The pressure-temperature-time path of migmatites from the Sikkim Himalaya

A combined metamorphic and isotopic study of lit-par-lit migmatites exposed in the hanging wall of the Main Central Thrust (MCT) from Sikkim has provided a unique insight into the pressure-temperature-time path of the High Himalayan Crystalline Series of the eastern Himalaya. The petrology and geochemistry of one such migmatite indicates that the leucosome comprises a crystallized peraluminous granite coexisting with sillimanite and alkali feldspar. Large garnet crystals (2-3 mm across) are strongly zoned and grew initially within the kyanite stability field. The melanosome is a biotite-garnet pelitic gneiss, with fibrolitic sillimanite resulting from polymorphic inversion of kyanite. By combining garnet zoning profiles with the NaCaMnKFMASHTO pseudosection appropriate to the bulk composition of a migmatite retrieved from c. 1 kin above the thrust zone, it has been established that early garnet formed at pressures of 10-12 kbar, and that subsequent decompression caused the rock to enter the melt field at c. 8 kbar and c. 750 degreesC, generating peritectic sillimanite and alkali feldspar by the incongruent melting of muscovite. Continuing exhumation resulted in resorption of garnet. Sm-Nd growth ages of garnet cores and rim, indicate pre-decompression garnet growth at 23 +/- 3 Ma and near-peak temperatures during melting at 16 +/- 2 Ma. This provides a decompression rate of 2 +/- 1 mm yr(-1) that is consistent with exhumation rates inferred from mineral cooling ages from the eastern Himalaya. Simple 1D thermal modelling confirms that exhumation at this rate would result in a near-isothermal decompression path, a result that is supported by the phase relations in both the melanosome and leucosome components of the migmatite. Results from this study suggest that anatexis of Miocene granite protoliths from the Himalaya was a consequence of rapid decompression, probably in response to movement on the MCT and on the South Tibetan detachment to the north.