Early Cenozoic thickening and reworking of the eastern Gangdese arc, south Tibet: constraints from the Oligocene granitoids

The Cenozoic tectonic evolution of the Gangdese arc, formed by juvenile crustal growth during the Mesozoic, remains ambiguous. Here, we conducted a petrological, geochronological and geochemical study of Oligocene (32–24 Ma) granitoids with protolith ages of 57–49 and 27 Ma from the eastern Gangdese arc. Geochemically, these rocks are divided into three groups. Group I has high CaO and Sr, and low REE contents, representing plagioclase-rich cumulates. Group II contains relatively high K₂O, Pb and REE, and low Na₂O contents, and is crystallized from evolved magmas. Group III has relatively high Al₂O₃ and Sr, and low MgO, Y, Yb, Cr and Ni concentrations, and is thickened lower-crust-derived adakitic rocks. The inherited zircon magmatic cores of these rocks have distinctly different ε_Hf(t) values (-9.88 to +8.50), whereas the magmatic rims have a narrow range of ε_Hf(t) values (-3.29 to +5.22). The Hf isotopic homogenization indicates intensive mixing of melts derived from the old and juvenile crustal materials. We concluded that the Cenozoic magmatic or sedimentary rocks were buried into the thickened lower crust and melted to generate the Oligocene granitoids, and that the Gangdese arc experienced lasting Paleogene crustal thickening and reworking.