Inverted metamorphism in the High Himalaya of Himachal Pradesh (NW India): Phase equilibria versus thermobarometry

The crystalline core of the Himalayan orogen in the Sutlej valley corresponds to a 9-km-thick, high-grade metamorphic sequence. This High Himalayan Crystalline (HHC) shows an inverted metamorphic zonation characterized, from the base to the top of the unit, by a gradual superposition of staurolite, kyanite, sillimanite, and migmatite Barrovian mineral zones. Phase equilibria constraints based on discontinuous reactions, as well as rim thermobarometry and P-T path results, indicate peak metamorphic conditions evolving trom T ≈ 580°C and P ≈ 750 MPa at the base of the HHC, to T ≈ 750°C and P ≈ 800 MPa at the top. These data suggest that the inverted metamorphic field gradient in the HHC is probably the consequence of diachronic equilibration at different temperatures but more or less constant pressure throughout the sequence. Rim thermobarometry determinations show, however, that final equilibration throughout the HHC occurred at an almost constant temperature around T ≈ 570°C. Comparable temperature profiles around T ≈ 600°C in other Himalayan sections have been interpreted as the result of a thermal buffering caused by anatexis, whereas isothermal pressure variations have been invoked to explain the inverted metamorphic donation observed in such sequences. In contrast, garnet zoning and P-T path data from the Sutlej section indicate that the apparent lack of thermal field gradient across the HHC is the consequence of retrograde reequilibration of high-grade mineral assemblages during exhumation and cooling.