Pb isotopes provide a unique tool for studying the age and genesis of magmatic rocks. We analysed carbonatites from Newania (NW India) and Sevattur (SW India) to constrain the age and geological history of these rocks.
Newania carbonatites (Udaipur district, Rajastan) consist mainly of dolomitic carbonatites (rauhargite) with subordinate ankeritic- and ferro-carbonatites. They are usually medium- to coarse-grained and intrusive into the Precambrian Untala granite-gneiss which itself is intrusive into pre-Aravalli gneisses and schists. The sequence of emplacement based on field relationships is dolomitic carbonatite, followed by ankeritic carbonatite and finally sövite in small dykelets.
The Proterozoic Sevattur carbonatite (Tamil Nadu) was emplaced contemporaneously with a large number of carbonatite complexes within the Precambrian gneissic terrane of the eastern Ghats Mobile Belt. The alkaline magmatic activity postdates the eastern Ghat orogeny. The Sevattur carbonatites show a wide variation in their mineralogy. The inner portion is occupied by dolomitic carbonatite (rauhargite) or parankeritic carbonatite surrounded by sövite.
All 14 analysed Newania whole-rock samples are characterised by extremely high Pb isotope ratios. For the main group the 206Pb/204Pb ratios lie between 60 and 176 and the 207Pb/204Pb ratios between 22 and 40, far beyond usual rocks and also for carbonatite compositions. One sample, New37, shows the extreme ratios of 206Pb/204Pb = 574 and 207Pb/204Pb = 73. This requires a µ2 value of about 2000. Compared with this the 208Pb/204Pb ratios lie in a normal range between 36 and 40. All samples regressed together yield an "errorchron" with an age of 1705±106 Ma which is basically defined by sample New37. However, if the samples are classified according to their petrographic/geochemical characteristics this results in an age of 1585±50 Ma for the ferro-carbonatites and of ×2.2 Ga for the main group. Two dolomites (6 samples) yield an isochron age of 2.23 Ga. This age is in agreement with the geological and petrographic results.
Concerning Sevattur, all 9 whole-rock samples together yield an isochron of 801±18 Ma. This isochron is mainly determined by ankeritic carbonatites with µ2 up to 1900. Taking only ankeritic rocks into account, the resulting age is 802±5 Ma. The 206Pb/204Pb- and 207Pb/204Pb-ratios of these samples are similar to the main group of Newania and far beyond the isotopic composition of usual igneous rocks.
The observed extremely high Pb isotope ratios require the existence of a very high-µ2 reservoirs within the upper mantle and because of the high 207Pb/204Pb-ratios differentiation must have occured very early (Archaean).