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 P b-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 f rom the Cretaceous (similar to 60 Ma) Amba Dongar complex, Gujarat (In dia) are relatively uniform. The isotopic ratios for the ferrocarbonat ites, however, are slightly more variable, in particular the initial P b-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 thous and) and delta(18)O (+ 8.80 to + 10.02 parts per thousand) isotopic ra tios for the calciocarbonatites plot within or close to the field defi ned for primary magmatic carbonatites, whereas the ferrocarbonatites a re 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) - va lues. Variations in Pb, C and O isotopic data from ferrocarbonatites a re attributed to fluid activity that is probaby related to formation o f 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 contam ination, and their ''mantle-like'' C and O isotopic ratios suggest tha t their isotopic ratios are probably inherited from their mantle sourc e. The initial Nd and Sr isotopic ratios from the Amba Dongar calcioca rbonatites indicate derivation from a Rb/Sr- and Nd/Sm-enriched mantle source quite different to most young (< 200 Ma) carbonatites which ha ve signatures that lie between HIMU and EM I mantle components. The is otopic ratios from the Amba Dongar calciocarbonatites are similar to p roximal alkaline complexes of similar age and the least contaminated D eccan 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 .