GEOCHEMICAL AND GEOCHRONOLOGICAL ANALYSIS OF LESSER AND HIGHER HIMALAYAN ROCKS, EASTERN AND CENTRAL BHUTAN

Abstract

The Bhutan Himalaya forms a unique portion along the orogenic front of collision of Indian and Eurasian plate because of the existence of several klippen, sequences of thrust fault and an apparently thickened exhumed core (the Greater Himalayan Sequence or GHS). Unravelling the tectonic history in this region is crucial for understanding heterogeneity of the Himalaya. In Central and Eastern Bhutan, the Lesser Himalayan sedimentary sequence is exposed much to the north due to the N-S trending antiformal Kuru Chu half window beneath the overthrust high grade metamorphics of the Great Himalayan Series (GHS) along the Main Cental Thrust (MCT). The LH mylonitized gneiss of the Shumar Formation contains abundant coarse grained plagioclase/orthoclase and recrystallized quartz megacrysts within highly mylonitized exceedingly fine grained foliated groundmass. The MCT and HHC samples have garnet and large number of plagioclase megacrysts with pressure shadow. Geochemical study is helpful to identify geodynamic setting of evolution, source rock characterization and . their characteristic rocks. The Lesser Himalaya contains metashale, metasandstone, and metacarbonates with uncommon igneous bodies of unspecified origins. These Cambrian to Ordovician igneous protoliths of the two-mica bearing Lesser Himalayan mylonitized Shumar gneisses in Bhutan have granodioritic composition and are rich in alkali elements with high K calkaline-shoshonitic affinity and peraluninous characteristics. These are fractionated S-type granitoids which are continental collisional granites and derived by partial melting of recycled crustal material with mantle and mid-crustal contributions. Depletion of HFSE and enrichment of LILE on the primitive mantle normalized trace element patterns imply 11 that the source rock were derived - from a mantle domain, enriched by earlier subduction processes or assimilation of continental crust. On the Si02 vs. Nb/Y diagram, it can be seen that source rock for these gneisses are predominantly granodiorite and generatted in the field of volcanic are granites. Y vs. Nb discrimination diagram and Y+Nb and Rb diagram and negative Nb anomaly suggest that the. rocks are derived in a subduction zone setting, which is now exposed in different lithotectonic units due to collision, thrusting and upliftment. The LH granitoids give Sm-Nd model ages ranging from 2.15 to 2.78 Ga, whereas model ages derived from the HHC samples are quite variable meaning thereby that the HHC granite generation from their sources is not constant and varies from 1.2 Ga and older. This means the granitoids of the HHC are derived by melting of older continental crust or its time of crustal separation from the mantle has a range of time, whereas the LH rocks has definite range are which are indicative of the age of the crustal separation from mantle domain. Epsilon Nd values in the Lesser Himalayan Sequence reveal that this package of rock is more juvenile than that of the Indian craton, which indicates that it is not Indian basement, and the data provide support for previous interpretations that the Lesser Himalayan Sequence is derived from an active continental margin. The HHC shows low values of €Nd, which indicates that they are overthrust, recrystallized Indian craton. 40Ar "Ar study of biotite and muscovite from the Central Bhutan indicates that the minerals have indistinguishable ages and reveal extremely fast cooling between 400 and 300°C. Reported data from the Eastern Bhutan does not reflect such fast cooling. Due to the presence of excess 40Ar in biotite, its ages are more than muscovite. However, in our case biotite and muscovite ages group between 12.0 ± 1.5 and 14.6 f 1.7 Ma. Cooling curve has been plotted by integrating the generated data of Rb-Sr mineral ages, Ar-Ar mineral ages and published apatite iii fission track ages. It suggests that exhumation rates in Eastern Bhutan is more than Central Bhutan from Present to about -5Ma time. Exhumation rate was nearly constant in both the sections from -5 to -10 Ma. At -12-13 Ma the central Bhutan rocks are exhumed vertically, and samples from MCT zone in Eastern Bhutan shows relatively slow rate of exhumation as compare to sample collected from HHC area near MCT. In Central Bhutan we can see that away from MCT the rate of exhumation increases. iv