Geol. Mag. 149 (3 ), 2012, pp. 483–506. c  Cambridge University Press 2011 483 doi:10.1017/S0016756811000811 The Liuyuan complex in the Beishan, NW China: a Carboniferous–Permian ophiolitic fore-arc sliver in the southern Altaids QIGUI MAO ∗ ‡, WENJIAO XIAO ‡†, BRIAN F. WINDLEY§, CHUNMING HAN ‡, JUNFENG QU ‡, SONGJIAN AO ‡, JI’EN ZHANG ‡ &QIANQIAN GUO ‡ ∗ Beijing Institute of Geology for Mineral Resources, Beijing 100012, China ‡State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China §Department of Geology, University of Leicester, Leicester LE1 7RH, UK (Received 27 October 2010; accepted 21 March 2011; first published online 6 October 2011) Abstract – The tectonic history and time of closure of the Palaeo-Asian ocean of the Altaids are issues of lively current debate. To address these issues, this paper presents detailed geological, petrological and geochemical data of the Liuyuan complex (LC) in the Beishan region in NW China, located in the southernmost Altaids, in order to constrain its age, origin and tectonic setting. The LC mainly comprises massive basalts, pillow basalts, basaltic breccias, gabbros and ultramafic rocks together with cherts and tuffs. Most prominent are gabbros and large volumes of basaltic lavas. These mafic rocks have high TiO 2 contents, flat rare earth element (REE) patterns and show high-field-strength elements (HFSEs) similar to those of mid-ocean ridge basalts (MORB). The mafic rocks exhibit positive ε Nd(t) (6.6–9.0) values, representing magmas derived from the mantle. But these basic rocks are also enriched in Th relative to REEs, and are systematically depleted in Nb–Ta–(Ti) relative to REEs. There is also a large range in initial 87 Sr/ 86 Sr (0.7037–0.7093). All these variables indicate that mantle-derived magma was contaminated by fluids and/or melts from a subducting lithospheric slab, and formed in a supra-subduction zone (SSZ) setting. A gabbro intruded in the complex was dated by LA-ICP-MS on 20 zircons that yielded a 206 Pb– 238 U weighted average age of 286 ± 2 Ma. Considering the fact that all these basalts are imbricated against Permian tuffaceous sediments and limestone, we propose that the LC formed as an ophiolite in a fore-arc in Carboniferous–Permian time. This indicates that the Palaeo-Asian ocean still existed at 286 ± 2 Ma in early Permian time, and thus the time of closure of the Palaeo-Asian ocean was in or after the late Permian. Keywords: Carboniferous–Permian, Liuyuan complex, accretionary orogenesis, Beishan, southern Altaids. 1. Introduction The Altaids, located between the Siberian, North China, Tarim and East European cratons, represent one of the most important sites of juvenile crustal growth in the world (¸ Sengör, Natal’in & Burtman, 1993; ¸ Sengör & Natal’in, 1996) (Fig. 1). After decades of study, it is widely accepted that the Altaids formed by successive accretions of island arcs, accretionary prisms, minor ophiolitic fragments and small continental blocks (Coleman, 1989; ¸ Sengör, Natal’in & Burtman, 1993; Dobretsov, Berzin & Buslov, 1995; ¸ Sengör & Natal’in, 1996; Gao et al. 1998; Buchan et al. 2002; Bazhenov et al. 2003; Xiao et al. 2003, 2004a,b, 2008; Windley et al. 2007). The closure of the Palaeo-Asian ocean ter- minated the accretionary history of the Altaids. How- ever, the timing of the final suture has been hotly de- bated (Coleman, 1989; Zuo et al. 1990, 1991; Shi et al. 1994; Han et al. 1997; Ma, Shu & Sun, 1997; Gao et al. 1998; Xiao et al. 2004b, 2008; Zhou et al. 2004; Mao et al. 2006, 2008; Windley et al. 2007; Zhang et al. 2007). †Author for correspondence: wj-xiao@mail.igcas.ac.cn The Beishan mountain range in NW China, located on the southern margin of the Altaids, is a key area for unravelling the evolution and accretionary processes of the southern Altaids, and the closure time of the Palaeo- Asian ocean (Fig. 1). In particular, a Permian magmatic belt, the Liuyuan complex (LC), contains important information about the final stages of tectonic evolution of the ocean (Zuo et al. 1990; Zhao et al. 2006; Jiang et al. 2007). Previous studies proposed diverse models to explain the geological evolution of the Beishan and adjacent areas based on different interpretations of the LC (Zuo et al. 1990, 1991; Liu & Wang, 1995; Ma, Shu & Sun, 1997; Nie et al. 2002a; Xiao et al. 2004b), but no consensus emerged. For example, a Permian continental rift was favoured by those who considered that the final closure of the Palaeo-Asian ocean was before the Carboniferous (GSBGMR, 1989; Zuo et al. 1990; XBGMR, 1993; Liu & Wang, 1995; Ma, Shu & Sun, 1997; Zuo, Liu & Liu, 2003; Zhao et al. 2006; Jiang et al. 2007). In contrast, others argued that the Palaeo-Asian ocean did not close until late Permian time or later (Shi et al. 1994; Zhu, 1997; Xiao et al. 2003, 2004b, 2008; Windley et al. 2007).