PB PB AGE-DETERMINATIONS ON THE NEWANIA AND SEVATTUR CARBONATITES OF INDIA - EVIDENCE FOR MULTISTAGE HISTORIES/

The ages of Indian carbonatites are still controversial. Most of the e arlier 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) t o constrain the age and geological history of these rocks. Newania car bonatites are intrusive into Precambrian Untala granite-gneiss and mai nly dolomitic in composition (rauhaugite) followed by a later phase of ankerite carbonatite, while thin calcite carbonatite (sovite) dykelet s are the youngest in the sequence. The analysed whole-rock samples ar e 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 compariso n to common igneous rocks and even for carbonatite compositions. One s ample, 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 1 550 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 extre me long age hiatus between dolomite carbonatite and ferrocarbonatite f ormation events raises severe problems for their petrologic interpreta tion. The Proterozoic Sevattur carbonatite complex (SCC, Tamil Nadu) w as emplaced contemporaneously with a large number of carbonatite compl exes within the Precambrian gneissic terrane of the Eastern Ghats Mobi le Belt. The main mass is composed of dolomite carbonatite (rauhaugite ) with a few dikes of calcite carbonatite (sovite) and ankerite carbon atite 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 ca rbonatites into account, the resulting age is 801 +/- 11 Ma. The Pb-20 6/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 commo n igneous rocks.Our investigations show that Fn carbonatitic rock syst ems extremely high lead isotropic ratios can be established due to the crystallization of uranium-rich mineral phases. In both cases the obs erved high to extremely high initial Pb isotope ratios require the res idence of the lead in intermediate high-mu reservoirs either within th e 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 le ad isotopic systematics. (C) 1997 Elsevier Science B.V.