Subduction of Indian continent beneath southern Tibet in the latest Eocene (~ 35 Ma): Insights from the Quguosha gabbros in southern Lhasa block

GR-01586; No of Pages 16 Gondwana Research xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Gondwana Research journal homepage: www.elsevier.com/locate/gr

Subduction of Indian continent beneath southern Tibet in the latest Eocene (~ 35 Ma): Insights from the Quguosha gabbros in southern Lhasa block Lin Ma a, Qiang Wang a,b,⁎, Zheng-Xiang Li c, Derek A. Wyman d, Jin-Hui Yang e, Zi-Qi Jiang a, Yong-sheng Liu f, Guo-Ning Gou a, Hai-Feng Guo a a

State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry,Chinese Academy of Sciences, Guangzhou 510640, China CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China Australian Research Council (ARC) Centre of Excellence for Core to Crust Fluid Systems (CCFS) and the Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University, Perth, WA 6845, Australia d School of Geosciences, The University of Sydney, NSW 2006, Australia e Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China f State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan, 430074, China b c

a r t i c l e

i n f o

Article history: Received 30 May 2015 Received in revised form 25 January 2016 Accepted 3 February 2016 Available online xxxx Keywords: Tibet Eocene Continental subduction Gabbro Metasomatism

a b s t r a c t Geophysical data illustrate that the Indian continental lithosphere has northward subducted beneath the Tibet Plateau, reaching the Bangong–Nujiang suture in central Tibet. However, when the Indian continental lithosphere started to subduct, and whether the Indian continental crust has injected into the mantle beneath southern Lhasa block, are not clear. Here we report new results from the Quguosha gabbros of southern Lhasa block, southern Tibet. LA-ICP-MS zircon U–Pb dating of two samples gives a ca. 35 Ma formation age (i.e., the latest Eocene) for the Quguosha gabbros. The Quguosha gabbro samples are geochemically characterized by variable SiO2 and MgO contents, strongly negative Nb–Ta–Ti and slightly negative Eu anomalies, and uniform initial 87 Sr/86Sr (0.7056–0.7058) and εNd(t) (−2.2 to −3.6). They exhibit Sr–Nd isotopic compositions different from those of the Jurassic–Eocene magmatic rocks with depleted Sr–Nd isotopic characteristics, but somewhat similar to those of Oligocene–Miocene K-rich magmatic rocks with enriched Sr–Nd isotopic characteristics. We therefore propose that an enriched Indian crustal component was added into the lithospheric mantle beneath southern Lhasa by continental subduction at least prior to the latest Eocene (ca. 35 Ma). We interpret the Quguosha mafic magmas to have been generated by partial melting of lithospheric mantle metasomatized by subducted continental sediments, which entered continental subduction channel(s) and then probably accreted or underplated into the overlying mantle during the northward subduction of the Indian continent. Continental subduction likely played a key role in the formation of the Tibetan plateau at an earlier date than previously thought. © 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction Continental subduction is a common process in collisional orogenic belts and has important implications for the recycle of crustal materials (e.g., Yin and Harrison, 2000; Wang et al., 2001, 2008a, 2008b; Zheng, 2012; Guo et al., 2014b). The subduction of continental crustal rocks to mantle depths of over 100 km has been demonstrated by the discoveries of coesite (Smith, 1984; Chopin, 1984) and diamond (Sobolev and Shatsky, 1990; Xu et al., 1992) in metamorphic supracrustal rocks, but the continental subduction process is still poorly understood. For instance, as the largest and highest topographic feature on Earth, the Tibetan Plateau was considered to have been created by the ⁎ Corresponding author at: State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China. E-mail address: [email protected] (Q. Wang).

Cenozoic collision between Indian and Asian continents and subsequent continental subduction (Harrison et al., 1992; Yin and Harrison, 2000; Tapponnier et al., 2001; Wang et al., 2001; Ding et al., 2003; Yin and Taylor, 2011; Xu et al., 2013a; 2013b; Jiang et al., 2014). Geophysical data also show that the current Indian continental lithosphere has subducted northward beneath the continental lithosphere to close to the Bangong–Nujiang suture (BNS) in central Tibet (Zhao and Nelson, 1993; Owens and Zandt, 1997; Kosarev et al., 1999; Tilmann and Ni, 2003; Schulte-Pelkum et al., 2005; Li et al., 2008; Nábělek et al., 2009). However, it has been unclear when the Indian continental lithosphere began to subduct and whether the Indian continental crust was ever subducted into the lithospheric mantle beneath the southern Lhasa block. Mantle-derived magmas have a potential to address this issue. Post-collisional (ca. 24–8 Ma) ultrapotassic–potassic lavas widely distributed within the Lhasa block of southern Tibet are characterized by relatively high contents of large ion lithophile elements (LILE) and

http://dx.doi.org/10.1016/j.gr.2016.02.005 1342-937X/© 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

2

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx

light rare earth element (LREE) (Fig. 5) and high (87Sr/86Sr)i and low (143Nd/144Nd)i (Fig. 6). This indicates that their parental magmas were mainly derived from an enriched mantle source (Williams et al., 2004; Ding et al., 2003; Zhao et al., 2009; Guo et al., 2013; Hébert et al., 2014; Liu et al., 2014b; Guo et al., 2015; Huang et al., 2015). Understanding the petrogenesis of the most primitive K-rich magmatic rocks can provide important constraints on mantle characteristics and deep geodynamic processes in such continent–continent collision zones (e.g., Arnaud et al., 1992; Turner et al., 1993). For more than 20 years, numerous studies on Cenozoic K-rich magmatic rocks in Tibet have been carried out in order to trace the mantle enrichment and deep geodynamic processes (e.g., Turner et al., 1996; Williams et al., 2001; Ding et al., 2003; Nomade et al., 2004; Mo et al., 2006; Gao et al., 2007; Zhao et al., 2009; Chen et al., 2010, 2012; Guo et al., 2013; Liu et al., 2014a; Wang et al., 2014a; Liu et al., 2014b; Guo et al., 2015; Huang et al., 2015; Tian et al., 2015). Three main genetic models have been proposed to account for their formation: (1) convective removal of previously thickened lithospheric mantle (e.g., Turner et al., 1993, 1996; Williams et al., 2001, 2004; Chung et al., 2005; Zhao et al., 2009; Liu et al., 2014b); (2) subduction of Indian continental lithosphere (e.g., Pearce and Mei, 1988; Arnaud et al., 1992; Tapponnier et al., 2001; Ding et al., 2003; Guo et al., 2013); and (3) break off of the subducted Indian continental lithosphere slab (e.g., Miller et al., 1999; DeCelles et al., 2002; Mahéo et al., 2002; Replumaz et al., 2010, 2013, 2014; Huang et al., 2015; Tian et al., 2015) or slab roll-back (e.g., Guo et al., 2013, 2015). However, how and when the enriched mantle source of post-collisional ultrapotassic rocks was formed remains highly controversial. Thus, understanding the relationship between the subduction of the Indian continent and the formation of an enriched mantle source beneath southern Tibet is critical to address the above issues. A large number of Paleocene–Eocene granitoids are found in southern Tibet, but few contemporary gabbros or basaltic rocks have been reported. In this study, we present detailed petrological, geochronological, and major and trace element, and Sr–Nd–Hf isotopic data for the Quguosha gabbros from the southern Gangdese batholith. Our zircon U–Pb analyses of the gabbros give a latest Eocene (~35 Ma) age, which is within the Late Eocene to Early Oligocene (ca. 40–30 Ma) magmatic gap in the Lhasa block (Chung et al., 2005). Moreover, their geochemical data show enriched Sr–Nd isotopic compositions different from those of the Jurassic to Eocene Gangdese magmatic rocks. Therefore, these gabbros provide a rare opportunity to examine a possible earlier subduction of the Indian continent and details of the continental subduction process. 2. Geologic background and petrographic characteristics The Tibetan plateau primarily consists of three Gondwana-derived continental fragments: from south to north, the Lhasa, Qiangtang and Songpan-Ganze–Hoh Xil blocks. They are separated from each other by the Bangong–Nujiang and the Jinsha sutures, representative of the relicts of the Meso- and Paleo-Tethys, respectively (e.g., Yin and Harrison, 2000). The Lhasa block was the last of a series of continental fragments to accrete onto southern Asia during the Phanerozoic before the collision of India and Asia (e.g., Yin and Harrison, 2000). Along the southern margin of the Lhasa block, Cretaceous–Paleogene subduction of Neo-Tethyan oceanic crust produced the Cretaceous–Early Tertiary Gangdese magmatic arc (e.g., Allégre et al., 1984; Coulon et al., 1986; Chung et al., 2005). The Indus–Yarlung Tsangpo suture (IYTS) marks the southern boundary of the Lhasa block (e.g., Klootwijk et al., 1992; Yin and Harrison, 2000; Ding et al., 2005; Cai et al., 2011; Chu et al., 2011; Yi et al., 2011; Hu et al., 2012; Decelles et al., 2014; Jiang et al., 2014; Wu et al., 2014; Zhang et al., 2014c; Hu et al., 2015;) (Fig. 1a). The southern Lhasa sub-block (the Gangdese area) represents the southernmost part of the Asian continent and is characterized by extensive Mesozoic–Cenozoic intrusive and volcanic rocks associated with Neo-Tethyan subduction and subsequent India–Asia continental collision. Based on their temporal–spatial distribution and different

geochemical characteristics, these magmatic rocks can be divided into three types. (1) The Mesozoic–Paleocene calc-alkaline rock suites, including the Late Triassic–Early Tertiary (205–43 Ma) gabbros and granitoids (Debon et al., 1986; Harris et al., 1990; Chung et al., 1998; Chung et al., 2005; Wen et al., 2008; Ji et al., 2009; Ma et al., 2013a; 2013b, Ma et al., 2013c, Zhang et al., 2013; Zhu et al., 2013; Ji et al., 2014; Zhang et al., 2014b; Ma et al., 2015), the Early Jurassic (190–174 Ma) Yeba Formation volcanic rocks (Zhu et al., 2008; Guo et al., 2014a), the Late Jurassic–Late Cretaceous (136–93 Ma) Sangri Group volcanic rocks (Zhu et al., 2009; Kang et al., 2010), and the Cretaceous–Tertiary (69–43 Ma) Linzizong Group terrestrial volcanic sequence (Coulon et al., 1986; Pearce and Mei, 1988; Mo et al., 2003, 2007, 2008; Lee et al., 2009, 2012). These magmatic rocks typically show depleted Nd–Hf isotope compositions (εNd(t) up to + 5.5 and εHf(t) up to +16.5) and arc-like geochemical characteristics with the enrichment in large ion lithophile elements (LILE) relative to high field strength elements (HFSE) and the strongly negative Ta–Nb–Ti anomalies. (2) The Oligocene–Miocene adakitic rocks (30–10 Ma) and (3) ultrapotassic rocks (25–8 Ma). These latter two types of rocks were developed after an apparent magmatic gap between ca. 40 and ca. 30 Ma in the Lhasa block (Chung et al., 2005). The adakitic rocks occur as small-volume plugs or dikes/sills, which intrude or crosscut the Gangdese batholith, the Linzizong volcanic successions and associated sedimentary formations, and extend ~ 1300 km across nearly the entire southern Tibet (Fig. 1). They display intermediate to silicic composition (SiO2 = 56–72 wt.%) and were interpreted to have been generated by partial melting of thickened lower crust (e.g., Chung et al., 2003; Hou et al., 2004; Guo et al., 2007; Chung et al., 2009; Hou et al., 2012; Ji et al., 2012; Ma et al., 2014; Zhang et al., 2014a) or subducted Indian continental crust (Xu et al., 2010; Jiang et al., 2014) or low-degree melting of enriched mantle (Gao et al., 2010). The ultrapotassic rocks crop out as small-volume lava flows, plugs and dike swarms within a series of north–south-trending rifts bounded by normal faults (e.g., Chung et al., 2005; Zhao et al., 2009; Guo et al., 2013, 2015) (Fig. 1). They have extremely radiogenic Sr (87Sr/86Sr(i) = 0.7107 to 0.7365) and Pb isotopes (206Pb/204Pb = 18.45–19.35, 207Pb/204Pb = 15.72–15.80, 208 Pb/204Pb = 39.44–40.17) with Nd isotopes (εNd(0) = − 7.6 to −15) and old Nd model ages (TDM = 2.1–1.3 Ga), which were considered to have originated from enriched mantle (Williams et al., 2004; Ding et al., 2003; Chung et al., 2005; Zhao et al., 2009; Guo et al., 2013, 2015; Huang et al., 2015). The Quguosha area is located in the Sangri County of Tibet and is only 10 km away from the Indus–Yarlung Zangbo suture zone. The Quguosha gabbro pluton intrudes the Eocene gneiss that was traditionally believed to be of Proterozoic (1290 Ma) age (Xie et al., 2007) (Fig. 1b). It consists of non-deformed amphibole gabbros (Fig. 2a). The gabbros are mainly of massive and medium- to fine-grained textures (Fig. 2a), and contain plagioclase (~ 30–35 vol.%), amphibole (~ 30–35 vol.%), clinopyroxene (~5–20 vol.%) and biotite (~10–15 vol.%) with Fe–Ti oxides, titanite, apatite and calcite. Amphiboles in the Quguosha gabbros consist of pargasites, magnesiohornblendes and tremolites. The pargasite grains are subhedral and exhibit variable colors, from dark-puce, dark-blue to opaque (Fig. 2f). The magnesiohornblende grains are xenomorphic and show light green to light yellow colors (Fig. 2e and f). Most of the xenomorphic magnesiohornblende grains surround coarse grains of pargasite, clinopyroxene and biotite where there are no reaction rims between them (Fig. 2c, e and f). Tremolites often occur within clinopyroxene grains (Fig. 2h). There are at least two types of feldspar crystals in the Quguosha gabbros (Fig. 2c–h). Some plagioclases are embedded in biotites (Fig. 2d). The other plagioclases are associated with clinopyroxene, amphibole and biotite grains (Fig. 2c–f and h). No reaction rims occur between plagioclase and clinopyroxene grains, indicating that they crystallized contemporaneously (Fig. 2c and h). Some plagioclase grains show the crystallization zonation and apatite grains cut across the crystallization zonation of the plagioclase grains (Fig. 2g). On the basis of textural relationships, the mineral crystallization sequence

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx

3

Fig. 1. (a) Geological map of the Lhasa block (modified from Chung et al., 2009 and Guo et al., 2015). (b) Geological map of the Sangri area, southern Tibet, showing the sampling locations.

may be summarized as follows: (1) early-stage crystalizing minerals: clinopyroxene + plagioclase (+ pargasite); (2) late-stage crystalizing minerals: plagioclase + magnesiohornblende + biotite; (3) accessory minerals: Fe–Ti oxides + apatite + titanite; and (4) metamorphic and altered minerals: tremolite + sericite. 3. Results The major and trace element and Sr–Nd–Hf isotope data are given in Tables 1 and 2, respectively. The analytical methods, LA–ICPMS zircon U–Pb geochronology, mineral composition data, and major and trace element for the Quguosha gabbros are given in Supplemental Files 1–4. We selected the least altered samples for geochemical and isotopic analyses. 3.1. Zircon U–Pb geochronology Two gabbro samples (09TB63 and 11SR10-1) were selected for zircon dating. The zircons in these samples have crystal lengths of ~ 150–300 μm and length/width ratios from 1:1 to 2:1 (Fig. 3), which are similar to those for gabbros (Grimes et al., 2008). The zircon U–Pb isotopic data are given in Supplemental File 2. The analyzed zircon grains from samples 09TB63 and 11SR10-1 have variable U (1633–7145 ppm

and 2614–17,260 ppm, respectively) and Th (1003–4225 ppm and 2391–38,976 ppm, respectively) contents, with Th/U ratios ranging from 0.39 to 2.26. Well-developed oscillatory zoning and high Th/U ratios of zircons from the Quguosha gabbros indicate a magmatic origin (Hoskin and Black, 2000). Fifteen and seventeen U–Pb spot analyses for samples 09TB63 and 11SR10-1 yield concordant 206Pb/238U ages of 35.6–36.1 Ma and 33.6–35.9 Ma, with mean ages of 35.9 ± 0.3 Ma (MSWD = 0.07) and 34.9 ± 0.4 Ma (MSWD = 0.15), respectively (Fig. 3a and b; Supplemental File 2). The consistent zircon U–Pb age data for the two samples suggest that the Quguosha gabbros were emplaced in the latest Eocene (ca. 35 Ma). 3.2. Mineral compositions Major oxide data for plagioclase, amphibole, clinopyroxene and biotite are listed in Supplemental File 3. Plagioclase in the Quguosha intrusive rocks mainly consists of labradorite (An53–66Ab47–34Or0–1) and andesine (An48–30Ab52–69Or0–1) and oligoclase (An9–29Ab71–91Or0–1) (Supplemental File 3). Some plagioclase grains reveal normal compositional zoning with labradorite cores and andesine mantles and oligoclase rims (Supplemental File 3) (Fig. 2g). The amphibole in the Quguosha gabbros is calcic and can be further classified as magnesiohornblende (the most common type), pargasite with minor edenite and tremolite

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

4

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx

a

b Pl Bt Bt Cpx Pl

Mag Pl

500µm

500µm

c

Bt

d Bt

Prg

Pl

Mag

Mag Mag Bt

Mag

Pl

Prg

Mag 500µm

500µm

e

f Ap

Cpx Pl

Pl

An=59

An=23

An=64

Tre

Ap Pl

An=61

500µm

g

An=23

Cpx Cpx

500µm

An=48

h

Fig. 2. Field geological characteristics and petrography (plane-polarized light) of the Quguosha intrusive rocks: (a) gabbroic pluton and felsic dykes; (b) the massive gabbros; (c) clinopyxene grains encompassed by amphibole grains; (d) biotites associated with plagioclases; (e) biotite grains encompassed by amphibole grains; (f) the compositional zoning of amphibole; (g) the compositional zoning of plagioclase; and (h) clinopyroxene grains. Abbreviation: Pl = plagioclase, Prg = pargasite, Mag = magnesiohornblende, Tre = tremolite, Bi = biotite, Cpx = clinopyroxene and Ap = apatite.

(Leake et al., 1997). The pargasite is mainly titaniferous pargasite with high TiO2 (up to 3.7 wt.%) contents (Supplemental File 3). Titaniferous pargasite is generally euhedral or subhedral, or embedded in larger

magnesiohornblende grains (Fig. 2e). Clinopyroxene is mainly diopside (Wo44–50En37–46Fs8–12). The biotite shows moderate to high TiO2 contents (1.5–3.4 wt.%; Supplemental File 3).

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx

5

Table 1 Sr and Nd isotope data for the Quguosha intrusive rocks. Sample

Group

87

Rb/86Sr

09TB61 11SR10-6 11SR10-7 09TB63 09TB64 11SR10-1 11SR10-4

Low-K Low-K Low-K High-K High-K High-K High-K

0.0994 0.0681 0.0436 0.0981 0.3693 0.2045 0.7447

87

Sr/86Sr ± 2σ

(87Sr/86Sr)i

147

Sm/144Nd

0.705768 ± 5 0.705822 ± 6 0.705803 ± 6 0.705752 ± 6 0.705760 ± 7 0.705789 ± 5 0.705933 ± 5

0.705719 0.705789 0.705782 0.705703 0.705577 0.705688 0.705565

0.1121 0.1269 0.1201 0.1075 0.1124 0.1034 0.1301

143

Nd/144Nd ± 2σ

(143Nd/144Nd)i

TDM (Ma)

εNd(t)

fSm/Nd

0.512451 ± 3 0.512436 ± 3

0.512425 0.512407

1052 1258

−3.28 −3.63

−0.43 −0.35

0.512476 ± 4 0.512504 ± 3 0.512493 ± 3 0.512492 ± 3

0.512452 0.512479 0.512470 0.512462

970 975 911 1204

−2.76 −2.23 −2.41 −2.55

−0.45 −0.43 −0.47 −0.34

The gabbros were classified as two groups in terms of their K2O contents (see text for details). 87 Rb/86Sr and 147Sm/144Nd are calculated using whole-rock Rb, Sr, Sm and Nd contents in Table 1. εNd(t) = [(143Nd/144Nd)s / (143Nd/144Nd)CHUR − 1] × 10,000. TDM = ln[(143Nd/144Nd)s − (143Nd/144Nd)DM] / [(143Sm/144Nd)s − (147Sm/144Nd)DM] / λ (DePaolo, 1981). In the calculation, (143Nd/144Nd)CHUR = 0.512638, (147Sm/144Nd)CHUR = 0.1967, (143Nd/144Nd)DM = 0.51315, (147Sm/144Nd)DM = 0.2136 and t = 34.8 Ma.

3.3. Major and trace element geochemistry The whole-rock major and trace element compositions are given in Supplemental File 4. On a total alkali versus silica (TAS) diagram, the samples mainly plot in the gabbro and gabbro–diorite field (Fig. 4a). The Quguosha gabbros have variable SiO2 (44.5–54.8 wt.%) and K2O (0.36–3.18 wt.%) contents. Apart from one sample (11SR10-4) with high K2O/Na2O (2.55) ratios, other samples show low K2O/Na2O (0.18–0.98) ratios (Supplemental File 4) and plot in the field of low-K tholeiitic to high-K calc-alkaline magmatic rocks (Fig. 4b). Based on their K2O and REE contents, the Quguosha gabbros can be divided into two groups: low-K suite (group I) gabbros with relatively low K2O (0.36–0.87 wt.%) and total REE contents of 97–163 ppm, and slightly negative Eu anomalies (Eu/Eu* = 0.7–0.8), and high-K suite (group II) gabbros with relatively high K2O (1.42–3.18 wt.%) and total REE contents (196–340 ppm), and negligible Eu anomalies (Eu/Eu* = 0.8–0.9) (Fig. 5a). On chondrite-normalized REE diagrams, all the gabbro samples display nearly parallel patterns with enriched ([La/Yb]N = 8.1–22.2) LREEs and relatively flat to depleted HREEs ([Gd/Yb]N = 3.2–4.5) (Fig. 5a). The primitive mantle-normalized trace element distribution patterns of the Quguosha gabbros are characterized by enrichments in LILEs and depletions in HFSEs (Fig. 5b). All the samples exhibit strongly to moderately negative Ta, Nb, Zr, Hf and Ti anomalies (e.g., [Nb/La]N = 0.17–0.47) and negative to positive Sr anomalies (Sr/Sr* = 0.5–2.9) (Fig. 5b). In addition, apart from the

two high-K samples (09TB64 and 11SR10-1) with relatively low Cr (118 and 66 ppm), Ni (48 and 25 ppm) and MgO (5.6 and 5.3 wt.%) contents, other rocks have high and variable Cr (123–1139 ppm) and Ni (156–455 ppm) and MgO (7.0–23.3 wt.%) contents (Supplemental File 4).

3.4. Sr–Nd–Hf isotope geochemistry The whole rock Sr–Nd isotope composition data for the Quguosha gabbros are listed in Table 1. All the samples show relative constant εNd(t) values (− 2.2 to − 3.6) and initial 87Sr/86Sr ratios (0.7056–0.7058) (Table 1). The Sr–Nd isotopic compositions of the Quguosha gabbros are significantly different from the Early Jurassic– Eocene magmatic rocks (such as the Yeba volcanic rocks, the Gangdese batholith and the Linzizong volcanic rocks) in southern Tibet with depleted mantle isotopic compositions (e.g., Ding et al., 2003; Chung et al., 2005; Wen et al., 2008; Zhu et al., 2008; Ji et al., 2009; Lee et al., 2012) (Fig. 6). It is noteworthy that the Quguosha gabbros display relatively enriched Sr–Nd isotopic compositions, which are somewhat similar to those of the post-collisional (Oligocene–Miocene) K-rich magmatic rocks but distinct from those of the Jurassic–Middle Eocene magmatic rocks with depleted isotopic composition (Fig. 6). The Ndisotope model ages (TDM) of the Quguosha gabbros range from 0.91 to 1.26 Ga (Table 1).

Table 2 Zircon Hf isotope data for the Quguosha intrusive rocks. Spot #

176

Yb/177Hf

09TB63 01 09TB63 02 09TB63 03 09TB63 04 09TB63 05 09TB63 06 09TB63 07 09TB63 08 09TB63 09 09TB63 10 09TB63 11 09TB63 12 09TB63 13 09TB63 14 09TB63 15

0.035835 0.037509 0.038695 0.018624 0.046088 0.048525 0.013325 0.045663 0.066290 0.035515 0.053906 0.042692 0.026527 0.022274 0.031623

176

Lu/177Hf

0.001626 0.001671 0.001708 0.000841 0.002025 0.002172 0.000636 0.002029 0.002930 0.001567 0.002352 0.001885 0.001197 0.001016 0.001412

176

Hf/177Hf

0.282969 0.282932 0.282937 0.282918 0.282982 0.282985 0.282924 0.282962 0.282960 0.282933 0.282963 0.282943 0.282943 0.282947 0.282965

±2σ

εHf(t)

±2σ

TDM (Ma)

fLu/Hf

0.000013 0.000013 0.000016 0.000014 0.000038 0.000016 0.000015 0.000017 0.000016 0.000016 0.000014 0.000016 0.000016 0.000014 0.000014

7.68 5.67 5.84 5.16 7.42 7.55 5.36 6.72 6.66 5.70 6.74 6.05 6.06 6.19 6.84

2.05 3.05 4.05 5.05 6.05 7.05 8.05 9.05 10.05 11.05 12.05 13.05 14.05 15.05 16.05

408 461 455 472 393 390 461 422 436 459 425 448 440 433 411

−0.95 −0.95 −0.95 −0.97 −0.94 −0.93 −0.98 −0.94 −0.91 −0.95 −0.93 −0.94 −0.96 −0.97 −0.96

[176Hf/177HfZ/176Hf/177HfCHUR(T) − 1] × 10,000. Hf/ HfCHUR(T) = 176Hf/177HfCHUR(0) − 176Lu/177HfCHUR × (eλT − 1). TDM = (1/λ) × ln[1 + (176Hf/177HfDM − 176Hf/177HfZ) / (176Lu/177HfDM − 176Lu/177HfZ)]. TCDM = TDM − (TDM − T) × [(fCC − fZ) / (fCC − fDM)]. fLu/Hf = 176Hf/177HfZ/176Lu/177HfCHUR − 1, where fCC, fZ and fDM are the fLu/Hf values of the continental crust, zircon sample and the depleted mantle; subscript Z = analyzed zircon sample, CHUR = chondritic uniform reservoir; DM = depleted mantle; T = 34.8 Ma, timing of the Quguosha gabbros crystallization; λ = 1.867 × 10−11 year−1, decay constant of 176Lu (Söderlund et al., 2004); 176Hf/177HfDM = 0.28325; 176Lu/177HfDM = 0.0384; present-day 176Hf/177HfCHUR(0) = 0.282772; 176Hf/177HfCHUR = 0.0332; 176Hf/177HfCC = 0.015. TDM represents the model age calculated from the measured 176Hf/177Hf and 176Lu/177Hf ratios of a zircon, giving a minimum limit for the crustal residence age of the hafnium in the zircon; whereas TCDM, a “crust Hf model age,” is derived from projecting the initial 176Hf/177Hf of a zircon with a Lu/Hf ratio corresponding to the continental crust back to the depleted mantle model growth curve. Thus, TCDM can represent the mean crustal residence age of the source material extracted from the depleted mantle. εHf(T) = 176 177

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

6

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx

0.007

40

11SR10−1

206

Pb/238U

0.006

30

38

34

0.005 30

0.004

206

(a) 0.02

0.03

0.04 207

0.05

0.06

0.07

37

gabbro

2

206

36

40

45

50

8 6

Shoshonites

4

High-K

(b)

35

0 40

Weighted Mean Pb/238Uage = 35.87 ± 0.32 (n = 15;MSWD = 0.069)

206

(b) 0.0052 0.032

34

0.034

0.036

0.038 207

0.040

0.042

60

Ultrapotassic

2 0.0054

55

Low–K suite High–K suite Mafic a rc magmatism Post-collisional in southern Lhasa K-rich magmatism in Lhasa

10

K2O (wt.%)

35

0.0056

diorite

gabbro

38

37

Pb/238U

4

12

09TB63 0.0058

gabbro

SiO2 (wt.%)

235

36

ijolite

6

0 35

Pb/ U

0.0060

syeno-diorite

8

(a)

Weighted Mean Pb/238Uage = 34.93 ± 0.40 (n=17;MSWD=0.15)

100µm 26

syenite

10

Na2O + K2O (wt.%)

42

45

50

Medium-K Low-K 55 60

SiO2 (wt.%) 0.044

235

Pb/ U

Fig. 3. Zircon U–Pb concordia diagram of the Quguosha gabbros in southern Tibet.

The zircon in situ Hf isotope data for sample 09TB63 are given in Table 2. The zircons have positive εHf(t) values (+ 5.2–+7.7), which are slightly lower than those (+ 6.5–+20.1) of the zircons from the Cretaceous mafic to felsic Gangdese intrusive rocks (e.g., Ji et al., 2009; Chu et al., 2011; Ma et al., 2013a, 2013b). In addition, the sample 09TB63 zircons also show young Hf model ages (0.47–0.39 Ga) (Table 2). 4. Discussion 4.1. Petrogenesis of the Quguosha gabbros 4.1.1. Crustal assimilation Crustal contamination is almost inevitable for mantle-derived melts during their ascent through continental crust or their evolution within a crustal magma chamber (e.g., Castillo et al., 1999). Given that crustal components generally contain distinctly low εNd(t), MgO, low (Nb/La)PM and Nb/Th values and high 87Sr/86Sr ratios (Rudnick and Fountain, 1995), any crustal assimilation that occurred during magma ascent would have caused an increase in (87Sr/86Sr)i and a decrease in εNd(t) in the magma suites (e.g., Rogers et al., 2000). The homogeneous εNd(t) values and (86Sr/87Sr)i ratios for high-K suite samples of the Quguosha gabbros, and the lack of correlations between 147Sm/144Nd and initial 143Nd/144Nd values (Fig. 7c; Table 1), argue against significant crustal contamination here (Vervoort and Blichert-Toft, 1999). This is further supported by the following observations. (1) The (87Sr/86Sr)i, εNd(t), (Nb/La)PM, and Nb/Th values of the Quguosha high-K gabbros show no

Fig. 4. (a) SiO2 versus K2O + Na2O plot (Middlemost, 1994). (b) SiO2 versus K2O plot (Peccerillo and Taylor, 1976). Data for Jurassic–Early Eocene mafic rocks of the southern Lhasa sub-block are provided for comparison (including the Linzizong mafic dykes and volcanic rocks, gabbros in the Gangdese batholith and Yeba Formation volcanic rocks). Data sources for the Jurassic–Early Eocene mafic rocks are from Zhu et al. (2008); Lee et al. (2012) and Ma et al. (2013a, 2013b). Data sources for the post-collisional K-rich magmatism in Lhasa are from Zhao et al. (2009) and Guo et al. (2013).

significant changes with increasing SiO2 and MgO contents (Fig. 7). (2) The depleted and relatively homogeneous zircon Hf isotopic compositions (εHf(t)zircon = +5.2–+7.7, weighted mean +6.3) for the high-K gabbro sample 09TB63 also indicate that the high-K mafic magmas did not undergo significant crustal contamination. (3) The [Nb/La]PM values of the Quguosha gabbros are mostly lower than the mean value of the lower continental crust ([Nb/La]PM = 0.6) and upper continental crust ([Nb/La]PM = 0.37) (Rudnick and Gao, 2003). Thus, the positive correlation between [Nb/La]PM and MgO for the high-K suite samples is difficult to be explained by crustal contamination (Fig. 7). Therefore, we conclude that crustal contamination did not play a significant role in the formation of the high-K Quguosha gabbros. On the other hand, similar trace element distribution patterns and approximately consistent isotopic composition for both the high-K and low-K samples suggest that they possibly originated from the same parental magmas. Nevertheless, the slightly lower εNd(t) and higher initial 87Sr/86Sr ratios for the low-K samples in comparison to the high-K samples indicate that the low-K mafic magmas likely underwent slightly more crustal assimilation during magma ascent. 4.1.2. Magma mixing Previous study proposed that some ultrapotassic rocks in southern Tibet were generated by the mixing between mantle- and crustderived magmas (Chen et al., 2012). However, it is unlikely that magma mixing could account for the formation of the Quguosha mafic

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx

10000

15

(a) Post-collisional K-rich mafic rocks

5

Dagze Diabases Yeba Volcanic Rocks

(T)

100

0 Linzizong Volcanic Rocks Post–collisional Adakites

-5

10

GLOSS

Early Jurassic mafic rocks ofYeba Formation La Ce Pr Nd

1000

(b)

Marine sediment

-10

1 10000

High–K suite gabbro Low–K suite gabbro 93 Ma Milin norite 94 Ma Zhengga gabbro 23- 8 Ma K–rich mafic rocks 30- 8 Ma adakitic rock

Yarlung Zangbo Ophiolite

10

Nd

Sample/chondrite

1000

Post–collisional Ultrapotassic Rocks

-15

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

(a) -20 0.70

High–K suite

0.71

0.72

0.73

0.74

20

Depleted arc lavas

15 10

100

5%

Oceanic (pelagic) sediments

5

(T)

10

1

2% 3%

5%

Sample 09TB63 Milin norite Zhengga gabbro Oligo-Miocene adakites Paleogene Gangdese Cretaceous Gangdese Miocene ultrapotassic rocks

30% 40%

10%

50% 20%

-20

Himalayan sediments

(b)

-25 -20

-15

the mantle array

4%

K=0.4 K=0.2

-15

4.1.3. Fractional crystallization and accumulation The variable major and trace element compositions of the Quguosha gabbros suggest that the varying degrees of fractional crystallization and/or crystal accumulation possibly played an important role in their formation. Thus, it is essential to assess effects of crystal accumulation for the Quguosha gabbros. The following observations suggest that the studied high-K gabbros were not significantly affected by crystal accumulation: (1) neither layered structure in the outcrop nor textural evidence for crystal accumulation in thin sections have been observed; (2) some plagioclase grains

K=1.01

5%

10%

10%

20%

-5 -10

rocks. Magma mixing generally requires mantle-derived basaltic and crust-derived felsic magma end-members (e.g., Streck et al., 2007; Sun. et al., 2010). As mentioned above, there is a magmatic gap during Eocene to Oligocene (40–30 Ma) in Gangdese area (Chung et al., 2005). Thus, in southern Lhasa, the coeval candidates for mantlederived basaltic and crust-derived felsic magma end-members are absent. The felsic or mafic enclaves and other igneous textures including needle-like apatite, oscillatory-zoned plagioclase, and local quartz, which are common in the classic magma mixing model (e.g., Barbarin and Didier, 1991; Yang et al., 2007), also have not been observed in the Quguosha gabbros. In contrast, the apatites as an accessory minerals displays thick euhedral crystals in the Quguosha gabbros (Fig. 2g). Moreover, the Quguosha gabbros are characterized by variable SiO2 and MgO contents but relatively uniform Sr–Nd isotopic compositions, which is inconsistent with the magma mixing model. Therefore, we suggest that the Quguosha high-K and low-K gabbro suits could not have been generated by the mixing between mantle- and crust-derived magmas.

1%

0

Rb Ba Th U Nb Ta La Ce Sr NdSm Zr Hf Eu Ti Tb Y Yb Lu Fig. 5. (a) Chondrite-normalized rare earth element (REE) patterns and (b) primitive mantle-normalized multi-element patterns of the Quguosha gabbros. The postcollisional K-rich mafic rocks data are from Guo et al. (2013). The Yeba Formation basalts data are from Zhu et al. (2008). The melange average composition is from Marschall and Schumacher (2012). The chondrite and primitive mantle normalization values are from Sun and McDonough (1989).

0.75

(87Sr/86Sr)i

Low–K suite Melange average

Hf

Sample/Primitive mantle

7

-10

-5

0

5

10

(T)

Nd

Fig. 6. (a) εNd(t) versus (87Sr/86Sr)i and (b) zircon εHf(t) versus εNd(t) diagrams (after Chu et al., 2011) for the Quguosha gabbros. Data sources: (a) IYTS ophiolites are from Mahoney et al. (1998) and Zhang et al. (2005); Himalayan leucogranites are from Guo and Wilson (2012); Yeba volcanic rocks are from Zhu et al. (2008); Linzizong volcanic rocks are from Lee et al. (2012), post-collisional adakites are from Chung et al. (2003) and Hou et al. (2004); Marine sediments are from Ben Othman et al. (1989) and McLennan et al. (1990); post-collisional ultrapotassic rocks are from Zhao et al. (2009); Milin norites are from Ma et al. (2013b); Zhengga gabbros are from Ma et al. (2013a); K-rich mafic rocks are from Guo et al. (2013, 2015) and Zhao et al. (2009); and 30–8 Ma Indian crust-derived adakites are from Xu et al. (2010) and Jiang et al. (2011). The sample 09TB79 (gabbro) representing the pre-collisional Gangdese lithospheric mantle wedge (εNd(T) = +3.8, (87Sr/86Sr)i = 0.7045, Nd = 6.16 ppm and Sr = 852.4 ppm) are from Ma et al. (2013a). The sample N-702 (leucogranite) representing the Himalayan sediments (εNd(T) = −15.3, (87Sr/86Sr)i = 0.7448, Nd = 14.3 ppm, Sr = 108.4 ppm) are from Guo and Wilson (2012). The GLOSS (εNd(T) = − 8.6, (87Sr/86Sr)i = 0.7170, Nd = 27 ppm, Sr = 327 ppm) are from Plank and Langmuir (1998). (b) The Hf isotopic data of OligoMiocene adakites, Paleocene Gangdese, Cretaceous Gangdese and Miocene ultrapotassic rocks are from Chu et al. (2011). Field of MORB, the mantle array, and oceanic and Himalayan sediments are from Chauvel et al. (2008) and field of depleted arc lavas are from Marini et al. (2005). The compositions of end-members or components assumed for mixing calculations are: (1) the Gangdese mantle wedge: εHf(T) = + 15.0, εNd(T) = + 7.0, Hf = 0.172 ppm and Nd = 1.16 ppm; (2) the Gangdese mafic lower crust (represented by a gabbro sample): εHf(T) = + 12.0, εNd(T) = + 4.0, Hf = 1.72 ppm and Nd = 11.6 ppm; (3) the Indian continental crust (represented by average Himalayan sediments): εHf(T) = −20.0, εNd(T) = −13.4, Hf = 4.06 ppm and Nd = 27 ppm. The mixing curves were constructed using different Nd/Hf elemental ratios, K = (Nd/Hf)Gangdese/(Nd/ Hf)Indian crust, of respective mixing components.

show normal compositional zoning with labradorite cores, andesine mantles and oligoclase rims, suggesting a magmatic evolution process rather than crystal accumulation. In addition, compared with typical cumulative gabbros, which are An-rich (NAn60; Beard, 1986), plagioclases in the Quguosha samples are characterized by highly variable anorthite of An9–66. Furthermore, the Quguosha high-K samples are characterized by moderate negative Eu anomalies (Eu/Eu* as low as 0.72), which

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

8

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx

0.5126

(143Nd/144Nd)i

0.5125 2

R =0.1138

143

144

( Nd/ Nd)i

0.5126

0.5124

0.5125 2

R =0.3325

0.5124

(a) 0.5123 0

(b) 5

10

15

20

0.5123 45

25

47

49

MgO (wt.%)

51

53

55

57

SiO2 (wt.%)

0.5126

0.7060

( Sr/ Sr)I

R2=0.0049

R2=0.0291

0.7056

143

87

86

144

( Nd/ Nd)i

0.7058 0.5125

0.5124 0.7054

(c) 0.5123 0.10

(d) 0.11

0.12 147

0.7052 45

0.13

45

144

Sm/ Nd

49

53

51

55

57

SiO2 (wt.%))

-1

0.5

0.4

[Nb/La]PM

Nd(t)

-2 R2=0.0061

0.3 2

R =0.7211

-3 0.2

(e)

(f)

-4

0.1 0.1

0.2

0.3

0.4

0.5

[Nb/La]PM

0

5

10

15

20

25

MgO ( wt.%)

Fig. 7. Plots of (a) (143Nd/144Nd)i versus MgO; (b) (143Nd/144Nd)i versus SiO2; (c) (143Nd/144Nd)i versus 147Sm/144Nd; (d) (87Sr/86Sr)i versus SiO2; (e) εNd(t) versus [Nb/La]PM; and (f) [Nb/La]PM versus MgO for the Quguosha mafic intrusive rocks. Data sources and symbols are the same as for Fig. 4.

contrast with the pronounced positive Eu anomaly in cumulative gabbros (Beard, 1986; Turner, 1996); and (3) the presence of large amounts of biotites in the Quguosha high-K samples suggests that these biotites are magmatic rather than near-solidus origin (Fig. 2c–e). The high Rb contents (62–125 ppm) of the high-K gabbros also support this inference (Supplemental File 4). On the other hand, because Ni is compatible in olivine and Cr is compatible in clinopyroxene and spinel, clinopyroxene-dominate fractionation is inferred for the high-K samples, given their variable Cr contents (66–610 ppm) and a positive correlation between Cr and MgO.

Given the similar Sr–Nd isotopic compositions and subparallel REE patterns, the high-K and low-K mafic samples in the Quguosha area were likely derived from similar mantle sources but were probably affected by accumulation or fractional crystallization to varying degrees. The Cr and Ni contents exhibit positive correlations with the MgO contents for the low-K gabbros, indicating fractional crystallization/ accumulation of olivine and clinopyroxene. The Quguosha low-K samples may have been subjected to olivine-dominated (with spinel and minor clinopyroxene) fractionation/accumulation, given their more variable Ni contents with a relatively narrow range of Cr

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx

(Table 1). In addition, the high and nearly constant Mg# (76–81) values with variable SiO2 (44.5–52.0 wt.%) contents for the Quguosha low-K gabbros suggest that they were affected by olivine-dominated accumulation. The highest Cr (1139 ppm) and high CaO (13.1 wt.%) and relatively low Ni (189 ppm) contents for the low-K sample (11SR10–6) probably suggest the accumulation of clinopyroxene and minor olivine and spinel. The occurrence of more clinopyroxene in the low-K gabbros may also support this inference (Fig. 2h). Trace element modeling was also used to evaluate the effect of fractional crystallization and accumulation on the Quguosha mafic magmas. The high-K gabbro sample 11SR10-4, with low SiO2 (45.4 wt.%), high MgO (14.6 wt.%) and Mg# (73.1), and a relatively weakly negative Eu anomaly (Eu/Eu* = 0.78), was assumed to closely resemble the composition of the initial melts. The modeling of olivine accumulation and Rayleigh fractionation (20% olivine + 65% clinopyroxene + 15% plagioclase) successfully reproduced the rare earth element compositional variation of the Quguosha gabbros (Fig. 8). The low-K Quguosha gabbros, with relatively low REE contents and high Mg# (76–81) values, show less negative Eu anomalies and more variation for Ni contents (Fig. 8b). The result indicates variable degrees of olivine accumulation for the low-K gabbros (Fig. 8), because Ni is compatible in olivine (Pedersen, 1979; Adam and Green, 2006). In summary, the high-K gabbros were most likely generated by olivine and minor clinopyroxene fractionation from mafic magmas that had experienced fractional crystallization of plagioclase and insignificant crustal contamination. The low-K gabbros were most plausibly produced by olivine-dominant accumulation combined with minor crustal assimilation from mafic magmas that were geochemically similar to that of the high-K gabbros. The lower K2O, Al2O3 and CaO contents (Table 1) of the low-K gabbros also support this inference. 4.1.4. Refertilized lithosphere mantle source The Quguosha gabbros have relatively high initial 87Sr/86Sr values of 0.7056–0.7058 and negative εNd(t) values of − 2.2 to − 3.6, distinct from those of the basalts in the Jurassic–Cretaceous Tethyan ophiolites (Mahoney et al., 1998; Zhang et al., 2005) and the pre-Cenozoic subcontinent lithospheric mantle (SCLM)-derived mafic magmatic rocks in southern Tibet (e.g., the Jurassic Yeba basalts and the Late Cretaceous Zhengga gabbros) (Fig. 6a) with depleted Sr–Nd isotopic characteristics (Zhu et al., 2008; Ma et al., 2013a). Their enriched Sr–Nd isotope and trace components are similar to those of post-collisional (Oligocene– Miocene) K-rich magmatic rocks widely occurring in the Lhasa block (Figs. 5 and 6) (e.g., Miller et al., 1999; Williams et al., 2004; Zhao et al., 2009; Guo et al., 2015). Given the distinctive elemental and Sr– Nd–Pb–O isotopic signatures of the Oligocene–Miocene ultrapotassic rocks in western Gangdese, it is nearly a consensus that they are were

derived from an extremely enriched upper mantle source below southern Tibet (e.g., Miller et al., 1999; Ding et al., 2003; Williams et al., 2004; Chung et al., 2005; Zhao et al., 2009; Guo et al., 2013, 2015; Huang et al., 2015; Tian et al., 2015). However, how and when the enriched materials entered the mantle source of the Quguosha gabbros remain controversial. The proposed metasomatic agents that have been proposed for producing enriched mantle beneath the Lhasa block include (1) a mature continental crust within the Lhasa block (Liu et al., 2014b, 2015); (2) melts and fluids derived from subducted Tethyan oceanic sediments (e.g., Williams et al., 2004; Gao et al., 2007; Liu et al., 2014b; Huang et al., 2015); and (3) subducted Indian continental materials (e.g., Mahéo et al., 2002; Ding et al., 2003; Williams et al., 2004; Zhao et al., 2009; Guo et al., 2013, 2015). Based on extensive U–Pb and Lu–Hf isotope investigations of Mesozoic–Early Tertiary magmatic rocks sampled along four north– south traverses across the block, Zhu et al. (2011) concluded that ancient crustal basement rocks of Proterozoic and Archean ages exist only in the central Lhasa sub-block. Thus, given the depleted mantlederived Sr–Nd–Hf isotopic compositions of Late Mesozoic mafic magmatic rocks in southern Lhasa sub-block and the lack of a plausible mechanism to bring the basement rocks of central sub-block to the mantle beneath the southern sub-block during the Cenozoic (Ma et al., 2014), the basement rocks of the central Lhasa sub-block could not be responsible for the mantle enrichment beneath the southern Lhasa sub-block. The Quguosha gabbro samples fall on Sr–Nd isotopic binary trends between global subducting sediments (GLOSS) and the Mesozoic Gangdese lithosphere mantle (Fig. 6). However, in order to achieve the observed Nd–Hf isotopic composition of the Quguosha gabbros, an assimilation of 50%–60% subducted pelagic sediments is required. Involvement of such a high percentage of pelagic sediments cannot be reconciled with the mafic composition of the Quguosha gabbros. In addition, the extremely low Nd–Hf isotopic ratios (143Nd/144Nd(i) = 0.5117–0.5120; 176 Hf/177Hf = 0.2823–0.2830) and particularly high Sr–Pb isotopic ratios (87Sr/86Sr(i) = 0.7107–0.7365; 206Pb/204Pb(i) = 18.30–18.92; 207 Pb/204Pb(i) = 15.65–15.87; 208Pb/204Pb(i) = 39.02–39.76) of the Oligocene–Miocene potassic and ultrapotassic rocks are distinctly different from those of marine sediments (Ben Othman et al., 1989; McLennan et al., 1990; Chauvel et al., 2008). This indicates that subducted marine sediments were not a suitable source for the extreme enrichment of the Oligocene–Miocene ultrapotassic rocks in the Lhasa block (e.g., Zhao et al., 2009; Chu et al., 2011; Guo et al., 2013). Given the depleted Sr–Nd–Hf isotopic compositions of the Late Mesozoic mafic rocks in the Gangdese area generated during the Neo-Tethys oceanic subduction (e.g., Zhu et al., 2008; Ma et al., 2013a; 2013b; Zhang et al., 2014b; Ma et al., 2015) (Fig. 6) and break off of the oceanic lithosphere

400

500

Fractionation of Ol0.2Cpx0.65Pl0.15

50%

300

(b)

nation Sp. fractio

400

Accumulation of Ol0.9Pl0.1

% 30

% 40

200

x. Cp

100

n tio na o i ct fra

50 %

100

300

Ol. frac tionatio n

150

15 %

5%

10% 20%

20 %

Accumulation of plagioclase

20% 10%

200

Ni (ppm)

30%

250

50 0.65

(a)

40%

10 %

350

9

0 0.70

0.75

0.80

Eu/Eu*

0.85

0.90

0.95

0

300

600

900

1200

Cr (ppm)

Fig. 8. (a) Plots of ΣREE versus Eu/Eu* and (b) Ni versus Cr diagram for the Quguosha mafic intrusive rocks. ΣREE is the sum of all rare earth elements. The sample 11SR10-4 is assumed as the initial melts before accumulation (90% olivine + 10% plagioclase) and Rayleigh fractionation (20% olivine + 65% clinopyroxene + 15% plagioclase). The numbers of the curves denote the degrees of accumulation and fractional crystallization in percent, using the partition coefficients of McKenzie and O'Nions (1991). The trace element concentrations of accumulated plagioclase are calculated from the assumed initial melts using the partition coefficients of McKenzie and O'Nions (1991). Abbreviations: Sp = spinel, Cpx = clinopyroxene, Ol = olivine, and Pl = plagioclase.

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx

from the Indian continental lithosphere at ca. 50 Ma (e.g., Lee et al., 2009; Zhu et al., 2013; Jiang et al., 2014; Ma et al., 2014; Replumaz et al., 2014; Zhu et al., 2015), subsequent mantle-derived magmas should contain a smaller component from subducted oceanic sediments than Late Mesozoic mafic rocks. Accordingly, it is difficult to imagine that previous subducted oceanic pelagic sediments were responsible for the mantle enrichment beneath the Lhasa block only during the latest Eocene to Miocene rather than since the Late Mesozoic. We suggest that the enriched isotopic characteristics of the latest Eocene to Miocene magmatic rocks require involvement of another “end-member” involved in their formation. In view of the above discussion, we propose that the enriched characteristics of mantle-derived rocks in southern Lhasa block were related to the subduction of the Indian continent (Fig. 10). This is supported by the following evidence. (1) In the 87Sr/86Sr versus εNd(t) and εHf(t) versus εNd(t) plots, the Quguosha gabbros show limited isotopic variations that can be attributed to a small degree of source contamination by the subducted Himalayan sediments represented by the Cenozoic Himalayan leucogranites (Fig. 6). The calculation of binary mixing between the Himalayan sediments and a depleted mantle wedge shows that an addition of ca. 5% Himalayan sediments via continental subduction into the mantle source can generate the isotopic compositions of the Quguosha gabbros (Fig. 6). Sample 09TB63 has negative εNd(t) (−2.8) and positive εHf(t) (+6.4), which could have been caused by the less Nd/Hf ratio of the Gangdese lithospheric mantle in comparison to the Indian crust, similar to that proposed for the Cenozoic adakite example (Chu et al., 2011). That is because Indian crustal melts with more abundant residual zircons can hold back Hf from partitioning into the melts and thus increase its Nd/Hf elemental ratio (Chu et al., 2011). Moreover, samples from the Oligocene–Miocene lower crust-derived adakites (e.g., Chung et al., 2003; Hou et al., 2004; Guo et al., 2007) and ultrapotassic rocks (e.g., Miller et al., 1999; Williams et al., 2004; Zhao et al., 2009; Guo et al., 2013, 2015) in the Lhasa block also plot on or around the simulated curves (Fig. 6), suggesting the involvement of ancient Indian continental crustal materials in their genesis. In addition, the modeling of multiple isotopic systems, including Sr–Nd–Pb–Hf–O isotopes, also supports the subduction of the Indian continent beneath the Lhasa block, leading to a highly contaminated mantle source (e.g., Zhao et al., 2009; Chu et al., 2011; Guo et al., 2013, 2015; Tian et al., 2015; this study). (2) Considerable geophysical data confirmed that parts of the Indian continental plate subduct beneath the Lhasa block (e.g., Zhao and Nelson, 1993; Owens and Zandt, 1997; Li et al., 2008; Nábělek et al., 2009; Zhao et al., 2010, 2011; Zhang et al., 2015a) and the Indian crust can be traced to 31°N (Nábělek et al., 2009). The distribution of ultrapotassic rocks in the Lhasa block agrees well with the extent of subducted Indian crust depicted by geophysical data (Nábělek et al., 2009). (3) Petrological evidence such as Indian crust-derived adakitic rocks in the southern Lhasa block and the Tethyan Himalaya also suggests that the Indian crust had subducted beneath Tibet by Oligocene (ca. 30 Ma) (e.g., Jiang et al., 2011; Hou et al., 2012; Jiang et al., 2014; Zhang et al., 2015b). (4) Decreased geothermal gradient of the supra-slab mantle triggered by continental subduction resulted in the reduction and even cessation of magmatism, which corresponded to the Gangdese magmatic gap during Eocene to Oligocene (40–30 Ma) in the Lhasa block, southern Tibet (Chung et al., 2005). Thus, the Quguosha gabbros were most possibly derived from a refertilized lithospheric mantle source metasomatized by subducted Indian continental sediment melts. Recent studies suggest that pyroxene-rich veins or layers could be the sources of some mafic (even ultrapotassic) magmas (e.g., Sobolev et al., 2005; Guo et al., 2015; Huang et al., 2015). Moreover, partial melts derived from mantle pyroxenite (without olivine) would generate magmas with moderate MgO and high SiO2 (up to 55%) and relatively high Ni/MgO ratios (Sobolev et al., 2005). However, the low SiO2 and Ni/MgO and high MgO for the Quguosha high-K gabbros in the Lhasa block, plus the significant positive correlation between Ni and MgO,

imply a peridotitic source, rather than a pyroxenite vein-plusperidotite type mantle or a pyroxenite mantle (Sobolev et al., 2005; Guo et al., 2015; Huang et al., 2015). Based on these observations, we suggest that the parental magmas of the Quguosha gabbros most likely originated from a refertilized peridotitic lithospheric mantle source that had been metasomatized by subducted Indian continental sediment melts. 4.1.5. Continental subduction erosion and mélange melting Subduction erosion is a major process involved in recycling components from a subducting slab and its overlying mantle wedge into the lower crust and mantle through subduction channels along active convergent plate boundaries (e.g., von Huene and Scholl, 1991; von Huene et al., 2004; Hacker et al., 2011; Stern, 2011; Marschall and Schumacher, 2012). This involves the formation of a mélange zone in a subduction channel on the top surface of the slab in which hydrated mantle rocks are mixed with material derived from the subducting slab, including trench and fore-arc basin sediments (Guo et al., 2015). The latest Eocene Quguosha high-K gabbros as well as the Oligocene–Miocene ultrapotassic rocks are characterized by significant enrichment in LILE and LREE relative to HFSE and HREE, with strongly negative Nb–Ta–Ti anomalies and positive Pb anomalies in primitive mantle-normalized incompatible trace element patterns (Fig. 5), which are consistent with an origin as subduction-related magmas (e.g., Gill, 1981; Pearce and Parkinson, 1993). The Pb enrichment in aqueous fluids is caused by their high capacity to dissolve and transport Pb and the HFSE depletion is due to the partial melting in the rutile stability field (Zheng, 2012). The primitive mantle-normalized incompatible trace element pattern of average subduction channel mélange rocks reported by Marschall and Schumacher (2012) is similar to those of the Quguosha high-K gabbros, characterized by the enrichment in LILE and significant depletion in Nb, Ta and Ti (Fig. 5), although overall abundances are lower in the mélange rocks. In addition, the HREE abundances, together with some trace element ratios (e.g., Na/La and Ce/Pb; Fig. 9), are similar between average subduction channel mélange rocks (Marschall and Schumacher, 2012), and the ultrapotassic magmas and the Quguosha gabbros in southern Tibet. Moreover, Zhang et al. (2015b) reported the Oligocene (34–26 Ma) metasedimentary rocks and migmatitic rocks of the Gangdese arc, which contained both preMesozoic inherited detrital zircon and young inherited magmatic zircon from pre- and syn-collisional Gangdese granitoids. These data are consistent with the involvement of a subducted continental crustal component in the formation of the Quguosha gabbros and the Oligocene ultrapotassic rocks in south Tibet (Guo et al., 2015). Furthermore, the 100.0

MORB

Arc basalts

OIB

10.0

Ce/Pb

10

Melange average 1.0

Ultrapotassic rocks in Lhasa

Bulk continental crust Low–K suite High–K suite

0.1 0.01

1.0

0.1

10.0

Nb/La Fig. 9. Plots of Ce/Pb versus Nb/La diagram for the Quguosha mafic intrusive rocks (modified from Guo et al., 2015). The mélange average is from Marschall and Schumacher (2012). Data for OIB and MORB are from Wilson (1989) and Sun and McDonough (1989). The compositions of bulk continental crust are from Rudnick and Gao (2003). Data of the post-collisonal ultrapotassic rocks from Guo et al. (2015).

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx

coeval (37–24 Ma) metamorphism (granulite facies and amphibolite facies) and anatexis indicated that the fore-arc sediments were transported into the middle–lower crust associated with the Indian continental subduction before ~37 Ma (Zhang et al., 2010; Xu et al., 2013b). Thus, combined with above evidence, we propose the occurrence of a subducted mélange zone in the subduction channel beneath southern Tibet, which likely provides the enriched Indian continental crustal materials for the mantle resource of the parental magmas of the Quguosha gabbros (Fig. 10). Although aqueous fluid played an insignificant role in the source enrichment of the Quguosha gabbro (Supplemental File 5), the presence of water and volatiles may still have played a key role in the formation of the Quguosha gabbros (Fig. 10). Some titaniferous pargasites, as magmatic crystallization core, are wrapped by magnesiohornblende rims in the Quguosha gabbros (Fig. 2e). Crystallization temperatures for titaniferous pargasite and magnesiohornblende calculated according to the titanium content of hornblende geothermometer (Otten, 1984) are 928–989 °C and 660–761 °C, respectively (Supplemental File 3). The crystallization temperature of hydrous mineral (i.e., titaniferous pargasite) is nearly equivalent to the temperature for hydrous mantlederived mafic magmas (Olafsson and Eggler, 1983; Gallagher and Hawkesworth, 1992), indicating high aqueous fluid contents during

the magma formation. In addition, the clinopyroxene grains in the Quguosha samples are typically embedded in amphiboles or display amphibole rims (Fig. 2c), which could have resulted from the replacement by hydrous minerals in water-rich conditions (Fagan and Day, 1997; Stalder et al., 1998). The occurrence of abundant hydrous minerals (magnesiohornblende + biotite) indicates high fluid contents during the late stages of magma evolution. Thus, the presence of titaniferous pargasite and the occurrence of hydrous mineral assemblage (magnesiohornblende + biotite) in the Quguosha gabbros suggest an abundance of hydrous fluids during mafic magma generation as well as at later stages of magma evolution. The high Ba contents (369–581 ppm) of high-K gabbros also support this inference. The abundance of water likely triggered melting of lithospheric mantle peridotites to produce basaltic magmas (Tatsumi and Eggins, 1995) (Fig. 10). As demonstrated by experiments and theoretical considerations, an addition of only 0.3 wt.% H2O to peridotites would lower its solidus by several hundred degrees (Olafsson and Eggler, 1983; Gallagher and Hawkesworth, 1992). The aqueous fluid occurs as hydrous minerals such as phengite, epidote and lawsonite in the deeply subducted continental crust. The decomposition of these hydrous minerals in continental subduction zones makes important contributions to the fluid regime in continental subduction zones

Accretionary Linzizong volcanics wedge

Himalaya Block

11

Gangdese adakites

(a) Lhasa Block

Indian crust

continental crust

Indian lithospheric mantle

Ca.51-46 Ma Accretionary wedge

Himalaya Block Indian crust

SCLM

st cru nic ea oc tle an an hy ric m he

Slab window created by slab break-off, asthenosphere upwelling, decompression melting

Ne olith Tet os p

Quguosha gabbros

(b) Lhasa Block continental crust

Indian lithospheric mantle Subduction channel Crustal rocks

Ca.35 Ma

Dehydration

Metasomatized SCLM

SCLM

sediments molten crust molten melange molten mantle

Fig. 10. Schematic illustration for the tectonic evolution and the petrogenesis of Eocene magmatic rocks in the southern Lhasa sub-block, Tibet. (a) 51–46 Ma: asthenosphere upwelling triggered by the break off of the subducted Neo-Tethyan slab created a hot and soft lithospheric mantle (Jiang et al., 2014; Ma et al., 2014); (b) ca. 35 Ma: the Indian continent has entered the mantle lithosphere beneath southern Lhasa sub-block and partial melting of lithospheric mantle metasomatized by the continental sediments generated the Quguosha mafic magmas.

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

12

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx

(e.g., Zheng, 2012). As a consequence, melting of hydrated peridotites will produce melts with higher SiO2 and lower FeO and CaO than those produced under anhydrous conditions. In addition, the hydrated minerals are kinetically stable in cold subduction zones (such as continental subduction) with low thermal gradients for UHP metamorphism (Zheng, 2012), which likely resulted in the high Ba abundances but clearly negative Ba anomalies of the Quguosha high-K gabbros and post-collisional ultrapotassic rocks. In summary, we propose that the primitive magmas of the Quguosha gabbros were generated by the following processes. (1) After the 65–55 Ma initial collision between the Indian and Asian continents (e.g., Klootwijk et al., 1992; Yin and Harrison, 2000; Jiang et al., 2014), an asthenosphere upwelling triggered by the break off of the subducted Neo-Tethyan slab at ca. 51–46 Ma provided the required heat for lithospheric mantle weakening during the Early Eocene (e.g., Lee et al., 2012; Jiang et al., 2014; Ma et al., 2014; Zhu et al., 2015); (2) Sedimentary materials mainly from the Indian continental crust possibly entered the continental subduction channel above the subducted Indian continental subduction after the break off of the subducted Neo-Tethyan slab; (3) The continental sedimentary materials in the subduction channel probably accreted or underplated into the hot and soft lithospheric mantle of the upper plate (e.g., Marschall and Schumacher, 2012); (4) Fluids derived from the decomposition of hydrous minerals in continental subduction channel triggered partial melting of the underplated mélanges and lithospheric peridotites in the upper plate.

4.2. Implications for geodynamic processes and tectonic evolution 4.2.1. Subduction of the Indian continent beneath southern Tibet The Quguosha gabbros are located in southern Lhasa block (Fig. 1), where the Jurassic–Cretaceous (ca. 180–70 Ma) igneous rocks (especially basalts and gabbros) are characterized by depleted mantle-derived Sr–Nd–Hf isotopic compositions (e.g., Wen, 2007; Wen et al., 2008; Zhu et al., 2008; Ji et al., 2009; Ma et al., 2013a; 2013b; Ji et al., 2014; Ma et al., 2015), indicating the occurrence of a juvenile middle–lower crust and depleted SCLM beneath the southern Lhasa during Late Mesozoic (Zhu et al., 2011; Ma et al., 2013a; Zhang et al., 2014b). However, compared with the Mesozoic and Middle Eocene mafic magmatic rocks in southern Gangdese area (e.g., Zhu et al., 2008; Lee et al., 2012; Ma et al., 2013a; 2013b; Zhang et al., 2014b; Ma et al., 2015), the latest Eocene Quguosha gabbros as well as Oligocene–Miocene ultrapotassic rocks are characterized by significant enrichment in LILE and LREE with strongly negative Nb–Ta–Ti anomalies and remarkably enriched Sr–Nd isotopic compositions (Figs. 5 and 6). Changes in the chemical and isotopic compositions of the resultant magmas generally record changes in tectonic processes (Chu et al., 2011). The mantle enrichment process, which was triggered by a dynamic mechanism closely associated with the formation of the plateau, likely occurred beneath the Lhasa block during the latest Eocene–Miocene (Harrison et al., 1992; Chung et al., 1998; Tapponnier et al., 2001; Chung et al., 2005; Zhao et al., 2009). The Sr–Nd–Hf isotope compositions of the latest Eocene Quguosha high-K gabbros as well as the post-collisional ultrapotassic rocks exhibit linear trends between the depleted SCLM and the Indian continental crust (Fig. 6). The extreme enrichment of the upper mantle below south Tibet was considered to have resulted from the addition of components derived from the subducted Indian continental crust to the overlying mantle wedge during northward underthrusting of the Indian continental lithosphere beneath the Lhasa block (Guo et al., 2015). The mixing calculation also shows that an addition of ca. 5% Himalayan sediments via continental subduction into the mantle source can generate the isotopic compositions of the Quguosha gabbros (Fig. 6). We propose that the formation of the Quguosha gabbros probably indicated the initial stage of mantle enrichment process beneath south Tibet.

Other evidence also supports the Indian continental subduction. Given the dramatic changes in the εNd(t) and εHf(t) values, the Indian continental components were identified in the Early Eocene lower crust-derived adakites in southern Lhasa block, suggesting the Indian continental crust materials were subducted into the middle-lower crust before the early Eocene (Chu et al., 2011; Ji et al., 2012; Ma et al., 2014). The narrow linear nature of the Eocene magmatic belt (Ma et al., 2014 and references therein) and synchronous asthenospherederived basaltic rocks (Lee et al., 2012) in southern Lhasa suggest that upwelling asthenosphere triggered by the break off of subducted NeoTethyan slab probably provided the required thermal conditions for lower crustal melting (Ma et al., 2014). Metamorphic rocks also provide crucial insights into the continental subduction process (Tonarini et al., 1993; Guillot et al., 1997; Ding et al., 2001). The coesite-bearing ultrahigh-pressure (UHP) eclogites found in the Tso Morari and Kaghan areas of the western Himalaya demonstrated that the continental crust of the entire northwestern part of the Indian plate was subducted beneath the Kohistan–Ladakh arc to a minimum depth of 90 km (O'Brien et al., 2001; Mukherjee et al., 2003). Moreover, in the eastern Himalaya, the high-pressure (HP) granulitefacies and medium-pressure (MP) amphibolite-facies metamorphic rocks with peak metamorphic age ranging from ~ 38 Ma to ~ 24 Ma also support that the Himalayan lithologies with overlying sediments were subducted into the middle–lower crust (Ding et al., 2001; Xu et al., 2010b; Zhang et al., 2010; Xu et al., 2013b; Zhang et al., 2015b). The seismic cross-section of Nábělek et al. (2009) provides the ideal images of the present configuration of subduction beneath the Lhasa block. They identified a flat-lying, ca.15 km thick layer above the seismic Moho, which they interpreted as subducted Indian lower continental crust (Nábělek et al., 2009). The extent of such subducted Indian lower crust agrees well with the distribution of ultrapotassic rocks in the Lhasa block (Fig. 1a). In addition, Wittlinger et al. (2009) and Schulte-Pelkum et al. (2005) both recognized eclogite-facies rocks in this region, which located in ca. 80 km depth and showed that it extends up to 150 km north of the ITS (~31°N) and more than 1000 km laterally beneath the Lhasa block (Wittlinger et al., 2009). In summary, the evidence the Indian continental subduction process beneath the Lhasa block. Following the Asia–India continental collision at Paleocene (ca. 62 Ma to 55 Ma), the Indian continental lithosphere started to subduct into the middle–lower crust beneath southern Lhasa. The break off of the subducted Neo-Tethyan slab during Eocene (51–48 Ma) probably caused a weakened lithospheric mantle providing the required condition for Indian continental subduction. Subsequently, subducted Indian sediments were likely responsible for the enriched characteristics of the latest Eocene Quguosha gabbros and Oligocene– Miocene ultrapotassic rocks, suggesting that the Indian continent crust had subducted into the SCLM level by at least ca. 35 Ma (Fig. 10). 4.2.2. Implications for Tibetan tectonic evolution The uplift of the Tibetan Plateau and its influence on global climate change have been the focus of numerous studies (e.g., Ruddiman and Kutzbach, 1991; Raymo and Ruddiman, 1992; Ruddiman, 1998; Dupont-Nivet et al., 2007). Various models have been proposed in the past decades to account for the mechanism of its uplift, two of which are of particular significance. The first attributes the surface uplift of the Tibetan Plateau to a dynamic response to convective removal or delamination of the lower portion of an overthickened Tibetan lithosphere (e.g., England and Houseman, 1986; Turner et al., 1993, 1996; Chung et al., 1998; Zhao et al., 2009; Liu et al., 2011). In the convective removal or lithospheric delamination model, the ultrapotassic rocks were commonly regarded as evidence of lithospheric mantle thinning, and are believed to indicate the time at which the plateau achieved its highest elevation (Molnar et al., 1993; Turner et al., 1993; 1996; Williams et al., 2001). According to this model, the Indian continental subduction could only have played a key role in forming the entire Tibetan plateau after the slab

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx

detachment or root foundering during the Oligocene (ca. 25 Ma), because ultrapotassic magmatism began in the Lhasa block at ~ 25 Ma (e.g., Chung et al., 2005, 2009; Zhao et al., 2009; Guo et al., 2015). This model suggests that these rocks formed in an intra-continental tectonic setting unrelated to subduction. The second model calls upon the northward underthrusting of the Indian continent and accompanying break off/delamination of the subducted Indian slab (e.g., Harrison et al., 1992; Tapponnier et al., 2001; Mahéo et al., 2002; Ding et al., 2003; Guo et al., 2013). The transition from the slightly enriched compositions of the latest Eocene Quguosha mafic potassic rocks to the highly enriched compositions of Oligocene–Miocene ultrapotassic rocks indicates an increasing contribution from the Indian continental crust and a progressive mantle enrichment process beneath the Lhasa block. This could best be explained if the continental crust started to underthrust beneath the mantle lithosphere of the southern Lhasa sub-block by at least ca. 35 Ma,which is earlier than the possible delamination or foundering of the orogenic root at 25–8 Ma. Moreover, the presence of thrust faults and contraction basins suggests that the Lhasa and Qiangtang blocks were undergoing crustal shortening during the Paleocene–Eocene (Yin and Harrison, 2000; Tapponnier et al., 2001; Wang et al., 2001, 2002; Kapp et al., 2005; Spurlin et al., 2005; Wang et al., 2014b). In addition, numerous new studies indicate that the Lhasa and Qiangtang blocks could have been at near-present elevation during the Middle–Late Eocene (e.g., Rowley and Currie, 2006; Wang et al., 2008a; 2008b; Polissar et al., 2009; Wang et al., 2010; Xu et al., 2013a; Ding et al., 2014; Wang et al., 2014b), which corresponds to northward underthrusting of the Indian continent beneath southern Tibet (Harrison et al., 1992; Yin and Harrison, 2000; Tapponnier et al., 2001; Ding et al., 2003; Kapp et al., 2005; Spurlin et al., 2005; Yin and Taylor, 2011; Jiang et al., 2014) and global cooling and Asian continental aridification beginning in the Eocene (Raymo and Ruddiman, 1992; Ruddiman, 1998; Dupont-Nivet et al., 2007). Accordingly, we propose that the underthrusting of the Indian continent has likely played a key role in the enrichment of the mantle beneath southern Tibet and the early uplift of the plateau by at least ca. 35 Ma. 5. Conclusions The Quguosha gabbros in the southern Lhasa sub-block were emplaced in the latest Eocene (~35 Ma). The Quguosha mafic magmas were generated by partial melting of the lithospheric mantle metasomatized by melts of Indian continental sediments accreted or underplated into the overlying mantle via continental subduction channel. The evolution from a Late Mesozoic depleted source to the slightly enriched latest Eocene source and highly enriched Oligocene–Miocene composition of mantle-derived mafic rocks indicates an increasing contribution of the Indian continent-derived components and a progressive mantle enrichment process beneath the Lhasa block. Our results suggest that the Indian continent had entered beneath the mantle lithosphere of the southern Lhasa sub-block by least 35 Ma, and the northward underthrusting of the Indian continent has played a key role in the enrichment of the mantle beneath southern Tibet and the formation of the Tibetan plateau. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.gr.2016.02.005. Acknowledgments Presented data are available by request from the corresponding author. We thank the editor-in-chief Prof. M. Santosh and the associate editor Professor Ze-Ming Zhang for their efforts in handling our manuscript. We are grateful to Professors Ze-Ming Zhang, Zheng-Fu Guo and Zhi-Dan Zhao for their constructive and helpful reviews. We appreciate the assistance of Yue-Heng Yang, Zhao-Chu Hu, ZhongYuan Ren, Jin-Long Ma, Guang-Qian Hu, Xiang-Lin Tu, Lie-Wen Xie

13

and Ying Liu for the zircon age and geochemical analyses. Financial support for this research was provided by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (grant no. XDB03010600), the National Natural Science Foundation of China (nos. 41402048, 41025006, 41421062, 41202040 and 41303017) and the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS 135 project Y234021001). This is the contribution no. IS-2225 from GIGCAS and contribution 802 from the ARC Centre of Excellence for Core to Crust Fluid Systems (http://www.ccfs.mq.edu.au). References Adam, J., Green, T., 2006. Trace element partitioning between mica- and amphibolebearing garnet lherzolite and hydrous basanitic melt: 1. Experimental results and the investigation of controls on partitioning behaviour. Contributions to Mineralogy and Petrology 152, 1–17. Allégre, C., Courtillot, V., Tapponnier, P., Hirn, A., Mattauer, M., Coulon, C., Jaeger, J., Achache, J., Schärer, U., Marcoux, J., 1984. Structure and evolution of the Himalaya– Tibet orogenic belt. Nature 307, 17–22. Arnaud, N.O., Vidal, P., Tapponnier, P., Matte, P., Deng, W.M., 1992. The high K2O volcanism of northwestern Tibet: geochemistry and tectonic implications. Earth and Planetary Science Letters 111, 351–367. Barbarin, B., Didier, J., 1991. Review of the main hypothesis proposed for the genesis and evolution of mafic microgranular enclaves. Developments in Petrology 13, 367–373. Beard, J.S., 1986. Characteristic mineralogy of arc-related cumulate gabbros: implications for the tectonic setting of gabbroic plutons and for andesite genesis. Geology 14, 848–851. Ben Othman, D., White, W.M., Patchett, J., 1989. The geochemistry of marine sediments, island arc magma genesis, and crust–mantle recycling. Earth and Planetary Science Letters 94, 1–21. Cai, F., Ding, L., Yue, Y., 2011. Provenance analysis of upper Cretaceous strata in the Tethys Himalaya, southern Tibet: implications for timing of India–Asia collision. Earth and Planetary Science Letters 305, 195–206. Castillo, P.R., Janney, P.E., Solidum, R.U., 1999. Petrology and geochemistry of Camiguin Island, southern Philippines: insights to the source of adakites and other lavas in a complex arc setting. Contributions to Mineralogy and Petrology 134, 33–51. Chauvel, C., Lewin, E., Carpentier, M., Arndt, N.T., Marini, J.C., 2008. Role of recycled oceanic basalt and sediment in generating the Hf–Nd mantle array. Nature Geoscience 1, 64–67. Chen, J.L., Xu, J.F., Wang, B.D., Kang, Z.Q., Jie, L., 2010. Origin of Cenozoic alkaline potassic volcanic rocks at Konglongxiang, Lhasa terrane, Tibetan Plateau: products of partial melting of a mafic lower-crustal source? Chemical Geology 273, 286–299. Chen, J.L., Zhao, W.X., Xu, J.F., Wang, B.D., Kang, Z.Q., 2012. Geochemistry of Miocene trachytes in Bugasi, Lhasa block, Tibetan Plateau: mixing products between mantle and crust-derived melts? Gondwana Research 21, 112–122. Chopin, C., 1984. Coesite and pure pyrope in high-grade blueschists of the Western Alps: a first record and some consequences. Contributions to Mineralogy and Petrology 86 (2), 107–118. Chu, M.-F., Chung, S.-L., O'Reilly, S.Y., Pearson, N.J., Wu, F.-Y., Li, X.-H., Liu, D., Ji, J., Chu, C.-H., Lee, H.-Y., 2011. India's hidden inputs to Tibetan orogeny revealed by Hf isotopes of Transhimalayan zircons and host rocks. Earth and Planetary Science Letters 307, 479–486. Chung, S.-L., Lo, C.-H., Lee, T.-Y., Zhang, Y., Xie, Y., Li, X., Wang, K.-L., Wang, P.-L., 1998. Diachronous uplift of the Tibetan plateau starting 40 Myr ago. Nature 394, 769–773. Chung, S.L., Liu, D., Ji, J., Chu, M.F., Lee, H.Y., Wen, D.J., Lo, C.H., Lee, T.Y., Qian, Q., Zhang, Q., 2003. Adakites from continental collision zones: melting of thickened lower crust beneath southern Tibet. Geology 31, 1021–1024. Chung, S.L., Chu, M.F., Zhang, Y., Xie, Y., Lo, C.H., Lee, T.Y., Lan, C.Y., Li, X., Zhang, Q., Wang, Y., 2005. Tibetan tectonic evolution inferred from spatial and temporal variations in post-collisional magmatism. Earth-Science Reviews 68, 173–196. Chung, S.L., Chu, M.F., Ji, J., O'Reilly, S.Y., Pearson, N., Liu, D., Lee, T.Y., Lo, C.H., 2009. The nature and timing of crustal thickening in Southern Tibet: geochemical and zircon Hf isotopic constraints from postcollisional adakites. Tectonophysics 477, 36–48. Coulon, C., Maluski, H., Bollinger, C., Wang, S., 1986. Mesozoic and Cenozoic volcanic rocks from central and southern Tibet: 39Ar–40Ar dating, petrological characteristics and geodynamical significance. Earth and Planetary Science Letters 79, 281–302. Debon, F., Fort, P.L.E., Sheppard, S.M.F., Sonet, J., 1986. The four plutonic belts of the Transhimalaya-Himalaya: A chemical, mineralogical, isotopic, and chronological synthesis along a Tibet-Nepal section. Journal of Petrology 27, 219–250. DeCelles, P.G., Robinson, D.M., Zandt, G., 2002. Implications of shortening in the Himalayan fold-thrust belt for uplift of the Tibetan Plateau. Tectonics 21 (6), 1062. http://dx.doi.org/10.1029/2001TC001322. DeCelles, P.G., Kapp, P., Gehrels, G.E., Ding, L., 2014. Paleocene–Eocene foreland basin evolution in the Himalaya of southern Tibet and Nepal: implications for the age of initial India–Asia collision. Tectonics 33 (5), 824–849. Depaolo, D.J., 1981. Neodymium isotopes in the Colorado Front Range and crust-mantle evolution in the Proterozoic. Nature 291, 193–196. Ding, L., Zhong, D., Yin, A., Kapp, P., Harrison, T.M., 2001. Cenozoic structural and metamorphic evolution of the eastern Himalayan syntaxis (Namche Barwa). Earth and Planetary Science Letters 192, 423–438. Ding, L., Kapp, P., Zhong, D., DENG, W., 2003. Cenozoic volcanism in Tibet: evidence for a transition from oceanic to continental subduction. Journal of Petrology 44, 1833–1865.

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

14

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx

Ding, L., Kapp, P., Wan, X., 2005. Paleocene–Eocene record of ophiolite obduction and initial India–Asia collision, south central Tibet. Tectonics 24, 1–18. Ding, L., Xu, Q., Yue, Y., Wang, H., Cai, F., Li, S., 2014. The Andean-type Gangdese Mountains: paleoelevation record from the Paleocene–Eocene Linzhou Basin. Earth and Planetary Science Letters 392, 250–264. Dupont-Nivet, G., Krijgsman, W., Langereis, C.G., Abels, H.A., Dai, S., Fang, X., 2007. Tibetan Plateau aridification linked to global cooling at the Eocene–Oligocene transition. Nature 445, 635–638. England, P., Houseman, G., 1986. Finite strain calculations of continental deformation: 2. Comparison with the India–Asia collision zone. Journal of Geophysical Research: Solid Earth 91, 3664–3676 (1978–2012). Fagan, T.J., Day, H.W., 1997. Formation of amphibole after clinopyroxene by dehydration reactions: implications for pseudomorphic replacement and mass fluxes. Geology 25, 395–398. Gallagher, K., Hawkesworth, C., 1992. Dehydration melting and the generation of continental flood basalts. Nature 358, 57–59. Gao, Y., Hou, Z., Kamber, B.S., Wei, R., Meng, X., Zhao, R., 2007. Lamproitic rocks from a continental collision zone: evidence for recycling of subducted Tethyan oceanic sediments in the mantle beneath southern Tibet. Journal of Petrology 48, 729–752. Gao, Y., Yang, Z., Santosh, M., Hou, Z., Wei, R., Tian, S., 2010. Adakitic rocks from slab meltmodified mantle sources in the continental collision zone of southern Tibet. Lithos 119, 651–663. Gill, J.B., 1981. Orogenic Andesites and Plate Tectonics. Springer-Verlag, Berlin Heidelberg, p. 390. Grimes, C.B., John, B.E., Cheadle, M.J., Wooden, J.L., 2008. Protracted construction of gabbroic crust at a slow spreading ridge: constraints from 206Pb/238U zircon ages from Atlantis Massif and IODP Hole U1309D (30°N, MAR). Geochemistry, Geophysics, Geosystems 9, Q08012. http://dx.doi.org/10.1029/2008GC002063. Guillot, S., Sigoyer, J.D., Lardeaux, J.M., Mascle, G., 1997. Eclogitic metasediments from the Tso Morari area (Ladakh, Himalaya): evidence for continental subduction during India–Asia convergence. Contributions to Mineralogy and Petrology 128, 197–212. Guo, Z., Wilson, M., Liu, J., 2007. Post-collisional adakites in south Tibet: products of partial melting of subduction-modified lower crust. Lithos 96, 205–224. Guo, Z., Wilson, M., 2012). The Himalayan leucogranites: constraints on the nature of their crustal source region and geodynamic setting. Gondwana Research 22, 360–376. Guo, Z., Wilson, M., Zhang, M., Cheng, Z., Zhang, L., 2013. Post-collisional, K-rich mafic magmatism in south Tibet: constraints on Indian slab-to-wedge transport processes and plateau uplift. Contributions to Mineralogy and Petrology 165, 1311–1340. Guo, L., Liu, Y., Liu, S., Cawood, P.A., Wang, Z., Liu, H., 2014a. Petrogenesis of Early to Middle Jurassic granitoid rocks from the Gangdese belt, Southern Tibet: implications for early history of the Neo-Tethys. Lithos 179, 320–333. Guo, Z., Wilson, M., Zhang, L., Zhang, M., Cheng, Z., Liu, J., 2014b. The role of subduction channel mélanges and convergent subduction systems in the petrogenesis of postcollisional K-rich mafic magmatism in NW Tibet. Lithos 198, 184–201. Guo, Z., Wilson, M., Zhang, M., Cheng, Z., Zhang, L., 2015. Post-collisional ultrapotassic mafic magmatism in South Tibet: products of partial melting of pyroxenite in the mantle wedge induced by roll-back and delamination of the subducted Indian continental lithosphere slab. Journal of Petrology 56, 1365–1406. Hacker, B.R., Kelemen, P.B., Behn, M.D., 2011. Differentiation of the continental crust by relamination. Earth and Planetary Science Letters 307, 501–516. Harris, N., Inger, S., Ronghua, X., 1990. Cretaceous plutonism in Central Tibet: an example of post-collision magmatism? Journal of Volcanology and Geothermal Research 44, 21–32. Harrison, T.M., Copeland, P., Kidd, W., Yin, A., 1992. Raising Tibet. Science 255, 1663–1670. Hébert, R., Guilmette, C., Dostal, J., Bezard, R., Lesage, G., Bédard, É., Wang, C., 2014. Miocene post-collisional shoshonites and their crustal xenoliths, Yarlung Zangbo Suture Zone southern Tibet: geodynamic implications. Gondwana Research 25, 1263–1271. Hoskin, P., Black, L., 2000. Metamorphic zircon formation by solid state recrystallization of protolith igneous zircon. Journal of Metamorphic Geology 18, 423–439. Hou, Z.Q., Gao, Y.F., Qu, X.M., Rui, Z.Y., Mo, X.X., 2004. Origin of adakitic intrusives generated during mid-Miocene east–west extension in southern Tibet. Earth and Planetary Science Letters 220, 139–155. Hou, Z.-Q., Zheng, Y.-C., Zeng, L.-S., Gao, L.-E., Huang, K.-X., Li, W., Li, Q.-Y., Fu, Q., Liang, W., Sun, Q.-Z., 2012. Eocene–Oligocene granitoids in southern Tibet: constraints on crustal anatexis and tectonic evolution of the Himalayan orogen. Earth and Planetary Science Letters 349, 38–52. Hu, X.M., Sinclair, H.D., Wang, J.G., Jiang, H.H., Wu, F.Y., 2012. Late Cretaceous–Palaeogene stratigraphic and basin evolution in the Zhepure Mountain of southern Tibet: implication for the timing of India–Asia initial collision. Basin Research 24, 520–543. Hu, X., Garzanti, E., Moore, T., Raffi, I., 2015. Direct stratigraphic dating of India–Asia collision onset at the Selandian (middle Paleocene, 59 ± 1 Ma). Geology 43, 859–862. Huang, F., Chen, J.L., Xu, J.F., Wang, B.D., Li, J., 2015. Os–Nd–Sr isotopes in Miocene ultrapotassic rocks of southern Tibet: partial melting of a pyroxenite-bearing lithospheric mantle? Geochimica et Cosmochimica Acta 163, 279–298. Ji, W.Q., Wu, F.Y., Chung, S.L., Li, J.X., Liu, C.Z., 2009. Zircon U–Pb geochronology and Hf isotopic constraints on petrogenesis of the Gangdese batholith, southern Tibet. Chemical Geology 262, 229–245. Ji, W.Q., Wu, F.Y., Liu, C.Z., Chung, S.L., 2012. Early Eocene crustal thickening in southern Tibet: new age and geochemical constraints from the Gangdese batholith. Journal of Asian Earth Sciences 53, 82–95. Ji, W.Q., Wu, F.Y., Chung, S.L., Liu, C.Z., 2014. The Gangdese magmatic constraints on a latest Cretaceous lithospheric delamination of the Lhasa terrane, southern Tibet. Lithos 210, 168–180.

Jiang, Z.Q., Wang, Q., Wyman, D.A., Tang, G.J., Jia, X.H., Yang, Y.H., Yu, H.X., 2011. Origin of 30 Ma Chongmuda adakitic intrusive rocks in the southern Gangdese region, southern Tibet: partial melting of the northward subducted Indian continent crust? Geochimica 40, 126–146 (in Chinese with English abstract). Jiang, Z.-Q., Wang, Q., Wyman, D.A., Li, Z.-X., Yang, J.-H., Shi, X.-B., Ma, L., Tang, G.-J., Gou, G.-N., Jia, X.-H., 2014. Transition from oceanic to continental lithosphere subduction in southern Tibet: evidence from the Late Cretaceous–Early Oligocene (~91–30 Ma) intrusive rocks in the Chanang–Zedong area, southern Gangdese. Lithos 196–197, 213–231. Kang, Z.Q., Xu, J.F., Chen, J.L., Wang, B.D., Dong, Y.H., 2010. The geochronology of Sangri Group volcanic rocks in Tibet constraints from later Mamen intrusions. Geochimica 39, 520–530. Kapp, P., Yin, A., Harrison, T.M., Ding, L., 2005. Cretaceous–Tertiary shortening, basin development, and volcanism in central Tibet. Geological Society of American Bulletin 117 (7), 865–878. Klootwijk, C.T., Gee, J.S., Peirce, J.W., Smith, G.M., McFadden, P.L., 1992. An early India–Asia contact: paleomagnetic constraints from Ninetyeast ridge, ODP Leg 121. Geology 20, 395–398. Kosarev, G., Kind, R., Sobolev, S., Yuan, X., Hanka, W., Oreshin, S., 1999. Seismic evidence for a detached Indian lithospheric mantle beneath Tibet. Science 283, 1306–1309. Leake, B.E., Woolley, A.R., Arps, C.E., Birch, W.D., Gilbert, M.C., Grice, J.D., Hawthorne, F.C., Kato, A., Kisch, H.J., Krivovichev, V.G., 1997. Nomenclature of amphiboles; Report of the Subcommittee on Amphiboles of the International Mineralogical Association, Commission on New Minerals and Mineral Names. American Mineralogist 82, 1019–1037. Lee, H.Y., Chung, S.L., Lo, C.H., Ji, J., Lee, T.Y., Qian, Q., Zhang, Q., 2009. Eocene Neotethyan slab breakoff in southern Tibet inferred from the Linzizong volcanic record. Tectonophysics 477, 20–35. Lee, H.Y., Chung, S.L., Ji, J.Q., Qian, Q., Galleta, S., Lo, C.H., Lee, T.Y., Zhang, Q., 2012. Geochemical and Sr–Nd isotopic constraints on the genesis of the Cenozoic Linzizong volcanic successions, southern Tibet. Journal of Asian Earth Science 53, 96–114. Li, C., Van der Hilst, R.D., Meltzer, A.S., Engdahl, E.R., 2008. Subduction of the Indian lithosphere beneath the Tibetan Plateau and Burma. Earth and Planetary Science Letters 274, 157–168. Liu, C.Z., Wu, F.Y., Chung, S.L., Zhao, Z.D., 2011. Fragments of hot and metasomatized mantle lithosphere in Middle Miocene ultrapotassic lavas, southern Tibet. Geology 39, 923–926. Liu, C.Z., Wu, F.Y., Chung, S.L., Li, Q.L., Sun, W.D., Ji, W.Q., 2014a. A ‘hidden’ 18O-enriched reservoir in the sub-arc mantle. Scientific Reports 4, 4232-4232. Liu, D., Zhao, Z., Zhu, D.-C., Niu, Y., DePaolo, D.J., Harrison, T.M., Mo, X., Dong, G., Zhou, S., Sun, C., Zhang, Z., Liu, J., 2014b. Postcollisional potassic and ultrapotassic rocks in southern Tibet: mantle and crustal origins in response to India–Asia collision and convergence. Geochimica et Cosmochimica Acta 143, 207–231. Liu, D., Zhao, Z., Zhu, D.C., Niu, Y., Widom, E., Teng, F.Z., Depaolo, D.J., Ke, S., Xu, J.F., Wang, Q., 2015. Identifying mantle carbonatite metasomatism through Os–Sr–Mg isotopes in Tibetan ultrapotassic rocks. Earth and Planetary Science Letters 430, 458–469. Ma, L., Wang, Q., Wyman, D.A., Jiang, Z.Q., Yang, J.H., Li, Q.L., Gou, G.N., Guo, H.F., 2013a. Late Cretaceous crustal growth in the Gangdese area, southern Tibet: petrological and Sr–Nd–Hf–O isotopic evidence from Zhengga diorite–gabbro. Chemical Geology 349, 54–70. Ma, L., Wang, Q., Li, Z.X., Wyman, D.A., Jiang, Z.Q., Yang, J.H., Gou, G.N., Guo, H.F., 2013b. Early Late Cretaceous (ca. 93 Ma) norites and hornblendites in the Milin area, eastern Gangdese: lithosphere–asthenosphere interaction during slab roll-back and an insight into early Late Cretaceous (ca. 100–80 Ma) magmatic “flare-up” in southern Lhasa (Tibet). Lithos 172, 17–30. Ma, L., Wang, Q., Wyman, D.A., Li, Z.-X., Jiang, Z.-Q., Yang, J.-H., Gou, G.-N., Guo, H.-F., 2013c. Late Cretaceous (100–89Ma) magnesian charnockites with adakitic affinities in the Milin area, eastern Gangdese: Partial melting of subducted oceanic crust and implications for crustal growth in southern Tibet. Lithos 175, 315–332. Ma, L., Wang, B.D., Jiang, Z.Q., Wang, Q., Li, Z.X., Wyman, D.A., Zhao, S.R., Yang, J.H., Gou, G.N., Guo, H.F., 2014. Petrogenesis of the Early Eocene adakitic rocks in the Napuri area, southern Lhasa: partial melting of thickened lower crust during slab break-off and implications for crustal thickening in southern Tibet. Lithos 196, 321–338. Ma, L., Wang, Q., Wyman, D.A., Jiang, Z.Q., Wu, F.Y., Li, X.H., Yang, J.H., Gou, G.N., Guo, H.F., 2015. Late Cretaceous back-arc extension and arc system evolution in the Gangdese area, southern Tibet: geochronological, petrological, and Sr–Nd–Hf–O isotopic evidence from Dagze diabases. Journal of Geophysical Research Solid Earth 120, 6159–6181. Mahéo, G., Guillot, S., Blichert-Toft, J., Rolland, Y., Pêcher, A., 2002. A slab breakoff model for the Neogene thermal evolution of South Karakorum and South Tibet. Earth and Planetary Science Letters 195, 45–58. Mahoney, J.J., Frei, R., Tejada, M., Mo, X., Leat, P., Nägler, T., 1998. Tracing the Indian Ocean mantle domain through time: isotopic results from old West Indian, East Tethyan, and South Pacific seafloor. Journal of Petrology 39, 1285–1306. Marini, J.-C., Chauvel, C., Maury, R.C., 2005. Hf isotope compositions of northern Luzon arc lavas suggest involvement of pelagic sediments in their source. Contributions to Mineralogy and Petrology 149, 216–232. Marschall, H.R., Schumacher, J.C., 2012. Arc magmas sourced from mélange diapirs in subduction zones. Nature Geoscience 5, 862–867. McKenzie, D., O'nions, R.K., 1991. Partial melt distributions from inversion of rare earth element concentrations. Journal of Petrology 32, 1021–1091. McLennan, S., Taylor, S., McCulloch, M., Maynard, J., 1990. Geochemical and Nd–Sr isotopic composition of deep-sea turbidites: crustal evolution and plate tectonic associations. Geochimica et Cosmochimica Acta 54, 2015–2050. Middlemost, E.A., 1994. Naming materials in the magma/igneous rock system. Earth-Science Reviews 37, 215–224.

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx Miller, C., Schuster, R., Klötzli, U., Frank, W., Purtscheller, F., 1999. Post-collisional potassic and ultrapotassic magmatism in SW Tibet: geochemical and Sr–Nd–Pb–O isotopic constraints for mantle source characteristics and petrogenesis. Journal of Petrology 40, 1399–1424. Mo, X., Zhao, Z., Deng, J., Dong, G., Zhou, S., Guo, T., Zhang, S., Wang, L., 2003. Response of volcanism to the India–Asia collision. Earth Science Frontiers 10, 135–148. Mo, X.X., Zhao, Z.D., Deng, J.F., Martin, F., Yu, X.H., Luo, Z.H., Li, Y.G., Zhou, S., Dong, G.C., Zhu, D.C., Wang, L.L., 2006. Petrology and geochemistry of postcollisional volcanic rocks from the Tibetan Plateau: implications for lithosphere heterogeneity and collision-induced asthenospheric mantle flow. Geological Society of America Special Papers 409, 507–530. Mo, X., Hou, Z., Niu, Y., Dong, G., Qu, X., Zhao, Z., Yang, Z., 2007. Mantle contributions to crustal thickening during continental collision: evidence from Cenozoic igneous rocks in southern Tibet. Lithos 96, 225–242. Mo, X., Niu, Y., Dong, G., Zhao, Z., Hou, Z., Zhou, S., Ke, S., 2008. Contribution of syncollisional felsic magmatism to continental crust growth: a case study of the Paleogene Linzizong volcanic succession in southern Tibet. Chemical Geology 250, 49–67. Molnar, P., England, P., Martinod, J., 1993. Mantle dynamics, uplift of the Tibetan Plateau, and the Indian Monsoon. Reviews of Geophysics 31 (4), 357–396. Mukherjee, B.K., Sachan, H.K., Ogasawara, Y., Muko, A., Yoshioka, N., 2003. Carbonatebearing UHPM rocks from the Tso-Morari Region, Ladakh, India: petrological implications. International Geology Review 2003, 49–69. Nábělek, J., Hetényi, G., Vergne, J., Sapkota, S., Kafle, B., Jiang, M., Su, H., Chen, J., Huang, B.S., 2009. Underplating in the Himalaya–Tibet collision zone revealed by the Hi-CLIMB experiment. Science 325, 1371–1374. Nomade, S., Renne, P.R., Mo, X., Zhao, Z., Zhou, S., 2004. Miocene volcanism in the Lhasa block, Tibet: spatial trends and geodynamic implications. Earth and Planetary Science Letters 221, 227–243. O'Brien, P.J., Zotov, N., Law, R., Khan, M.A., Jan, M.Q., 2001. Coesite in Himalayan eclogite and implications for models of India–Asia collision. Geology 29, 435–438. Olafsson, M., Eggler, D.H., 1983. Phase relations of amphibole, amphibole–carbonate, and phlogopite–carbonate peridotite: petrologic constraints on the asthenosphere. Earth and Planetary Science Letters 64, 305–315. Otten, M.T., 1984. The origin of brown hornblende in the Artfjället gabbro and dolerites. Contributions to Mineralogy and Petrology 86, 189–199. Owens, T.J., Zandt, G., 1997. Implications of crustal property variations for models of Tibetan Plateau evolution. Nature 387, 37–43. Pearce, J.A., Mei, H.J., 1988. Volcanic rocks of the 1985 Tibet geotraverse: Lhasa to Golmud. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences 327, 169–201. Pearce, J.A., Parkinson, I.J., 1993. Trace element models for mantle melting: application to volcanic arc petrogenesis. Geological Society, London, Special Publications 76, 373–403. Peccerillo, A., Taylor, S.R., 1976. Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastamonu area, northern Turkey. Contributions to Mineralogy and Petrology 58, 63–81. Pedersen, A.K., 1979. Basaltic glass with high-temperature equilibrated immiscible sulphide bodies with native iron from Disko, central West Greenland. Contributions to Mineralogy and Petrology 69, 397–407. Plank, T., Langmuir, C.H., 1998. The chemical composition of subducting sediment and its consequences for the crust and mantle. Chemical Geology 145, 325–394. Polissar, P.J., Freeman, K.H., Rowley, D.B., McInerney, F.A., Currie, B.S., 2009. Paleoaltimetry of the Tibetan Plateau from D/H ratios of lipid biomarkers. Earth and Planetary Science Letters 287 (1), 64–76. Raymo, M.E., Ruddiman, W.F., 1992. Tectonic forcing of Late Cenozoic climate. Nature 359, 117–122. Replumaz, A., Negredo, A.M., Villaseñor, A., Guillot, S., 2010. Indian continental subduction and slab break-off during Tertiary collision. Terra Nova 22, 290–296. Replumaz, A., Guillot, S., Villaseñor, A., Negredo, A.M., 2013. Amount of Asian lithospheric mantle subducted during the India/Asia collision. Gondwana Research 24, 936–945. Replumaz, A., Capitanio, F.A., Guillot, S., Negredo, A.M., Villaseñor, A., 2014. The coupling of Indian subduction and Asian continental tectonics. Gondwana Research 26, 608–626. Rogers, N., Macdonald, R., Fitton, J.G., George, R., Smith, M., Barreiro, B., 2000. Two mantle plumes beneath the East African rift system: Sr, Nd and Pb isotope evidence from Kenya Rift basalts. Earth and Planetary Science Letters 176, 387–400. Rowley, D.B., Currie, B.S., 2006. Palaeo-altimetry of the late Eocene to Miocene Lunpola basin, Central Tibet. Nature 439 (7077), 677–681. Ruddiman, W., 1998. Early uplift in Tibet? Nature 394, 723–725. Ruddiman, W.F., Kutzbach, J.E., 1991. Plateau uplift and climatic change. Scientific American 264 (3), 66–74. Rudnick, R.L., Fountain, D.M., 1995. Nature and composition of the continental crust: a lower crustal perspective. Reviews of Geophysics 33, 267–309. Rudnick, R., Gao, S., 2003. Composition of the continental crust. Treatise on geochemistry 3, 1–64. Schulte-Pelkum, V., Monsalve, G., Sheehan, A., Pandey, M.R., Sapkota, S., Bilham, R., Wu, F., 2005. Imaging the Indian subcontinent beneath the Himalaya. Nature 435, 1222–1225. Smith, D.C., 1984. Coesite in clinopyroxene in the Caledonides and its implications for geodynamics. Nature 310, 641–644. Sobolev, N., Shatsky, V., 1990. Diamond inclusions in garnets from metamorphic rocks: a new environment for diamond formation. Nature 343, 742–746. Sobolev, A.V., Hofmann, A.W., Sobolev, S.V., Nikogosian, I.K., 2005. An olivine-free mantle source of Hawaiian shield basalts. Nature 434, 590–597. Söderlund, U., Patchett, P.J., Vervoort, J.D., Isachsen, C.E., 2004. The 176 Lu decay constant determined by Lu–Hf and U–Pb isotope systematics of Precambrian mafic intrusions. Earth and Planetary Science Letters 219, 311–324.

15

Spurlin, M.S., Yin, A., Horton, B.K., Zhou, J., Wang, J., 2005. Structural evolution of the Yushu–Nangqian region and its relationship to syncollisional igneous activity, east– central Tibet. Geological Society of American Bulletin 117 (9), 1293–1317. Stalder, R., Foley, S., Brey, G., Horn, I., 1998. Mineral-aqueous fluid partitioning of trace elements at 900–1200 °C and 3.0–5.7 GPa: new experimental data for garnet, clinopyroxene, and rutile, and implications for mantle metasomatism. Geochimica et Cosmochimica Acta 62, 1781–1801. Stern, C.R., 2011. Subduction erosion: rates, mechanism, and its role in arc magmatism and the evolution of the continental crust and mantle. Gondwana Research 20, 284–308. Streck, M.J., Leeman, W.P., Chesley, J., 2007. High-Mg andesite from Mount Shasta: a product of magma mixing and contamination, not a primitive mantle melt. Geology 35, 351–354. Sun, S.S., McDonough, W., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Geological Society, London, Special Publications 42, 313–345. Sun, J.-F., Yang, J.-H., Wu, F.-Y., Li, X.-H., Yang, Y.-H., Xie, L.-W., Wilde, S.A., 2010. Magma mixing controlling the origin of the Early Cretaceous Fangshan granitic pluton, North China Craton: in situ U–Pb age and Sr-, Nd-, Hf-and O-isotope evidence. Lithos 120, 421–438. Tapponnier, P., Zhiqin, X., Roger, F., Meyer, B., Arnaud, N., Wittlinger, G., Jingsui, Y., 2001. Oblique stepwise rise and growth of the Tibet Plateau. science 294, 1671–1677. Tatsumi, Y., Eggins, S.M., 1995. Subduction zone magmatism. Blackwell, Cambridge. Tian, S.-H., Yang, Z.-S., Hou, Z.-Q., Mo, X.-X., Hu, W.-J., Zhao, Y., Zhao, X.-Y., 2015. Subduction of the Indian lower crust beneath southern Tibet revealed by the post-collisional potassic and ultrapotassic rocks in SW Tibet. Gondwana Research http://dx.doi.org/ 10.1016/j.gr.2015.09.005. Tilmann, F., Ni, J., 2003. Seismic imaging of the downwelling Indian lithosphere beneath central Tibet. Science 300, 1424–1427. Tonarini, S., Villa, I.M., Oberli, F., Meier, M., Spencer, D.A., Pognante, U., Ramsay, J.G., 1993. Eocene age of eclogite metamorphism in Pakistan Himalaya: implications for India– Eurasia collision. Terra Nova 5, 13–20. Turner, S., 1996. Petrogenesis of the late-Delamerian gabbroic complex at Black Hill, South Australia: implications for convective thinning of the lithospheric mantle. Mineralogy and Petrology 56, 51–89. Turner, S., Hawkesworth, C., Liu, J.Q., Rogers, N., Kelley, S., Van Calsteren, P., 1993. Timing of Tibetan uplift constrained by analysis of volcanic rocks. Nature 364, 50–54. Turner, S., Arnaud, N., LIU, J., Rogers, N., Hawkesworth, C., Harris, N., Kelley, S., Van Calsteren, P., Deng, W., 1996. Post-collision, shoshonitic volcanism on the Tibetan Plateau: implications for convective thinning of the lithosphere and the source of ocean island basalts. Journal of Petrology 37, 45–71. Vervoort, J.D., Blichert-Toft, J., 1999. Evolution of the depleted mantle: Hf isotope evidence from juvenile rocks through time. Geochimica et Cosmochimica Acta 63, 533–556. von Huene, R.V., Scholl, D.W., 1991. Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust. Reviews of Geophysics 29, 279–316. von Huene, R.V., Ranero, C.R., Vannucchi, P., 2004. Generic model of subduction erosion. Geology 32, 913–916. Wang, J.H., Yin, A., Harrison, T.M., Grove, M., Zhang, Y.Q., Xie, G.H., 2001. A tectonic model for Cenozoic igneous activities in the eastern Indo–Asian collision zone. Earth and Planetary Science Letters 188 (1–2), 123–133. Wang, C., Liu, Z., Yi, H., Liu, S., Zhao, X., 2002. Tertiary crustal shortening and peneplanation in the Hoh Xil region: implications for the tectonic history of the northern Tibetan Plateau. Journal of Asian Earth Sciences 20 (3), 211–223. Wang, C., Zhao, X., Liu, Z., Lippert, P.C., Graham, S.A., Coe, R.S., Yi, H., Zhu, L., Liu, S., Li, Y., 2008a. Constraints on the early uplift history of the Tibetan Plateau. Proceedings of the National Academy of Sciences 105, 4987–4992. Wang, Q., Wyman, D.A., Xu, J., Dong, Y., Vasconcelos, P.M., Pearson, N., Wan, Y., Dong, H., Li, C., Yu, Y., Zhu, T., Feng, X., Zhang, Q., Zi, F., Chu, Z., 2008b. Eocene melting of subducting continental crust and early uplifting of Central Tibet: evidence from central–western Qiangtang high-K calc-alkaline andesites, dacites and rhyolites. Earth and Planetary Science Letters 272 (1), 158–171. Wang, Q., Wyman, D.A., Li, Z.X., Sun, W.D., Chung, S.L., Vasconcelos, P.M., Zhang, Q.Y., Dong, H., Yu, Y.S., Pearson, N., Qiu, H.N., Zhu, T.X., Feng, X.T., 2010. Eocene north– south trending dikes in central Tibet: new constraints on the timing of east–west extension with implications for early plateau uplift? Earth and Planetary Science Letters 298, 205–216. Wang, B.D., Chen, J.L., Xu, J.F., Wang, L.Q., 2014a. Geochemical and Sr–Nd–Pb–Os isotopic compositions of Miocene ultrapotassic rocks in southern Tibet: petrogenesis and implications for the regional tectonic history. Lithos s 208–209, 237–250. Wang, C., Dai, J., Zhao, X., Li, Y., Graham, S.A., He, D., Ran, B., Meng, J., 2014b. Outwardgrowth of the Tibetan Plateau during the Cenozoic: a review. Tectonophysics 621, 1–43. Wen, D.R., 2007. The Gangdese Batholith, Southern Tibet: Ages, Geochemical Characteristics and Petrogenesis. National Taiwan University, Taibei, p. 140. Wen, D.R., Liu, D., Chung, S.L., Chu, M.F., Ji, J., Zhang, Q., Song, B., Lee, T.Y., Yeh, M.W., Lo, C.H., 2008. Zircon SHRIMP U–Pb ages of the Gangdese Batholith and implications for Neotethyan subduction in southern Tibet. Chemical Geology 252, 191–201. Williams, H., Turner, S., Kelley, S., Harris, N., 2001. Age and composition of dikes in Southern Tibet: New constraints on the timing of east-west extension and its relationship to postcollisional volcanism. Geology 29, 339. Williams, H., Turner, S., Pearce, J., Kelley, S., Harris, N., 2004. Nature of the source regions for post-collisional, potassic magmatism in southern and northern Tibet from geochemical variations and inverse trace element modelling. Journal of Petrology 45, 555–607. Wilson, M., 1989. Igneous petrogenesis. Unwin Hyman, London.

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005

16

L. Ma et al. / Gondwana Research xxx (2016) xxx–xxx

Wittlinger, G., Farra, V., Hetényi, G., Vergne, J., Nabelek, J., 2009. Seismic velocities in Southern Tibet lower crust: a receiver function approach for eclogite detection. Geophysical Journal International 177, 1037–1049. Wu, F.Y., Ji, W.Q., Wang, J.G., Liu, C.Z., Chung, S.L., Clift, P.D., 2014. Zircon U–Pb and Hf isotopic constraints on the onset time of India–Asia collision. American Journal of Science 314 (2), 548–579. Xie, R.W., Peng, X.J., Duan, G.X., Jampa-Tashi, Helo-Namgyal, Cheng, D,Q., Sha, S.L., Peng, D.P., Chen, Y.M., Bao, J.Y., He, S.H., 2007. 1:250,000 geological report of Zedang County with geological map. Xizang Province Geology Survey (unpublished, in Chinese). Xu, S.T., Okay, A.I., Ji, S.Y., Sengor, A.M.C., Su, W., Liu, Y.C., Jiang, L.L., 1992. Diamond from the Dabie Shan metamorphic rocks and its implication for tectonic setting. Science 256, 80–82. Xu, W.C., Zhang, H.F., Guo, L., Yuan, H.L., 2010. Miocene high Sr/Y magmatism, south Tibet: product of partial melting of subducted Indian continental crust and its tectonic implication. Lithos 114, 293–306. Xu, Q., Ding, L., Zhang, L., Cai, F., Lai, Q., Yang, D., Liu-Zeng, J., 2013a. Paleogene high elevations in the Qiangtang Terrane, central Tibetan Plateau. Earth and Planetary Science Letters 362, 31–42. Xu, W.C., Zhang, H.F., Harris, N., Guo, L., Pan, F.B., 2013b. Rapid Eocene erosion, sedimentation and burial in the eastern Himalayan syntaxis and its geodynamic significance. Gondwana Research 23, 715–725. Yang, J.-H., Wu, F.-Y., Wilde, S.A., Xie, L.-W., Yang, Y.-H., Liu, X.-M., 2007. Tracing magma mixing in granite genesis: in situ U–Pb dating and Hf-isotope analysis of zircons. Contributions to Mineralogy and Petrology 153, 177–190. Yi, Z., Huang, B., Chen, J., Chen, L., Wang, H., 2011. Paleomagnetism of early Paleogene marine sediments in Southern Tibet, China: implications to onset of the India–Asia collision and size of greater India. Earth and Planetary Science Letters 309, 153–165. Yin, A., Harrison, T.M., 2000. Geologic evolution of the Himalayan–Tibetan orogen. Annual Review of Earth and Planetary Sciences 28, 211–280. Yin, A., Taylor, M.H., 2011. Mechanics of V-shaped conjugate strike-slip faults and the corresponding continuum mode of continental deformation. Geological Society of America Bulletin 123, 1798–1821. Zhang, S.Q., Mahoney, J., Mo, X.X., Ghazi, A., Milani, L., Crawford, A., Guo, T.Y., Zhao, Z.D., 2005. Evidence for a widespread Tethyan upper mantle with Indian-Ocean-type isotopic characteristics. Journal of Petrology 46, 829–858. Zhang, Z.M., Zhao, G.C., Santosh, M., Wang, J.L., Dong, X., Liou, J.G., 2010. Two stages of granulite facies metamorphism in the eastern Himalayan syntaxis, south Tibet: petrology, zircon geochronology and implications for the subduction of Neo Tethys and the Indian continent beneath Asia. Journal of Metamorphic Geology 28, 719–733. Zhang, Z.M., Dong, X., Xiang, H., Liou, J.G., Santosh, M., 2013. Building of the Deep Gangdese Arc, South Tibet: Paleocene Plutonism and granulite-facies metamorphism. Journal of Petrology 54, 2547–2580. Zhang, L.Y., Ducea, M.N., Ding, L., Pullen, A., Kapp, P., Hoffman, D., 2014a. Southern Tibetan Oligocene–Miocene adakites: a record of Indian slab tearing. Lithos 210-211, 209–223.

Zhang, Z.M., Dong, X., Xiang, H., He, Z., Liou, J., 2014b. Metagabbros of the Gangdese arc root, South Tibet: implications for the growth of continental crust. Geochimica et Cosmochimica Acta 143, 268–284. Zhang, Z.M., Dong, X., Santosh, M., Zhao, G.C., 2014c. Metamorphism and tectonic evolution of the Lhasa terrane, Central Tibet. Gondwana Research 25, 170–189. Zhang, H., Zhao, D., Zhao, J., Liu, H., 2015a. Tomographic imaging of the underthrusting Indian slab and mantle upwelling beneath central Tibet. Gondwana Research 28, 121–132. Zhang, Z.M., Dong, X., Xiang, H., Ding, H., He, Z., Liou, J.G., 2015b. Reworking of the Gangdese magmatic arc, southeastern Tibet: post-collisional metamorphism and anatexis. Journal of Metamorphic Geology 33, 1–21. Zhao, W., Nelson, K., 1993. Deep seismic reflection evidence for continental underthrusting beneath southern Tibet. Nature 366, 557–559. Zhao, Z., Mo, X., Dilek, Y., Niu, Y., DePaolo, D.J., Robinson, P., Zhu, D., Sun, C., Dong, G., Zhou, S., 2009. Geochemical and Sr–Nd–Pb–O isotopic compositions of the post-collisional ultrapotassic magmatism in SW Tibet: petrogenesis and implications for India intra-continental subduction beneath southern Tibet. Lithos 113, 190–212. Zhao, J., Yuan, X., Liu, H., Kumar, P., Pei, S., Kind, R., Zhang, Z., Teng, J., Ding, L., Gao, X., 2010. The boundary between the Indian and Asian tectonic plates below Tibet. Proceedings of the National Academy of Sciences 107, 11229–11233. Zhao, W., Kumar, P., Mechie, J., Kind, R., Meissner, R., Wu, Z., Shi, D., Su, H., Xue, G., Karplus, M., 2011. Tibetan plate overriding the Asian plate in central and northern Tibet. Nature Geoscience 4, 870–873. Zheng, Y.F., 2012. Metamorphic chemical geodynamics in continental subduction zones. Chemical Geology 328, 5–48. Zhu, D.C., Pan, G.T., Chung, S.L., Liao, Z.L., Wang, L.Q., Li, G.M., 2008. SHRIMP zircon age and geochemical constraints on the origin of Lower Jurassic volcanic rocks from the Yeba Formation, southern Gangdese, South Tibet. International Geology Review 50, 442–471. Zhu, D.C., Zhao, Z.D., Pan, G.T., Lee, H.Y., Kang, Z.Q., Liao, Z.L., Wang, L.Q., Li, G.M., Dong, G.C., Liu, B., 2009. Early cretaceous subduction-related adakite-like rocks of the Gangdese Belt, southern Tibet: products of slab melting and subsequent meltperidotite interaction? Journal of Asian Earth Sciences 34, 298–309. Zhu, D.C., Zhao, Z.D., Niu, Y., Mo, X.X., Chung, S.L., Hou, Z.Q., Wang, L.Q., Wu, F.Y., 2011. The Lhasa terrane: record of a microcontinent and its histories of drift and growth. Earth and Planetary Science Letters 301, 241–255. Zhu, D.-C., Zhao, Z.-D., Niu, Y., Dilek, Y., Hou, Z.-Q., Mo, X.-X., 2013. The origin and preCenozoic evolution of the Tibetan Plateau. Gondwana Research 23, 1429–1454. Zhu, D.C., Wang, Q., Zhao, Z.D., Chung, S.L., Cawood, P.A., Niu, Y., Liu, S.A., Wu, F.Y., Mo, X.X., 2015. Magmatic record of India–Asia collision. Scientific Reports 5, 14289. http://dx.doi.org/10.1038/s\rep14289.

Please cite this article as: Ma, L., et al., Subduction of Indian continent beneath southern Tibet in the latest Eocene (~35 Ma): Insights from the Quguosha gabbros in southern Lhasa block, Gondwana Research (2016), http://dx.doi.org/10.1016/j.gr.2016.02.005