Post-collisional adakites in south Tibet: Products of partial melting of subduction-modified lower crust Zhengfu Guo a,b, ⁎ , Marjorie Wilson b , Jiaqi Liu a a Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, PR China b School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Received 9 December 2005; accepted 5 September 2006 Available online 29 December 2006 Abstract Post-collisional (26.2 to 10.1 Ma) adakites occur within the Lhasa terrane of the southern Tibetan Plateau in an E–W trending, 1500 km long, magmatic belt. Outcrops are small and restricted within N–S-trending rift zones (grabens); they include both extrusive and intrusive facies. The adakites have high SiO 2 (59–70 wt.%), Al 2 O 3 (15–18 wt.%) and Sr (317–1133 ppm) contents and Sr/Y ratios (44–162), and low Y (4.2–12.9 ppm) and HREE (e.g. Yb b 0.9 ppm) concentrations. Their MORB-normalised incompatible element patterns exhibit strong enrichments in large ion lithophile elements (LILE) relative to high field strength elements (HFSE). The combined trace element and Sr–Nd–Pb isotope characteristics of the adakites suggest that their source was mafic-intermediate lower crust formed during a preceding stage (153–40 Ma) of active continental margin magmatism. Lower crustal melting was primarily induced by the conduction of heat from contemporaneous potassic–ultrapotassic magmas produced by partial melting in the asthenosphere or lower lithosphere. Trace element modelling calculations suggest that the adakites are the products of 5–10% partial melting of garnet-bearing amphibolite facies meta-igneous rocks. Differences in the LILE contents and Sr–Nd isotope compositions of adakites sampled to the east and west of 86° E can be attributed to variable degrees of partial melting of the lower crust and mixing between potassic–ultrapotassic magmas and lower crustal melts. Extensional collapse of the Tibetan Plateau may have contributed to partial melting of the lower crust and the formation of the potassic–ultrapotassic magmatism by decompression melting of a thin asthenospheric mantle wedge above a subducted slab of Indian continental margin lithosphere. The oldest age of the post-collisional adakites and contemporaneous potassic–ultrapotassic magmatism in the Lhasa terrane may, therefore, provide constraints on the timing of initiation of tectonic collapse in the southern part of the plateau. © 2006 Elsevier B.V. All rights reserved. Keywords: Tibetan Plateau; Adakite; Subduction-modified lower crust; Trace element model; Uplift 1. Introduction Uplift of the Tibetan Plateau, post-dating India–Asia collision, is considered to be one of most important geological events worldwide. Understanding the cause of post-collisional magmatism in the Tibetan Plateau can provide important constraints on both the mechanism of plateau uplift and the nature of the source region of the magmas (Pearce and Mei, 1988; Turner et al., 1993, 1996; Miller et al., 1999; Williams et al., 2001, 2004; Guo et al., 2005, 2006). In addition, such understanding may provide greater insights into the nature of the magmatic plumbing system in a young orogenic belt. Lithos 96 (2007) 205 – 224 www.elsevier.com/locate/lithos ⁎ Corresponding author. Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, PR China. Tel.: +86 10 62007334; fax: +86 10 62032495. E-mail address: zfguo@mail.iggcas.ac.cn (Z. Guo). 0024-4937/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2006.09.011