Petrology of the Carbonatites and Associated Rocks of Sung Valley, Jaintia Hills District, Meghalaya, India

Abstract

In Sung Valley, (Lat : 25°31' -25°36'N : Long; 92°05' -92°10'E) the Shillong Series rocks comprising mainly quartzites and phyllites are intruded by a N-S trending, roughly oval shaped body of pyroxenite covering an area of about 35 km² and into which the following genetically related rock types have been emplaced in order of decreasing age and abundance. Peridotites (stock-like bodies), ijolites (ring dyke and plug like), carbonatites (incomplete cone sheets) and syenites (sheets, dykes and veins). Minor dykes and veins of magnetite-serpentine rock, ijolite and melilite-bearing types with a rough radial disposition are also present. Fenitization, due to both ijolites and carbonatites, has affected older rocks resulting in wollastonite-rich aegirine-augite bearing rocks, K-feldspar-aegirine-augite veins and oversaturated potassic syenites.

Carbonatites are mainly sovites with minor beforsite and types rich in magnetite and apatite are present. Average abundances of trace elements (values in ppm) characterizing carbonatites such as Bu (549), Sr (3134), La (50-100), Ce (489), Y (50), Nb (372), Zr (179) and P₂O₅ (5.7) wt% are comparable to those found in reported intrusive carbonatites. The weathered mantle of the carbonatites and the contact zones between carbonatites and pyroxenites have resulted in soils rich in pyrochlore, apatite and magnetite. It is postulated that the carbonatites may be differentiates of an ijolitic magma, in turn derived (liquid immiscibility?) from a parental mela-nephelinite or alkali picritic magma. The ultramafic suite could represent the cumulates from such a picritic source.