Newly identified rhyolite-biotite monzogranite (A2-type granite)-norite belt from the Bangong-Nujiang collision zone in Tibet Plateau: Evidence for the slab break-off beneath the Lhasa Terrane

Abstract

The processes and related rock types in post-collision setting are important to fully understand the geodynamic evolution of a collision zone. Bangong-Nujiang collision zone is the oldest collision zone in the Tibet Plateau. Slab break-off is one of the primary processes during post-collision, and is considered to have occured in the Bangong-Nujiang collision zone. Here, we report a newly identified rhyolite-biotite monzogranite (A2-type granite)-norite belt from this collision zone, which trend E-W for nearly 1000 km, and formed 115–104 Ma. The felsic rocks include rhyolite and biotite monzogranite, who share whole-rock geochemical similarities with A2-type granites. These felsic rocks were likely generated from a juvenile mafic crust melting at low pressure and high-temperature. The juvenile crust was derived from melting of metasomatized mantle wedge. The norites were generated from low degree (1–5%) melting of remnant enriched lithospheric mantle mixed with upwelling asthenospheric mantle at low pressure (~1.0 GPa) and high melting temperature (up to 1244 °C). A slab break-off beneath the Lhasa Terrane accounts for this post-collisional high-temperature extensional environment. The results provide important implications for the evolution of the Bangong-Nujiang collisional zone, including the subducting polarity, and timing of the Bangong-Nujiang ocean closure.