Single-Stage Synchronous India-Asia Collision Model Revealed by Himalayan High-Pressure Metamorphic Rocks

Despite a half-century of intense research, the timing and diachroneity of initial collision between India and Asia remain highly debated, largely due to different definitions of “initial collision” and correspondingly different methods adopted. This study focuses on high-pressure pelitic granulites of the Eastern Himalayan Syntaxis (EHS) to elucidate their metamorphic evolution and provide new constraints on the timing of initial India-Asia collision. Petrological examination and phase equilibria modeling show that high-pressure pelitic granulites have undergone four metamorphic stages, with the peak assemblage of garnet + K-feldspar + kyanite + biotite ± plagioclase ± rutile + ilmenite + quartz at P-T conditions of 13.1–15.7 kbar and 790–850°C. Clockwise P-T paths suggest that the Indian continent underwent tectonometamorphic processes of initial collision and subsequent continent subduction. Zircon and monazite dating results indicate that the metamorphic ages of pelitic granulites range from 60 to 15 Ma, with the oldest ones clustered at 60–58 Ma. The oldest metamorphic ages of high- to ultrahigh-pressure Himalayan metamorphic rocks can provide an upper age limit of the initial collision. Therefore, the initial India-Asia collision must have occurred before 60–58 Ma in the EHS, roughly consistent with ca. 57 Ma in the western Himalaya and 63–60 Ma in the central Himalaya. Collectively, we conclude that the northern margin of India most likely underwent a single-stage synchronous collision with the southern margin of Asia initially at around 60 Ma along the entire Yarlung-Tsangpo suture zone.