喜马拉雅中段高压麻粒岩变质作用、地球化学与年代学

The studied high pressure mafic granulite (HP-granulite) outcrops in the Greater Himalayas 40 kilometers north to Yadong City in the southern Tibetan Plteau. It exposes in the footwall of the southern Tibetan detachment system (STDS) as the enclaves of the parageiness. The HP-granulite is marked by the early mineral assemblages garnet (Grt) + clinopyroxene (Cpx) Plgioclase (Pl) and the later assemblages amphibole + Plgioclase + biotite(Bio) orthopyroxene(Opx). The metamorphic reaction textures indicate the following reactions: Grt + Cpx + Qtz = Opx + Pl, Grt + Qtz = Opx + Pl, Grt + Cpx + L = Hbl + Pl + Bio + Mt, Cpx + L = Hbl + Mt. Thermobarmetric estimates based on equilibrium texture and mineral chemistry indicate that the early mineral assemblages formed at P = c. 1. 2 ∼ 1.5 GPa, T = c. 780 ∼ 850°C with the geothermal gradient of 15°C ∼ 18°C/km, and the later retrograde assemblages experienced near-isothermal decomposition to P = 4 ∼ 6kbar, T = 730 ∼ 760°C with the geothermal gradient of 38°C ∼ 50°C/km. Metamorphic texture, P and T evolution shows HP-granulite had experienced a rapid change of tectonic background from thicken crust to thinning or heating crust. The U-Pb SHRIMP ages of zircons collected from the HP-granulite show most ages are around 17 Ma, and other mainly around 98 Ma. Because the estimated metamorphic temperatures are higher than the closure temperature of zircon U-Pb chronological system, the later age of 17 Ma should be the cooling age during rapid exhumation related to the activity of STDS and MCT in the Greater Himalayas. The Greater Himalayan belt as well as the lesser Himalayan belt is regarded to be the part of Indian continent. The HP-granulite must have been the continental thoelitic basalt metamorphosed due to the collision between Indian and Eurasian continents. The HP-granulite provides a new clue to study the process of collision and convergence between India and Asia, and how and why the Greater Himalayas uplift.