Zircon U-Pb geochronology and geochemistry of Early Jurassic granodiorites in Sumdo area, Tibet: Constraints on petrogenesis and the evolution of the Neo-Tethyan Ocean

The Gangdese magmatic belt is oriented E-W and located between the Indus-Yarlung Tsangpo and Bangong-Nujiang suture zones in Tibet. It records the subduction of the Neo-Tethyan oceanic crust, Indian-Eurasian collision, and uplift of the Tibetan Plateau. Here we present new zircon U-Pb geochronology, Hf isotopic compositions, and whole-rock geochemistry of the granodiorites in the Sumdo area of the Gangdese Belt. Our aim was to identify the petrogenesis of granitoid rcoks and to clarify the early-stage evolution of the Neo-Tethyan Ocean during the Late Triassic to Early Jurassic. The granodiorites yield zircon U-Pb ages of 193-201 Ma, indicating an Early Jurassic magmatic event. They are metaluminous, with medium- to high-K calc-alkaline geochemistry, and show characteristics of I-type granitoids. They show enrichment in light rare earth elements (LREEs) and depletion in high-field-strength elements (HFSEs) in primitive-mantle-normalized spider diagrams, indicating that their initial magma was generated in a subduction-related active continental margin. They have negative εHf(t) values (−11.3 to −4.1) and crustal model ages (TC DM) of 1.50-1.95 Ga. Hf isotopes and whole-rock geochemistry indicate that the granodiorites were derived from partial melting of ancient lower-crust material mixed with mantle material. Our data, in combination with published geochronological and geochemical data, show that the Late Triassic-Early Jurassic arc magmatism in the Gangdese magmatic belt was formed by the northward subduction of the Neo-Tethyan oceanic crust beneath the Lhasa Terrane.