Subduction of the Indian lower crust beneath southern Tibet revealed by the post-collisional potassic and ultrapotassic rocks in SW Tibet

Abstract

New major and trace elemental, Sr–Nd–Pb isotope, and zircon U–Pb geochronological and Hf–O isotope data of post-collisional potassic and ultrapotassic volcanic rocks (PVRs and UPVs, respectively) along with geochemical data of PVRs, UPVs, and Mg-rich potassic rocks (MPRs) in the literature are used to constrain their mantle source and genesis. The PVRs, UPVs, and MPRs share similar geochemical features but with some discrepancies, suggesting that they were derived from subcontinental lithospheric mantle (SCLM) with isotopic heterogeneity resulting from the varying contributions of subducted Indian lower crust into the mantle source (ca. 6–20%, ca. 8–30%, and ca. 9–30%, respectively). The zircon Hf–O isotopic compositions of these rocks can be classified into two groups, including Group I rocks with high δ18O (6.7–11.3‰), low εHf(t) (−17.0 to −12.0), and old Hf crustal model ages (1.87–2.19Ga) that indicate an ancient SCLM source, and Group II rocks with δ18O values of 6.8–10.7‰, εHf(t) values of −11.8 to −6.3, and younger Hf crustal model ages (1.50–1.86Ga). The negative correlation defined by δ18O and εHf(t) of Group II samples suggests a two-component mixing between mantle- and crust-derived melts, in which the latter would be the subducted Indian lower crust as indicated by the similar negative εHf(t) values between Group II samples (−11.8 to −6.3) and the High Himalayan gneiss (−14.2 to +0.3). Thus we propose two enrichment events to account for the Hf–O isotopic compositions of the PVRs and UPVs/MPRs: the first involves the enrichment of the overlying SCLM that was metasomatized by fluids derived from dehydration of the subducted Indian lower crust, and the second invokes the enrichment of the overlying SCLM metasomatized by melts of the already dehydrated different proportions of the Indian lower crust. We argue that break-off of the northwards subducted Indian Plate in the early Miocene caused the asthenospheric upwelling under the Indian plate through slab window, resulting in varying degrees of partial melting of the overlying metasomatized heterogeneous SCLM to produce the primitive magmas of the PVRs, UPVs, and MPRs in an extensional setting. These observations and interpretations imply that the Indian lower crust was subducted beneath the Lhasa terrane in the Early–Middle Miocene.