The crustal record in Rajasthan

Age determinations mostly by Rb/Sr whole rock isochrons of the Precambrian rocks of Rajasthan in northwest India are summarized and discussed. On present sampling and subject to its possible bias, the following conclusions can be made. The Untala Granite believed to be intrusive into the gneissic terrain (bgc) east of Udaipur has the oldest age, 2.95 b.y. yet measured for a granite in Rajasthan. This, coupled with the lead isochron age of 3.5 b.y. for detrital zircon from the Aravalli schists by Vinogradov and others extends the basement of Rajasthan well into the Archaean. The time equivalence of thebgc east of Udaipur with the Berach Granite dated only at 2.55 b.y. is not tenable. No satisfactory radiometric age control exists for the onset and duration of the Aravalli Supergroup, believed to be an early Proterozoic linear belt. Heron’s original Delhi Supergroup has recorded acid magmatism widely separate in space and time. The earliest activity between 1700 and 1500 m.y. is recorded mainly in the Alwar basin in northeastern Rajasthan while the younger activity between 850 and 750 m.y. is represented by the ‘Erinpura type’ granites in the central and southern Aravalli sector. This younger event not only has let its thermal overprinting on the older Alwar rocks but also marks the onset of emplacement of the Malani Igneous suite in the trans-Aravalli terrain. This raises the new possibility that the Delhi rocks of Heron represent atleast two chronologically independent sequences with varying geographical extent. The trans-Aravalli terrain is most probably floored by partly reworked, crystalline basement and developed along linear rift zones which acted as loci for high heat flow and igneous activity since about 800 m.y. ago.