Magnetic susceptibility, mineral chemistry, and geothermobarometry of granitoids from Lohit Plutonic Complex, Arunachal Trans-Himalaya, Northeast India: Implications on emplacement and crystallization conditions of oxidized calc alkaline magmatic arc system

Abstract

The Lohit Plutonic Complex (LPC) of Arunachal Trans-Himalaya represents the northeast extension of Trans-Himalayan magmatic arc system located in the north of Indus Tsangpo Suture Zone (ITSZ). Field relation, magnetic susceptibility (MS), and phase petrology on the granitoids of LPC was conducted in order to assess the granite series (magnetite, oxidized vs. ilmenite, reduced types), and physico-chemical conditions of the LPC granitoid magmas. The studied granitoids are well-exposed in the Dibang and Lohit valleys, and their MS values indicate a bi-modal patterns corresponding to ilmenite (reduced) series (71%) and magnetite (oxidized) series (29%) granites. The variation of MS in the LPC granitoids is related to the alteration of ferromagnetic minerals, and later tectonic and deformational processes that acted upon them. The amphiboles from the LPC granitoids are calcic (Ca_B >1.5, Si = 6.30–7.06 apfu) and exhibit tschermak substitutions typical to their evolution in a calc alkaline, metaluminous (I-type) felsic magmas. Al-in-hornblende rims estimate the emplacement of quartz diorite and granodiorite magmas at shallow (~5 km) and mid (~16 km) crustal depths. Geothermometric results point to a regime of magmatic crystallization (940–837°C for quartz diorite; 882–829°C for granodiorite) sufficiently above the solidus of respective melts. Biotites from LPC granitoids are primary to re-equilibrated, and transitional between magnesio- and ferri-biotites. Quartz diorite and granodiorite biotites evolved under oxidizing magmas (log ƒO₂⁻¹⁴ to log ƒO₂⁻¹³) in a temperature range of ~750–950°C, typical to their formation in a calc alkaline magma of subduction zone environment. However, the biotites from leucogranite appear to have evolved under a mildly reducing magma environment, most likely attained in a collisional setting. The obtained results suggest that the oxidized nature of calc alkaline, subduction-related magmatic arc rocks of the LPC is largely modified and reduced by post-magmatic, and later tectonothermal and deformational events that operated during Himalayan and Trans-Himalayan orogenesis.