Discovery of granulitized eclogite in North Sikkim expands the Eastern Himalaya high-pressure province

Abstract

Eclogite occurrences are very few in the Himalayas and mainly consist of two types (Lombardo and Rolfo 2000): the coesite-bearing eclogites of the western Himalaya (Kaghan valley: O’Brien et al. 2001; Tso Morari: Sachan et al. 2004) and the granulitized eclogites of the eastern Himalaya (Ama Drime range: Lombardo et al. 1998). Unlike the “western type” eclogites, where post-peak decompression under constant to decreasing temperature suggests exhumation favoured by thrusting associated with accretion of the metamorphic pile onto the base of the over-riding plate, the “eastern type” Ama Drime eclogites were probably exhumed relatively slowly and were subjected to thermal relaxation in the thickened continental crust producing a strong granulite-facies overprint at intermediate crustal levels.A new finding of granulitized eclogites in North Sikkim, east of Mount Kangchendzonga and some 120 km southeast of the Ama Drime, extends the eclogite province in eastern Himalaya.Mount Kangchendzonga sits at the western flank of the Ranjit Tectonic Window (RTW), a regional structure cutting deeply through the Himalayan nappe pile. At low structural levels, the RTW exposes low grade Lesser Himalayan schists overlying unmetamorphosed Permo-Carboniferous sedimentary rocks of Gondwanian affinity. At higher structural levels, the northern end of the RTW shows an increase in metamorphic grade through the Main Central Thrust zone up to migmatites with garnet-bearing leucosomes in the Higher Himalayan Crystallines (HHC). In north Sikkim, along the Zemu Glacier east of Kangchendzonga, HHC main lithologies are granite to granodiorite orthogneiss with intercalations of biotite-rich paragneiss and very rare metabasites. Rolfo et al. (2006) gave a preliminary description of this geologic transect, where the main fabric is folded by two systems of later folds trending NNE-SSW and WNW-ESE with steeply dipping axial planes. The RTW can be the result of the interference pattern between these two later system of folds. Compressive and extensional shear zones with a top both to the SW and SE sense of shear respectively affect the main foliation. Top to the SE shear zones can be correlated with normal shear zones recognized in the nearmost western Bhutan (Carosi et al. 2006), developed during the extrusion of the HHC.The lithological associations of the HHC in the Zemu Glacier area are comparable to those described south of Mt. Kangchendzonga by Vezzoli et al. (2005). Among metabasites collected in the drainage of Zemu Glacier, a few samples are very similar to the granulitized eclogites first described in eastern