Gondwana Research 21 (2012) 88–99 Contents lists available at ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr 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.