Exhumation history of a section of the Sikkim Himalayas, India: records in the metamorphic mineral equilibria and compositional zoning of garnet

Abstract

The exhumation history of pelitic migmatite samples from the High Himalayan Crystalline Complex (HHC) near the South Tibetian Detachment System in the Sikkim-Darjeeling section has been determined on the basis of thermo-barometric analyses, retrograde breakdown reactions and compositional zoning of garnet. The peak metamorphic condition is estimated to be ~10.4 kbar, 800°C from thermo-barometric and phase equilibrium constraints. The observed retrograde breakdown of garnet to spinel and cordierite requires near isothermal and, hence, extremely rapid (~15 mm/yr) exhumation up to the depth of ~15 km. Numerical modeling suggests that the initial rapid exhumation must have been followed by a much slower process, ~2 mm/yr, up to at least ~5 km depth, to lead to the development of the observed compositional zoning of garnet. The dramatic change of the exhumation velocity (V_z) might reflect a process of tectonic thinning followed by erosion and/or horizontal flow at shallow depth. Assuming that the V_z did not change significantly below 5 km depth, these results suggest that the HHC samples studied in this work exhumed from a depth of ~34 km within ~8 Ma.