Chemical mass transport during deformation and metamorphism: Insights from the Main Central Thrust and its footwall of Western Arunachal Himalaya, NE India

Abstract

The Main Central Thrust (MCT) is one of the major tectonic discontinuities stretching about ~2500 km along the length of the Himalaya and it plays important role during the tectono-metamorphic evolution of Himalayan orogen. This contribution aims to find out the element mobility and loss or gain of chemical mass during mylonitization and migmatization of rocks and their relation with deformation and metamorphism within the Main Central Thrust Zone (MCTz) and its immediate footwall along the Bomdila-Dirang-Seppar section of the Western Arunachal Himalaya, India. Based on meso- and microstructural observations, we divided the Dirang Formation in the footwall of the MCT into 3 units: Lower Mylonite Zone (LMZ), Protomylonite Zone (PMZ) and Upper Mylonite Zone (UMZ), with increasing structural distance from Dirang thrust towards NW. Deformation of the Dirang Formation results in enrichment of Na, K and Si with significant gain in mass of ~55% in the UMZ relative to the PMZ. We inferred that shear strain localization during mylonitization in the UMZ induces textural transitions causing chemical alterations at peak metamorphism conditions of 0.62–0.66 GPa and 690°-710 °C. Migmatites of Lower Structural Position (LSP) show enrichment of Si, Na, K, Ca, Mn and P with depletion of Mg and Fe in the leucosome + mafic selvage (LM) relative to the paleosome with ~13% chemical mass gain. On the other hand, migmatites of Upper Structural Position (USP) are characterized by enrichment of Si, Mn and K and depletion of Ca, Mg, Na, P and Fe in the LM relative to the paleosome with ~17% chemical mass gain. Such chemical changes occur during near peak metamorphic conditions of 0.52–0.63 GPa and 680°-720 °C and it is accompanied by the appearance of sillimanite. This study improves our understanding of the chemical modification of rocks in the MCT and its footwall, which were subjected to deformation and metamorphism during its evolution in Himalayan orogen. The emplacement of the GHS along the MCT buried the Dirang Formation to a depth of ~23–25 km. Interaction of fluids, derived from dewatering of the underthrusted Dirang Formation, induced chemical alteration in the UMZ. We interpret that chemical mass transfer in migmatite zones directly influences the metamorphic reactions that occur during deformation and crustal anatexis in intracontinental shear zones, such as the MCT.