Petrology of the Proterozoic Alkaline Carbonatite Complex of Samalpatti, District Dharmapuri, Tamil Nadu

The gneissic rocks of Samalpatti area are intruded by the Proterozoic carbonatites, alkaline rocks, pyroxenites and minor dunites. Mineralogically carbonatites show wide variations in their composition but chemically most of them are silico-carbonatites. Presence of hornfelsic textures and metamorphic minerals like chlorite, scapolite, diopside and grossularite, suggest metamorphism of the present carbonatites. Alkaline rocks are either syenites or lamprophyres. Both carbonatites and alkaline rocks are generally enriched in large-ion lithophile elements (LILE) and high-field strength elements (HFSE) relative to the primordial mantle. Although carbonatite samples have lower concentration of LILE and HFSE than the average values of known carbonatites, they fall within the range of magmatic carbonatite concentrations. Lower concentrations of these elements are a result of hydrothermal metamorphism by hot fluids.

Carbonatites are also enriched in δ¹³C and δ¹⁸O-values. The Rayleigh fractionation together with post-magmatic secondary processes, particularly exchange with magmatic fluids of high δ¹⁸O content, are the best explanation for the C and 0 isotope enrichments. The carbonatites and alkaline rocks have probably crystallized from the carbonate and silicate melts separated from a postulated carbonated nephelinitic magma by Iiquid immiscibility at shallow levels.