New magmatic record of Indian continental subduction at ca. 50 Ma: Evidence from ultrapotassic–potassic lamprophyre dikes in the Gangdese Batholith, Southern Tibet

Abstract

Geophysical and geochemical evidence confirmed the northward-directed Indian continental lithosphere subduction beneath the Gangdese orogen, however, the geodynamic processes responsible for the transition from the Neo-Tethyan subduction to the Indian continental subduction, particularly magma generation mechanisms regarding the 53–42 Ma magmatic flare-up events, remain poorly constrained. Here we have conducted a comprehensive investigation combining geochemical and geochronological studies of newly discovered Early Eocene (ca. 50–47 Ma) ultrapotassic–potassic lamprophyre dikes in the Lhasa terrane, Tibet. The ultrapotassic lamprophyres revealed highly evolved Sr–Nd–Pb isotopes and high concentrations of incompatible elements, and are thought to have originated from the Indian lithospheric mantle. In contrast, the potassic lamprophyres, displaying relatively depleted Sr–Nd–Pb isotopes and lower MgO contents and K₂O/Na₂O ratios than the ultrapotassic lamprophyres, show high Sr/Y and low Y geochemical signatures, thereby demonstrating that they formed due to contamination of the ultrapotassic lamprophyre magma by adakitic melts. We, therefore, propose that the lamprophyres represent a new magmatic record of the Indian lithosphere, suggesting that the subduction of Indian plate beneath the Lhasa terrane was already operating at ca. 50 Ma. The data gained here in conjunction with published data further suggest that the upwelling asthenosphere associated with the Neo-Tethyan slab breakoff caused the significant melting of both mantle and crust.