Disjunctive Grade Variation from Greenschist to Granulite Facies, Siyom Valley, Eastern Arunachal Pradesh, India

Abstract:

The Siyom Valley section in eastern Arunachal Pradesh exposes an inverted metamorphic succession (Nandini & Thakur, 2011), metapelitic assemblages increasing in grade northwards from chlorite, through biotite, garnet-staurolite and kyanite-bearing schist to kyanite-sillimanite migmatite. Grade changes are mostly controlled by shallowly north, and northwest-dipping fault structures. Two textural stages of garnet growth can be identified in the ilmenite-bearing amphibolite facies rocks, staurolite having formed late in, or after, deformation responsible for the main penetrative foliation (S2). Kyanite and rutile inclusions in garnet indicate that their growth in migmatite preceded that of matrix sillimanite, ilmenite and cordierite, though unrecrystallized kyanite is also common in the feldspathic matrix. Preliminary data indicate the pronounced tectonic thinning of metasedimentary protoliths during exhumation, and the probability of a pronounced step in grade in the middle part of the river section. Similarities with sections in the Sikkim (Dasgupta et al., 2004) and western Arunachal Pradesh (Goswami et al., 2009) Himalaya reflect the lateral continuity of the south-vergent thrusts that controlled the exhumation of the high-grade rocks, with debate concerning the location and significance of the Main Central Thrust zone begging protolith and metamorphic age data.

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Goswami, S., Bhowmik, S.K. & Dasgupta, S., 2009. Petrology of a non-classical Barrovian inverted metamorphic sequence from the western Arunachal Himalaya, India. Journal of Asian Earth Sciences, 36, 390–406.

Nandini, P. & Thakur, S.S., 2011. Metamorphic evolution of the Lesser Himalayan Crystalline Sequence, Siyom Valley, NE Himalaya, India. Journal of Asian Earth Sciences, 40, 1089–1100