Crustal Thickening Prior to 38Ma in Southern Tibet: Evidence From

Gondwana Research 21 (2012) 88–99

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Crustal thickening prior to 38 Ma in southern Tibet: Evidence from lower crust-derived adakitic magmatism in the Gangdese Batholith

Qi Guan a,b, Di-Cheng Zhu a,⁎, Zhi-Dan Zhao a, Guo-Chen Dong a, Liang-Liang Zhang a, Xiao-Wei Li a,c, Min Liu a, Xuan-Xue Mo a, Yong-Sheng Liu d, Hong-Lin Yuan e a State Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China b College of Resources, Shijiazhuang University of Economics, Shijiazhuang 050031, China c School of Earth and Space Sciences, Peking University, Beijing 100871, China d State Key Laboratory of Geological Processes and Mineral Resources, and Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China e State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China article info abstract

Article history: The petrogenesis and geodynamic implications of the Cenozoic adakites in southern Tibet remain topics of debate. Received 17 March 2011 Here we report geochronological and geochemical data for host granites and maficenclavesfromWolonginthe Received in revised form 24 June 2011 eastern Gangdese Batholith, southern Tibet. Zircon LA-ICP-MS dating indicates that the Wolong host granites and Accepted 3 July 2011 enclaves were synchronously emplaced at ca. 38 Ma. The host granites are medium- to high-K calc-alkaline, Available online 14 July 2011 metaluminous (A/CNK=0.93–0.96), with high Al2O3 (15.47–17.68%), low MgO (0.67–1.18%), very low abundances of compatible elements (e.g., Cr=3.87–8.36 ppm, Ni=3.04–5.71 ppm), and high Sr/Y ratios Keywords: – fi – – Zircon U–Pb geochronology (127 217), similar to those typical of adakite. The ma cenclaves(SiO2 =51.08 56.29%) have 3.83 5.02% MgO # Eocene adakite and an Mg of 48–50, with negative Eu anomalies (δEu=0.59–0.79). The Wolong host granites and enclaves have 87 86 Mafic enclave similar Sr–Nd isotopic compositions (initial Sr/ Sr=0.7053–0.7055, εNd(t)=−2.7 to −1.4), with varying

Gangdese Batholith zircon εHf(t) values, ranging from +6.0 to +12.6. A comprehensive study of the data available for adakitic rocks Southern Tibet from the Gangdese Batholith indicates that the Wolong adakitic host granites were derived from partial melting of a thickened lower crust, while the parental magmas of the maﬁc enclaves were most likely derived from lithospheric mantle beneath southern Tibet. The Wolong granitoids are interpreted as the result of mixing between the thickened lower crust-derived melts and lithospheric mantle-derived maﬁc melts, which are likely the protracted magmatic response to the break-off of the Neo-Tethyan oceanic slab at about 50 Ma. Our results suggest that the crustal thickening in southern Tibet occurred prior to ~38 Ma, and support the general view that the India–Asia collision must have occurred before 40 Ma. © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction dominant view is that most Cenozoic adakitic rocks (30–10 Ma) were derived from partial melting of lower crust in a post-collisional As part of the Tibetan Plateau, the Lhasa Terrane is widely accepted setting (Chung et al., 2003, 2009; Guo et al., 2007; Xu et al., 2010; as a Mesozoic Andean-type convergent margin associated with the Zhang et al., 2010a; Jiang et al., 2011). However, Gao et al. (2007, northward subduction of the Neo-Tethyan oceanic lithosphere and an 2010), assuming that the India–Asia collision did not occur until the archetype of a Cenozoic collisional orogen related to the India–Asia late Eocene (Aitchison et al., 2007), argued that these adakites collision (Maluski et al., 1982; Xu et al., 1985; Coulon et al., 1986; Yin originated from partial melting of upper mantle that had been and Harrison, 2000; Mo et al., 2007, 2008; Zhu et al., 2009a, 2011a; metasomatized by slab-derived melts. The Cenozoic adakites reported Aitchison et al., 2011; Xia et al., 2011). Although Cenozoic adakites previously are mostly of Miocene ages (30–9 Ma) (cf. Chung et al., have been recognized in the Gangdese Batholith in the southern Lhasa 2003, 2005, 2009; Hou et al., 2004; Qu et al., 2004; Guo et al., 2007; Xu Terrane for several years, their petrogenesis and geodynamic et al., 2010; Zhang et al., 2010a; Jiang et al., 2011), making them coeval implications remain subjects of much debate. For example, the with the potassic–ultrapotassic magmatism in the Lhasa Terrane (cf. Zhao et al., 2009) but after the waning of the Linzizong magmatism at about 40 Ma (cf. Mo et al., 2007, 2008; Lee et al., 2009). The oldest Cenozoic adakites reported have been considered to offer important ⁎ Corresponding author at: State Key Laboratory of Geological Processes and Mineral petrological evidence for the timing of both the initiation of tectonic Resources, China University of Geosciences (Beijing), 29# Xue-Yuan Road, Haidian District, Beijing 100083, China. Tel./fax: +86 10 8232 2094(O). collapse (ca. 26 Ma; Guo et al., 2007) and crustal thickening in E-mail address: [email protected] (D.-C. Zhu). southern Tibet (ca. 30 Ma; Chung et al., 2009).

1342-937X/$ – see front matter © 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2011.07.004 Q. Guan et al. / Gondwana Research 21 (2012) 88–99 89

Here we report zircon U–Pb geochronological and whole-rock 2011). The granitoids consist of granites and mafic (gabbroic to dioritic) geochemical data for host granites and enclaves identified for the first enclaves (Fig. 2a and b). Ten samples, including six host rocks and four time in the Gangdese Batholith at Wolong (Fig. 1a). Our new data mafic enclaves, were collected west of Wolong town along the main indicate that adakitic magmatism was already active at about 38 Ma, road from Nang to Mainling. The rocks are generally fresh, without which provides important constraints not only on the petrogenesis of visible alteration. The host rocks are coarse-grained and are composed Cenozoic adakitic magmatism but also on the timing of crustal mainly of plagioclase (30–40%), quartz (20–25%), K-feldspar (15–20%), thickening in southern Tibet. The potential implications of the Wolong biotite (5–15%), and amphibole (5–10%) (Fig. 2c), with accessory granitoids for the timing of the India–Asia collision are also discussed. minerals including zircon and Fe-Ti oxides (b 1%). The mafic enclaves exhibit fine-grained igneous texture (Fig. 2d) and contain plagioclase (50–75%), amphibole (10–15%), and biotite (5–10%). Distinct petro- 2. Geological setting and samples graphical features are observed in the mafic enclaves, such as the presence of K-feldspar and quartz megacrysts, typical back vein, a The Tibetan Plateau is essentially composed of four continental quenched margin (Fig. 2a and b), and acicular apatites (Fig. 2d). Samples ‘ ’ – blocks, or terranes . From north to south, these are the Songpan Ganzi ML18-2 and ML18-4, a mafic enclave–host rock pair, were subjected to fl fi ysch complex, followed by the Qiangtang, Lhasa, and nally Tethyan zircon separation for LA-ICPMS U–Pb dating. Himalaya terranes, which are separated by the Jinsha, Bangong-Nujiang, and Yarlung Zangbo suture zones, representatives of the Meso- and Neo-Tethyan relicts, respectively (Yin and Harrison, 2000)(Fig. 1a). The 3. Analytical methods Lhasa Terrane, which is recently thought to have detached from Australian Gondwana rather than Indian Gondwana (Zhu et al., Zircons were separated from samples ML18-2 and ML18-4 using 2011b) and then drifted northward, joining with the Qiangtang Terrane standard density and magnetic separation techniques at the Special in the Early Cretaceous (Yin and Harrison, 2000; Kapp et al., 2005), Laboratory of the Geological Team of Hebei Province, China. Cathodo- consists primarily of Paleozoic–Paleogene sedimentary strata and luminescence (CL) images were used to check the internal structures of associated igneous rocks (Yin and Harrison, 2000; Zhu et al., 2011a). individual zircon grains and to select positions for analysis. In situ zircon The latter include a series of volcanic sequences (e.g., Early Jurassic U–Pb dating and trace element analysis of zircon was conducted by laser volcanic rocks of the Yeba Formation, Zhu et al., 2008; Late Jurassic– ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at Early Cretaceous volcanic rocks of the Sangri Group and Zenong Group, the State Key Laboratory of Geological Processes and Mineral Resources, Zhu et al., 2006, 2011a; and the Linzizong volcanic successions, Mo et al., China University of Geosciences, Wuhan. Detailed operating conditions 2007, 2008) and voluminous Cretaceous granitoids in the central and for the laser ablation system, ICP-MS instrument and data reduction are northern Lhasa Terrane (Zhu et al., 2009b,c, 2011a) and Gangdese described by Liu et al. (2008a, 2010a, 2010b). The common Pb correction Batholith in the southern Lhasa Terrane (cf. Ji et al., 2009a; Zhu et al., followed the ComPbCorr#3-151 procedure (Andersen, 2002). Data 2011a). Recently, the Gangdese Batholith has been dated as Late Triassic processing was performed using Isoplot/Ex_ver3 (Ludwig, 2003). Zircon to Miocene (~205–9Ma;Chu et al., 2006; Wen et al., 2008b; Chung et al., trace-element and U–Pb isotope data are given in Tables 1 and 2. 2009; Ji et al., 2009a,b; Zhu et al., 2011a). Major elements were measured by XRF at the State Key Laboratory The Wolong granitoids investigated in this study are exposed in the of Continental Dynamics (SKLCD), Northwest University, Xi'an, China. eastern Gangdese Batholith between Nang and Mainling (Fig. 1b). They The analytical uncertainty is usuallyb5%. Trace elements were occur as apophyses within the Late Cretaceous granitoids that intruded analyzed with an Agilent 7500a ICP-MS at the State Key Laboratory into the Meso-Neoproterozoic metamorphic rocks (Fig. 1b) with of Geological Processes and Mineral Resources (GPMR), China metamorphic overprinting during the Mesozoic and Cenozoic (Wang University of Geosciences, Wuhan, China. For the detailed sample- et al., 2008, 2009; Dong et al., 2010; Zhang et al., 2010d, 2011; Guo et al., digestion procedure for ICP-MS analysis, and analytical precision and

80° Qaidam 36° Tarim Songpan Pt2-3 b a -Ganzi JSSZ 04km

Qiangtang 500 km 29°10' 29°10' BNSZ 32° 32° Lhasa Terrane Fig. b 40 38.5 Lhasa Dagze YZSZ 38 37.4 Yarl Yardoi un N Wolong toMa Himalayas 43 gZan W E 28° to Nang in gb l in oR g S i 90° ver JSSZ = Jinsha Suture Zone Gangdese Batholith: Wolong BNSZ = Bangong-Nujiang Suture Zone Eocene granitoids YZSZ = Yarlung Zangbo Suture Zone Meso- to Neoproterozoic Late Cretaceous Pt2-3 Nyingchi Group granitoids Zircon U-Pb age 37.4 Age data reported data (this study) 40 in literature

Sample localities Road 93°45'

Fig. 1. (a) Tectonic outline of the Tibetan Plateau showing the study area and the Eocene magmatic rocks reported in Dagze (Gao et al., 2008) and Wolong (this study) in the Gangdese Batholith and Yardoi in the Tethyan Himalaya (Zeng et al., 2011). (b) Simpliﬁed geological map of the Wolong region (modiﬁed from Yin et al., 2003). 90 Q. Guan et al. / Gondwana Research 21 (2012) 88–99

ab ML18-2 host granite (37.4 Ma) K-feldspar megacryst

back vein

ML18-4 dioritic enclave (38.5 Ma)

200 µm 200 µm (c)c Ap d

Bt Kfs Pl Amp Qtz

Pl Kfs Amp

Amp ML18-2 ML18-4

Fig. 2. Field observations (a–b) and photomicrographs (c–d) of the Wolong granitoids. Abbreviations: Amp = amphibole; Kfs = K-feldspar; Pl = plagioclase; Ap = apatite.

Table 1 Zircon trace element data of the Wolong host granite (ML18-2) and dioritic enclave (ML18-4).

Spot La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu δEu Hf Y Ta P Ti Nb

ML18-2 01 0.02 15.8 0.03 0.75 1.87 1.10 12.5 4.34 55.7 22.5 126 30.3 352 77.5 0.70 10312 835 0.53 98 2.88 3.04 ML18-2 02 0.02 16.1 0.03 0.72 2.00 1.16 11.4 3.82 47.4 18.6 100 24.1 272 59.1 0.74 10938 686 0.50 103 2.04 2.69 ML18-2 03 0.01 21.7 0.04 0.73 2.49 1.38 15.7 4.83 64.0 24.8 136 32.5 366 78.4 0.67 9918 899 0.40 153 2.22 2.66 ML18-2 04 0.00 24.7 0.05 0.85 2.44 1.39 14.0 4.37 51.8 19.3 103 24.3 272 57.4 0.73 9616 705 0.52 129 2.42 2.83 ML18-2 05 0.00 21.9 0.05 0.72 2.15 1.19 12.6 4.02 52.3 21.7 120 29.0 330 70.9 0.70 9666 793 0.54 123 2.67 3.44 ML18-2 06 0.01 19.2 0.01 0.48 1.91 1.12 14.1 4.94 67.3 27.5 155 38.6 437 98.3 0.66 10551 1043 0.56 122 2.93 4.07 ML18-2 07 0.58 15.9 0.23 1.50 2.07 0.96 10.6 3.37 42.5 17.2 95 24.0 279 62.2 0.63 11565 648 0.53 1076 1.85 2.59 ML18-2 08 0.01 16.0 0.02 0.59 1.58 0.99 10.8 3.39 45.7 19.5 108 27.4 324 74.2 0.73 9200 726 0.23 168 2.45 2.07 ML18-2 09 0.02 28.3 0.05 1.14 3.32 1.69 17.2 5.17 59.2 22.5 117 26.9 292 63.3 0.68 10320 833 0.70 117 3.33 3.94 ML18-2 10 0.00 26.0 0.05 0.77 2.43 1.40 15.1 4.74 58.8 22.4 119 28.5 317 67.0 0.71 10179 808 0.61 114 2.03 3.48 ML18-2 11 0.01 22.0 0.02 0.84 2.08 1.17 14.1 4.80 64.9 27.0 152 37.1 418 90.9 0.66 9565 998 0.59 161 2.65 4.17 ML18-2 12 0.01 24.0 0.03 1.07 2.77 1.47 16.6 5.77 74.0 29.5 162 38.7 438 97.0 0.66 10622 1081 0.60 129 2.34 4.06 ML18-2 13 0.01 36.1 0.09 1.46 4.28 2.38 23.7 7.43 87.8 30.3 143 29.5 285 53.7 0.72 7408 986 0.42 167 5.91 2.06 ML18-2 14 0.07 21.7 0.06 0.94 2.50 1.47 13.1 4.47 54.0 20.3 109 25.6 293 60.4 0.78 10786 723 0.48 115 5.60 2.76 ML18-2 15 0.01 14.6 0.02 0.67 1.51 0.82 8.6 2.88 35.5 13.9 78 18.7 215 48.0 0.69 10962 527 0.55 95 2.33 2.11 ML18-2 16 0.00 26.7 0.06 1.22 2.74 1.47 16.7 5.15 64.4 25.2 135 32.0 357 75.2 0.66 9580 908 0.62 127 3.12 4.10 ML18-2 17 0.25 24.3 0.23 2.56 3.47 1.83 18.5 6.01 73.6 28.2 150 35.7 399 86.1 0.70 10746 1031 0.67 139 62.3 5.13 ML18-2 18 0.00 13.3 0.03 0.59 1.57 1.02 11.2 4.00 53.3 22.4 129 32.7 393 85.4 0.75 10714 842 0.30 143 2.04 2.09 ML18-4 01 1.13 34.3 0.52 3.41 3.70 1.96 16.9 5.10 60.7 22.1 122 27.0 300 60.4 0.76 9093 797 0.55 448 2.97 3.79 ML18-4 02 0.02 15.5 0.07 1.02 1.63 0.94 10.8 3.79 48.6 19.6 118 27.5 334 69.7 0.68 9909 730 0.31 215 4.03 2.12 ML18-4 03 0.06 13.3 0.11 0.86 1.30 0.87 9.2 3.33 40.9 17.2 104 24.3 289 63.1 0.77 9489 643 0.28 191 22.2 2.06 ML18-4 04 0.00 21.8 0.06 0.93 1.98 1.21 13.1 4.58 59.7 24.9 150 34.5 395 83.5 0.73 9298 921 0.42 155 3.24 3.60 ML18-4 05 0.13 23.8 0.07 1.19 2.53 1.42 13.4 4.35 52.7 19.5 109 23.8 270 57.6 0.75 9510 697 0.53 197 2.34 2.95 ML18-4 06 0.02 16.5 0.03 0.72 1.84 1.04 11.0 3.89 50.1 20.8 121 28.4 321 69.5 0.71 9318 749 0.32 174 2.70 2.52 ML18-4 07 0.01 11.9 0.03 0.38 1.22 0.74 7.6 2.90 39.4 16.8 100 23.9 282 62.3 0.74 10407 624 0.43 97 1.72 2.39 ML18-4 08 0.04 23.1 0.06 1.01 1.98 1.37 13.8 4.81 63.9 26.1 159 36.6 419 91.3 0.80 9360 995 0.58 123 2.62 4.01 ML18-4 09 0.00 19.2 0.06 0.60 2.14 1.18 13.6 4.97 68.3 28.9 176 40.9 482 106.5 0.67 10796 1088 0.68 121 1.90 4.30 ML18-4 10 0.00 15.3 0.01 0.37 1.93 0.92 10.4 3.78 51.2 20.6 125 30.0 359 78.6 0.63 9682 787 0.43 125 1.97 2.93 ML18-4 11 0.01 26.6 0.03 0.93 2.32 1.23 14.6 5.70 73.9 30.5 179 40.5 449 98.1 0.65 9388 1104 0.70 155 3.10 4.86 ML18-4 12 0.00 23.8 0.02 0.76 2.17 1.39 14.3 4.43 54.4 21.1 120 27.4 311 66.2 0.76 10261 767 0.59 99 2.79 3.40 ML18-4 13 0.22 21.6 0.08 1.16 2.10 1.16 11.2 3.92 48.9 19.4 110 25.4 285 61.9 0.73 9437 700 0.47 233 2.25 2.60 ML18-4 14 0.24 15.3 0.15 0.69 1.63 0.91 8.6 2.51 34.2 13.4 80 18.8 221 49.5 0.74 11053 524 0.42 1304 1.03 2.18 ML18-4 15 0.08 21.0 0.04 0.81 1.93 1.13 12.4 4.28 57.6 24.1 144 34.1 391 85.4 0.71 9742 912 0.42 286 2.82 3.47 ML18-4 16 0.01 21.4 0.04 0.58 2.29 1.35 12.0 4.14 49.2 19.2 111 25.3 290 62.0 0.79 10525 717 0.64 154 2.77 3.17 ML18-4 17 0.00 21.8 0.03 0.78 2.31 1.27 12.5 4.55 57.9 23.1 139 33.1 383 86.3 0.72 9078 898 0.31 164 2.49 2.61 ML18-4 18 0.01 25.4 0.03 0.63 2.47 1.24 15.0 5.38 72.1 29.6 174 40.1 455 96.2 0.70 9925 1102 0.56 179 3.59 4.49

1/2 δEu=EuN/(SmN ×GdN) , N is chondrite-normalized (Sun and McDonough, 1989). Q. Guan et al. / Gondwana Research 21 (2012) 88–99 91

Table 2 Zircon LA-ICPMS U–Pb data of the Wolong host granite (ML18-2) and dioritic enclave (ML18-4).

Spot Pb Th U Th/U 207Pb*/206Pb* 207Pb*/235U* 206Pb*/238U* 208Pb*/232Th 207Pb*/206Pb* 207Pb*/235U 206Pb*/238U

ppm ppm ppm Ratio ± 1σ Ratio ± 1σ Ratio ± 1σ Ratio ± 1σ Age ± 1σ Age ± 1σ Age ± 1σ

ML18-2, granite, 17 spots (without spot 15), mean=37.4±0.1 Ma, MSWD=1.0 1 12 359 2032 0.18 0.04663 0.00126 0.03799 0.00105 0.00591 0.00005 0.00176 0.00005 30 44 38 1.0 38 0.3 2 17 563 2921 0.19 0.04850 0.00095 0.03903 0.00077 0.00583 0.00004 0.00197 0.00015 124 34 39 0.7 38 0.2 3 14 560 2391 0.23 0.04708 0.00106 0.03773 0.00086 0.00581 0.00006 0.00200 0.00006 53 35 38 0.8 37 0.4 4 14 782 2366 0.33 0.04681 0.00102 0.03739 0.00082 0.00578 0.00004 0.00176 0.00003 40 37 37 0.8 37 0.2 5 11 494 1929 0.26 0.04657 0.00112 0.03740 0.00087 0.00583 0.00005 0.00178 0.00004 27 36 37 0.8 38 0.3 6 14 469 2414 0.19 0.04710 0.00106 0.03762 0.00086 0.00577 0.00005 0.00175 0.00005 54 38 38 0.8 37 0.3 7 17 534 2954 0.18 0.04692 0.00103 0.03749 0.00084 0.00577 0.00005 0.00187 0.00004 45 36 37 0.8 37 0.3 8 10 433 1663 0.26 0.05351 0.00232 0.04343 0.00187 0.00587 0.00007 0.00196 0.00008 351 77 43 2.0 38 0.4 9 27 1322 4467 0.30 0.04718 0.00092 0.03765 0.00075 0.00577 0.00004 0.00182 0.00003 58 34 38 0.7 37 0.3 10 14 666 2275 0.29 0.04749 0.00107 0.03780 0.00085 0.00576 0.00004 0.00174 0.00003 74 40 38 0.8 37 0.3 11 12 470 1947 0.24 0.04589 0.00124 0.03728 0.00100 0.00588 0.00004 0.00177 0.00004 −8 39 37 1.0 38 0.3 12 19 777 3196 0.24 0.04800 0.00102 0.03874 0.00078 0.00585 0.00004 0.00182 0.00004 99 35 39 0.8 38 0.3 13 2 291 352 0.83 0.04982 0.00259 0.03883 0.00192 0.00576 0.00008 0.00185 0.00006 187 89 39 2.0 37 0.5 14 13 488 2194 0.22 0.04665 0.00181 0.03752 0.00151 0.00583 0.00011 0.00221 0.00009 31 53 37 1.0 38 0.7 15 16 568 2653 0.21 0.04806 0.00136 0.03967 0.00104 0.00599 0.00005 0.00188 0.00005 102 46 40 1.0 39 0.3 16 13 900 2128 0.42 0.04863 0.00107 0.03930 0.00087 0.00585 0.00005 0.00182 0.00004 130 36 39 0.9 38 0.3 17 18 677 3013 0.22 0.04640 0.00103 0.03734 0.00089 0.00582 0.00005 0.00177 0.00004 18 34 37 0.9 37 0.3 18 14 416 2440 0.17 0.04814 0.00155 0.03798 0.00136 0.00579 0.00016 0.00176 0.00006 106 40 38 1.0 37 1.0

ML18-4, syenodiorite, 17 spots (without spot 10), mean=38.5±0.3 Ma, MSWD=2.3 1 18 1159 2867 0.40 0.04726 0.00132 0.03721 0.00103 0.00570 0.00006 0.00176 0.00004 62 45 37 1.0 37 0.4 2 9 348 1476 0.24 0.04656 0.00237 0.03696 0.00194 0.00575 0.00010 0.00187 0.00008 27 80 37 2.0 37 0.7 3 9 320 1461 0.22 0.04729 0.00218 0.04008 0.00177 0.00618 0.00009 0.00185 0.00009 64 71 40 2.0 40 0.6 4 11 431 1757 0.25 0.04714 0.00164 0.03836 0.00129 0.00594 0.00006 0.00188 0.00006 57 56 38 1.0 38 0.4 5 13 676 2117 0.32 0.04389 0.00136 0.03604 0.00112 0.00596 0.00006 0.00192 0.00005 −76 49 36 1.0 38 0.4 6 10 369 1532 0.24 0.04658 0.00165 0.03882 0.00140 0.00604 0.00007 0.00207 0.00007 28 56 39 1.0 39 0.5 7 8 271 1306 0.21 0.04617 0.00199 0.03816 0.00160 0.00600 0.00007 0.00194 0.00008 7 66 38 2.0 39 0.4 8 12 478 1903 0.25 0.04707 0.00208 0.03778 0.00155 0.00586 0.00007 0.00192 0.00008 53 68 38 2.0 38 0.4 9 16 566 2901 0.20 0.04640 0.00143 0.03792 0.00128 0.00604 0.00020 0.00188 0.00005 18 32 38 1.0 39 1.0 10 11 392 1829 0.21 0.04074 0.00256 0.03374 0.00215 0.00599 0.00008 0.00206 0.00008 −247 118 34 2.0 39 0.5 11 13 570 2129 0.27 0.04573 0.00184 0.03827 0.00154 0.00604 0.00006 0.00200 0.00009 −16 66 38 2.0 39 0.4 12 17 777 2706 0.29 0.04578 0.00129 0.03813 0.00107 0.00604 0.00006 0.00201 0.00005 −14 38 38 1.0 39 0.4 13 11 492 1802 0.27 0.04584 0.00145 0.03869 0.00122 0.00612 0.00006 0.00203 0.00006 −10 46 39 1.0 39 0.4 14 17 655 2751 0.24 0.04668 0.00160 0.03914 0.00123 0.00610 0.00007 0.00213 0.00007 33 48 39 1.0 39 0.4 15 12 491 2018 0.24 0.04674 0.00136 0.03833 0.00113 0.00593 0.00005 0.00199 0.00005 36 47 38 1.0 38 0.3 16 18 835 2932 0.28 0.04687 0.00111 0.03873 0.00093 0.00598 0.00005 0.00194 0.00005 42 38 39 0.9 38 0.3 17 12 579 1875 0.31 0.04581 0.00142 0.03796 0.00116 0.00602 0.00006 0.00181 0.00005 −12 42 38 1.0 39 0.4 18 13 577 2192 0.26 0.04931 0.00157 0.03904 0.00121 0.00576 0.00007 0.00192 0.00006 163 50 39 1.0 37 0.5

*Radiogenic lead, isotopic ratios and ages were corrected by common lead, following the methods reported by Andersen (2002).

accuracy, see Liu et al. (2008b). Whole-rock Sr and Nd isotopic ratios 4. Results were measured by a Triton thermal ionization mass spectrometer at GPMR. Sr and Nd isotopic fractionations were normalized to 4.1. Zircon U–Pb geochronology 86Sr/88Sr=0.1194 and 146Nd/144Nd=0.7219, respectively. During analysis, NBS987 and La Jolla standards yielded 87Sr/86Sr=0.71025± Zircons in both the host rocks and the enclaves are mostly subhedral 4(2σ) and 143Nd/144Nd=0.511853±9 (2σ), respectively; the BCR-2 to euhedral, with crystal lengths of 50–200 μm and length-to-width standard gave 143Nd/144Nd=0.512638±5 (2σ). For details of the Sr ratios from 1:1 to 2:1. Most zircons are transparent, colorless to slightly and Nd isotopic analytical procedures, see Zhang et al. (2002). Whole- brown. CL images of zircon exhibit oscillatory magmatic growth-zoning rock major, trace-element, and Sr and Nd isotopic data are listed in (Fig. 3a and b). Zircons from both the host granite (Fig. 3c) and the Table 3. dioritic enclave (Fig. 3d) exhibit fractionated REE patterns of HREE In situ Hf isotope measurements were subsequently performed on enrichment and LREE depletion, with strong positive Ce (Fig. 3candd) the dated spots using LA-MC-ICPMS at SKLCD. The laser spot was 44 μm and insigniﬁcant negative Eu anomalies (δEu=0.63–0.80; Table 1). in diameter. Helium was used as the carrier gas transporting the ablated These features, together with Th/U ratiosN0.1 (Table 2), indicate that sample from the ablation cell to the ICP-MS torch. Zircon 91500, GJ-1 the analyzed zircons are magmatic in origin (Hoskin and Schaltegger, and Monastery were used as reference standards. The analytical 2003). procedures are described in Yuan et al. (2008).Thedecayconstant Seventeen analyses from the host rock (ML18-2) yield concordant used for 176Lu was 1.867×10−11 year−1 (Söderlund et al., 2004). Initial zircon 206Pb/238U ages ranging from 37.0±0.6 Ma to 38.5±0.6 Ma, 176Hf/177Hf was calculated according to the 206Pb/238U age, and the with a weighted mean of 37.4±0.2 Ma (2σ; MSWD=1.0) (Fig. 3e). value of εHf(t) was calculated relative to the chondritic reservoir, which Seventeen analyses from the enclave (ML18-4) give concordant has a 176Hf/177Hf ratio of 0.282772 and 176Lu/177Hf of 0.0332 (Blichert- zircon 206Pb/238U ages ranging from 36.7±0.8 Ma to 39.3±0.8 Ma,

Toft and Albarède, 1997). Hf model ages (TDM) were calculated relative with a weighted mean of 38.5±0.3 Ma (2σ; MSWD=2.3; Fig. 3f). We to the depleted mantle, which has a 176Hf/177Hf ratio of 0.283250 and interpret these two ages as the timing of zircon crystallization and 176Lu/177Hf of 0.0384 (Grifﬁn et al., 2000). Zircon Hf isotope data are thus the emplacement timing of the host granite and dioritic enclave, listed in Table 4. respectively. 92 Q. Guan et al. / Gondwana Research 21 (2012) 88–99

Table 3 Whole-rock major, trace element and Sr–Nd isotopic data of the Wolong host granites and maﬁc enclaves.

Sample ML18-1 ML18-2 ML18-3 ML18-4 ML18-5 ML18-6 ML18-7 ML18-8 ML18-9 ML18-10

Rock type gabbro granite granodiorite syenodiorite syenodiorite syenite granodiorite syenite granite granodiorite

XRF — major element(wt.%)

SiO2 51.08 71.03 66.59 54.80 56.29 55.97 66.35 65.26 68.07 65.91

TiO2 0.82 0.12 0.33 0.79 0.74 0.56 0.32 0.26 0.27 0.34

Al2O3 16.50 15.47 16.74 17.24 16.76 15.71 16.92 17.68 16.45 17.08

TFe2O3 10.42 1.75 3.07 8.56 8.19 7.73 3.24 2.75 2.53 3.28 MnO 0.26 0.04 0.06 0.17 0.18 0.19 0.06 0.05 0.05 0.06 MgO 5.02 0.67 1.12 4.35 3.83 3.91 1.18 1.03 0.89 1.18 CaO 6.69 2.51 3.46 5.14 5.49 4.51 3.39 2.85 3.06 3.33

Na2O 4.55 4.93 5.50 4.96 4.80 2.85 5.46 5.08 5.53 5.43

K2O 2.44 3.22 2.41 2.53 2.36 6.89 2.63 4.54 2.84 3.01

P2O5 0.79 0.09 0.17 0.46 0.70 0.44 0.18 0.16 0.17 0.19 LOI 1.21 0.45 0.63 0.91 1.00 0.77 0.63 0.44 0.38 0.48 Total 99.78 100.28 100.08 99.91 100.34 99.53 100.36 100.10 100.24 100.29 Mg# 49.07 43.37 42.18 50.41 48.33 50.29 42.14 42.83 41.30 41.84

ICP-MS — trace element (ppm) Be 4.91 1.85 2.51 3.53 3.43 2.06 2.15 1.99 2.25 2.28 Sc 21.6 2.65 4.36 10.6 13.2 15.5 4.53 3.88 3.50 4.43 V 184 33.5 59.6 152 144 145 59.1 53.2 47.6 63.9 Cr 85.2 3.87 6.21 97.3 66.0 61.7 7.14 5.94 8.36 7.06 Co 24.5 3.56 6.07 20.9 19.1 17.7 6.23 5.56 4.83 6.38 Ni 27.5 3.04 5.09 95.8 26.1 20.7 5.71 4.97 4.86 5.62 Cu 169 10.3 19.5 23.6 54.0 22.2 13.8 17.1 21.7 19.5 Zn 206 34.3 55.0 165 154 146 56.6 50.8 44.8 59.4 Ga 30.45 16.86 19.69 25.81 25.64 20.30 19.18 19.02 18.58 20.13 Rb 143 82.1 72.2 154 139 212 75.2 110 79.0 85.4 Sr 675 961 1106 859 944 868 1203 1309 1212 1223 Y 16.7 4.43 8.42 6.97 13.0 14.1 8.16 7.87 8.09 9.59 Zr 291 89.4 127 113 243 166 132 133 137 157 Nb 12.2 4.71 8.87 6.45 9.66 9.62 7.54 7.62 7.99 9.56 Sn 2.38 0.49 0.86 1.38 1.64 1.95 0.80 0.76 0.76 0.89 Cs 8.73 2.00 2.11 10.7 12.4 5.28 1.99 2.59 2.34 2.63 Ba 370 1094 811 503 545 2801 1027 3120 1344 1441 La 139 24.7 43.7 49.7 128 72.9 46.8 51.8 43.2 54.6 Ce 239 43.7 78.7 88.7 217 130 82.6 91.6 77.6 95.7 Pr 25.7 4.89 8.83 9.62 23.4 14.9 9.31 10.1 8.77 10.8 Nd 83.2 16.6 30.5 32.2 75.5 52.5 32.1 34.4 30.7 37.5 Sm 10.7 2.46 4.79 4.31 9.42 7.93 4.82 4.95 4.70 5.68 Eu 1.76 0.72 1.14 0.78 1.57 1.81 1.17 1.50 1.20 1.38 Gd 6.94 1.64 3.21 2.90 5.97 5.60 3.20 3.25 3.13 3.79 Tb 0.74 0.20 0.38 0.32 0.63 0.67 0.39 0.38 0.38 0.45 Dy 3.20 0.91 1.76 1.39 2.63 2.99 1.68 1.61 1.72 2.04 Ho 0.58 0.15 0.32 0.25 0.47 0.53 0.31 0.30 0.30 0.34 Er 1.58 0.42 0.78 0.65 1.21 1.34 0.77 0.75 0.75 0.91 Tm 0.24 0.07 0.12 0.10 0.20 0.22 0.12 0.12 0.12 0.14 Yb 1.53 0.39 0.74 0.62 1.10 1.22 0.67 0.66 0.70 0.84 Lu 0.27 0.06 0.11 0.11 0.20 0.18 0.11 0.10 0.11 0.12 Hf 8.12 2.51 3.42 2.92 6.29 4.37 3.34 3.32 3.52 4.07 Ta 0.52 0.29 0.52 0.14 0.37 0.51 0.46 0.46 0.50 0.59 Pb 37.1 43.1 41.1 33.2 32.8 66.0 41.1 53.4 44.2 46.3 Th 73.4 14.3 24.8 23.1 68.6 36.2 25.9 30.1 26.3 30.8 U 7.50 2.33 4.01 2.05 5.99 4.21 2.68 3.16 3.99 3.41 87Rb/86Sr 0.6298 0.1945 0.5355 0.4404 0.7279 0.1864 0.2081 87Sr/86Sr 0.705717 0.705458 0.705600 0.705729 0.705779 0.705581 0.705593 ±2σ 0.000006 0.000004 0.000006 0.000006 0.000006 0.000006 0.000007 87 86 ( Sr/ Sr)i 0.70538 0.70535 0.70531 0.70549 0.70539 0.70548 0.70548 147Sm/144Nd 0.0776 0.0950 0.0811 0.0754 0.0913 0.0907 0.0917 143Nd/144Nd 0.512473 0.512529 0.512538 0.512468 0.512520 0.512511 0.512495 ±2σ 0.000002 0.000005 0.000001 0.000002 0.000006 0.000005 0.000005 143 144 ( Nd/ Nd)i 0.51245 0.51251 0.51252 0.51245 0.51250 0.51249 0.51247 εNd(t) −2.6 −1.6 −1.4 −2.7 −1.8 −2.0 −2.3

tDM(Ga) 0.76 0.80 0.70 0.75 0.79 0.79 0.82

# 2+ 2+ 2+ LOI=loss on ignition. Mg =100×molar Mg /(Mg +TFe ), calculated by assuming TFeO=0.9×TFe2O3. Corrected formula as follows: 87 86 87 86 87 86 λt − 11 − 1 143 144 143 144 147 144 λt ( Sr/ Sr)i =( Sr/ Sr)sample – Rb/ Sr (e — 1), λ =1.42×10 a ;( Nd/ Nd)i =( Nd/ Nd)sample – ( Sm/ Nd)sample ×(e — 1); εNd(t)= 143 144 143 144 4 143 144 λt 143 144 147 144 [( Nd/ Nd)sample/( Nd/ Nd)CHUR(t)−1]× 10 ;( Nd/ Nd)CHUR(t)=0.512638–0.1967×(e — 1); TDM =1/λ×ln{1+[(( Nd/ Nd)sample−0.51315)/(( Sm/ Nd)sample −12 −1 −0.21317)]}, λSm-Nd=6.54×10 a .

4.2. Whole-rock geochemistry Mg# of 41–43. They are medium- to high-K calc-alkaline metaluminous

rocks (Fig. 4b and c). The enclaves are gabbroic to dioritic, with SiO2 of The host rocks belong to the granodiorite–granite–syenite suite in 51.08–56.29% (Fig. 4a). These rocks are characterized by high MgO # composition (Fig. 4a), with SiO2 of 65.26–71.03%, Al2O3 of 15.47–17.68%, (3.83–5.02%) and Mg (48–50) relative to the host rocks, and one is Na2Oof4.93–5.53%, Na2O/K2Oof1.1–2.3, and MgO of 0.67–1.18%, with particularly enriched in K2O (up to 6.89%; Table 3). Q. Guan et al. / Gondwana Research 21 (2012) 88–99 93

Table 4 Zircon Hf isotopic compositions of the Wolong host granite (ML18-2) and the dioritic enclave (ML18-4).

176 177 176 177 176 177 176 177 C No. Age (Ma) Yb/ Hf Lu/ Hf Hf/ Hf 2σ Hf/ Hft εHf(0) εHf(t) TDM (Ma) TDM (Ma) fLu/Hf

ML18-2, host granite, 37.4±0.1 Ma, εHf(t)=+6.0 to +12.6 (17 analyses) 1 38 0.021510 0.001077 0.282923 0.000036 0.282922 5.3 6.1 467 723 −0.97 2 37.5 0.016491 0.000815 0.283037 0.000031 0.283037 9.4 10.2 302 462 −0.98 3 37.3 0.023248 0.001128 0.282919 0.000037 0.282919 5.2 6.0 473 731 −0.97 4 37.1 0.023157 0.001116 0.283017 0.000037 0.283016 8.7 9.5 334 510 −0.97 5 37.5 0.019985 0.000934 0.282997 0.000038 0.282996 7.9 8.7 361 556 −0.97 6 37.1 0.026155 0.001279 0.283032 0.000033 0.283031 9.2 10.0 313 475 −0.96 7 37.1 0.015334 0.000741 0.283059 0.000032 0.283059 10.2 11.0 271 413 −0.98 8 37 0.028321 0.001363 0.283040 0.000033 0.283039 9.5 10.3 303 458 −0.96 9 37.1 0.020488 0.000958 0.283012 0.000032 0.283012 8.5 9.3 339 520 −0.97 10 37.7 0.019080 0.000923 0.282977 0.000033 0.282976 7.2 8.1 389 600 −0.97 11 37.8 0.025477 0.001219 0.283029 0.000039 0.283028 9.1 9.9 317 482 −0.96 12 37.6 0.024706 0.001195 0.283099 0.000031 0.283098 11.6 12.4 217 322 −0.96 13 37 0.024945 0.001033 0.283106 0.000041 0.283106 11.8 12.6 206 306 −0.97 14 37.5 0.018320 0.000872 0.283103 0.000032 0.283102 11.7 12.5 210 313 −0.97 15 37.6 0.022080 0.001033 0.283055 0.000035 0.283054 10.0 10.8 279 422 −0.97 16 37.4 0.025174 0.001234 0.283096 0.000033 0.283095 11.5 12.2 222 330 −0.96 17 37 0.021099 0.000979 0.282990 0.000039 0.282990 7.7 8.5 370 570 −0.97

ML18-4, syenodiorite enclave, 38.5±0.3 Ma, εHf(t)=+6.5 to +10.5 (16 analyses) 1 36.7 0.014399 0.000706 0.283045 0.000061 0.283044 9.6 10.4 291 446 −0.98 2 36.9 0.022037 0.001068 0.283042 0.000047 0.283042 9.6 10.3 297 452 −0.97 3 39.7 0.020453 0.001011 0.283046 0.000037 0.283046 9.7 10.5 291 441 −0.97 4 38.2 0.026366 0.001256 0.283016 0.000052 0.283015 8.6 9.4 337 512 −0.96 5 38.3 0.025093 0.001209 0.283038 0.000044 0.283037 9.4 10.2 305 462 −0.96 6 38.8 0.023151 0.001131 0.282965 0.000048 0.282964 6.8 7.7 408 626 −0.97 7 38.6 0.015362 0.000771 0.282969 0.000045 0.282969 7.0 7.8 398 617 −0.98 8 37.7 0.029257 0.001422 0.282937 0.000070 0.282936 5.8 6.6 452 692 −0.96 9 39 0.025843 0.001285 0.283018 0.000062 0.283017 8.7 9.5 334 507 −0.96 10 38.8 0.028656 0.001411 0.283017 0.000061 0.283016 8.7 9.5 336 509 −0.96 11 38.8 0.020962 0.001028 0.282990 0.000057 0.282989 7.7 8.5 372 570 −0.97 12 39.3 0.025926 0.001281 0.282980 0.000054 0.282979 7.4 8.2 388 593 −0.96 13 38.1 0.025037 0.001232 0.282964 0.000040 0.282963 6.8 7.6 411 631 −0.96 14 38.4 0.020551 0.001006 0.282985 0.000047 0.282984 7.5 8.3 379 582 −0.97 15 38.7 0.032136 0.001541 0.283013 0.000042 0.283012 8.5 9.3 343 518 −0.95 16 37 0.022555 0.001103 0.282933 0.000057 0.282932 5.7 6.5 453 701 −0.97

176 177 176 177 λt 176 177 176 177 λt 176 177 176 177 176 177 εHf(t)=10000×{[( Hf/ Hf)S – ( Lu/ Hf)S ×(e – 1)]/[( Hf/ Hf)CHUR,0 – ( Lu/ Hf)CHUR ×(e – 1)] – 1}. TDM=1/λ×ln{1+[( Hf/ Hf)S – ( Hf/ Hf)DM]/[( Lu/ Hf)S – 176 177 C 176 177 176 177 ( Lu/ Hf)DM]}. TDM =TDM – (TDM – t)×[(fcc – fs)/(fcc – fDM)]. fLu/Hf =( Lu/ Hf)S/( Lu/ Hf)CHUR – 1. −11 176 177 176 177 176 177 176 177 Where, λ=1.867×10 /a (Söderlund et al., 2004); ( Lu/ Hf)S and ( Hf/ Hf)S are the measured values of the samples; ( Lu/ Hf)CHUR =0.0332, ( Hf/ Hf)CHUR,0 =0.282772 176 177 176 177 176 177 176 177 176 177 (Blichert-Toft and Albarède, 1997); ( Lu/ Hf)DM =0.0384, ( Hf/ Hf)DM =0.28325 (Grifﬁn et al., 2000); ( Lu/ Hf)mean crust =0.015; fcc =[( Lu/ Hf)mean crust/( Lu/ Hf)CHUR] – 176 177 176 177 1; fs =fLu/Hf; fDM =[( Lu/ Hf)DM/( Lu/ Hf)CHUR] – 1; t=crystallization time of zircon.

The host granites have low concentrations of heavy rare earth 5. Discussion elements (HREE) and Y (e.g., Yb=0.39–0.85 ppm, Y=4.4–9.6 ppm) (Table 3). These characteristics, together with high Sr abundance 5.1. Adakitic magmatism in the Gangdese Batholith through time (961–1309 ppm) and Sr/Y ratios (127–217), allow classification of the host granites as adakites, as defined by Defant and Drummond (1990) Existing studies indicate that adakitic rocks with a broad age (Fig. 5a). The mafic enclaves exhibit negative Eu anomalies range (136–9 Ma) are common in the Gangdese Batholith (cf. Chung (δEu=0.59–0.79), which are unclear (and certainly less extreme) in et al., 2003, 2005, 2009; Hou et al., 2004; Qu et al., 2004; Guo et al., the host granites (δEu=0.84–1.07) (Fig. 6a). Both the host granites 2007; Zhu et al., 2009a; Guan et al., 2010; Xu et al., 2010; Zhang et al., and the mafic enclaves show strong enrichment in large-ion lithophile 2010a, 2010c; Jiang et al., 2011). These rocks document tectonomag- elements (LILE) relative to high-field-strength elements (HFSE), matic events in response to the subduction of the Neo-Tethyan pronounced negative Nb-Ta, and positive Pb anomalies in primitive- oceanic slab, continent–continent collision, and post-collision tec- mantle-normalized incompatible element patterns (Fig. 6b). tonics. For example, the Early Cretaceous adakitic rocks (ca. 137 Ma) 87 86 Initial Sr/ Sr and εNd(t) values are calculated at 38 Ma. The host are most likely derived from partial melting of the subducting Neo- 87 86 granites have initial Sr/ Sr values of 0.7054–0.7055 and εNd(t) Tethyan slab, and subsequent reaction with peridotite in the mantle values of −2.3 to −1.6 (Table 3). Similar Sr–Nd isotopic compositions wedge (Zhu et al., 2009a), which have recently been interpreted as are also observed in the mafic enclaves, which exhibit initial 87Sr/86Sr documenting the magmatic response to the initiation of northward values of 0.7053–0.7055 and εNd(t) values of −2.7 to −1.4 (Fig. 7). subduction of the Neo-Tethyan oceanic slab (Zhu et al., 2011a). The Late Cretaceous adakitic rocks (100–80 Ma) have been interpreted as 4.3. Zircon Hf isotope data being derived from partial melting of a newly underplated mafic lower crust during low-angle northward subduction of the Neo- Seventeen Hf isotope analyses from the host granite (ML18-2) yield Tethyan ocean slab (Wen et al., 2008a, 2008b), or from partial 176 177 Hf/ Hf ratios of 0.282919 to 0.283106, corresponding to εHf(t) melting of a subducted ocean slab during subduction of the Neo- values ranging from +6.0 to +12.6 (Table 4). Sixteen analyses of Tethyan mid-ocean ridge (Guan et al., 2010; Zhang et al., 2010c). zircons from the dioritic enclave (ML18-4) have 176Hf/177Hf ratios of Chung et al. (2003) first reported the presence of the Miocene – 0.282933 to 0.283046, corresponding to εHf(t) values of +6.5 to +10.5 adakitic rocks (26 10 Ma) in the southern Lhasa Terrane. These rocks (Table 4; Fig. 8). have variously been attributed to: (i) partial melting of a thickened 94 Q. Guan et al. / Gondwana Research 21 (2012) 88–99

37.1 ± 0.4 37.1 ± 0.6 a 6.0 b 38.0 ± 0.6 10.2 37.5 ± 0.6 10.4 7.7 37.3 ± 0.8 10.0 10.5 10.2 6.5 10.3 9.4 9.5 8.7 36.7 ± 0.4 37.5 ± 0.4 39.7 ± 0.6 38.3 ± 0.4 36.9 ± 0.7 38.2 ± 0.4 38.8 ± 0.5 37.1 ± 0.6 10.3 37.8 ± 0.6 9.5 8.1 7.8 9.3 9.5 8.5 6.6 37.7 ± 0.8 37.0 ± 0.6 37.6 ± 0.6 11.0 9.9 37.7 ± 0.4 38.8 ± 0.4 38.6 ± 0.4 38.8 ± 0.4 12.4 8.2 37.1 ± 0.6 12.6 12.2 39.0 ± 1.0 10.8 6.5 37.5 ± 1.4 39.3 ± 0.4 12.5 37.6 ± 0.6 7.6 8.3 37.0 ± 0.5 8.5 39.2 ± 0.4 9.3 100 100 m 37.0 ± 1.0 37.4 ± 0.6 37.0 ± 2.0 38.1 ± 0.3 38.4 ± 0.3 38.7 ± 0.4

10000 10000 c d 1000 1000

100 100

10 10

1 ML18-2 1 ML18-4

0.1 Host granite (37 Ma) 0.1 Dioritic enclave (38 Ma) Samples/Chondrite Samples/Chondrite 0.01 0.01

0.001 0.001 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

40 0.0062 ef0.0066 42 Dioritic enclave Host granite 39 0.0064 41 (ML18-2) (ML18-4) 0.0060

U 40 38 U 0.0062 238 238 39 0.0058 37 0.0060 38 Pb / Pb / 206 206 0.0058 37 0.0056 36 36 Mean = 37.4 ± 0.2 Ma 0.0056 Mean = 38.5 ± 0.3 Ma 35 0.0054 MSWD=1.0,n=17 MSWD = 2.3, n = 17 0.0054 0.030 0.034 0.038 0.042 0.046 0.050 0.030 0.034 0.038 0.042 0.046 207Pb / 235U 207Pb / 235U

Fig. 3. Cathodoluminescence (CL) images (a–b), chondrite-normalized REE patterns (c–d), and concordia diagrams (e–f) for zircons from the Wolong granitoid samples. Solid and dashed circles indicate the locations of LA-ICP-MS U–Pb analyses and Hf-isotope analyses, respectively. Zircon U–Pb ages and εHf(t) values are given for each analyses.

mafic lower crust in a continent–continent collisional zone in exploring the origin and generation of the Wolong granitoids response to convective removal of thickened lithosphere (cf. Chung reported here. et al., 2003, 2005, 2009; Hou et al., 2004); (ii) partial melting of subduction-modified lower crust in response to extensional collapse 5.2. Generation of the Wolong granitoids in the southern part of the Tibetan Plateau (Guo et al., 2007); and (iii) partial melting of the mafic lower crust of the Indian continent (Xu et 5.2.1. Host granites al., 2010; Jiang et al., 2011). Following Aitchison et al. (2007), Gao et As shown in Fig. 6a and b, the abundances of trace elements in the al. (2007, 2010) argued that the Indian continent may have collided mafic enclaves are higher than those of the host granites. This with Eurasia in the late Eocene, and thus proposed that the Miocene difference suggests that the host granitic melts cannot be derived adakites in the Gangdese Batholith were generated by partial melting from the mafic melts through fractional crystallization but must of an upper mantle source metasomatized by slab-derived melts instead have been derived from a different source. In fact, several lines during Neo-Tethyan subduction. However, we consider this possi- of evidence indicate that the Wolong adakitic host granites are most bility unlikely because numerous lines of evidence indicate that the likely derived from partial melting of thickened lower crust: India–Asia collision occurred in the early Cenozoic (see below). The adakitic rocks (136–9Ma)identified in the Gangdese Batholith can (1) Compared to the adakitic rocks derived from partial melting of be classified as either related to subduction of the Neo-Tethyan slab, the subducting oceanic crust, the Wolong host granites are or derived from the lower crust. Subduction-related rocks are relatively silica-rich and potassium-rich (Fig. 4b), with lower A/ identified as geochemically distinct from lower-crust-derived rocks NK ratios, similar to those of lower-crust-derived adakitic rocks by their low K2O(Fig. 4b), high A/NK [=molar Al2O3/(Na2O+K2O)] (Fig. 4c). (Fig. 4c), high Mg# (Fig. 4d), and high abundances of compatible (2) The MgO contents (0.67–1.18%) and Mg# (41–43) of the elements (Fig. 9a and b). Such differences provide important clues for Wolong adakitic host granites differ significantly from the Q. Guan et al. / Gondwana Research 21 (2012) 88–99 95

Host granites Mafic enclaves 137 Ma adakites ca. 90 Ma adakites 80 Ma adakites 30 Ma adakites 26-10 Ma adakites

16 9 a Lower crust-derived b 14 nepheline 8 adakites Alkalic syenite Shoshonite 12 7 syenite syenite 6 10 alkali granite 5

O(wt.%) High-K syenite-diorite Subduction-related 2 8 granite adakites

O(wt.%) 4 ijolite quartz gabbro 2

O+K 6 diorite K 2 3 gabbro diorite (granodiorite) Na 4 2 Medium-K gabbro 2 Sub-Alkalic 1 Low-K 0 0 40 50 60 70 45 50 55 60 65 70 75 80

SiO2(wt.%) SiO2 (wt.%)

3.0 80 Subduction-related c adakites d Subduction-related Mantle adakites 2.6 70 melts

Peraluminous C 2.2 60 r u stal

1.8 50 AF C Mg# A/NK

1.4 40 Linzizong volcanics Metaluminous 1.0 30 Lower crust-derived adakites Lower crust-derived adakites 0.6 20 0.7 0.8 0.9 1.0 1.1 1.2 1.3 45 50 55 60 65 70 75

A/CNK SiO2 (wt.%)

Fig. 4. Selected major-element plots for the Wolong granitoids. (a) Total alkalis vs. silica diagram (Wilson, 1989); (b) K2O vs. SiO2 diagram (Rollinson, 1993); (c) A/NK vs. A/CNK # diagram (Maniar and Piccoli, 1989); (d) Mg vs. SiO2 diagram. Data sources: 137 Ma adakites (Zhu et al., 2009a); 90 Ma adakites (Zhang et al., 2010c); 80 Ma adakites (Wen et al., 2008a; Guan et al., 2010); 30 Ma adakites (Jiang et al., 2011); 26–10 Ma adakites (Chung et al., 2003; Hou et al., 2004; Guo et al., 2007; Gao et al., 2010; Xu et al., 2010; Zhang et al., 2010a); ﬁeld of Linzizong volcanic succession (LVS) (Mo et al., 2007, 2008); crustal AFC (Stern and Kilian, 1996).

subduction-related adakitic rocks but are comparable to those slab. The discernable concave-upward Dy–Ho–Er–Tm-depleted of the lower-crust-derived adakitic rocks (Fig. 4d). This patterns (Fig. 6a) observed in the Wolong host granites indicate difference is also supported by the very low abundances of that they were most likely generated by partial melting of compatible elements (Cr=3.87–8.36 ppm, Ni=3.04– basaltic lower crust under variable water fugacity, leaving a 5.71 ppm), signiﬁcantly lower than those of the subduction- garnet-bearing, amphibole-rich residue (Fig. 5b) (Petford and related adakitic rocks (Fig. 9). All these features indicate that Atherton, 1996; Zhu et al., 2008). the Wolong adakitic granites are unlikely to be derived from (4) Sr and Nd isotopic data also support a lower crustal origin for

partial melting of the subducted Neo-Tethyan oceanic slab and the Wolong adakitic host granites. Although the positive εHf(t) interaction with mantle peridotite but are consistent with a values (+6.0 to +12.6) indicate a juvenile crustal origin, the Sr thickened lower crustal origin. and Nd isotopic compositions point to a trend towards lower (3) Trace element signatures for the Wolong adakitic host granites crust that is similar to the trend deﬁned by the Miocene ﬁt with generation from lower crust rather than subducted adakitic rocks (26–10 Ma) (Fig. 7).

150 400 abHost granites 137 Ma adakites Eclogite 350 ~90 Ma adakites Adakite Adakite 80 Ma adakites 300 30 Ma adakites 100 N 250 26-10 Ma adakites 30% garnet amphibolite

Sr/Y 200 7% garnet amphibolite (La/Yb) 150 50 100 Arc magmatic rocks Amphibolite 50

0 0 0102030400 5101520 Y YbN

Fig. 5. Plots of Sr/Y vs. Y (a) and (La/Yb)N vs. YbN (b) for the Wolong granitoids. Fields of adakite and arc magmatic rocks are from Petford and Atherton (1996) and Defant and Drummond (1990). N denotes normalized to chondrite composition (Sun and McDonough, 1989). Data sources are as in Fig. 4. 96 Q. Guan et al. / Gondwana Research 21 (2012) 88–99

1000 1000 abHost granites Mafic enclaves 100 100

10 1 Samples/Chondrite Samples/Primitive mantle Samples/Primitive Ba U Nb La Pb Sr P Hf EuGd Dy Ho Tm Lu 1 0.1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Rb Th K Ta CePr Nd Zr Sm Ti Tb Y Er Yb

Fig. 6. Chondrite-normalized REE (a) and primitive-mantle-normalized trace element patterns (b) for the Wolong granitoids. Chondrite and primitive mantle values are from Sun and McDonough (1989).

5.2.2. Mafic enclaves equilibrium is attained more rapidly than chemical equilibrium The mafic enclaves are generally thought to represent either during magma mixing (Lesher, 1990; Poli et al., 1996). residual material (restite), which unmixes from the melt during the The presence of a gabbroic enclave (sample ML18-1) suggests that rise of a crystal mush from its source region (e.g., Chappell and the precursor of the Wolong mafic enclaves could be basaltic in Simpson, 1984; Chappell and White, 1992), or remnants of a mafic composition. The mafic enclaves are enriched in LILE and depleted in component added to granitic magma (magma mixing) (e.g., Didier, HFSE, with negative Nb, Ta and Ti anomalies (Fig. 6b). Such features 1987; Holden et al., 1987). A critical feature of the unmixing process can be interpreted as resulting from: (i) melting of an enriched required by the restite model is the presence of linear chemical lithospheric mantle source, or (ii) melting of a depleted mantle source variations in many calc-alkaline plutonic suites (e.g., Chappell et al., mixed with melts from subducted sediments. The negative εNd(t) 1987; Chappell and White, 1992). However, this is not the case for the values (−2.7 to −1.4) of the mafic enclaves are different from those Wolong suite (Fig. 4a, b and d). In addition, the mafic enclaves have of the coeval Dagze mafic rocks (+5.2 to +3.6) of 40–38 Ma, 200 km typical igneous textures, distinct from the metamorphic textures to the west, which likely originated from an enriched asthenospheric expected of restites. The isotopic compositions of the host granites mantle source (Gao et al., 2008; Fig. 1a). However, they are and enclaves are similar, which seems more consistent with the comparable to those of the coeval basaltic dykes (−3.9 to −0.3) of restite model. However, petrographical observations, such as the 40–42 Ma in the Gaoligong–Tengliang belt, which were likely derived presence of K-feldspar and quartz megacrysts, quenched margins, and from continental lithospheric mantle (Xu et al., 2008). Therefore, we acicular apatites within the enclaves (Fig. 2), together with the suggest that the parental magmas of the Wolong mafic enclaves were identical emplacement ages (ca. 38 Ma) of the host granite and mafic derived from lithospheric mantle beneath the southern Lhasa Terrane. enclave, are indicative of magma mixing. Thus, the similarity in It is widely accepted that the Gangdese Batholith is derived from whole-rock Sr and Nd isotopic compositions between the host partial melting of juvenile crustal materials with predominantly positive granites and the mafic enclaves can be interpreted as isotopic zircon εHf(t)values(cf.Chu et al., 2006; Chung et al., 2009; Ji et al., equilibrium, which can be readily achieved, as radiogenic isotopic 2009a; Mo et al., 2009; Guan et al., 2010; Zhu et al., 2011a). Such an interpretation is also supported by our data on the Wolong host granites,

which exhibit positive zircon εHf(t)values(+6.0to+12.6)(Table 4) (Fig. 8). However, the negative εNd(t) values of the host granites (−2.3 12 to −1.6) suggest that ancient crustal materials may also have Yarlung Zangbo MORB Host granites Mafic enclaves contributed to the generation of the Wolong adakitic rocks. The negative 8 Subduction-related 137 Ma adakites correlation deﬁned by the whole-rock Sr and Nd isotopic data (Fig. 7) adakites 80 Ma adakites 30 Ma adakites 26-10 Ma adakites 20 4 Linzizong Dagze basaltic rocks Depleted mantle volcanics Gaoligong-Tengliang

(t) 15 basaltic dykes Nd 0 Lower crust-derived 10 adakites

-4 5 (t)

Amdo orthogneiss Hf CHUR 0 -8 Ancient lower crust 0.704 0.708 0.712 0.716 -5 137 Ma adakites 80 Ma adakites (87Sr /86Sr)t -10 Host granites 30 Ma adakites Mafic enclaves 26-10 Ma adakites 87 86 Fig. 7. εNd(t) vs. initial Sr/ Sr values diagram for the Wolong granitoids. Data sources: 137 Ma adakites (Zhu et al., 2009a); 80 Ma adakites (Wen et al., 2008a, our unpublished -15 0 30 6090 120 150 data); 30 Ma adakites (Jiang et al., 2011); 26–10 Ma adakites (Hou et al., 2004; Gao et al., 2010; Xu et al., 2010; Zhang et al., 2010a); Dazi Eocene (40–38 Ma) basaltic rocks Age (Ma) (Gao et al., 2008); Eocene (42–40 Ma) basaltic dykes in the Gaoligong–Tengliang belt, eastern Tibet (Xu et al., 2008); ﬁeld of Linzizong volcanic succession (LVS) (Mo et al., Fig. 8. Plots of zircon εHf(t) vs. U–Pb ages for the Wolong granitoids. Data sources: 2007, 2008); Ancient lower crust (Miller et al., 1999); Amdo orthogneiss (Harris et al., 137 Ma adakites (Zhu et al., 2009a); 80 Ma adakites (Wen et al., 2008a); 30 Ma adakites 1988). (Jiang et al., 2011); 26–10 Ma adakites (Chung et al., 2009; Xu et al., 2010). Q. Guan et al. / Gondwana Research 21 (2012) 88–99 97

1000 150 Host granites abSubduction-related Dioritic enclaves Subduction-related adakites adakites 137 Ma adakites 120 100 ~90 Ma adakites 80 Ma adakites 30 Ma adakites 90 26-10 Ma adakites 10 60 Ni (ppm) Ni (ppm) Lower crust-derived 1 adakites Lower crust-derived 30 adakites

0 0 0 1 10 100 1000 25 35 45 55 65 75 Cr (ppm) Mg#

Fig. 9. Ni vs. Cr and Ni vs. Mg# plots for the Wolong granitoids. Data sources: 137 Ma adakites (Zhu et al., 2009b); 90 Ma adakites (Zhang et al., 2010c); 80 Ma adakites (Wen et al., 2008a; Guan et al., 2010); 30 Ma adakites (Jiang et al., 2011); 26–10 Ma adakites (Guo et al., 2007; Gao et al., 2010; Xu et al., 2010).

further corroborates this suggestion, falling on a trend towards the (Fig. 10). This inference is consistent with the suggestion that the postulated ancient lower crust of the Lhasa microcontinent (Miller et al., adakites were produced by partial melting of garnet-bearing mafic 1999; Zhu et al., 2011a). We therefore conclude that the Wolong rocks at depths of over ~50 km in the lower part of the thickened granitoids are the result of mixing between thickened lower crust- crust (Chung et al., 2003; Guo et al., 2007). This suggests that the derived melts and lithospheric mantle-derived mafic melts beneath the southern Lhasa Terrane had a much thicker crust when the Wolong southern Lhasa Terrane. adakites were emplaced (ca. 38 Ma), in good agreement with the conclusions of Mo et al. (2007). The voluminous Linzizong volcanic succession and coeval plutonic 5.3. Implications rocks with abundant mafic enclaves within the Gangdese Batholith have been attributed to the break-off of the Neo-Tethyan Ocean slab at about Mo et al. (2007) suggested that the volcanic rocks in the 50 Ma (Wen et al., 2008b; Chung et al., 2009; Ji et al., 2009a; Lee et al., Dianzhong (emplaced at 64–60 Ma) and Nianbo (emplaced at 2009; Zhu et al., 2011a). Numerical modeling indicates that slab break- ~54 Ma) formations of the lower and middle Linzizong volcanic off will trigger localized asthenospheric upwelling and the eduction and succession were emplaced in crust with an ordinary thickness of exhumation of buoyant crustal material in the subduction channel ~35 km, while the high-K calc-alkaline to shoshonitic volcanic rocks beneath the orogen (cf. Duretz et al., 2011). As a result, decompression in the Pana Formation of the upper Linzizong volcanic succession melts from the chemically enriched sub-slab asthenosphere will not – (emplaced at 50 40 Ma) were built upon a thickened crust, with only intrude/underplate the overriding plate but will also likely flow garnet as a residual phase in the source. Existing studies indicate that into the subducting plate, accompanying the buoyant crustal material the La/Yb ratios of rocks from the Andes can be used as a geochemical through the subduction channel (Zhu et al., submitted for publication). proxy for crustal thickness (see Chung et al., 2009, and references In this case, magmatism induced by mantle-derived melts in response to therein). This led Chung et al. (2009) to propose that the crust slab break-off will simultaneously occur on both the overriding and beneath the southern Lhasa Terrane had been thickened toN50 km subducting plates, as observed in the Tibetan Plateau (Zhu et al., before the adakitic rocks began to be emplaced at ~30 Ma. The submitted for publication). Therefore, a protracted magmatic response presence of the Wolong adakitic host granites reported here indicates to the slab break-off of the Neo-Tethyan oceanic lithosphere is that the crust of the southern Lhasa Terrane was already thickened to advocated here for the generation of magmatism at about 40 Ma, – 60 70 km, as inferred from the La/Yb ratios (Chung et al., 2009) documented in both the Gangdese Batholith (Mo et al., 2007, 2008; Gao et al., 2008; Wen et al., 2008b; Xu et al., 2008; Lee et al., 2009; Ji et al., 2009a; this study) and the Tethyan Himalaya (i.e., Yardoi granites; Zeng Inferred La/Yb Host granites et al., 2011)(Fig. 1a). Under this geodynamic regime, lithospheric crustal mantle-derived magmas provide not only mantle-derived materials but thickness 137 Ma adakites 80 ~90 Ma adakites also a heat supply that could induce melting of the mafic lower crust to 80 Ma adakites generate the Wolong adakitic host granites and melting of the mid- 30 Ma adakites upper crust to generate the Pana volcanic rocks of the upper Linzizong 26-10 Ma adakites 60 volcanic succession. ~50-55 Linzizong volcanic succession Recent studies on the structural geology and geochemistry of the km Dala and Yardoi adakitic granites (~43 Ma) (Fig. 1a) indicate that the 40 Tethyan Himalayan crust was also thickened to over 50 km before 44 Ma (cf. Aikman et al., 2008; Zeng et al., 2011). The coeval magmatism (~43 Ma) and significant crustal thickening documented ~40 km 20 in both the Gangdese Batholith (overriding plate; Mo et al., 2007;this ~30 km study) and the Tethyan Himalaya (subducting plate; Aikman et al., 2008; Zeng et al., 2011) is best interpreted as the tectonomagmatic response to the India–Asia continental collision. This in turn indicates 0 30 60 90 120 that the India–Asia collision occurred prior to ~43 Ma, which is Age (Ma) consistent with the general view that this collision took place in the – Fig. 10. Plot of La/Yb vs. U–Pb ages for the Cenozoic magmatic rocks in the Gangdese early Cenozoic (~65 55 Ma; Searle et al., 1987; Lee and Lawver, Batholith. Data sources are as in Fig. 4. 1995; Yin and Harrison, 2000; Mo et al., 2003, 2007, 2008; Ding et al., 98 Q. Guan et al. / Gondwana Research 21 (2012) 88–99

2005; Chung et al., 2009; Chen et al., 2010; Najman et al., 2010; Zhang Chung, S.L., Chu, M.F., Zhang, Y.Q., Xie, Y.W., Lo, C.H., Lee, T.Y., Lan, C.Y., Li, X.H., Zhang, Q., Wang, Y.Z., 2005. Tibetan tectonic evolution inferred from spatial and temporal et al., 2010b; Zhu et al., 2011a), rather than in the Oligocene variations in post-collisional magmatism. Earth-Science Reviews 68, 173–196. (Aitchison et al., 2007)orat~43Ma(Tan et al., 2010). Chung, S.L., Liu, D.Y., Ji, J.Q., 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. 6. Conclusions Coulon, C., Maluski, H., Bollinger, C., Wang, S., 1986. Mesozoic and cenozoic volcanic rocks from central and southern Tibet: 39Ar/40Ar dating, petrological characteristics fi – (1) The Wolong host granites and dioritic enclaves in the Gangdese and geodynamical signi cance. Earth and Planetary Science Letters 79, 281 302. Defant, M.J., Drummond, M.S., 1990. Derivation of some modern arc magmas by Batholith, southern Tibet, were synchronously emplaced in the melting of young subducted lithosphere. Nature 347, 662–665. Late Eocene (~38 Ma). The host granites show geochemical Didier, J., 1987. Contribution of enclave studies to the understanding of origin and affinities with adakite. evolution of granitic magmas. Geologische Rundschau 76, 41–50. Ding, L., Kapp, P., Wan, X., 2005. Paleocene–Eocene record of ophiolite obduction and (2) The Wolong adakitic host granites were most likely derived initial India–Asia collision, south central Tibet. Tectonics 24 (TC3001). from partial melting of thickened lower crust, while the Dong, X., Zhang, Z.M., Santosh, M., 2010. Zircon U–Pb chronology of the Nyingtri Group, parental magmas of the mafic enclaves were likely derived southern Lhasa terrane, Tibetan Plateau: implications for Grenvillian and Pan- African provenance and Mesozoic–Cenozoic metamorphism. Journal of Geology from lithospheric mantle beneath the southern Lhasa Terrane. 118, 677–690. (3) The Wolong granitoids are interpreted as the result of mixing Duretz, T., Gerya, T.V., May, D.A., 2011. Numerical modelling of spontaneous slab between thickened lower crust-derived melts and lithospheric breakoff and subsequent topographic response. Tectonophysics 502, 244–256. fi Gao, Y.F., Hou, Z.Q., Kamber, B.S., Wei, R.H., Meng, X.J., Zhao, R.S., 2007. Adakite-like mantle-derived ma c melts, representing the protracted porphyries from the southern Tibetan continental collision zones: evidence for slab magmatic response to break-off of the Neo-Tethyan oceanic melt metasomatism. Contributions to Mineralogy and Petrology 153, 105–120. lithosphere that occurred at about 50 Ma. Gao, Y.F., Wei, R.H., Hou, Z.Q., Tian, S.H., Zhao, R.S., 2008. Eocene high-MgO volcanism in (4) The presence of the Wolong granitoids supports the general southern Tibet: new constraints for mantle source characteristics and deep processes. Lithos 105, 63–72. view that the India–Asia collision occurred at prior to ~43 Ma Gao, Y.F., Yang, Z.S., Santosh, M., Hou, Z.Q., Wei, R.H., Tian, S.H., 2010. Adakitic rocks rather than in the Oligocene or at ~43 Ma. from slab melt-modified mantle sources in the continental collision zone of southern Tibet. Lithos 119, 651–663. Griffin, W.L., Pearson, N.J., Belousova, E., Jackson, S.E., van Achterbergh, E., O'Reilly, S.Y., Acknowledgments Shee, S.R., 2000. The Hf isotope composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysts in kimberlites. Geochimica et Cosmochimica Acta 64, 133–147. We thank Profs. Zeming Zhang and M. Santosh for inviting this Guan, Q., Zhu, D.C., Zhao, Z.D., Zhang, L.L., Liu, M., Li, X.W., Yu, F., Liu, M.H., Mo, X.X., contribution to the special volume of the journal. We are grateful for 2010. Late Cretaceous adakites from the eastern segment of the Gangdese Belt, constructive comments by two anonymous reviewers, and editorial Southern Tibet: products of Neo-Tethyan mid-ocean ridge subduction. Acta fi Petrologica Sinica 26, 2165–2179 (in Chinese with English abstract). handling by Prof. Zeming Zhang. This research was nancially co- Guo, L., Zhang, H.F., Harris, N., Parrish, R., Xu, W.C., Shi, Z.L., 2011. Paleogene crustal supported by the National Key Project for Basic Research of China anatexis and metamorphism in Lhasa terrane, eastern Himalayan syntaxis: (Project 2011CB403102 and 2009CB421002), the National Natural evidence from U–Pb zircon ages and Hf isotopic compositions of the Nyingchi Science Foundation of China (40830317, 40973026, and 40873023), Complex. Gondwana Research. doi:10.1016/j.gr.2011.03.002. Guo, Z.F., Wilson, M., Liu, J.Q., 2007. Post-collisional adakites in south Tibet: products of the New Century Excellent Talents in University (NCET-10-0711), the partial melting of subduction-modified lower crust. Lithos 96, 205–224. Fundamental Research Funds for the Central Universities (2010ZD02), Harris, N.B.W., Ronghua, X., Lewis, C.L., Hawkesworth, C.J., Yuquan, Z., 1988. Isotope the Program for Changjiang Scholars and Innovative Research Team in Geochemistry of the 1985 Tibet Geotraverse, Lhasa to Golmud. Philosophical Transactions of the Royal Society of London A327, 263–285. University of Ministry of Education of China (PCSIRT), and the Holden, P., Halliday, A.N., Stephens, W.E., 1987. Neodymium and strontium isotope Programme of the China Geological Survey (1212011121260 and content of microdiorite enclaves points to mantle input to granitoid production. 1212011121066). We also thank Benjamin M. for English polishing, Nature 330, 53–56. and Jian-Qi Wang, Hai-Hong Chen for helping with whole-rock Hoskin, P.W.O., Schaltegger, U., 2003. The composition of zircon and igneous and metamorphic petrogenesis. Reviews in Mineralogy and Geochemistry 53, geochemical analysis. 27–62. Hou, Z.Q., Gao, Y.F., Qu, X.M., Rui, Z.Y., Mo, X.X., 2004. Origin of adakitic intrusives References generated during mid-Miocene east-west extension in southern Tibet. Earth and Planetary Science Letters 220, 139–155. – Aikman, A.B., Harrison, T.M., Ding, L., 2008. Evidence for Early (N 44 Ma) Himalayan Ji, W.Q., Wu, F.Y., Chung, S.L., Li, J.X., Liu, C.Z., 2009a. Zircon U Pb geochronology and Hf crustal thickening, Tethyan Himalaya, southeastern Tibet. Earth and Planetary isotopic constraints on petrogenesis of the Gangdese batholith, southern Tibet. Science Letters 274, 14–23. Chemical Geology 262, 229–245. Aitchison, J.C., Ali, J.R., Davis, A.M., 2007. When and where did India and Asia collide? Ji, W.Q., Wu, F.Y., Liu, C.Z., Chung, S.L., 2009b. Geochronology and petrogenesis of Journal of Geophysical Reseach 112, B05423. granitic rocks in Gangdese batholith, southern Tibet. Science in China Series D: Aitchison, J.C., Xia, X., Baxter, A.T., Ali, J.R., 2011. Detrital zircon U–Pb ages along the Earth Sciences 52, 1240–1261. Yarlung-Tsangpo suture zone, Tibet: implications for oblique convergence and Jiang, Z.Q., Wang, Q., Wyman, D.A., Tang, G.J., Jia, X.H., Yang, Y.H., Yu, H.X., 2011. Origin collision between India and Asia. Gondwana Research. doi:10.1016/j.gr.2011.04.002. of ~30 Ma Chongmuda adakitic intrusive rocks in the southern Gangdese region, Andersen, T., 2002. Correction of common lead in U–Pb analyses that do not report southern Tibet: partial melting of the northward subducted Indian continent crust? 204Pb. Chemical Geology 192, 59–79. Geochimica 40, 126–146 (in Chinese with English abstract). Blichert-Toft, J., Albarède, F., 1997. The Lu–Hf isotope geochemistry of chondrites and the Kapp, P., Yin, A., Harrison, T.M., Ding, L., 2005. Cretaceous-Tertiary shortening, basin evolution of the mantle-crust system. Earth and Planetary Science Letters 148, 243–258. development, and volcanism in central Tibet. Geological Society of America Bulletin Chappell, B.W., Simpson, P.R., 1984. Source rocks of I- and S-Type granites in the 117, 865–878. Lachlan Fold Belt, Southeastern Australia [and Discussion]. Philosophical trans- Lee, H.Y., Chung, S.L., Lo, C.H., Ji, J.Q., Lee, T.Y., Qian, Q., Zhang, Q., 2009. Eocene actions of the Royal Society of London. Series A. Mathematical and Physical Sciences Neotethyan slab breakoff in southern Tibet inferred from the Linzizong volcanic 310, 693–707. record. Tectonophysics 477, 20–35. Chappell, B.W., White, A.J.R., 1992. I- and S-type granites in the Lachlan Fold Belt. Lee, T.Y., Lawver, L.A., 1995. Cenozoic plate reconstruction of Southeast Asia. Transactions of the Royal Society of Edinburgh: Earth Sciences 83, 1–26. Tectonophysics 251, 85–138. Chappell, B.W., White, A.J.R., Wyborn, D., 1987. The importance of residual source Lesher, C.E., 1990. Decoupling of chemical and isotopic exchange during magma material (Restite) in granite petrogenesis. Journal of Petrology 28, 1111–1138. mixing. Nature 344, 235–237. Chen, J.S., Huang, B.C., Sun, L.S., 2010. New constraints to the onset of the India–Asia Liu, Y.S., Gao, S., Hu, Z.C., Gao, C.G., Zong, K.Q., Wang, D.B., 2010a. Continental and collision: paleomagnetic reconnaissance on the Linzizong Group in the Lhasa Block, oceanic crust recycling-induced Melt–Peridotite interactions in the Trans-North China. Tectonophysics 489, 189–209. China Orogen: U–Pb dating, Hf isotopes and trace elements in zircons from Mantle Chu, M.F., Chung, S.L., Song, B., Liu, D.Y., O'Reilly, S.Y., Pearson, N.J., Ji, J.Q., Wen, D.J., Xenoliths. Journal of Petrology 51, 537–571. 2006. Zircon U–Pb and Hf isotope constraints on the Mesozoic tectonics and crustal Liu, Y.S., Hu, Z.C., Gao, S., Günther, D., Xu, J., Gao, C.G., Chen, H.H., 2008a. In situ analysis evolution of southern Tibet. Geology 34, 745–748. of major and trace elements of anhydrous minerals by LA-ICP-MS without applying Chung, S.L., Chu, M.F., Ji, J.Q., O'Reilly, S.Y., Pearson, N.J., Liu, D.Y., Lee, T.Y., Lo, C.H., 2009. an internal standard. Chemical Geology 257, 34–43. The nature and timing of crustal thickening in Southern Tibet: geochemical and Liu, Y.S., Hu, Z.C., Zong, K.Q., Gao, C.G., Gao, S., Xu, J., Chen, H.H., 2010b. Reappraisement zircon Hf isotopic constraints from postcollisional adakites. Tectonophysics 477, and refinement of zircon U–Pb isotope and trace element analyses by LA-ICP-MS. 36–48. Chinese Science Bulletin 55, 1535–1546. Q. Guan et al. / Gondwana Research 21 (2012) 88–99 99

Liu, Y.S., Zong, K.Q., Kellemen, P.B., Gao, S., 2008b. Geochemistry and magmatic history Xu, W.C., Zhang, H.F., Guo, L., Yuan, H.L., 2010. Miocene high Sr/Y magmatism, south of eclogites and ultramafic rocks from the Chinese continental scientific drill hole: Tibet: product of partial melting of subducted Indian continental crust and its Subduction and ultrahigh-pressure metamorphism of lower crustal cumulates. tectonic implication. Lithos 114, 293–306. Chemical Geology 247, 133–153. Xu, Y.G., Lan, J.B., Yang, Q.J., Huang, X.L., Qiu, H.N., 2008. Eocene break-off of the Neo- Ludwig, K.R., 2003. Isoplot v. 3.0: a geochronological toolkit for Microsoft Excel. Tethyan slab as inferred from intraplate-type mafic dykes in the Gaoligong Berkeley Geochronology Center, Special Publication, No. 4, pp. 1–70. orogenic belt, eastern Tibet. Chemical Geology 255, 439–453. Maluski, H., Proust, F., Xiao, X.C., 1982. 39Ar/40Ar dating of the trans-Himalayan calc- Yin, A., Harrison, T.M., 2000. Geologic evolution of the Himalayan–Tibetan Orogen. alkaline magmatism of southern Tibet. Nature 298, 152–154. Annual Review of Earth and Planetary Sciences 28, 211–280. Maniar, P.D., Piccoli, P.M., 1989. Tectonic discrimination of granitoids. Geological Yin, G.H., Chen, Y.M., Su, X.J., Bao, J.Y., Hou, S.Y., Lv, Y., Duan, G.X., Zhang, J.Y., Liu, Z., Xiao, Society of America Bulletin 101, 635–643. L., 2003. 1: 250, 000 geological report of Linzhi County with geological map. Miller, C., Schuster, R., Klotzli, U., Frank, W., Purtscheller, F., 1999. Post-collisional Yunnan Province Geology Survey (unpublished, in Chinese). potassic and ultrapotassic magmatism in SW Tibet: geochemical and Sr–Nd–Pb–O Yuan, H.L., Gao, S., Dai, M.N., Zong, C.L., Günther, D., Fontaine, G.H., Liu, X.M., Diwu, C.R., isotopic constraints for mantle source characteristics and petrogenesis. Journal of 2008. Simultaneous determinations of U–Pb age, Hf isotopes and trace element Petrology 40, 1399–1424. compositions of zircon by excimer laser-ablation quadrupole and multiple- Mo, X.X., Dong, G.C., Zhao, Z.D., Zhu, D.C., Zhou, S., Niu, Y.L., 2009. Mantle input to the collector ICP-MS. Chemical Geology 247, 100–118. crust in southern Gangdese, Tibet, during the Cenozoic: zircon Hf isotopic evidence. Zeng, L.S., Gao, L.E., Xie, K.J., Zeng, J.L., 2011. Mid-Eocene high Sr/Y granites in the Journal of Earth Science 20, 241–249. Northern Himalayan Gneiss Domes: melting thickened lower continental crust. Mo, X.X., Hou, Z.Q., Niu, Y.L., Dong, G.C., Qu, X.M., Zhao, Z.D., Yang, Z.M., 2007. Mantle Earth and Planetary Science Letters 303, 251–266. contributions to crustal thickening during continental collision: evidence from Zhang, H.F., Gao, S., Zhong, Z.Q., Zhang, B.R., Zhang, L., Hu, S.H., 2002. Geochemical and Cenozoic igneous rocks in southern Tibet. Lithos 96, 225–242. Sr–Nd–Pb isotopic compositions of Cretaceous granitoids: constraints on tectonic Mo, X.X., Niu, Y.L., Dong, G.C., Zhao, Z.D., Hou, Z.Q., Zhou, S., Ke, S., 2008. Contribution of framework and crustal structure of the Dabieshan ultrahigh-pressure metamorphic syncollisional felsic magmatism to continental crust growth: a case study of the belt, China. Chemical Geology 186, 281–299. Paleogene Linzizong volcanic Succession in southern Tibet. Chemical Geology 250, Zhang, H.F., Harris, N., Guo, L., Xu, W.C., 2010a. The significance of Cenozoic magmatism 49–67. from the western margin of the eastern syntaxis, southeast Tibet. Contributions to Mo, X.X., Zhao, Z.D., Deng, J.F., Dong, G.C., Zhou, S., Guo, T.Y., Zhang, S.Q., Wang, L.L., Mineralogy and Petrology 160, 83–98. 2003. Response of volcanism to the India–Asia collision. Earth Science Frontiers 10, Zhang, K.X., Wang, G.C., Ji, J.L., Luo, M.S., Kou, X.H., Wang, Y.M., Xu, Y.D., Chen, F.N., Chen, 135–148 (in Chinese with English abstract). R.M., Song, B.W., Zhang, J.Y., Liang, Y.P., 2010b. Paleogene–Neogene stratigraphic Najman, Y., Appel, E., Boudagher-Fadel, M., Bown, P., Carter, A., Garzanti, E., Godin, L., realm and sedimentary sequence of the Qinghai-Tibet Plateau and their response to Han, J., Liebke, U., Oliver, G., Parrish, R., Vezzoli, G., 2010. Timing of India–Asia uplift of the plateau. Science in China Series D: Earth Sciences 53, 1271–1294. collision: geological, biostratigraphic, and palaeomagnetic constraints. Journal of Zhang, Z.M., Zhao, G.C., Santosh, M., Wang, J.L., Dong, X., Shen, K., 2010c. Late Cretaceous Geophysical Research 115, B12416. charnockite with adakitic affinities from the Gangdese batholith, southeastern Petford, N., Atherton, M., 1996. Na-rich partial Melts from Newly Underplated Basaltic Tibet: evidence for Neo-Tethyan mid-ocean ridge subduction? Gondwana Research Crust: the Cordillera Blanca Batholith, Peru. Journal of Petrology 37, 1491–1521. 17, 615–631. Poli, G., Tommasini, S., Halliday, A.N., 1996. Trace element and isotopic exchange during Zhang, Z.M., Zhao, G.C., Santosh, M., Wang, J.L., Dong, X., Liou, J.G., 2010d. Two-stages of acid–basic magma interaction processes. Geological Society of America Special granulite-facies metamorphism in the eastern Himalayan syntaxis, south Tibet: Papers 315, 225–232. petrology, zircon geochronology and implications for the subduction of Neo-Tethys Qu, X.M., Hou, Z.Q., Li, Y.G., 2004. Melt components derived from a subducted slab in and the Indian continent beneath Asia. Journal of Metamorphic Geology 28, late orogenic ore-bearing porphyries in the Gangdese copper belt, southern Tibetan 719–733. plateau. Lithos 74, 131–148. Zhang, Z.M., Dong, X., Santosh, M., Liu, F., Wang, W., Yiu, F., He, Z.Y., Shen, K., 2011. Rollinson, H.R., 1993. Using geochemical data: evaluation, presentation, interpretation. Petrology and geochronology of the Namche Barwa Complex in the eastern Longman Group UK Ltd, New York, pp. 1–352. Himalayan syntaxis, Tibet: constraints on the origin and evolution of the north- Söderlund, U., Patchett, P.J., Vervoort, J.D., Isachsen, C.E., 2004. The 176Lu decay constant eastern margin of the Indian Craton. Gondwana Research. doi:10.1016/j. determined by Lu–Hf and U–Pb isotope systematics of Precambrian mafic gr.2011.02.002. intrusions. Earth and Planetary Science Letters 219, 311–324. Zhao, Z.D., Mo, X.X., Dilek, Y., Niu, Y.L., DePaolo, D.J., Robinson, P., Zhu, D.C., Sun, C., Searle, M.P., Windley, B.F., Coward, M.P., Cooper, D.J.W., Rex, A.J., Rex, D., LI, T.D., Xiao, X. Dong, G., Zhou, S., Luo, Z., Hou, Z.Q., 2009. Geochemical and Sr–Nd–Pb–O isotopic C., Jan, M.Q., Thakur, V.C., Kumar, S., 1987. The closing of Tethys and the tectonics of compositions of the post-collisional ultrapotassic magmatism in SW Tibet: the Himalaya. Geological Society of America Bulletin 98, 678–701. petrogenesis and implications for India intra-continental subduction beneath Stern, C.R., Kilian, R., 1996. Role of the subducted slab, mantle wedge and continental southern Tibet. Lithos 113, 190–212. crust in the generation of adakites from the Andean Austral Volcanic Zone. 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, Contributions to Mineralogy and Petrology 123, 263–281. G.C., Liu, B., 2009a. Early cretaceous subduction-related adakite-like rocks of the Sun, S.S., McDonough, W.F., 1989. Chemical and isotope systematics of oceanic basalts: Gangdese Belt, southern Tibet: products of slab melting and subsequent melt- Implications for mantle composition and processes. In: Saunders, A.D. (Eds.), peridotite interaction? Journal of Asian Earth Sciences 34, 298–309. Magmatism in ocean Basins. Geological Society Publication 42, 313–345. Zhu, D.C., Mo, X.X., Niu, Y.L., Zhao, Z.D., Wang, L.Q., Liu, Y.S., Wu, F.Y., 2009b. Geochemical Tan, X., Gilder, S., Kodama, K.P., Jiang, W., Han, Y., Zhang, H., Xu, H., Zhou, D., 2010. New investigation of Early Cretaceous igneous rocks along an east–west traverse throughout paleomagnetic results from the Lhasa block: Revised estimation of latitudinal the central Lhasa Terrane, Tibet. Chemical Geology 268, 298–312. shortening across Tibet and implications for dating the India–Asia collision. Earth Zhu, D.C., Mo, X.X., Wang, L.Q., Zhao, Z.D., Niu, Y.L., Zhou, C.Y., Yang, Y.H., 2009c. and Planetary Science Letters 293, 396–404. Petrogenesis of highly fractionated I-type granites in the Zayu area of eastern Wang, J.L., Zhang, Z.M., Shi, C., 2008. Anatexis and dynamics of the southeastern Lhasa Gangdese, Tibet: constraints from zircon U–Pb geochronology, geochemistry and terrane. Acta Petrologica Sinica 24, 1539–1551 (in Chinese with English abstract). Sr–Nd–Hf isotopes. Science in China Series D: Earth Sciences 52, 1223–1239. Wang, J.L., Zhang, Z.M., Dong, X., Liu, F., Yu, F., Wang, W., Shen, K., 2009. Discovery of late Zhu, D.C., Pan, G.T., Chung, S.L., Liao, Z.L., Wang, L.Q., Li, G.M., 2008. SHRIMP zircon age Cretaceous garnet two-pyroxene granulite in the southern Lhasa terrane, Tibet and and geochemical constraints on the origin of lower Jurassic volcanic rocks from the its tectonic significances. Acta Petrologica Sinica 25, 1695–1706 (in Chinese with Yeba formation, Southern Gangdese, south Tibet. International Geology Review 50, English abstract). 442–471. Wen, D.R., Chung, S.L., Song, B., Iizuka, Y., Yang, H.J., Ji, J.Q., Liu, D.Y., Gallet, S., 2008a. Zhu, D.C., Pan, G.T., Mo, X.X., Wang, L.Q., Liao, Z.L., Zhao, Z.D., Dong, G.C., Zhou, C.Y., Late Cretaceous Gangdese intrusions of adakitic geochemical characteristics, SE 2006. Late Jurassic–Early Cretaceous geodynamic setting in middle–northern Tibet: Petrogenesis and tectonic implications. Lithos 105, 1–11. Gangdese: new insights from volcanic rocks. Acta Petrologica Sinica 22, 534–546 Wen, D.R., Liu, D.Y., Chung, S.L., Chu, M.F., Ji, J.Q., Zhang, Q., Song, B., Lee, T.Y., Yeh, M.W., (in Chinese with English abstract). Lo, C.H., 2008b. Zircon SHRIMP U–Pb ages of the Gangdese Batholith and Zhu, D.C., Zhao, Z.D., Niu, Y.L., Dilek, Y., Hou, Z.Q., Mo, X.X., submitted for publication. implications for Neotethyan subduction in southern Tibet. Chemical Geology 252, Origin and evolution of the Tibetan Plateau. Gondwana Research. 191–201. Zhu, D.C., Zhao, Z.D., Niu, Y.L., Mo, X.X., Chung, S.L., Hou, Z.Q., Wang, L.Q., Wu, F.Y., Wilson, M., 1989. Igneous petrogenesis. Unwim Hyman, London, pp. 1–366. 2011a. The Lhasa Subterrane: record of a microcontinent and its histories of drift Xia, L.Q., Li, X.M., Ma, Z.P., Xu, X.Y., Xia, Z.C., 2011. Cenozoic volcanism and tectonic and growth. Earth and Planetary Science Letters 301, 241–255. evolution of the Tibetan plateau. Gondwana Research 19, 850–866. Zhu, D.C., Zhao, Z.D., Niu, Y.L., Dilek, Y., Mo, X.X., 2011b. Lhasa Terrane in southern Tibet Xu, R.H., Schärer, U., Allègre, C.J., 1985. Magmatism and metamorphism in the Lhasa came from Australia. Geology 39, 727–730. Block (Tibet): a geochronological study. Journal of Geology 93, 41–57.