Geochemistry and Tectonic Implications of the Trans-Himalayan Lohit Plutonic Complex, Eastern Arunachal Pradesh

The Trans-Himalayan Lohit Plutonic Complex occurs to the east of the Eastern Syntaxis in eastern Arunachal Pradesh and the investigations were carried out mainly along the Lohit River section. The plutonic complex intrudes the metavolcanics of probable arc affinity in the west and high-grade metamorphic rocks of the Asian continental margin in the east. It can be sub divided into western and eastern belts, separated by Walong Thrust. The complex consists of multiple intrusions and they arc:: (1) an early phase of metaluminous, tholeitic to calc- alkaline gabbro and quartz diorites with minor diorites and hybrid rocks; (2) an intermediate phase of calc-alkaline, metaluminous, leucotonalite of high Al-trondhjemitic or adakitic composition; (3) a last phase of peraluminous leucogranite and associated veins of pegmatite and aplite. The enrichment of large-ion-lithophile elements (LILE) relative to high field strength elements (HFSE) of these rock types suggest that they were emplaced in a subduction related environment and derived from different source materials. The major and trace element variations in the early gabbro-quartz diorite, along with the low initial ⁸⁷Sr/⁸⁶Sr ratio (0.703-0.704) suggest that they were derived from a parental tholeitic magma, from which the intrusives have been differentiated under increasing water pressure conditions. The geochemical characteristics of the trondhjemites, such as the high Na₂O, AI₂O₃, Sr, Sr/Y ratios and low K₂0, Y and depletion of heavy rare earth elements (HREE) are similar to those of itdakite and Archaean high Al- trondhjemite. They might have been derived either by melting of the subducted oceanic: crust or by partial melting of newly under- plated basaltic crust beneath the thickened continental arc leaving residual ;garnet and amphibole. The leucogranites with high K₂0, Rb, K_O/Na₂O, K/Rb, Rb/Sr ratios, and high initial ⁸⁷Sr/⁸⁶ Sr ratio (0.713), along with depletion in HREE and absence of negative Eu anomaly, suggest that they were derived from anatectic melting of a mixture of leucotonalite and rnetasedimentary rocks, induced by thrust related thickening during collision. The collision related convergence has resulted in the development of major intra-continental thrusts, which at present steeply dipping in the Lohit Plutonic Complex. Possibly during Miocene these thrusts have been reactivated into right-lateral strike-slip faults.