Magmatic and metamorphic history of the Proterozoic Lesser Himalayan Crystallines from Bomdila area, Arunachal Pradesh, NE Lesser Himalaya, India: Constraints from whole rock and mineral chemistry

Abstract

The excellent exposures of plentiful Proterozoic granitic bodies in the north‐east Himalaya offer a chance to explore the evolution of continental crust in the Early Proterozoic period. The present comprehensive study from Bomdila area, western Arunachal Pradesh, NE India documents petrogenesis of Palaeoproterozoic Bomdila granite gneiss and metamorphic history of the region using the whole rock and mineral chemistry, and suitable mineral thermobarometry. The gneisses, with quartz, k‐feldspar, plagioclase, and mica mineralogy, are characterized by enrichment in rare earth elements (total REE up to 290 ppm), Rb, Ba, Th, and depletion in mafic elements (Ni, Cr, V, Sc). The gneisses are subalkaline and peraluminous with A/CNK (Al2O3/CaO + Na2O + K2O) > 1.1, indicating S‐type nature of the granites. The gneisses show fractionated REE patterns (LaN/YbN = up to 22) with LREE enrichment and HREE depletion. Geochemical characteristics such as enriched SiO2, Al2O3, alkalis, LREE, high incompatible element ratios (Rb/Sr and Ba/Sr), and depleted mafic contents with noticeable negative Eu anomalies (Eu/Eu* = 0.2 – 0.6) suggest an origin of gneisses through high‐temperature (>800°C) shallow crustal melting of a pelitic source similar to the metasedimentary enclaves within the gneisses. Mineral thermobarometric calculations suggest a tightly constrained P–T condition of 7.5–8.6 kbar and 560–590°C for the gneiss and schist from the study area. Almost identical P–T conditions of metamorphism from different lithologies attests that the Bomdila area may have attained an upper amphibolitic facies condition, possibly concomitant to the Himalayan Orogeny. Though rudimentary, temperature estimates from Ti‐in biotite thermometer suggest an increase in metamorphic grade towards the Main Central Thrust.