Microstructures in quartz and feldspars of the Bomdila Gneiss from western Arunachal Himalaya, Northeast India: Implications for the geotectonic evolution of the Bomdila mylonitic zone

Abstract

Microstructures of the Bomdila Gneiss (BG) that intruded the metapelites of the Lesser Himalayan Crystallines (LHC) in the western Arunachal Himalaya were studied to understand the deformation mechanisms and temperatures during deformation. The LHC is delimited at the base by the Bomdila Thrust (BT) and the Main Central Thrust (MCT) at the top. The microstructural features of quartz and feldspar in the BG indicate that the rocks underwent an early stage of deep level ductile deformation superimposed by shallow level brittle deformation during a later stage of deformation. Grain boundary migration and subgrain rotation recrystallization are the main deformation mechanisms for recrystallization of quartz and feldspars. Pressure solution and solution transfer were also important at the base of the Bomdila mylonitic zone. Both the feldspars (plagioclase and k-feldspar) and quartz show dynamic recrystallization with growth of myrmekite at the upper level while the development of quartz subgrains and fractured in feldspars are dominate at the lower level. The deformation microstructures of BG suggest that the Bomdila mylonitic zone shows an inverted thermal profile towards the MCT, i.e. increase in deformation temperature from ∼400 °C at the lower zone to ∼500–600 °C at the upper zone of the gneiss. The microstructural variations and decreasing temperatures of deformation from west to east indicate that the LHC, initially buried below Higher Himalayan Crystallines (HHC) along the MCT, have been exhumed along the BT. Shear criteria indicates top-to-the-SE displacement, in agreement with the kinematics of the LHC along the BT. Producing of an inverted thermal profile in the study area may be related to imbrication and post metamorphic shearing.