Middle Triassic back-arc rifting in central China: Evidence from geochronology, geochemistry and Hf isotopes of basic-intermediate dykes in the Gonghe basin

In this paper, we present an integrated study of petrology, zircon U-Pb geochronology, whole-rock geochemistry, and zircon Lu-Hf isotopes for the basic-intermediate dykes in the western Gonghe basin, northeastern Qinghai-Tibetan Plateau. LA-MC-ICP-MS zircon U-Pb dating indicates that these dykes were emplaced in the Middle Triassic (240 +/- 1.6 Ma). Geochemically, these basic-intermediate dykes are characterized by enrichments in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs; e.g. K, Rb, and Pb), coupled with depletions in high-field-strength elements (HFSEs; e.g. Nb, Ta, and Ti). These geochemical features, along with the relatively low zircon epsilon(Hf) values (-7.2 to 1.6), suggest that these mafic dykes were originated from enriched mantle metasomatized by subduction-related fluids in a back-arc extensional environment. Literature investigations indicate that Middle Triassic mafic dykes are widespread in central China and are geochemically of arc affinities. Therefore, a regional back-arc extensional event occurred in central China during Middle Triassic time, which was likely caused by rollback of the subducting Kunlun-Animaqin Paleo-Tethyan slab.