Petrogenesis and geochemical characteristics of plagiogranites from Naga Ophiolite Belt, northeast India: Fractional crystallization of MORB-type magma

The Naga Ophiolite Belt is a part of the Naga-Arakan-Yoma flysch trough that occurs along the Indo-Myanmar border. It is represented by peridotites, mafic-ultramafic cumulates, mafic volcanics, mafic dykes, plagiogranites, pelagic sediments and minor felsic to intermediate intrusives. Minor plagiogranites, gabbros and thin serpentinite bands occur juxtaposed near Luthur, with the slate-phyllite-metagreywacke sequence (Phokpur Formation) adjacent to the contact. The development of tonalites, trondhjemites and diorites in the oceanic crust, which is grouped as plagiogranites, offers an opportunity to study the process of formation of silicic melts from mafic crust. Plagiogranites from Naga Ophiolite Belt contains moderate SiO2 (51.81-56.71 wt.%), low K2O (0.08-1.65 wt.%) and high Na2O (4.3-5.03 wt.%). The Naga Ophiolite Belt plagiogranites like ocean-ridge granites contain low K2O, high Na2O and CaO. The rocks investigated from Naga Ophiolite Belt contain TiO2 concentrations above the lower limit for fractionated Mid Oceanic Ridge Basalt which is above 1 wt% of TiO2 and the ternary plots of A (Na2O + K2O) F(FeOT) M(MgO) and TiO2-K2O-SiO2/50 indicate that the plagiogranite are tholeiitic in character and gabbro samples are calc-alkaline in nature. The plagiogranites are enriched in Rb, Ba, Th, U, Nb and Sm against chondrite with negative anomalies on Sr and Zr whereas Y and Yb are depleted to Mid Oceanic Ridge Basalt. The chondrite normalized REE patterns of the plagiogranite display enrichments in LREE (LaN/SmN: 2.37-3.62) and flat HREE (Eu/Eu*: 0.90-1.06). The Mid Oceanic Ridge Basalt normalization of gabbro is characterized by strong enrichment of LILE like Ba and Th. The REE pattern is about 50-100 times chondrite with slight enrichment of LREE (LaN/SmN = 2.21-3.13) and flat HREE (Eu/Eu*: 0.94-1.19). The major-element and trace element data of the NOB plagiogranites and their intrusive nature with host gabbroic rock suggest that the plagiogranites were produced by fractional crystallization of basaltic parental magmas at Mid Oceanic Ridge.