Enriched subcontinental upper mantle beneath southern India: Evidence from Pb, Nd, Sr, and C-O isotopic studies on Tamil Nadu carbonatites

The major, trace and rare earth element distributions as well as the Nd, Sr, Pb, C and O isotope geochemistry of four carbonatite occurrences (Sevattur, Fogipatti, Samalpatti, Pakkanadu) from Tamil Nadu, southern India, have been investigated. The majority of these carbonatites are highly enriched in light ran earth elements (LREE) and exhibit stable isotope patterns similar to those reported for primary mantle-derived carbonatitic melts. In Sevattur, a well-defined lead/lead isochron yields an age of 801 +/- 11 Ma for ankeritic carbonatites. Most of the samples from the other Tamil Nadu carbonatites define an array in the lead system which is interpreted as a mixing line between two reservoirs, namely a depleted mantle and a U-enriched component (either crustal lead or an enriched mantle reservoir). The carbonatites are characterized by very low present-day epsilon(Nd) values between -8.8 and -20.1 and high Sr-87/Sr-86 ratios (0.7045-0.7054). At 800 Ma, the isotopic signature was similar ta EM I, but somewhat more enriched In contrast to the lend darn, the Nd-Sr isotope data shaw a clear signature of a mantle source excluding crustal contamination. The data are compatible with either an EM I-like mantle component 800 my ago or a mixing process between two mantle reservoirs.