Coexistence of MORB and OIB-type mafic volcanics in the Manipur Ophiolite Complex, Indo-Myanmar Orogenic Belt, northeast India: Implication for heterogeneous mantle source at the spreading zone

Abstract

This paper presents the first report of the coexistence of tholeiitic mid-ocean ridge basalt (MORB) – type and alkaline ocean island basalt (OIB) – type mafic volcanics from the Manipur Ophiolite Complex (MOC), Indo-Myanmar Orogenic Belt, northeast India. The MORB-types have comparatively lower TiO2 concentrations (0.6–1.6wt.%), show almost flat REE patterns with depleted LREEs [(La/Sm)N=0.62–1.03]. However, few samples in the MORB group show enrichment in LREE [(La/Sm)N=2.83–2.95] which is the typical composition of P-MORB. Alkaline OIB-types are characterized by high concentration of TiO2 (1.7–3.5wt.%) with highly enriched LREE pattern as compared to their HREE [(La/Sm)N=2.27–3.44, (Sm/Yb)N=2.56–3.29]. Such geochemical variation implies more than two sources. Possibly, one for OIBs (enriched mantle) and several ones for MORBs; from depleted MORB mantle (DMM) source to significantly enriched DMM source by OIB-type components. Petrogenetic modeling suggests that 20% partial melting of depleted mantle within spinel stability facies zone (shallow depth) is responsible for generation of MORB tholeiites and 5–10% partial melting of enriched mantle or plume material at garnet facies stability zone (deeper depth) is responsible for production of alkaline OIB-type. Geochemical signatures of variably enriched MORB and P-MORB samples further suggest possible scenario of mixing of depleted N-MORB and enriched OIB melt. It is therefore likely that mafic volcanics of the MOC were derived from chemically heterogeneous mantle sources erupted at the sea floor spreading zone as MORB generated by partial melting of depleted upper mantle and as OIB generated by partial melting of enriched mantle or a plume source, which was proximal to the spreading axis. Later, due to prolonged subduction of the Indian plate beneath the Myanmar plate and afterward collisional activity, they might have accreted along the Indo-Myanmar Orogenic Belt as upthrust ocean crust.