Origin of Tibetan post-collisional high-K adakitic granites: Anatexis of intermediate to felsic arc rocks

Abstract

Abstract Potassium-rich adakitic rocks have been used to infer high-pressure (HP) melting of thickened or foundered mafic lower crust in post-collisional settings. However, their origin remains debated because of their potassic rather than sodic nature. We address this debate by investigating the source of post-collisional Oligocene high-K adakitic granites in southern Tibet, which are widely attributed to melting of overthickened mafic lower crust. Our new data provide evidence for the generation of these high-K adakitic granites by anatexis of exposed migmatitic intermediate to felsic arc orthogneisses. These granites contain high-Th/U Oligocene magmatic zircons (30−22 Ma), and inherited zircons (66−48 Ma) with the same εHf(t) (0 to +12) as the coeval migmatites. The migmatites have in turn low-Th/U metamorphic Oligocene zircon rims around 66−48 Ma magmatic zircon cores recording the anatectic event at 29−25 Ma. Phase equilibrium modeling yields an anatectic temperature of ~740 °C and pressure (P) of ~0.9 GPa and shows that garnet is stable at P > 0.7 GPa in the melt-present field. The results indicate that high-K adakitic magmas are derived from melting of older intermediate to felsic arc rocks at intermediate pressures, in the garnet stability field, without any involvement of HP melting of metabasaltic rocks. We propose that hybridization between such purely crustal magmas and subcontinental lithospheric mantle–derived shoshonitic magmas results in the hybridized post-collisional Oligocene–Miocene mafic microgranular enclave–bearing potassic adakitic granitic rocks in southern Tibet.