Metamorphic evolution of the Lesser Himalayan Crystalline Sequence, Siyom Valley, NE Himalaya, India

Abstract

The paper reports classical Barrovian inverted metamorphism in the Lesser Himalayan Crystalline Sequence, Siyom Valley, NE Himalaya. The sequence is thrust-bounded and constitutes a part of the western limb of the Siang antiform. Six metamorphic zones are delineated with increasing grade from biotite zone through garnet-, staurolite-, kyanite-, sillimanite- to sillimanite–K-feldspar zone with increasing structural level. Granulite-facies pelitic assemblages, marked by the occurrence of garnet + cordierite occur in the sillimanite–K-feldspar zone. Three phases of folding, F 1, F 2 and F 3 and two generations of planar structures, S 1 and S 2 are recognised. Peak metamorphic mineral growths are dominantly syn-F 2. Metamorphic isograds and boundary thrusts of the sequence are co-folded around the Siang antiform by F 3 deformation. Garnet shows prograde growth zoning in lower grade rocks and retrograde diffusion zoning in higher grade rocks. Pelites of the sillimanite–K-feldspar zone show textural evidence of decompression such as corona of cordierite around kyanite. Breakdown reactions of muscovite and biotite in this zone are attributed to decompression. Geothermobarometric calculations show gradual increase of temperature from garnet zone to sillimanite–K-feldspar zone, whereas pressure increases sharply from garnet zone to staurolite zone and remains nearly constant thereafter. Peak metamorphism reached temperature of >750 °C and pressure of ∼10 kbar in the sillimanite–K-feldspar zone. P–T estimates, decompression reactions and pseudosection topologies suggest a clockwise path with steep decompression for the evolution of the sillimanite–K-feldspar zone in the Siyom Valley.