Abstract
Initial 206 Pb 204 Pb (19.05–19.19), 207 Pb 204 Pb (15.72–15.74) and 208 Pb 204 Pb (39.72–40.16) ratios, and ϵ Sr ( + 15.1 to + 19.2) and ϵ Nd (−1.6 to −0.8) -values for the calciocarbonatites from the Cretaceous ( ∼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 206 Pb 204 Pb (18.94–19.28), 207 Pb 204 Pb (15.66–15.81) and 208 Pb 204 Pb (39.55–40.05) ratios. δ 13C (−4.20 to −3.58‰) and δ 18O ( + 8.80 to + 10.02‰) 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 δ 13C (−4.29 to −1.62‰) and 8'80 ( + 13.73 to +24.67%o) -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 Réunion hot spot responsible for the Deccan flood basalts may have had some bearing on producing the parental melt that generated the Amba Dongar carbonatite.
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