U-PB ZIRCON GEOCHRONOLOGY, THERMOBAROMETRY, AND GEOCHEMISTRY OF LATE CRETACEOUS-AGED GRANITOIDS IN THE NORTHERN INDO-BURMA RANGES, NE INDIA; INSIGHTS INTO NORTHERN GANGDESE BELT ROCKS IN THE EASTERNMOST HIMALAYAN OROGEN

The >1600-km-long Gangdese batholith was generated during Neo-Tethyan subduction-related magmatism prior to Indo-Asia collision, and consists of a Late Cretaceous-aged northern belt and Early Cenozoic-aged southern belt within the Lhasa terrane. Although previous studies have shown plutonic rocks of Gangdese affinity extend around the eastern Himalayan syntaxis into the northern Indo-Burma Ranges (i.e., Lohit batholith), crystallization ages and geochemical data remain sparse in the region. In this study, we present new U-Pb zircon ages coupled with thermobarometric and geochemical constraints to the crystallization environment of granitoids from the Dibang and Lohit river valleys of eastern Arunachal Pradesh, NE India. Zircons from five samples range in age from 93-115 Ma, with one 154 Ma sample. Application of the hornblende-plagioclase thermobarometer and Ti-in-hornblende thermometer yielded crystallization P-T conditions of 4.9 kbar and 627°C, corresponding to depths of ~18 km. Zircon HfO₂ wt% and Zr/Hf ratios range from 1.13-1.31 and 85-100, respectively, suggesting initial crystallization under relatively high and constant temperatures. U-Pb ages clustered at ~100 Ma, volcanic arc trace element signatures, low Th/Y and La/Yb ratios, and existing positive ε_Hf(T) data suggest the Lohit batholith represents the southward continuation of the Late Cretaceous-aged northern Gangdese belt. Absence of Early Cenozoic-aged plutonic rocks of the southern Gangdese belt can be explained by cessation of Neo-Tethyan subduction and associated arc magmatism along the eastern margin of the southern Lhasa terrane by Cenozoic time.