Early Eocene ( c . 50 Ma) collision of the Indian and Asian continents: Constraints from the North Himalayan metamorphic rocks, southeastern Tibet

Despite several decades of investigations, the nature and timing of the India–Asia collision remain debated. In the western Himalaya, the leading edge of the Indian continent was deeply subducted to mantle depths and experienced ultrahigh-pressure metamorphism in the Eocene at c. 50 Ma. In this paper, however, we demonstrate that the North Himalayan metamorphic rocks in the eastern Himalaya underwent Early Eocene (48–45 Ma) medium-pressure (MP) metamorphism due to shallow subduction of the Indian continent beneath southeastern Tibet. The studied garnet–kyanite–staurolite schists occur in the core of the Yardoi gneiss dome, the easternmost North Himalayan Gneiss Dome, and represent the upper structural level of the Higher Himalayan Crystallines (HHC). Petrology and phase equilibria modeling show that these rocks have mineral assemblages of Grt + Pl + Bt + Qz ± Ky ± St ± Ms that were formed under conditions of 7–8 kbar and 630–660 °C. Zircon U–Pb chronology shows that these rocks have peak-metamorphic ages of 48–45 Ma and protracted zircon growth, indicating that the collision between Indian and Asian continents must have occurred at c. 50 Ma in southeastern Tibet. Combining with available data, we suggest that the HHC represents a crustal section of the subducted and subsequently exhumed Indian continent. Due to shallow subduction of the continent during the Eocene, the middle to lower crust of the continent was subducted into depths of 40–60 km and underwent high-pressure (HP) and high-temperature (HT) granulite-facies metamorphism and intense anatexis, whereas the upper crust was buried to shallower depths of 20–30 km and witnessed MP metamorphism and intrusion of leucogranites derived from the lower structural level of the HHC.