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

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 thermobarome tric analyses, retrograde breakdown reactions and compositional zoning of g arnet. The peak metamorphic condition is estimated to be similar to 10.4 kb ar, 800 degreesC from thermo-barometric and phase equilibrium constraints. The observed retrograde breakdown of garnet to spinel and cordierite requir es near isothermal and, hence, extremely rapid (similar to 15 mm/yr) exhuma tion up to the depth of similar to 15 km. Numerical modeling suggests that the initial rapid exhumation must have been followed by a much slower proce ss, similar to2 mm/yr, up to at least similar to5 km depth, to lead to the development of the observed compositional zoning of garnet. The dramatic ch ange of the exhumation velocity (V-z) might reflect a process of tectonic t hinning followed by erosion and/or horizontal flow at shallow depth. Assumi ng that the V-z did not change significantly below 5 km depth, these result s suggest that the HHC samples studied in this work exhumed from a depth of similar to 34 km within similar to8 Ma. (C) 2000 Elsevier Science B.V. All rights reserved.