Post-collisional adakites in south Tibet: Products of partial melting of subduction-modified lower crust

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 2O 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 < 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.