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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

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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 Gondwana Research 41 (2017) 9–28 Contents lists available at ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr 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 involve- ment 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 pres- sure (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).
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