[S-GC37] Volatiles in the Earth - from Surface to Deep Mantle

[SGC37-P05] Geochemistry and geodynamic implications of High-Mg diorites from the Lohit Plutonic Complex of eastern Arunachal Trans-Himalaya, north eastern India

*LEIPHRAKPAM LEMBA SINGH¹、RK. BIKRAMADITYA SINGH¹ (1.DEPARTMENT OF GEOLOGY, BANARAS HINDU UNIVERSITY, VARANASI, INDIA)

キーワード：Trans Himalaya, Lohit Plutonic Complex, High-Mg diorites, slab rollback

The Lohit plutonic complex (LPC) in the Trans Himalayan part of the eastern Arunachal Himalaya which marks as a linking point between the Kohistan-ladakh-Gangdese arc of Trans Himalaya and the Wutho-Popa arc of Myanmar is a key region to decipher the tectono-magmatic evolution of the Neotethyan oceanic subduction. The LPC is divided into the western belt and eastern belt by the Walong thrust. The western belt consists dominantly of granodiorites, diorites, gabbros, trondhjemite, hornblendites, basaltic dykes and pegmatites. On the other hand, eastern belt is comprising of garnet-sillimanite gneiss at the base followed by a complex zone of granodiorites, granites and microdiorites intruded by leucogranites, applites and pegmatites. The widely distributed early Late Cretaceous diorite rocks in the Lohit plutonic complex can provide comprehensive data for understanding its crustal evolution. Here, we present the detailed whole rock geochemical data for these diorites to interpret the petrogenesis and the tectonic environment of their formation. The Lohit diorites display intermediate SiO₂ (52.02-58.81 wt.%) and K₂O (1.33-1.85 wt.%), but high MgO (4.56-7.17), Al₂O₃ (16.06-19.65 wt.%), Na₂O (3.65-5.30 wt.%) and Mg# values of 53 to 69, implying they are high-Mg diorites. They are also characterized by high Ba (165-407 ppm) Th (1.3-2.3) and Sr (421-935 ppm) contents, but depleted in HREE, and high (La/Yb)N ratios of 2.61 to 4.61 and slightly negative to weak positive Eu anomalies (Eu/Eu*= 0.82-1.09). The Lohit diorite samples also exhibit a mantle signature with high Mg# values (53-69), Cr (36 to 364 ppm) and Ni (23 to 67 ppm) contents, but enriched in LILE, pointing to an enriched mantle source. Based on the trace element geochemical features, the metasomatic agents were suggested to be the subduction related fluids as well as the sediment melts. Therefore, the formation of the Lohit diorites can be explained by a two-stage process. Initially, fluids released from the subducting oceanic slab interacted with the mantle peridotite, leading to its metasomatism. This was followed by reheating and partial melting of the subducted sediments and their subsequent interaction with the overlying metasomatized mantle peridotite, which was triggered by the upwelling of the asthenospheric mantle resulting from the rollback of the oceanic slab during the Late Cretaceous, providing the conditions necessary for the generation of these high Mg-diorites.