Rb-Sr isotope dating of Neoproterozoic (Malani Group) magmatism from Southwest Rajasthan, India: Evidence of younger Pan-African thermal event by Ar-40-Ar-39 studies

This paper reports W-Sr isotope ages of the Neoproterozoic volcanics and as sociated granitoids of the trans-Aravalli belt of northwestern India. All t hese rocks along with the earlier reported 779 +/- 10 Ma old felsic volcani cs from Diri and Gurapratap Singh of Pall district, Rajasthan, constitute t he Malani Group. The study indicates that different rock suites belonging t o the Malani Group represent a polyphase igneous activity which spanned for about 100 Ma ranging from 780 to 680 Ma. The granitoids of the Malani Grou p, i.e. peraluminous Jalore type and peralkaline Siwana type, were emplaced around 730 and 700 Ma ago, respectively. These plutonic suites represent t wo different magmatic episodes within a short time interval. The initial Sr ratios of these granitoids suggest lower crustal derivation of the magma. The peralkaline granitoids and the associated peralkaline rhyolites (pantel lerites) are coeval and cogenetic. The ultrapotassic rhyolite exposed at Ma nihari of Pali district represents the youngest magmatic activity at 681 +/ - 20 Ma, having a very high initial Sr ratio of 0.7135 +/- 0.0033. The high initial Sr ratio of these rocks may be due to incorporation of radiogenic Sr-87 from the country rock, by assimilation or fusion, into the residual f raction of the magma in the crust which gave rise to other differentiated r ocks of the Group. Ar-40-Ar-39 studies of two Jalore granite samples indicate presence of post crystallisation thermal disturbance between 500-550 Ma ago. The timing of this thermal overprinting on the Malani rocks is related to the widespread Pan-African thermo-tectonic event which is witnessed and magmatically manif ested in different part of the Indian shield.