Petrogenesis of Lohit Plutonic Complex with Special Reference to Hayuliang Granitoids, Arunachal Pradesh, India

Study of the evolution of Himalayas and Trans-Himalayas has attracted the attention of researchers worldwide. The Himalayas formed as a result of the collision between the Indian plate and Eurasian plate during the Cenozoic. The present study focuses on the Trans-Himalayan Belt. The Trans-Himalayan belt is divided into the Tuting-Tidding suture zone (Ophiolite melange zone) and Lohit Plutonic Complex (LPC). LPC is again divided into the Eastern belt and the Western belt by the Walong Thrust. Our study area lies in the western belt. The common plutonic rocks occurring in the LPC include granodiorites, quartz-diorites, diorites and gabbros. Occasionally, gabbroic (mafic) enclaves are also found in some places. In this study, the petrographical and geochemical features of granitoids and enclaves are presented. SiO₂ vs. FeO^t plus MgO plot suggests both I-type and A-type nature of the granitoids. SiO₂ vs. K₂O and the AFM plots reveal a calc-alkaline environment (only a few of the rocks display a tholeiitic signature of formation). The A/NK value ranges from 1.2 to 3.8 while the A/CNK value ranges from 0.6 to 1.1. Their metaluminous nature is suggested by the A/NK vs. A/CNK plot. Presence of high diopside, hypersthene and magnetite as normative minerals points to a tholeitic or calc-alkaline aflnity. Enrichment of LILE’s and depletion of HFSE (NS) suggest a subduction related environment. Enrichment of the LILE’s-Ba, K & Rb indicates possible metasomatic processes or crustal contamination. Petrotectonic discrimination diagrams indicate a mixed tectonic setting, ranging from a pre-collision environment to post-collision environment, ultimately leading to a late-orogenic stage. Field observations, along with mineralogical and geochemical evidence, suggest that the Hayuliang granitoids, part of the Lohit Plutonic Complex (LPC) in the eastern Himalaya, represent a composite intrusive suite formed through multiple magmatic pulses involving mantle derived mafic magmas and crust derived felsic melts. The geochemical affinity of gahbros suggests derivation from a metasomatised lithospheric mantle wedge influenced by subduction-related fluids.