Neoproterozoic adakitic rocks from Mopanshan in the western Yangtze Craton: Partial melts of a thickened lower crust Xiao-Long Huang ⁎, Yi-Gang Xu ⁎, Jiang-Bo Lan, Qi-Jun Yang, Zheng-Yu Luo Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China abstract article info Article history: Received 11 September 2008 Accepted 17 March 2009 Available online 28 March 2009 Keywords: Hf isotope Neoproterozoic Adakitic rocks Lower crust Western Yangtze Craton South China The tectonic setting of South China during the Neoproterozoic remains a subject of debate. Neoproterozoic adakites from Xuelongbao and Datian along the western margin of the Yangtze Craton have been used to argue for an arc setting assuming that these rocks are the melting products of a subducted oceanic slab (Zhou, M.F., Yan, D.P., Wang, C.L., Qi, L., Kennedy, A., 2006a. Subduction-related origin of the 750 Ma Xuelongbao adakitic complex (Sichuan Province, China): Implications for the tectonic setting of the giant Neoproterozoic magmatic event in South China. Earth and Planetary Science Letters 248, 286–300.; Zhao, J.H., Zhou, M.F., 2007. Neoproterozoic Adakitic Plutons and Arc Magmatism along the Western Margin of the Yangtze Block, South China. The Journal of Geology 115, 675–689.). However, the slab-related petrogenetic model is unsuitable for the Neoproterozoic (782 ± 6 Ma) adakites from Mopanshan. The Mopanshan adakites are characterized by low MgO, Cr and Ni contents, high Rb/Sr and Sr/Y ratios and negative ε Nd (t) values (-2.06 to -0.43). These features, plus the presence of relict zircons (1.16–0.83 Ga) and Mesoproterozoic zircon Hf model ages (1.06– 1.27 Ga), are inconsistent with a slab-melting origin, but favour an origin by melting of a thickened continental lower crust. It is likely that the crust associated with magma generation may have experienced continuous growth and reworking since the Sibao Orogeny. Sharing the main geochemical features with the Mopanshan rocks, the Xuelongbao and Datian adakites may also have been derived from a thickened lower crust. This interpretation casts doubts on the Neoproterozoic arc setting proposed for south China. Instead, the Neoproterozoic in south China may represent an intra-continental extensional setting. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Adakites, characterized by high Sr/Y and low HREE, have been interpreted as typical melting products of subducted oceanic slabs (Defant and Drummond, 1990). If this interpretation is correct, adakites can be used as an indicator for tectonic setting. Unfortunately, it has been demonstrated that adakitic rocks can also be generated by partial melting of thickened lower crusts (e.g., Petford and Atherton, 1996; Chung et al., 2003; Wang et al., 2005) or delaminated lower crust (Xu et al., 2002; Wang et al., 2006a). Moreover, adakites can also be formed via fractionation of mafic magmas (Castillo, 2006). The potentially multiple origin of adakitic rocks suggests (e.g., Castillo, 2006; Wang et al., 2006a; Li et al., 2007a) that adakitic rocks cannot be used as an effective tectonic indicator unless there are independent lines of evidence. In recent years, the Neoproterozoic tectonic setting in south China has been a subject of debate (e.g., Li et al., 2004a; Wang et al., 2004a,b; Li et al., 2006a; Zhou et al., 2006b;Wu et al., 2006a,b; Zhou et al., 2007; Li et al., 2007a; Munteanu and Yao, 2007; Zhang et al., 2008). Three competing models have been proposed. (1) Li et al. (1999, 2002a, 2003) attributed the extensive Neoproterozoic magmatism in South China to a mantle plume associated with the break-up of Rodinia supercontinent. (2) Zheng et al. (2007, 2008) proposed a plate-rift model that assumes the Neoproterozoic magmatism resulted from lithospheric extension in response to the tectonic evolution from super-continental rift to breakup. (3) Zhou et al. (2002) suggested that the magmatism was associated with two major arcs around the Yangtze Block: the Jiangnan arc to the east and the Hannan–Panxi arc to the west (Zhou et al., 2002, 2006a,b, 2007). To strengthen their arguments, Zhou and co-workers (Zhou et al., 2006a; Zhao and Zhou, 2007) used adakitic rocks occurring in Xuelongbao (750 Ma) and Datian (760 Ma), along the western margin of the Yangtze Craton, as supporting evidence. They interpreted these adakitic rocks as products of partial melting of a subducted oceanic slab. This interpretation, however, has been questioned by Li et al. (2007a) who instead argued for an intra-plate origin. Given these controversies, additional studies are needed to decipher the petrogenesis of the Neoproterozoic adakitic rocks in the western Yangtze Craton. In this paper, we report geochronological and geochemical data on an adakitic complex in Mopanshan along the western margin of the Yangtze Craton (Fig. 1). New zircon U–Pb isotopic dating reveals Lithos 112 (2009) 367–381 ⁎ Corresponding authors. E-mail addresses: xlhuang@gig.ac.cn (X.-L. Huang), yigangxu@gig.ac.cn (Y.-G. Xu). 0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.03.028 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos