Petrology of the Basic Intrusive Rocks Around the Moulabhanja Hills in the Dhenkanal District of Orissa

The Moulabhanja hills and the adjoining areas are underlain by Precambrian granite gneiss which is intersected by a variety of gabbroic dykes and stocks. Mineral assemblages and textural relationships point recognition of four types of gabbroic rocks which also represent critical stages of fractional crystallization, e,g., (i) olivine gabbro with essential olivine, pigeonite augite and bytownite (ii) hypersthene gabbro with essential hypersthene, augite and labradorite (iii) doleritic gabbro with essential augite and labradorite, and (iv) normal gabbro with augite and less calcic labradorite.

The chemical and normative mineral composition of the basic magma is tholeiitic. The magma seemingly differentiated into different types of gabbroic rocks by fractional crystallization and gradual enrichment in iron. Two analyses of the olivine gabbro lie on the transitional zone between tholeiitic and calc-alkali suite. The closing temperature ofcrystallization of the basic magma and the corresponding oxygen fugacity are estimated to be 600°C and 18.4 (-Log₁₀ FO₂) respectively.