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. Mineralog ically carbonatites show wide variations in their composition but chem ically most of them are silico-carbonatites. Presence of hornfelsic te xtures and metamorphic minerals like chlorite, scapolite, diopside and grossularite, suggest metamorphism of the present carbonatites. Alkal ine rocks are either syenites or lamprophyres. Both carbonatites and a lkaline rocks are generally enriched in large-ion lithophile elements (LlLE) and high-field strength elements (HFSE) relative to the primord ial mantle. Although carbonatite samples have lower concentration of L ILE and HFSE than the average values of known carbonatites, they fall within the range of magmatic carbonatite concentrations. Lower concent rations of these elements are a result of hydrothermal metamorphism by hot fluids. Carbonatites are also enriched in delta(13)C and delta(18 )O-values. The Rayleigh fractionation together with post-magmatic seco ndary processes, particularly exchange with magmatic fluids of high de lta(18)O content, are the best explanation for the C and O isotope enr ichments. The carbonatites and alkaline rocks have probably crystalliz ed from the carbonate and silicate melts separated from a postulated c arbonated nephelinitic magma by liquid immiscibility at shallow levels .