Petrology of the Intrusive Alkalic Suite of Koraput, Orissa

The late Pre-Cambrian intrusive suite of Koraput, Orissa comprises of alkali-gabbro-calc-alkali syenite-perthite syenite (perthosite)-nepheline syenite, arranged in the sequence of emplacement. Alaskitic granite develops as a direct offshoot from the hydrous alkali-gabbro magma during fractional crystallisation.

Chemical, optical and X-ray data for the rock forming minerals have been furnished, and the trend of mineralogical variation during fractionation of the magma has been traced. Structural states and geothermometric significance of the feldspars are investigated. The alkali feldspafs in the suite reveal a compositional variation tending to approach the temperature minimum on Ab-Or join. Nephelines both in syenite and pegmatite show a marked variation from the ideal composition. The major mafic silicates, namely pyroxene and amphibole, essentially show a parallel trend of variation being progressively richer in ferrous iron at the expense of magnesium while the entry of calcium in the chain structures is not much affected. The trend of increasing Fe/Mg ratio with fractionation is also imprinted on biotite. The iron-titanium oxides indicate the change of physicochemical conditions in fractionating magma.

Variations of major elements and of some trace elements as related to respective diadochic major elements have been discussed. The alkalic suite shows a progressive increase in alkali metals with advancing differentiation as reflected through increasing differentiation index though peralkalinity is never reached but in granite. Magmatic evolution in the development of the alkalic complex has been elucidated.