Metamorphic phase relations in orthopyroxene-bearing granitoids: implication for high-pressure metamorphism and prograde melting in the continental crust

Abstract

The lesser Himalayan sequence in Sikkim exposes an unusually complete inverted Barrovian sequence of metapelitic rocks. A number of different bulk compositions are interlayered along the prograde sequence, providing an excellent natural laboratory for studying the controls of pressure, temperature and composition on stability of mineral assemblages. Using three different approaches for determining pressures and temperatures, and making use of the bulk chemical constraints, we show that all three methods yield consistent estimates of pressures and temperatures once the strengths and weaknesses inherent to each approach are critically evaluated. The metamorphic field gradient along two separate traverses are found to be positive (60 °C/kbar in east Sikkim and 70 °C/kbar in north Sikkim) - pressure as well as temperature increases continually up to the highest grade. The various bulk compositions allow different assemblages to be used to establish the coherence of the sequence. In combination these place tight constraints on the permissible tectonic mechanisms for the production of this inverted metamorphic sequence. Neither older hot iron type models, nor more recent models that invoke thrusting (post- or syn- metamorphic) as a mechanism of inversion can produce such a coherent package inverted in pressure as well as temperature. On the other hand, the observations are more consistent with the predictions of channel flow type tectonics. In particular, a recent geodynamic model of subduction-collision that takes into account high heat generation (as found in these Himalayan metapelites) and incorporates the effects of the resulting melting (reduction of viscosity and enhancement of buoyancy) on the tectonic evolution, predicts such inversion of coherent blocks as an unavoidable consequence. Such melting triggered inversion and exhumation as a coherent, fault bounded block explains many features of the inverted sequence in Sikkim and it is not necessary to invoke inversion by multiple thrusting events.