Pb/Pb age determinations on the Newania and Sevattur carbonatites of India: evidence for multi-stage histories

The ages of Indian carbonatites are still controversial. Most of the earlier datings were done by K/Ar methods. We therefore analysed Pb/Pb ratios in carbonatites from carbonatite-alkalic complexes of Newania (NW India, Rajasthan State) and Sevattur (SW India, Tamil Nadu State) to constrain the age and geological history of these rocks. Newania carbonatites are intrusive into Precambrian Untala granite-gneiss and mainly dolomitic in composition (rauhaugite) followed by a later phase of ankerite carbonatite, while thin calcite carbonatite (sovite) dykelets are the youngest in the sequence. The analysed whole-rock samples are characterised by Pb-206/Pb-204 ratios between 60 and 176 and Pb-207/Pb-204 ratios between 22 and 40, which are extremely high in comparison to common igneous rocks and even for carbonatite compositions. One sample, New 37, shows the extreme ratios of Pb-206/Pb-204 = 574 and Pb-207/Pb-204 = 73. This requires a mu-value of about 2000 for the last 1550 Ma. If the samples are classified according to their petrographic/geochemical characteristics this results in an isochron age of 1551 +/- 46 Ma for the ankerite carbonatites (six samples). The dolomites (6 samples) yield an isochron age of 2.27 Ga. Although these results fit quite well into the geological evolution scheme of the area, the extreme long age hiatus between dolomite carbonatite and ferrocarbonatite formation events raises severe problems for their petrologic interpretation. The Proterozoic Sevattur carbonatite complex (SCC, Tamil Nadu) was emplaced contemporaneously with a large number of carbonatite complexes within the Precambrian gneissic terrane of the Eastern Ghats Mobile Belt. The main mass is composed of dolomite carbonatite (rauhaugite) with a few dikes of calcite carbonatite (sovite) and ankerite carbonatite within it. All eight samples together yield an isochron of 805 +/- 10 Ma. This isochron is mainly determined on ankerite carbonatites with mu-values up to 1900 for the last 800 Ma. Taking only ankerite carbonatites into account, the resulting age is 801 +/- 11 Ma. The Pb-206/Pb-206 and Pb-207/Pb-204 ratios of these samples are similar to the main group of Newania and far beyond the isotopic composition of common igneous rocks. Our investigations show that Fn carbonatitic rock systems extremely high lead isotropic ratios can be established due to the crystallization of uranium-rich mineral phases. In both cases the observed high to extremely high initial Pb isotope ratios require the residence of the lead in intermediate high-mu reservoirs either within the upper mantle or the crust prior to the carbonatite formation. A high-temperature event, which completely reset the Rb/Sr and K/Ar isotopic systems of Nevania carbonatites, seems to have no influence on the lead isotopic systematics. (C) 1997 Elsevier Science B.V.