et al., 2011; Hou et al., 2013)(Figs. 3–5). with previously published age data using K–Ar and Ar–Ar dating The YPVR are mainly characterized by high SiO2 (61.34–64.48 wt.%), methods (10.83 ± 0.27 Ma, 10.84 ± 0.17 Ma; Li et al., 1992; Zhou low MgO (0.84–2.80 wt.%), FeOt (3.70–4.91 wt.%) and low contents of et al., 2010). Furthermore, potassic magmatism in the Yangying volcanic Cr (53.1–79.2 ppm) and Ni (27.2–36.4 ppm) and other compatible field is coeval with the late stage of the ultrapotassic magmatism in the elements (Table S1 in the Supplementary data), implying that they western Lhasa terrane (25–8 Ma; e.g., Miller et al., 1999; Williams et al., might result from either partial melting of the mafic lower crust or 2001, 2004; Ding et al., 2003; Zhao et al., 2009; J.L. Chen et al., 2010; assimilation and fractional crystallization (AFC) of mantle-derived Chen et al., 2012; Guo et al., 2013). primitive melts. The negative Eu anomalies (δEu = 0.55–0.68) in Chondrite- 5.3. Zircon Hf–Oisotopes normalized REE patterns of the YPVR (Fig. 4a) indicate fractional crystallization of plagioclase, which differs from lower crust-derived In situ zircon Hf–Oisotopicdatawereanalyzedfor45zircon adakitic rocks with positive or no Eu anomalies in the Lhasa terrane grains (samples YY-08 and YY-12) that have been previously dated by (e.g., Guo et al., 2007; Chen et al., 2011). Furthermore, the YPVR are 176 177 87 86 207 204 208 SIMS. Magmatic zircons exhibit relatively uniform Hf/ Hf ratios characterized by relatively higher ( Sr/ Sr)i,( Pb/ Pb)i,( Pb/ 204 206 204 143 144 (0.282647–0.282774) and negative ƐHf(t) values (−4.79 to −0.17). In Pb)i,( Pb/ Pb)i and lower ( Nd/ Nd)i relative to those of contrast, xenocrystic zircons have relatively variable 176Hf/177Hf ratios adakitic rocks in the Lhasa terrane (Fig. 5), indicating different magma 18 (0.282850–0.282996) and positive ƐHf(t) values (3.65–8.11). The δ O sources for potassic and adakitic rocks which questions the hypothesis values of the magmatic zircons range from 5.51‰ to 7.22‰ and that potassic magma of the Lhasa terrane originated from the mafic xenocrystic zircons have similar δ18O value (5.62–6.57‰), which are lower crust (e.g., J.L. Chen et al., 2010; D. Liu et al., 2014). 16 L. Zhang et al. / Gondwana Research 41 (2017) 9–28

143 144 87 86 87 86 206 204 143 144 206 204 207 204 206 204 208 204 206 204 Fig. 5. (a) ( Nd/ Nd)i vs. ( Sr/ Sr)i.(b)( Sr/ Sr)i vs. ( Pb/ Pb)i.(c)( Nd/ Nd)i vs. ( Pb/ Pb)i.(d)( Pb/ Pb)i vs. ( Pb/ Pb)i.(e)( Pb/ Pb)i vs. ( Pb/ Pb)i. Lower continental crust (LCC) and upper continental crust (UCC) are from Zartman and Haines (1988). Data for the average composition of subduction channel mélange rocks are calculated from Bulle et al. (2010). The GLOSS (Global Subducting Sediment; Plank and Langmuir, 1998), NHRL (Northern Hemisphere Reference Line; Hart, 1984), EMI and EMII (enriched mantle end-members; Zindler and Hart, 1986; Hofmann, 1997; Zou et al., 2000), and MORB and OIB ﬁelds (Wilson, 1989; Hofmann, 1997) are shown for comparison. The potassic and ultrapotassic rocks from North Tibet are from Turner et al. (1993, 1996), Deng (1998); Liu (1999); Ding et al. (2003, 2007), Guo et al. (2006) and references therein. Data for the Higher Himalayan Crystalline Sequence (HHCS) are from Allègre and Ben Othman (1980); Vidal et al. (1982, 1984); Deniel et al. (1987); Inger and Harris (1993); Parrish and Hodges (1996); Harrison et al. (1999); Whittington et al. (1999); Ahmad et al. (2000); Robinson et al. (2001); Richards et al. (2005); Guo and Wilson (2012) and references therein. Compositions of the Gangdese batholith in the Lhasa terrane are from Jiang et al. (1999); Dong et al. (2008);andZ.D. Zhao et al. (2011). Linzizong volcanic rocks in Lhasa terrane are from Dong (2002) and Zhang (1996) and Lee et al. (2007, 2009, 2012). Adakitic rocks in Lhasa terrane are from Guo et al. (2007); Chen et al. (2011); Hou et al. (2004, 2013) and references therein. Lower crust in South Tibet is from Miller et al. (1999). Data sources of the potassic and ultrapotassic rocks in the Lhasa terrane are as in Fig. 3.

Compared with ultrapotassic magmatic rocks with MgO ≥ 6wt.% contaminants. The YPVR have lower contents of LILE (e.g., Rb, Ba, Sr) from the Lhasa terrane, the bulk potassic magmatic rocks (including and LREE than those of the ultrapotassic rocks in the Lhasa terrane 87 86 143 144 the YPVR) display slightly lower ( Sr/ Sr)i and higher ( Nd/ Nd)i (Fig. 4), indicating that only fractional crystallization of the ultrapotassic ratios, and moreover the potassic rocks plot in linear trends between magmas could not account for geochemical characteristics of the YPVR mantle-derived maﬁc ultrapotassic magmas and relatively depleted and crustal contaminant must be involved. Additionally, the maﬁc crustal contaminants from the Lhasa terrane (Fig. 5), suggesting close ultrapotassic rocks (MgO ≥ 6 wt.%) in the Lhasa terrane have extremely 87 86 206 204 links with ultrapotassic magmas and the relatively depleted crustal high ( Sr/ Sr)i (0.7117–0.7393), ( Pb/ Pb)i (18.284–18.965), L. Zhang et al. / Gondwana Research 41 (2017) 9–28 17

207 204 208 204 ( Pb/ Pb)i (15.660–15.839) and ( Pb/ Pb)i (39.092–40.025) (Fig. 8a), which might be interpreted as assimilation of the ultrapotassic 143 144 ratios and low ( Nd/ Nd)i (0.5117–0.5212) ratios (e.g., Miller magma by crustal materials during fractional crystallization. 87 86 et al., 1999; Williams et al., 2001, 2004; Liao et al., 2002; Jiang, 2003; Albeit with broader ranges of ( Sr/ Sr)i and SiO2 for ultrapotassic Gao et al., 2007b; Zhao et al., 2009; Ding et al., 2006; Chen et al., 2012; and potassic magmatic rocks (Fig. 8b), their correlations might be Tian et al., 2012; Guo et al., 2013), further enriched relative to the explained by AFC processes of ultrapotassic magma involving variable whole-rock continental crust (Fig. 5; Zartman and Haines, 1988). crustal contaminants (from an enriched one to a depleted one) Hence, ultrapotassic and potassic magmatic rocks including the YPVR (DePaolo, 1981; Handley et al., 2010), giving rise to potassic magma 143 144 87 86 display a positive correlation between ( Nd/ Nd)i and SiO2 with higher SiO2 (51.41–76.30 wt.%) and broad ( Sr/ Sr)i (0.710749–

Fig. 6. Cathodoluminescence (CL) images of zircons of the YPVR samples YY08 (a) and YY12 (b). Cathodoluminescence (CL) images are taken for inspecting internal structures of individual zircons. Solid circles and dashed ellipses indicate the locations of in situ U–Pb dating (yellow–dashed ellipses) and Hf–O isotope analyses (green—solid circles), respectively. The U–Pb ages (yellow number), Hf isotopic values (green number) and O isotopic values (white number) and the grain numbers for measured zircon (blue number) are also shown for each analyses spot. Tera-Wasserburg diagrams (lower intercept age) and weighted-mean age diagrams (206Pb/238U age) of zircons from the YPVR are shown for samples YY08 (c) and YY12 (d). 18 (e) Zircon δ Ovs.ƐHf(t) for the YPVR. (f) Zircon ƐHf(t) vs. U–Pb age for the YPVR, Gangdese batholith, adakitic rocks and Linzizong volcanic rocks from the Lhasa terrane. The Zircon Hf isotope and U–Pb ages of Gangdese batholith, adakitic rocks and Linzizong volcanic rocks from Lhasa terrane are from Lee et al. (2007, 2009, 2012); Wen et al. (2008); Ji et al. (2009b, 2012) and Hou et al. (2013). 18 L. Zhang et al. / Gondwana Research 41 (2017) 9–28

Fig. 6 (continued).

0.737638). Therefore, we suggest that the YPVR might result from assim- geodynamic modeling for subduction kinetics (Lee and Lawver, 1995; ilation and fractional crystallization of the mantle-derived ultrapotassic Mahéo et al., 2002; Husson et al., 2012). Partial melting of previously 87 86 magma involving depleted crustal contaminant with lower ( Sr/ Sr)i enriched mantle wedge induced by decompression and hot astheno- 143 144 and higher ( Nd/ Nd)i in the Lhasa terrane. The processes responsi- spheric corner ﬂow during rollback of northward subducted Indian ble for evolution of the Yangying potassic magmas will be discussed in slab would gave rise to formation of the post-collisional K-rich magmas detail below based on energy-constrained assimilation and fractional (Guo et al., 2013). However, the nature of enriched component added to crystallization (EC–AFC) model. mantle source of the K-rich magmas remains a matter of heated debate (Miller et al., 1999; Gao et al., 2007b; Guo et al., 2013). 6.2. Petrogenesis of the ultrapotassic magmatic rocks in the Lhasa terrane Previous studies (e.g., Gao et al., 2007b) have focused on the role of subducted sediments from the Neo-Tethyan oceanic lithosphere in 207 204 Convective thinning of the over-thickened Tibetan lithosphere enrichment of the mantle source. However, the low ( Pb/ Pb)i and 208 204 (e.g., Miller et al., 1999; Williams et al., 2001; Zhao et al., 2009; D. Liu ( Pb/ Pb)i ratios of the Neo-Tethyan oceanic subducted sediments 87 86 206 204 et al., 2014) has been proposed to explain the genesis of the post- cannot account for the extremely high ( Sr/ Sr)i, ( Pb/ Pb)i, 207 204 208 204 143 144 collisional K-rich magmatic rocks in South Tibet. However, this model ( Pb/ Pb)i and ( Pb/ Pb)i ratios and low ( Nd/ Nd)i ratios might be ineffective to account for E–W trending linear distribution of of the ultrapotassic rocks in South Tibet (Fig. 5; Zhao et al., 2009). ultrapotassic and potassic rocks in the Lhasa terrane and younging Whole-rock Sr–Nd–Pb isotopic compositions exhibit linear trends trend of the K-rich magmatism from north to south (Ding et al., 2003; between depleted MORB-source mantle (DMM) and Indian continental Nomade et al., 2004; Guo et al., 2013). Although break-off of the north- crust (Fig. 5). Enrichment of mantle source for the post-collisional ward subducted Indian continental lithosphere (e.g., Mahéo et al., 2002; K-rich magmas might have close afﬁnity with subducted Indian conti- Replumaz et al., 2010, 2013, 2014) can explain the E–W trending K-rich nental crust added to depleted MORB-source mantle during northward magmatic belt in the Lhasa terrane, it is inconsistent with the southward underthrusting of the Indian continental lithosphere beneath the Lhasa younging trend and systematic variations on proportions of metasomatic terrane (Mahéo et al., 2002). Using Sr–Nd–Pb–O isotopes and trace components added to the mantle source of the K-rich magmatic rocks elements ratios from Higher Himalayan Crystalline Sequence (HHCS), from Xuruco lake–Dangre Yongcuo lake rift in the central Lhasa terrane many previous studies (e.g., Ding et al., 2003; Zhao et al., 2006, 2009; (Guo et al., 2013). Considering the temporal, spatial and compositional Guo et al., 2013, 2015) have pointed out that the ultrapotassic lavas variations of the post-collisional K-rich magmas in the Lhasa terrane, can be well explained by underthrusting of the Himalayan continental rollback and breakoff of northward subducted Indian slab might materials into the mantle source beneath the southern Tibet. In Figs. 5 be reasonable geodynamic mechanism responsible for the post- and 10, the ultrapotassic samples lie along a clear linear trend between collisional K-rich volcanism (Guo et al., 2013, 2015), which is supported depleted MORB-source mantle (DMM; Workman and Hart, 2005)and by Cenozoic plate motion reconstruction of Indian and Asian plates and the Indian continental margin sediments (HHCS; e.g., Najman et al., L. Zhang et al. / Gondwana Research 41 (2017) 9–28 19

Fig. 7. Mineral compositions of major phenocrysts in the YPVR. (a) Pyroxenes plotted on the enstatite–ferrosilite–diopside–hedenbergite quadrilateral diagram after Morimoto (1988). (b) Feldspars plotted on ternary diagram after Smith (1974). (c) Micas plotted on the Al–Mg–Fe diagram after Sheppard and Taylor (1992). Data for phlogopite of the ultrapotassic rocks in the Lhasa terrane are from Miller et al. (1999); Zhao et al. (2009); Gao et al. (2007a) and C.Z. Liu et al. (2011). Data for the Sailipu mantle xenolith are from C.Z. Liu et al. (2011). Data for kamafugite of the West Qinling are from Yu (1994).

2010; Robinson et al., 2001; Harris et al., 2004; Zhao et al., 2009; Guo Workman and Hart (2005). In addition, modeling curves 1 and 2 in and Wilson, 2012; Guo et al., 2013 and references therein), implying Fig. 9b represent upper and lower limitation of the simulation curves the involvement of the Indian plate-derived materials due to northward between HHCS and DMM (Table 3). subduction of the Indian lithosphere beneath the Tibetan Plateau. Previous studies (e.g., Johnson and Plank, 1999; Woodhead et al., Therefore, we propose a two-component mixing model to account 2001) demonstrated that ratios of fluid/melt-mobile trace elements to for the enrichment of asthenospheric mantle source of the mafic fluid/melt-immobile trace elements can effectively reflect the impor- ultrapotassic magmas beneath the Lhasa terrane (Fig. 9b). As shown tance of subduction-induced fluids/melts in the mantle source of in Fig. 9b, Sr–Nd isotopic compositions of the mantle-derived subduction-related magmas, and particularly these elements ratios are ultrapotassic magmas could be explained by a simple two-component insignificantly fractionated during partial melting (Class et al., 2000). mixing between DMM and HHCS. The HHCS are represented by the The combination of incompatible element and Sr–Nd isotopic ratios metamorphic rocks in the Higher Himalaya (e.g., Najman et al., 2010; has been effectively used as a fingerprint in identifying metasomatic Robinson et al., 2001; Harris et al., 2004; Richards et al., 2005; Guo components in the mantle source of the subduction-related magmas and Wilson, 2012), while the composition of the DMM is taken from (e.g., Hawkesworth et al., 1997; Turner and Hawkesworth, 1997; Class

143 144 87 86 Fig. 8. (a) ( Nd/ Nd)i vs. SiO2 and (b) ( Sr/ Sr)i vs. SiO2. Data sources and the symbols of the potassic and ultrapotassic rocks in the Lhasa terrane are as in Fig. 3. 20 L. Zhang et al. / Gondwana Research 41 (2017) 9–28

et al., 2000; Guo et al., 2005, 2006, 2013, 2014a, 2015). Therefore, we use Ba/La, Ba/Th and Th/Nd ratios to constrain the involvement of fluids and melts in generation of the ultrapotassic magmas. High Ba/La, Ba/Th and Th/Nd ratios in the ultrapotassic rocks in the Lhasa terrane reflect presence of the slab-derived fluids and melts in the mantle source region (Fig. 10). Recently, subduction-derived mélange rocks, mixture of hydrous fluids and melts derived from subducted slab and mantle peridotite formed at the slab–mantle interface, have been introduced to illustrate the physico-chemical process responsible for recycling of crustal component to the mantle-source region of arc volcanic rocks (Marschall and Schumacher, 2012; Guo et al., 2014a). The ultrapotassic magmatic rocks in the Lhasa terrane and the mélange rocks both display enrichment in LILE, LREE and depletion in HFSE, e.g., positive Pb anomaly and negative Nb, Ta and Ti anomalies (Fig. 4), suggesting that the mé- lange rocks might be potential source rocks for the mantle-derived ultrapotassic magmatic rocks. This inference is in good agreement with comparable Nb/La and Ce/Pb ratios between ultrapotassic igneous rocks and mélange rocks (Fig. 11a).

Positively-correlated Ba/Th and (La/Sm)N ratios of the ultrapotassic magmatic rocks in the Lhasa terrane suggest involvement of subduction-related ﬂuids and melts in magma generation (Fig. 11b), which could be well explained by characteristics of the mélange rocks (Marschall and Schumacher, 2012; Guo et al., 2014a). Th/La and Sm/La ratios of ultrapotassic magmatic rocks in the Lhasa terrane show a positive correlation (Fig. 12a), suggesting that Th and Sm enrichments could be linked to the involvement of the subducted Indian mélanges which might contain zoisite/epidote and lawsonite (Guo et al., 2014a, 2015). Ultrapotassic magmatic rocks in the Lhasa terrane are characterized by lower Dy/Dy* than those of magmas derived from oceanic plate subduction and oceanic island basalts (Fig. 12b; Davidson et al., 2013), implying the addition of the Indian continental subducted sediments to the mantle source and the different evolution trend of magma at continental and oceanic seduction zones.

6.3. Shallow crustal magma chamber processes

Clinopyroxene-liquid thermobarometers are key tools in understanding processes of storage, cooling, and fractionation of magmas 87 86 143 144 87 86 (Putirka et al., 1996, 2003; Putirka, 2008). There are several steps that Fig. 9. (a) ( Sr/ Sr)i vs. Sr and (b) ( Nd/ Nd) vs. ( Sr/ Sr)i plots for the potassic and ultrapotassic rocks in the Lhasa terrane, showing two-component mixing line of Higher should be followed before calculating temperature and pressure using Himalayan Crystalline Sequence (HHCS) and depleted MORB-source Mantle (DMM) and the clinopyroxene-liquid thermobarometers. – the AFC and EC AFC processes. For two-component mixing trends with the F = 10% First, choose the suitable melts which are in equilibrium with increment. The AFC trends are displayed as green curves with 5% increment in F = M / m clinopyroxenes. The most commonly used melts are represented by com- M0 (Mm, present mass of magma; M0, initial mass of magma). EC–AFC trends (red curve) for equilibration temperatures (Teq) of 900 °C are shown. Abbreviations are as positions of whole rock, groundmass and glass (e.g., Shaw and Klügel, follows: A, crustal assimilant; P, primitive magma. 2002; Putirka and Condit, 2003; Putirka et al., 2003; Dahren et al., 2012). Considering that clinopyroxene crystallizes at the early stage during magma evolution, whole rock components of the YPVR are chosen to be liquid composition (Putirka et al., 1996, 2003; Putirka, 2008). Second, check whether the clinopyroxene grains are in equilibrium with the host melt. Clinopyroxenes of the YPVR (Fig. 2)arecommonly Table 3 euhedral, exhibiting no clear petrographic indications of disequilibrium 87 86 144 143 Sr and Nd concentration (ppm) and initial ( Sr/ Sr)i and ( Nd/ Nd)i of the High with their host melt (e.g., reaction rim texture, corrosion structure). Himalayan Crystalline Sequence (HHCS) and depleted MORB-source mantle (DMM). Based on major elements of the clinopyroxene analyses (Table S6 in HHCS the Supplementary data), we use the Fe–Mg exchange coefﬁcients End-member parameter DMM cpx-liq Min Max KD(Fe–Mg) to test for the clinopyroxene-melt equilibrium (Putirka et al., 2003), as is shown by the equation below: Sr (ppm) 2 253.5 7.66 87 86 ( Sr/ Sr)i 0.705442 1.198001 0.702626 liq cpx − Mg Fe Nd (ppm) 1.1 1847 0.58 K ðÞFe−Mg cpx liq ¼ ¼ 0:27 0:03 144 143 D cpx liq ( Nd/ Nd)i 0.511591 0.512571 0.513106 Mg Fe Sr and Nd concentration and their initial isotopic ratios of the HHCS are taken from Vidal et al. (1982), Deniel et al. (1987), France-Lanord et al. (1993), Inger and Harris (1993), Third, estimate H2O content of the magma before eruption. H2O Parrish and Hodges (1996), Whittington et al. (1999), Ahmad et al. (2000); Najman et al. content is a very inﬂuential parameter for the clinopyroxene- (2010); Robinson et al. (2001); Harris et al. (2004), Richards et al. (2005), Guo and Wilson liquid thermobarometers (e.g., Putirka, 2008; Dahren et al., 2012). (2012). Compositions of depleted MORB-source mantle wedge (DMM) are taken from Workman and Hart (2005). The HHCS and DMM are age-corrected to 10 Ma based on the Based on the thermal model proposed by Melekhova et al. (2013),the age of the Yangying potassic volcanic rocks. compositional distribution of derivative magma varies as a function L. Zhang et al. / Gondwana Research 41 (2017) 9–28 21

87 86 87 86 143 144 143 144 Fig. 10. (a) Ba/Th vs. Th/Nd. (b) Ba/La vs. Th/Nd. (c) ( Sr/ Sr)i vs. Ba/La. (d) ( Sr/ Sr)i vs. Th/Nd. (e) ( Nd/ Nd)i vs. Ba/La and (f) ( Nd/ Nd)i vs. Th/Nd. The ultrapotassic rocks (MgO ≥ 6%, ﬁlled circle) in the Lhasa terrane form linear arrays, suggesting binary mixing (brown dashed-line with double-sided arrows) between DMM and HHCS-derived components with higher Ba/La (ﬂuids) and Th/Nd (melts) ratios relative to the bulk HHCS (Fig. 10c–f). Data sources and symbols of the K-rich rocks are as in Figs. 3 and 5.

of water content of the magmatic system, which is related with the ﬂux data). Using the equation D (km) = 4.02 + 3.03 × P (kbar) proposed by and duration of magma input. In order to minimize the effect of H2O Ave Lallemant et al. (1980), we calculated storage depth of the magma content, we use a new equation of clinopyroxene-liquid chamber which ranges from 11.60 km to 16.44 km, consistent with thermobarometers proposed by Masotta et al. (2013) for alkaline the changing depth of the clinopyroxene crystallization due to broad VI IV differentiated magmas. The pre-eruptive H2O content we used in range of Al /Al ratios. Previous studies have demonstrated that pres- clinopyroxene-liquid thermobarometer is estimated at 3%. ence of a low-velocity anomaly zone with depth of ~15 km beneath Based on the above analyses, we calculated pressure (P) and the Yadong-Gulu rift which was interpreted as partially temperature (T) conditions under which clinopyroxenes crystallize in molten middle crust (Nelson et al., 1996; Brown et al., 1996; Wei the magma chamber beneath the Yangying volcanic ﬁeld in South et al., 2001). Our estimations are consistent with the depth of the Tibet. The results of the clinopyroxene-liquid thermobarometers Miocene Yangying magma chamber, implying that the low-velocity display a relatively concentrated pressure (P = 2.5–4.1 kbar) and tem- anomaly zone might be remnant of the Late Miocene potassic magmas perature (T = 1029.4–1082.9 °C) range (Table S6 in the Supplementary since eruption. 22 L. Zhang et al. / Gondwana Research 41 (2017) 9–28

Fig. 11. (a) Ce/Pb vs. Na/La plot for the potassic and ultrapotassic rocks in the Lhasa terrane. (b) Ba/Th vs. (La/Sm)N plot for the potassic and ultrapotassic rocks in the Lhasa terrane. Data for the average composition of subduction channel mélange rocks are from Marschall and Schumacher (2012) and references therein. The Global Subducting Sediment (GLOSS) data are from Plank and Langmuir (1998). Upper continental crust (UCC) data are from Zartman and Haines (1988).MORBandOIBdataarefromSun and McDonough (1989). Data sources and symbols of the K-rich rocks are as in Figs. 3 and 5.

6.4. Energy-constrained assimilation and fractional crystallization (EC– consistent with ƐHf(t) (3.65–8.11) and ages (67–59 Ma) of xenocrystic AFC) modeling zircons in the YPVR. Following these considerations, the adakitic rocks should be ruled out because their extrusive/intrusive ages (26–8 Ma; Crustal contamination and fractional crystallization are considered e.g., Hou et al., 2004, 2013; Guo et al., 2007) postdate the U–Pb ages as important processes contributing to magmatic differentiation of the (67–59 Ma) of these xenocrystic zircons (Fig. 6f). Hence, the Gangdese evolved magma (DePaolo, 1981; Foland et al., 1993; Douce, 1999; batholith and/or Linzizong volcanic rocks might be the most appropri- Spera and Bohrson, 2001, 2004; Barnes et al., 2005; Guo et al., 2006; ate crustal material mixed to mantle-derived ultrapotassic melts in Farahat et al., 2007). Based on the available geochemical data of the the middle crust, because (1) they have the proper Sr–Nd–Pb isotope mafic ultrapotassic (MgO ≥ 6 wt.%) and potassic rocks in the Lhasa ratios required for assimilation of the mantle-derived ultrapotassic terrane, we present EC–AFC model to quantitatively illustrate physical melts to generate YPVR magmas (Fig. 5; Zhang, 1996; Jiang et al., and chemical characteristics of the magma–country rock interactions 1999; Dong, 2002; Guo et al., 2007; Lee et al., 2007, 2009, 2012; Dong in YPVR, which could provide rigorous constraints on evolution of the et al., 2008; Z.D. Zhao et al., 2011); (2) they have consistent intrusive mantle-derived ultrapotassic magma in crustal magma chamber ages and zircon Hf isotopic ratios (Fig. 6f) with the studied xenocrystic beneath the Yangying volcanic field. Reasonable ranges of initial zircons from YPVR (Lee et al., 2007, 2009, 2012; Wen et al., 2008; Ji thermal and compositional parameters used in the EC–AFC model are et al., 2009b, 2012); (3) Guo et al. (2007) have indicated that the from Bohrson and Spera (2007) as shown in Table 4. Linzizong magmas underplate the lower crust of the Lhasa terrane, Potential crustal materials involved in petrogenesis of the YPVR and result in the lower crustal thickening. The middle crust of Lhasa 87 86 143 144 are characterized by low ( Sr/ Sr)i and high ( Nd/ Nd)i ratios terrane also might be affected by the underplating of the Linzizong (Figs. 5 and 8), similar to those of relatively depleted crustal materials magmas, suggesting the component of the Lhasa terrane middle crust from the Lhasa terrane, e.g., adakitic rocks, Linzizong volcanic rocks consistent with the Linzizong magmas. Furthermore, Mo et al. (2003) and Gangdese batholith (Fig. 5a). Moreover, the best-fit lithology for have provided evidence to demonstrate that the composition of the crustal contamination of the mafic ultrapotassic magmas must also be Linzizong magmas could be regarded as the component of middle-

Fig. 12. (a) Sm/La vs. Th/La plot for the YPVR and the potassic and ultrapotassic rocks in Lhasa terrane. Continental Crust (Rudnick and Gao, 2003) and Global Subducting Sediment (GLOSS; Plank and Langmuir, 1998) are reported as proxies of crustal recycled components. MORB and OIB data are from Sun and McDonough (1989). (b) Dy/Yb vs. Dy/Dy* plot for the YPVR and the potassic–ultrapotassic rocks in Lhasa terrane. Fields for MORB and OIB and continent crust are from Davidson et al. (2013). Primitive mantle (PM) is from Sun and McDonough (1989); depleted mantle (DM) is from Salters and Stracke (2004). GLOSS average are calculated from Plank and Langmuir (1998). Data sources and symbols of the K-rich rocks are as in Fig. 3. L. Zhang et al. / Gondwana Research 41 (2017) 9–28 23

Table 4 melts from the Gangdese batholith and/or the Linzizong volcanic rocks Assumed parameters and best-ﬁt calculation results of the EC-AFC model calculations. with depleted Sr–Nd isotopic compositions) of mantle-derived Yangying potassic ultrapotassic magmas, which gains support from petrographic evidence Modeling processes volcanic ﬁelds (Fig. 2). For example, YPVR shows that plagioclase appears to have expe- Thermal parameters rienced stages of growth assimilation and overgrowth (Fig. 2), implying

Magma liquidus temperature (Tl,q, °C) 1200 that the Yangying magmas underwent complicated EC–AFC processes. In Magma initial temperature (Tm°, °C) 1200 addition, the AFC calculation proposed by DePaolo (1981) was also per- Assimilant liquidus temperature (Tl,a, °C) 1000 formed, and the results (Fig. 9b) show that the YPVR have undergone a – Assimilant initial temperature (Ta°, °C) 230 300 Ṁ Ṁ Ṁ Solidus temperature (T , °C) 800 very high rate of assimilation (r = a/ c =0.6; a, the mass of assim- s Ṁ Equilibration temperature (Teq, °C) 900 ilation; c, the mass of fractional crystallization) which is nearly consis- ﬁ ⁎ ⁎ Isobaric speci c heat of magma (Cp,m, J/kg per K) 1200 tent with the results displayed by the EC–AFC (Ma/Mc = 0.675; Ma,the Isobaric speciﬁc heat of Assimilant (C , J/kg per K) 900 p,a amount of assimilant partial melt; Mc, the mass of cumulate). Crystallization enthalpy (Δhcry, J/kg) 396,000 The thermal and compositional parameters are generally used to Fusion enthalpy (Δhcry, J/kg) 354,000 explain the site at which EC–AFC processes occurred (Guo et al., Compositional parameters 2006). Chan et al. (2009) proposed a geothermal gradient (16 °C/km) Magma initial Sr content (C °, ppm) 985.8 sr,m for crust of the Lhasa terrane based on the xenolith temperature Magma initial Nd content (CNd,m°, ppm) 191.2 estimations. Therefore, the depth at which the EC–AFC process occurred Magma Sr isotope ratio (ɛsr,m) 0.720952

Magma Nd isotope ratio (ɛNd,m) 0.511982 for the YPVR ranges from 14.40 to 18.75 km, similar to depth of the ﬁ Magma bulk distribution coef cient for Sr (Dsr,m) 0.8 crustal magma chamber provided by the clinopyroxene-liquid thermo- ﬁ Magma bulk distribution coef cient for Nd (DNd,m) 1.45 barometers (Fig. 13). Based on the depth of crustal magma chamber and Assimilant initial Sr content (C °, ppm) 551.32 sr,a crustal thicknesses in the Lhasa terrane (Zhang and Klemperer, 2005; Assimilant initial Nd content (CNd,a°, ppm) 15.57

Assimilant Sr isotope ratio (ɛsr,a) 0.704056 Searle et al., 2011), we suggest that the assimilation and fractional crys- ɛ Assimilant Nd isotope ratio ( Nd,a) 0.512849 tallization process took place in the middle crust of the Lhasa terrane fi Assimilant bulk distribution coef cient for Sr (Dsr,a) 0.1 (11.60–18.75 km). Assimilant bulk distribution coefficient for Nd (DNd,a) 0.2 The thermal parameters are taken from Bohrson and Spera (2007). The compositional 6.5. Petrogenetic model of the Yangying potassic volcanic rocks parameters of primitive magmas are from Guo et al. (2013). The assimilants are the Gangdese batholith with average compositions reported by Dong et al. (2008). Based on the above discussions, we present a two-stage petrogenetic model for the Yangying potassic volcanic rocks (Fig. 13). The first stage of the model involves formation of the mafic ultrapotassic magmas. The upper crust of the Lhasa terrane. Thus, the component of the Linzizong subducted Indian continent crustal materials and mantle wedge were magmas can be used as that of the middle-upper crust beneath the mechanically mixed in subduction channel forming the Indian mé- Lhasa terrane; and (4) the whole-rock Sr–Nd–Pb isotopic compositions langes at the slab-mantle interface as a consequence of the northward exhibit linear trends between enriched mantle-derived mafic subduction of Indian continental lithosphere followed the Asia–India ultrapotassic magmas and relatively depleted crustal contaminants collision during 55–25 Ma (Guo et al., 2013, 2014a, 2015). Then, the (e.g., the Gangdese batholith and/or Linzizong volcanic rocks) in the enriched Indian mélanges rose buoyantly from the surface of the Lhasa terrane. Indian subducting slab and underplated beneath the base of the Compositions of the crustal assimilant in the Lhasa terrane are listed Tibetan lithosphere (Marschall and Schumacher, 2012; Guo et al., in Table 4. As indicated by the calculated results, the liquidus tempera- 2014a, 2015). India–Asia continent convergence rate decreased during ture, solidus temperature and initial temperature of the crustal the period of 25–8Ma(Lee and Lawver, 1995; Chung et al., 2005; Guo assimilant (i.e., the Gangdese batholith and/or the Linzizong volcanic et al., 2013), leading to rollback of the subducted Indian slab rocks) are 1000 °C, 800 °C and 230–300 °C, respectively. In addition, (Fig. 13a), and more importantly, decompression partial melting of the liquidus temperature, initial temperature of the primitive magma the Indian mélanges and the generation of the post-collisional and the equilibration temperature between magma and the country ultrapotassic maficmagmas(Fig. 13b). Moreover, age of E–W extension rocks are 1200 °C, 1200 °C and 900 °C, respectively. In consideration of of the Yadong-Gulu rift in South Tibet has been constrained to be ~12– the ultrapotassic magmatic rocks have not been discovered in Yangying 8Ma(Harrison et al., 1995; Edwards and Russell, 1998; Wu et al., 1998), volcanic field, we select some mafic ultrapotassic rocks with the highest which is in accordance with the ages of the slab rollback (25–8 Ma), im- MgO, Cr and Ni content in the western Lhasa terrane as the primitive plying that lithospheric extensional tectonics beneath the Lhasa terrane magmas for the EC–AFC model simulation (Table 4). As shown in might be closely related to slab rollback process. These can also explain Fig. 9, the YPVR can be explained by an EC–AFC model with a best-fit the spatial correlation between the post-collisional potassic– primitive K-rich magma with Sr = 985.8 ppm, Nd = 191.2 ppm, ultrapotassic magmatism and the NS trending rifts in South Tibet (Guo 87 86 143 144 ( Sr/ Sr)i = 0.720952, and ( Nd/ Nd)i =0.511982. et al., 2013, 2015). Before the country rocks were heated up to solidus temperature The second stage of the model involves the AFC processes of (800 °C), Sr content of the mantle-derived ultrapotassic magmas firstly the mantle-derived ultrapotassic magmas in middle crust (11.60– 87 86 increased whereas ( Sr/ Sr)i remained constant as indicated by a flat 18.75 km) beneath the Lhasa terrane (Fig. 13c). During this stage, trajectory in Fig. 9a, suggesting that fractional crystallization mantle-derived ultrapotassic mafic magmas were contaminated by (e.g., clinopyroxene and phlogopite) dominated this stage before addi- the Gangdese batholith and/or Linzizong volcanic rocks with depleted tion of melts from country rocks into the mantle-derived ultrapotassic Sr–Nd isotopic compositions (Fig. 9), accompanied by fractional crystal- magmas. When the temperatures reached solidus of the country rocks lization processes, giving rise to formation of the Yangying potassic 87 86 (800 °C), Sr content continued to increase while ( Sr/ Sr)i decreased magmas (Fig. 13c). The linear trends between mantle-derived dramatically due to involvement of partial melts derived from the coun- ultrapotassic magmas and the Gangdese batholiths and/or Linzizong 87 86 try rocks with relatively less radiogenic ( Sr/ Sr)i ratios. By the end of volcanics in the Sr–Nd–Pb isotope diagrams are consistent with this the magma evolution, Sr content of the contaminated primitive AFC inference (Fig. 5). Geophysical studies (e.g., Zhou and Murphy, ultrapotassic magma decreased in response to the fractional crystalliza- 2005; Li et al., 2008; Zhao et al., 2010; W. Zhao et al., 2011; J. Zhao tion of Sr-rich plagioclase (Fig. 9a). Therefore, YPVR can be well ex- et al., 2014; Q. Xu et al., 2015; Zhang et al., 2015) have indicated the plained by fractional crystallization and contamination (by partial continuous presence of a northward dipping subducted slab of Indian 24 L. Zhang et al. / Gondwana Research 41 (2017) 9–28

Fig. 13. Petrogenetic model for post-collisional potassic magmatism in the Yangying volcanic field of the Lhasa terrane, South Tibet. The diagram shows an NS cross section and is modified after studies of Guo et al. (2007, 2013, 2014a); Marschall and Schumacher (2012) and Cashman and Sparks (2013). Abbreviations are as follows: BNS, Bangong-Nujiang suture; ITS, Indus- Tsangpo suture; MBT, the Main Boundary thrust; MCT, the Main Central thrust; STD, South Tibetan detachment; SCLM, subcontinental lithospheric mantle; P, potassic magmatism; UP, ultrapotassic magmatism. Outcrops of ultrapotassic volcanic rocks in Yangying volcanic field have not been discovered, as shown by the question mark.

continental lithosphere beneath the Lhasa terrane, and a low-velocity as a result of AFC processes of the mantle-derived primitive anomaly zone with depth of ~15 km beneath the Yadong-Gulu rift is ultrapotassic magmas in middle crust of the Lhasa terrane. clearly imaged which has been interpreted as partial melts in middle crust (e.g., Nelson et al., 1996; Brown et al., 1996; Wei et al., 2001). We suggest that the residual magma beneath the Yadong-Gulu rift can provide continuous heat for extensive hydrothermal activities at Acknowledgments present (e.g. hot springs, soil microseepage; Guo et al., 2014b; L.H. Zhang et al., 2014) in Yangying and Yangbajing, which are releasing This study was supported by the Strategic Priority Research Program (B) of Chinese Academy of Sciences (Grant No. XDB03010600). We are considerable amounts of volatiles (e.g., CO2; Guo et al., 2014b; L.H. Zhang et al., 2014). Moreover, hydrothermal gases released from the grateful to Drs. Xianhua Li, Qian Mao, Qiuli Li and Xiaoxiao Ling for their Yadong-Gulu rift are characterized by “mantle signature” (3He/4He = assistance in laboratory. Drs. Wenfeng Guo and Yanan Yang are thanked for helpful discussion. 0.1–0.14 RA; Yokoyama et al., 1999; Hoke et al., 2000; L.H. Zhang et al., 2014), suggesting that the excess 3He might have close afﬁnities with residual mantle-derived magmas in middle crust beneath the Yadong- Appendix A. Supplementary data Gulu rift in the Lhasa terrane. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.gr.2015.11.007. 7. Conclusions References Based on the whole-rock major and trace elements and Sr–Nd–Pb Ahmad, T., Harris, N., Bickle, M., Chapman, H., Bunbury, J., Prince, C., 2000. Isotopic isotopic compositions of the Yangying volcanic rocks, and comprehen- constraints on the structural relationships between the Lesser Himalayan Series sive mineralogical and petrological data, combined with in situ zircon and the High Himalayan Crystalline Series, Garhwal Himalaya. Geological Society of U–Pb dating and Hf–O isotopic compositions, we conclude that: America Bulletin 112, 467–477. Aitchison, J.C., Xia, X., Baxter, A.T., Ali, J.R., 2011. Detrital zircon U–Pb ages along the Yarlung–Tsangpo suture zone, Tibet: implications for oblique convergence and (1) SIMS zircon U–Pb dating analyses yield ages of 10.61 ± 0.10 Ma collision between India and Asia. Gondwana Research 20, 691–709. Allègre, C.J., Ben Othman, D., 1980. Nd–Sr isotopic relationship in granitoid rocks and con- and 10.70 ± 0.18 Ma (weighted mean ages); their ƐHf(t) values – range from −4.79 to −0.17, combined with O isotope (5.51– tinental crust development: a chemical approach to orogenesis. Nature 286, 335 342. Arnaud, N., Vidal, P., Tapponnier, P., Matte, P., Deng, W., 1992. The high K2O volcanism of 7.22‰), imply an addition of crustal material in their source. northwestern Tibet: geochemistry and tectonic implications. Earth and Planetary (2) Based on the EC–AFC model and the clinopyroxene-liquid Science Letters 111, 351–367. thermobarometers, the assimilation and fractional crystallization Ave Lallemant, H., Mercier, J., Carter, N., Ross, J., 1980. Rheology of the upper mantle: inferences from peridotite xenoliths. Tectonophysics 70, 85–113. processes of the mantle-derived ultrapotassic magmas might Balintoni, I., Balica, C., Ducea, M.N., Stremţan, C., 2011. Peri-Amazonian, Avalonian-type have taken place in middle crust with the depth ranging from and Ganderian-type terranes in the South Carpathians, Romania: the Danubian – 11.60 km to 18.75 km beneath the Lhasa terrane. domain basement. Gondwana Research 19, 945 957. Barnes, C.G., Prestvik, T., Sundvoll, B., Surratt, D., 2005. Pervasive assimilation of carbonate (3) A two-stage model for the petrogenesis of the YPVR was and silicate rocks in the Hortavær igneous complex, north-central Norway. Lithos 80, proposed, which involves (a) formation of the ultrapotassic 179–199. maﬁc magmas due to partial melting of the Indian mélanges Bohrson, W.A., Spera, F.J., 2007. Energy-constrained recharge, assimilation, and fractional crystallization (EC–RAχFC): a visual basic computer code for calculating trace beneath base of the Tibetan lithosphere during rollback of the element and isotope variations of open-system magmatic systems. Geochemistry, subducted Indian lithosphere, and (b) occurrence of the YPVR Geophysics, Geosystems 8, Q11003. http://dx.doi.org/10.1029/2007GC001781. L. Zhang et al. / Gondwana Research 41 (2017) 9–28 25

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