Present status of the geochronology of the Precambrian rocks of Rajasthan

Abstract

Age determinations, mostly by Rb---Sr analyses, of the Precambrian rocks of Rajasthan by us and by others are summarized and discussed. Broad periods of acid magmatism at (1) 3000-2900 m.y., (2) 2600-2500 m.y., (3) 2000-1900 m.y., (4) 1700-1500 m.y. and (5) 850-750 m.y. were identified. The oldest rocks in the area are the yet undated banded gneisses (BGC) east of Udaipur, intruded by the Untala granite dated at 2950 m.y. and hence of mid-Archean age. The basal status hitherto attributed to the Berach granite dated at 2600 m.y. is no longer tenable. The radiometric control on the beginning and duration of the overlying Aravalli Supergroup is not yet satisfactory, though a lower limit at 2000 m.y. is indicated. Heron's original Delhi rocks have recorded two magmatic events widely separate in space and time. While the earliest granitic activity at 1600 m.y. is recorded only in the Alwar basin in the northeast, the younger activity between 850-750 m.y. is widespread, as shown by the nearly concordant ages of "Erinpura-type" granites along the Aravalli mountain Range and the Malani rhyolites in the western plains of the Aravalli Range.