Origin of the Sung Valley Carbonatite Complex, Meghalaya, India: Major Element Geochemistry Constraints

The Sung Valley carbonatite complex is situated in the Shillong Plateau, Meghalaya and includes an array of silicate rocks along with sovitic carbonatite. The silicate rocks are represented by serpentinized peridotite, pyroxenite, melilitolite, ijolite and nepheline syenite and occupy more than 99% of the total outcrop. Variation in major element composition divides the silicate rocks into two distinct groups: i) alumina-alkali-poor, represented by peridotite, pyroxenite and melilitolite and ii) alumina-alkali-rich consisting of ijolite and nepheline syenite. Perceptible variation in chemical composition can also be noted among the individual group members. The carbonatite is dominantly CaCO₃-rich and poor in alkalies. Present study suggests that the parent magma in the Sung Valley complex was of melilitic composition. In the initial stages this magma produced peridotite and pyroxenite by fractional crystallization. With progressive crystallization the parent magma gradually became enriched in Ca and generated carbonatitic melt by liquid immiscibility. The residual silicate melt, after separation of immiscible carbonate melt, produced ijolite and nepheline syenite by fractional crystallization.