Origins and tectonic implications of Late Cretaceous adakite and primitive high-Mg andesite in the Songdo area, southern Lhasa subterrane, Tibet.

Late Cretaceous igneous rocks in the southern Lhasa subterrane, Tibet, include primitive high-Mg andesites and adakites, which provide important constraints on the tectonic evolution of the Neo-Tethys Ocean. Here, we present detailed zircon U Pb and Hf isotopic and whole-rock geochemical data for granodioritic and dioritic porphyry samples from the Songdo area in the southern Lhasa subterrane. Zircon U Pb dating indicates that the granodiorite crystallized at 88 Ma, whereas the diorite yields ages of 68 and 66 Ma. The granodiorite has adakite-like geochemical characteristics, including high Sr (801–1005 ppm) and low Y (6.8–15.2 ppm) and Yb (0.6–1.3 ppm) concentrations, and high Sr/Y (62–145) and La/Yb (39–93) ratios. We infer that the adakitic granodiorites formed through partial melting of subducted oceanic crust. The dioritic porphyry has intermediate moderate SiO2 (53–58 wt%) and high MgO (5.6–8.2 wt%) contents, and high Mg# (66.4–69.5) values, and is therefore classified as a primitive high-Mg andesite that was derived from interaction between subducted sediment and mantle. The presence of coeval adakite and charnockite, as well as high-temperature granulite-facies metamorphism, indicates that mid-ocean ridge subduction occurred at 100–80 Ma, followed by a 10 Myr hiatus in magmatism and subsequent rollback of the Neo-Tethys slab at 68 Ma. These processes resulted in significant crustal growth within the Lhasa terrane. Unlabelled Image • The ocean ridge subduction of the Neo-Tethys occurred at 100–80 Ma. • The slab rollback of the Neo-Tethys was at 68 Ma and formed sanukitoid. • There was significant crustal growth in Lhasa terrane during the Late Cretaceous. [ABSTRACT FROM AUTHOR]