Constraining the conditions of Barrovian metamorphism in Sikkim, India: P -T -t paths of garnet crystallization in the Lesser Himalayan Belt

Garnet crystallization in metapelites from the Barrovian garnet and staurolite zones of the Lesser Himalayan Belt in Sikkim is modelled utilizing Gibbs free energy minimization, multi-component diffusion theory and a simple nucleation and growth algorithm. The predicted mineral assemblages and garnet-growth zoning match observations remarkably well for relatively tight, clockwise metamorphic P–T paths that are characterized by prograde gradients of ∼30 °C kbar−1 for garnet-zone rocks and ∼20 °C kbar−1 for rocks from the staurolite zone. Estimates for peak metamorphic temperature increase up-structure toward the Main Central Thrust. According to our calculations, garnet stopped growing at peak pressures, and protracted heating after peak pressure was absent or insignificant. Almost identical P–T paths for the samples studied and the metamorphic continuity of the Lesser Himalayan Belt support thermo-mechanical models that favour tectonic inversion of a coherent package of Barrovian metamorphic rocks. Time-scales associated with the metamorphism were too short for chemical diffusion to substantially modify garnet-growth zoning in rocks from the garnet and staurolite zones. In general, the pressure of initial garnet growth decreases, and the temperature required for initial garnet growth was reached earlier, for rocks buried closer toward the MCT. Deviations from this overall trend can be explained by variations in bulk-rock chemistry.