Petrology of the Alkalic Suite of Sivamalai, Coimbatore, Tamil Nadu

The 'Sivamalai Series' of Holland comprise ferrosyenite, perthite syenite and a group of nepheline syenites. The main intrusion of Sivamalai forms a thick lensoid body tilted to the northeast. The ferrosyenite is characterised by abundant mesoperthite whereas the mafic phases, olivine and clinopyroxene, are appreciably rich in ferrous iron. The clinopyroxene, a hypersolvus ferroaugite, unmixed during subsolidus cooling. The major petrographic types in Sivamalai intrusion are (1) hornblende nepheline syenite (2) biotite hastingsite nepheline syenite (3) perthite syenite (perthosite) (4) leucocratic nepheline syenite and pegmatite with clots of iron-titanium oxides. Alkali feldspar, unmixed to mesoperthite or antiperthite, is the dominant constituent of these hypersolvus alkali syenites. Chemical and X-ray data indicate the primary feldspar to be a sodium rich monoclinic phase. The nepheline in leucocratic syenite and pegmatite shows deviation from the ideal composition with respect to Si and K contents. With progressive fractional crystallisation, both amphibole and biotite show relative enrichment in iron, the latter particularly in ferric iron. The coexisting iron-titanium oxides indicate a low temperature of crystallisation for the late syenitic liquid.

Differentiation index of the Sivamalai series increases progressively from ferrosyenite to leucocratic syenite, the composition of the magma descends down from near the feldspar join towards the temperature minimum in the undersaturated part of Petrogeny's residua system. The index of agpaicity in these rocks is normally below unity except for the locally developed peralkalic facies. The trend of evolution of syenitic magma in Sivamalai series has been compared with that in adjoining alkalic complexes.