Enriched Subcontinental Upper Mantle Beneath Southern India: Evidence from Nd, Sr, Pb and C-O Isotopic Studies on Tamil Nadu Carbonatites

H. Schleicher Universität Hamburg, Hamburg, Germany

U. Kramm Techn.Universität Aachen, Aachen, Germany

E. Pernicka Max Planck-Institut Heidelberg, Germany

M. Schidlowski Max Planck-Institut, Mainz, Germany

F. Schmidt Universität Heidelberg, Heidelberg, Germany

W. Todt Max Planck-Institut, Mainz, Germany

S. G. Viladkar St. Xavier's College, Bombay, India

Several carbonatite complexes (Sevattur, Samalpatti, Jogipatti, Pakkanadu) in Tamil Nadu, southern India, have been
investigated by means of major, trace and rare earth element as well as by Nd, Sr, Pb, C and O isotope geochemistry. The samples show highly enriched LREE contents and C-O stable isotopic patterns as reported for primary mantle derived carbonatitic melts (Keller and Hoefs, 1995). For Sevattur, a well defined lead/lead isochron yields an age of 802 ± 5 Ma for ankeritic carbonatites (Todt et al., 1996), which are the geologically youngest in the carbonatite sequence. Most of the other samples in the lead system arrange between a depleted mantle and an enriched mantle / lithospheric component, the latter being comparable to EM II some 800 Ma resp. 1500 Ma ago. This array is interpreted as a mixing line between two source reservoirs; the nature of the enriched one is discussed.

In the Nd-Sr isotopic system, all of the carbonatite samples are characterized by very low eNd values between -8.8 and -20.1 and high 87Sr/86Sr ratios (0,7045 - 0,7054), comparable to Leucite Hills alkaline rocks and pointing towards an enriched mantle composition. Analogously to the lead data, the Nd-Sr isotopic data can be interpreted as the result of a mixing process between a depleted mantle and an enriched subcontinental mantle / lithospheric component, either 800 or around 1500 Ma ago. The todays Nd-Sr isotopic compositions of the carbonatites were established by an isotopic evolution of the Nd system only but not of the Sr system due to the extremely low Rb/Sr ratios within the carbonatites.


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