Rb-Sr whole-rock and mineral isochron ages of the Tirodi gneiss, Sausar group, Bhandara district, Maharashtra

Six whole rock samples of the Tirodi gneiss belonging to the basal part of the Sausar Group rocks in the Dongri Buzurg area yield a good Rb-Sr isochron corresponding to an age of 1525 ± 70 Ma and an initial Sr ratio of 0.7148 ± 0.0033. Mineral isochrons for two of these rocks give concordant but younger ages at 860 Ma in good agreement with the previously reported K/Ar ages on minerals separated from the Sausar Group rocks. The 1525 Ma event is interpreted to mark the main phase of regional metamorphism of the Sausar Group of rocks including the granitization of the basal Tirodi unit. The deposition of the sediments of the Sausar Group and their basement must therefore be even older. The good agreement of the Rb/Sr and K/Ar mineral ages at about 860 Ma indicates the latest thermal overprint on the Sausar rocks. The regional metamorphism of the Tirodi gneiss at about 1500 Ma ago is also reflected in the Rb/Sr and K/Ar mineral ages of the Amgaon gneiss and Dongargarh granite to the east of the Tirodi gneiss. (author)