Tibetan Oligocene adakitic granites sourced from metasedimentary rocks in thickened lower crust

Abstract

Widespread Late Cenozoic granites in the southern Tibetan Plateau mostly show adakite-like geochemical signatures and have been commonly considered to derive from partial melting of mafic rocks in thickened juvenile lower crust. Here, we show that Late Oligocene adakitic granites from the eastern Gangdese arc, southeastern Tibet, contain an unequivocal metasedimentary source. The studied granites are characteristically adakitic, with variable but high SiO₂, K₂O and Sr (222–802 ppm), and high Sr/Y (16.1–195.2) and La/Yb (6.7–89.7). Most importantly, zircon with Late Oligocene (∼26 Ma) magmatic rims has inherited detrital cores with major age peaks at ∼1600, ∼1100, ∼550, and ∼ 300 Ma. Isotope compositions have high (⁸⁷Sr/⁸⁶Sr)_i (0.70588–0.71058) and low ε_Nd(t) (−5.8 to −2.3). These characteristics, together with phase equilibrium and radiogenic heating modeling results, indicate derivation of adakitic granites from partial melting of Carboniferous metasedimentary rocks in the thickened lower crust. Crustal thickening and associated radioactive heating during Cenozoic collisional orogeny triggered extensive partial melting of thickened lower crust. We suggest that the Late Cenozoic adakitic granites in southern Tibet were derived from diverse lower crustal sources, and have significantly distinct chemical compositions and metallogenic potentials. The coexistence of adakitic granites sourced both from metaigneous and metasedimentary rocks is a key indicator of Tibetan Plateau-like collisional orogens.