Petrogenesis and Geochemical Signature of Gabbros and Plagiogranites from the Naga Hills Ophiolite, Northeast India: Evidence for Fractional Crystallization in a Back-Arc Setting

Abstract

The remnants of Neotethyan oceanic crust have been preserved in north-south trending Indo-Myanmar ranges as Naga Hills ophiolite. It is associated with gabbros and plagiogranites which are formed by sequential fractional crystallization of a basaltic parental magma. The grain size decreases from top to bottom exhibiting progressive differentiation. The gabbros and plagiogranites are show intergranular texture and have moderate SiO2 (49.50 to 56.37 wt.%) and Al2O3 (12.50-13.66 wt.%), high Fe2O3 (14.01-27.15 wt.%), CaO (4.68-5.73 wt.%), Na2O (4.61-5.65 wt.%) and TiO2 (1.54-3.10 wt.%), and low K2O (0.54-1.63 wt.%) content. The presence of myrmekitic texture in gabbros and plagiogranites provide evidence for simultaneous late stage of plagioclase and quartz from a low-K magma. The gabbros and plagiogranites exhibit tholeiitic character in AFM diagram and are formed by fractional crystallization of a basaltic magma. The gabbros and plagiogranite are enriched in light rare earth elements (LREE) and have relatively flat middle rare earth elements (MREE) and heavy rare earth elements (HREE). Primitive mantle–normalized trace element patterns of the Naga Hills Ophiolite gabbros and plagiogranites display a positive large ion lithophile elements (LILE) enrichment with a positive Nb–Ta anomaly and a negative Zr–Hf anomaly. This geochemical signature explains that the asthenospheric melt is contaminated by the addition of slab-derived components through rutile breakdown/melting during slab dehydration and slab melting. This indicates the oceanic plagiogranite and gabbros are formed by fractional crystallization of a hydrous LILE, LREE and Nb-Ta enriched basaltic magma at a back-arc spreading ridge. Ilmenite mineral chemistry from the NHO gabbros and plagiogranites is characterized by low MgO, V2O5, and Cr2O3 contents, with elevated MnO content. This compositional trend suggests ilmenites are crystallized under relatively low-temperature conditions and reduced oxygen fugacity at the final stage of fractional crystallization. The occurrence of pyrite, chalcopyrite, and sphalerite in the Naga Hills Ophiolite plagiogranites reflects crystallization from a highly differentiated, hydrous, and oxidized magma, consistent with a subduction-influenced back-arc setting. The NHO plagiogranites and gabbros show a complex history of post-magmatic hydrothermal alteration in which clinopyroxenes are altered under greenschist facies to form actinolite and chlorite, and plagioclase are altered to form calcite, albite, sericite, and K-feldspar, and ilmenites are altered to form titanite.