High P-T polymetamorphism, dehydration melting, and generation of migmatites and granites in the higher Himalayan crystalline complex, Sikkim, India

Abstract

The Higher Himalayan Crystalline Complex (HHC) in Sikkim, India, consists of pelitic migmatites interlayered with calc-silicate rocks and minor metabasites. Microstructural relationships between the mineral phases and deformational fabric elements and zoning characteristics of garnet indicate a prolonged and complex polymetamorphic history for the HHC. The pelitic rocks in the upper part of the HHC contain the assemblage plagioclase + quartz ± garnet + K-feldspar + biotite + sillimanite and are devoid of muscovite. Most of the mineral phases grew syn- to post-tectonically. Mineral growth coincided with development of a pervasive fabric (S₂) during prograde metamorphism (M₂), in the early stages of the collisional event. Some garnet grains display texturally distinct cores and rims, which are separated by calcic plagioclase. This texture suggests an earlier metamorphic episode (M_l). M may represent pre-Himalayan metamorphism and decompression of the HHC. Later, the collisional event led to renewed burial of the HHC and M₂ reactions. M₂ is reflected by dehydration melting of muscovite and biotite to form granitic melts, which either crystallized in situ to form leucosomes, or migrated from their source regions to form larger granitic bodies. Geothermobarometric estimates for peak M₂ conditions indicate P=10-12 kbar, T=800-850°C. A subsequent metamorphic event (M₃) occurred because of 5 kbar of decompression. M₃ is recorded by the breakdown of porphyroblastic garnet in all HHC lithologies. Higher temperature and pressure estimates come from progressively higher structural levels of the complex. The thermal gradient of 5.5°C/km is anomalous, and may be a consequence of thermal buffering during melting. However, the pressure gradient of 0.25 kbar/km resembles a normal lithostatic gradient, which suggests that the HHC in Sikkim represents an inverted Barrovian sequence. This inverted zonation of the HHC is probably the result of large-scale structural inversion and/or tectonic juxtaposition because of ductile shearing.