Lithos 190–191 (2014) 328–348 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Age and geochemistry of western Hoh-Xil–Songpan-Ganzi granitoids, northern Tibet: Implications for the Mesozoic closure of the Paleo-Tethys ocean Li-Yun Zhang a,⁎, Lin Ding a, Alex Pullen b,c,QiangXua,De-LiangLiua, Fu-Long Cai a,Ya-HuiYuea, Qing-Zhou Lai a,Ren-DengShia, Mihai N. Ducea b,d,PaulKappb,AlanChapmane a Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, People's Republic of China b Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA c Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA d Universitatea Bucuresti, Facultatea de Geologie Geofizica, Str. N. Balcescu Nr 1., Bucuresti 010041, Romania e Department of Geological Sciences and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA article info abstract Article history: A geologic investigation was undertaken in the Hoh-Xil–Songpan-Ganzi (HXSG) complex, northern Tibet in Received 21 May 2013 order to better understand magma genesis and evolution during the late stages of Paleo-Tethys ocean closure. Accepted 23 December 2013 The HXSG complex is composed of vast accumulations of Middle–Upper Triassic marine gravity flow deposits Available online 3 January 2014 that were extensively intruded by igneous rocks. These early Mesozoic rocks exposed in this area record a rich history of accretionary tectonics during the amalgamation of the Tibetan Plateau terranes. Eight plutons sampled Keywords: from the western HXSG complex yield zircon U–Pb ages that range from 225 to 193 Ma. Muscovite 40Ar/39Ar ages Hoh-Xil–Songpan-Ganzi Tibet for the Hudongliang and Zhuonai Lake plutons yield ages of 210.7 ± 2.5 Ma and 212.7 ± 2.5 Ma, respectively. Granitoids These plutonic rocks can be subdivided into two geochemically distinct groups. Group 1 (221–212 Ma: Paleo-Tethys Dapeng Lake, Changhong Lake and Heishibei Lake plutons) is composed of high-K calc-alkaline rocks that Rollback have strongly fractionated REE patterns with high (La/Yb)N ratios (91–18) and generally lack Eu anomalies (Eu*/ Eu = 1.02–0.68). Rocks in Group 1 display pronounced negative Nb–Ta and Ti anomalies on primitive mantle- normalized spidergrams. Group 1 rocks exhibit high Sr (782–240 ppm) and low Y (6.3–16.0 ppm) contents with 87 86 high Sr/Y ratios (84–20). Based on Sr–Nd–Hf isotopic data ( Sr/ Sri = 0.7079–0.7090, εNd(t) = −7.7–−4.7, εHf(t) = −5.7–−0.8) and low MgO contents (MgO = 1.10–2.18%), Group 1 rocks are geochemically similar to adakitic rocks and were probably derived from partial melting of the downgoing Paleo-Tethys oceanic slab and overlying marine sediments. Group 2 plutons (225–193 Ma: Daheishan, Yunwuling, Zhuonai Lake, Malanshan and Hudongliang plutons) display lower P2O5 with increasing SiO2 and are medium-K to high-K I-type calc- alkaline bodies with low Sr (14–549 ppm) and high Y (22.3–10.5 ppm) contents. Group 2 rocks have variable fractionated REE patterns ((La/Yb)N =3–38) and negative Eu anomalies (Eu*/Eu = 0.02–0.86). Together with 87 86 Sr–Nd–Hf isotopes ( Sr/ Sri = 0.7072–0.7143, εNd(t) = −6.6–−2.0, εHf(t) = −0.6–+3.0), Group 2 rocks are most likely formed by partial melting of the juvenile crustal sources. Collectively, these data suggest that the Hoh-Xil turbidites were underlain by more continental arc crust than previously thought. We propose that rollback of the subducting Paleo-Tethys oceanic slab led to partial melting of overlying continental arc fragments which developed beneath the HXSG gravity flow deposits. © 2013 Elsevier B.V. All rights reserved. 1. Introduction deformed calciclastic and siliciclastic Middle–Upper Triassic deep- water gravity flow deposits widely regarded as turbidites or flysch The Hoh-Xil–Songpan-Ganzi (HXSG) complex is located in the derived from adjacent continental landmass (Bruguier et al., 1997; northern Tibet between the Kunlun arc terrane to the north, the South Ding et al., 2013; Enkelmann et al., 2007; Gu, 1994; Nie et al., 1994; China block to the east, and the Qiangtang terrane and Yidun arc to Zhang et al., 2008c; Zhou and Graham, 1996). These deposits are vast, the south (Fig. 1) and is widely considered as a ‘remnant’ of the Paleo- with an areal exposure of N200,000 km2 and thicknesses that range Tethys Ocean (Yin and Harrison, 2000; Zhou and Graham, 1996). from 5 to 15 km (Chang, 2000; Xu et al., 1992; Zhou and Graham, Rocks exposed in the HXSG complex mainly consist of moderately 1996). These turbidites were extensively intruded by Late Triassic– Early Jurassic granitoids (Fig. 1, Hu et al., 2005; Huang et al., 2003; ⁎ Corresponding author. Tel./fax: +86 1084097104. Roger et al., 2004; Shi et al., 2009; Weislogel, 2008; Xiao et al., 2007; E-mail address: [email protected] (L.-Y. Zhang). Yuan et al., 2010; Zhang et al., 2006, 2007) and minor volcanic rocks 0024-4937/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.lithos.2013.12.019 L.-Y. Zhang et al. / Lithos 190–191 (2014) 328–348 329 Fig. 1. Simplified tectonic map of the Tibetan Plateau showing major terranes and Late Triassic igneous rocks. Modified after Wang et al. (2011b) and Yin and Harrison (2000) and Zircon U–Pb isotope ages for Late Triassic to Early Jurassic magmatic rocks are from references Cai et al. (2010), Ding et al. (2011), Dai et al. (2013), Fu et al. (2010), Hu et al. (2005), Kapp et al. (2003), Liu et al. (2006), Lü et al. (2006), Reid et al. (2007), Roger et al. (2003, 2004), Shi et al. (2009), Wang et al. (2011a,b), Weislogel (2008), Xiao et al. (2007), Yuan et al. (2009, 2010), Zhai et al., 2012, Zhang et al. (2006, 2007, 2008a, 2011) and this study. were interbedded within the Late Triassic gravity flow deposits (Cai 240 Ma calcic/calc-alkaline plutonic and volcano-clastic rocks consid- et al., 2010; Wang et al., 2011b). The geodynamic significance of the ered to be the products of northward subduction of Paleo-Tethys eastern HXSG igneous rocks is widely debated (e.g. Pullen et al., 2008; oceanic lithosphere (Harris et al., 1988; Jiang et al., 1992; Matte et al., Roger et al., 2010; Şengör, 1984; Weislogel, 2008; Yuan et al., 2010; 1996; Yang et al., 1996). Harris et al. (1988) concluded that the Kunlun Zhang et al., 2006, 2007, 2008a,b). However, coeval magmas in western granitoids were derived from melting of a garnet-bearing source at HXSG complex are still poorly understood (Roger et al., 2004; Wang middle-lower crust above a subduction zone. These Permian to Triassic et al., 2011b). rocks are thought to be super imposed on an early Paleozoic arc (Jiang Models suggest that the HXSG turbidites were deposited in: (1) a et al., 1992; Şengör, 1984). Igneous rocks in the range of 230–190 Ma back-arc basin (Gu, 1994; Şengör, 1984); (2) a rift basin (Chang, 2000; exposed in the eastern Kunlun belt were likely emplaced in a post- Chen et al., 1987); (3) a remnant-ocean basin (Zhou and Graham, collisional setting (Dai et al., 2013; Ding et al., 2011; Harris et al., 1988; 1996); or (4) a Mediterranean-style rollback basin (Ding et al., 2013; Jiang et al., 2012; Liu et al., 2004). Other isotopic, paleontological and Pullen et al., 2008). Differentiating between these models and thus kinematic investigations suggest that the closure time of the northern improving our understanding of the final stages of ocean basin closure Paleo-Tethys ocean along the AKM suture did not occur later than and the initial stages of continent–continent collisions requires Middle Triassic (Bian et al., 2004; Elena et al., 2003). an improved understanding of the age, nature, and distribution of The Jinshajiang suture separates the HXSG complex from the magmatism in the HXSG terrane. To improve our understanding of the Qiangtang terrane to the south (Dewey et al., 1988). The basement of closure of the Paleo-Tethys ocean, we have analyzed samples from the Qiangtang terrane is likely composed of Cambro-Ordovician gneiss plutons which intruded into the turbidite deposits and generated zircon and siliciclastic metasedimentary mélange (Kapp et al., 2003; Li et al., U–Pb ages and muscovite 40Ar/39Ar ages. Whole-rock major and trace 1995; Xu et al., 1985). The Qiangtang terrane was part of Gondwana element abundances as well as the Sr–Nd–Hf isotopic compositions until Permian time when it began drifting northward across the Paleo- were determined to better define the magma sources and their tectonic Tethys towards Eurasia (Stampfli and Borel, 1992; Xu et al., 1985). The implications. Ganzi-Litang suture separates the eastern HXSG complex from the Yindun arc to the southwest. The Yidun arc is a micro-continent 2. Geological setting and sample description composed of Paleozoic carbonates and mafic volcanic rocks, Triassic mudstone and shale, and calc-alkaline granitoids and volcanic rocks 2.1. Geological setting (Fig. 1, Chen et al., 1987; Hou et al., 2007; Reid et al., 2007; Wang et al., 2011a). The Anyimaqin–Kunlun–Muztagh (AKM) suture separates the HXSG The northeast striking Longmen Shan thrust belt defines the eastern complex from the Kunlun arc terrane and Qaidam basin to the north limit of the HXSG complex (Fig. 1). Neoproterozoic basement rocks (Fig. 1). However, in the area of the West Qinling between the eastern similar to those of South China crop out along the Longmen Shan thrust Kunlun and Qinling-Dabie orogen, the AKM suture is widely inferred fault (Yan et al., 2003; Zhou et al., 2002, 2006).