Abstract:
The Sindreth Group exposed near Sirohi in southern Rajasthan, western India, is a volcanosedimentary sequence. Zircons from Sindreth rhyolite lavas and tuffs have yielded U-Pb crystallization ages of similar to 768-761 Ma, suggesting that the Sindreth Group is a part of the Malani magmatic event. Earlier Ar-40-Ar-39 studies of other Malani volcanic and plutonic rocks yielded disturbed argon release spectra, ascribed to a similar to 550 Ma thermal event possibly related to the Pan-African orogeny. To test and confirm this possibility, we dated two whole-rock and three feldspar separate samples of the Sindreth volcanics by the Ar-40-Ar-39 method. All samples yield disturbed argon release spectra suggesting radiogenic argon loss and with plateau segments at 550 Ma or 490 Ma. We interpret these as events of argon loss at 550-490 Ma related to an Ediacaran-Cambrian thermal event, possibly related to the Malagasy orogeny. The combined older and new Ar-40-Ar-39 results are significant in showing that whereas Ediacaran-Cambrian magmatic and metamorphic events are well known from many parts of India, they left thermal imprints in much of Trans-Aravalli Rajasthan as well. The overall evidence is consistent with a model of multiphase assembly of Gondwanaland from separate continental landmasses.