[S-MP25] Oceanic and Continental Subduction Processes

[SMP25-05] The geological evolution of the north-eastern Arunachal Himalaya, India: a testament to erstwhile subduction and ongoing collision between Indian and Eurasian plates

*TOKU OMEN ATTE¹, SNEHA SABU¹, AMITAVA CHATTERJEE¹ (1.Department of Earth Sciences, School of Physical, Chemical and Applied Sciences Pondicherry University, India)

Keywords:Arunachal Himalaya, Metamorphism, Elastic barometry, Subduction-collisional tectonics

The Himalaya acts as a natural laboratory to study the erstwhile subduction of the Indian plate, and the ongoing continent-continent collision between the Indian and Eurasian plates. Such subduction-collision tectonics of the Indian plate affect the entire crust along its northern margin, especially in terms of metamorphism and deformation. The nature of metamorphism and deformation near the eastern (Namche Barwa) and western (Nanga Parbat) syntaxes can be directly linked with the style of subduction-collision of the Indian plate. However, the area close to the eastern syntaxis from India, i.e., north-eastern Arunachal Himalaya remains relatively unexplored. The lack of studies in this region impedes our understanding of the geological evolution of one of the most tectonically active regions of the Himalayas, which, in turn, hinders our overall understanding of the Himalayan tectonics. Hence, the Tato-Monigong and Tato-Menchukha transects of Arunachal Pradesh, India, which are located within the Lesser Himalayan Crystallines (LHC) and Greater Himalayan Sequence (GHS), were targeted in the present study. The entire area not only remains unmapped, but also the metamorphic P-T-t evolution is not constrained. The present study establishes a comprehensive geological map of the study area, including mesoscopic lithological and structural information. To characterize the mineral chemistry and constrain the metamorphic condition, an electron probe micro-analysis was carried out, which shows occasional presence of silica-rich garnet (Si a.p.f.u 3.05-3.31) in metapelites. Interestingly, these porphyroblastic garnet grains contain radially fractured quartz as inclusions and kyanite and/or sillimanite in the matrix, which suggests that the rock experienced, at least, granulite facies metamorphism. In addition to conventional thermobarometry and thermodynamic modelling, elastic barometry (Quartz-in-garnet barometer) using the change in the Raman peak shift of the included mineral is also employed to constrain the metamorphic P-T more accurately. The entrapment pressure of quartz is calculated using the QuiG elastic barometer, which ranges between 11±0.4–9±0.1 kbar (at 750°C). Metamorphic P-T estimation can be helpful to validate or negate the presence of high-pressure and high-temperature metamorphic overprinting, which are present in the adjacent areas near the Namche Barwa syntaxis. Such metamorphic imprint(s) may provide additional clues regarding the nature of subduction-collisional tectonics of the Himalayas.