ISOTOPIC DATA FROM THE AMBA-DONGAR CARBONATITE COMPLEX, WEST-CENTRAL INDIA - EVIDENCE FOR AN ENRICHED MANTLE SOURCE

Initial Pb-206/Pb-204 (19.05-19.19), Pb-207/Pb-204 (15.72-15.74) and Pb-208/Pb-204 (39.72-40.16) ratios, and epsilon(Sr) (+ 15.1 to + 19.2) and epsilon(Nd) (- 1.6 to - 0.8) - values for the calciocarbonatites from the Cretaceous (similar to 60 Ma) Amba Dongar complex, Gujarat (India) are relatively uniform. The isotopic ratios for the ferrocarbonatites, however, are slightly more variable, in particular the initial Pb-206/Pb-204 (18.94-19.28), Pb-207/Pb-204 (15.66-15.81) and Pb-208/Pb-204 (39.55-40.05) ratios. delta(13)C (- 4.20 to - 3.58 parts per thousand) and delta(18)O (+ 8.80 to + 10.02 parts per thousand) isotopic ratios for the calciocarbonatites plot within or close to the field defined for primary magmatic carbonatites, whereas the ferrocarbonatites are characterized by much higher delta(13)C (- 4.29 to - 1.62 parts per thousand) and delta(18)O (+ 13.73 to + 24.67 parts per thousand) - values. Variations in Pb, C and O isotopic data from ferrocarbonatites are attributed to fluid activity that is probaby related to formation of associated massive fluorite deposits, known to be of low-temperature hydrothermal origin. In contrast, the high abundances of Nd (average 1191 ppm) and Sr (average 4261 ppm) from the calciocarbonatites, which buffer their initial Nd and Sr isotopic ratios against crustal contamination, and their ''mantle-like'' C and O isotopic ratios suggest that their isotopic ratios are probably inherited from their mantle source. The initial Nd and Sr isotopic ratios from the Amba Dongar calciocarbonatites indicate derivation from a Rb/Sr- and Nd/Sm-enriched mantle source quite different to most young (< 200 Ma) carbonatites which have signatures that lie between HIMU and EM I mantle components. The isotopic ratios from the Amba Dongar calciocarbonatites are similar to proximal alkaline complexes of similar age and the least contaminated Deccan flood basalts. We propose, therefore, that the Reunion hot spot responsible for the Deccan flood basalts may have had some bearing on producing the parental melt that generated the Amba Dongar carbonatite.