Origin and evolution of multi-stage felsic melts in eastern Gangdese belt: Constraints from U–Pb zircon dating and Hf isotopic composition

Abstract

This integrated study of whole rock geochemistry, zircon U–Pb dating and Hf isotope composition for seven felsic rocks from the Nyingchi Complex in eastern Himalayan syntaxis has revealed a complex magmatic history for the eastern Gangdese belt. This involves multiple melt sources and mechanisms that uniquely identify the tectonic evolution of this part of the Himalayan orogen. Our U–Pb zircon dating reveals five stages of magmatic or anatectic events: 165, 81, 61, 50 and 25Ma. The Jurassic granitic gneiss (165Ma) exhibits εHf(t) values of +1.4 to +3.5. The late Cretaceous granite (81Ma) shows variable εHf(t) values from −0.9 to +6.2, indicating a binary mixing between juvenile and old crustal materials. The Paleocene granodioritic gneiss (61Ma) has εHf(t) values of +5.4 to +8.0, suggesting that it originated from partial melting of a juvenile crustal material. The Eocene anatexis is recorded in the leucosome, which has Hf isotopic composition similar to that of the Jurassic granite, indicating that the leucosome could be derived from partial melting of the Jurassic granite. The late Oligocene biotite granite (25Ma) shows adakitic geochemical characteristics, with Sr/Y=49.3–56.6. The presence of a large number of inherited zircons and negative εHf(t) values suggest that it sourced from anatexis of crustal materials. In contrast to the Gangdese batholiths that are mainly derived from juvenile crustal source in central Tibet, the old crustal materials play an important role for the magma generation of the felsic rocks, suggesting the existence of a crustal basement in the eastern Gangdese belt. These correspond to specific magmatic evolution stages during the convergence between India and Asia. The middle Jurassic granitic gneiss resulted from the northward subduction of the Neo-Tethyan oceanic slab. The late Cretaceous magmatism is probably related to the ocean ridge subduction. The Paleocene–Eocene magmatism, metamorphism and anatexis are interpreted to result from roll-back and break-off of the subducted Neo-Tethyan slab that occurred in the early stage of the India–Asian collision, respectively. The late Oligocene adakitic rocks resulted from the break-off of the subducted Indian continental crust starting at ~25Ma.