Neoproterozoic age of oldest rocks from the Tom' inlier (Mountainous Shoriya): Implication of U-Pb, Sm-Nd, Rb-Sr, and Ar-Ar dating

Age of metamorphic and magmatic rocks from the Mountainous Shoriya is measu red using U-Pb, Sm-Nd, Rb-Sr, and Ar-Ar dating. As established, the oldest rocks of the region are metabasites of the Konzhin Complex (Sm-Nd isochron data T = 694 +/- 43 (2 sigma) Ma, epsilon(Nd)(T) = 7.1 +/- 0.1 (2 sigma), M SWD = 2.0), whose isotopic composition corresponds to that of abyssal and o cean island basalts. Geochronological dates obtained show that the crystall ine schists, and gneissoid to normal granites of the Tom' inlier, which hav e been attributed earlier to the deep Precambrian, range in age from the Ca rboniferous to Late Triassic. None of the geochronological methods revealed dates older than the Late Riphean. The same age constraint for oldest rock s from the Mountainous Shoriya is inferred from the Sm-Nd model ages T-Nd(D M-2st) = 590-920 Ma calculated for protoliths of granitoids. Maximum values epsilon(Nd)(T) = +6.0 are shown to be characteristic of the Early Cambrian plagiogranites of the ophiolite association, whereas low parameters epsilo n(Nd)(T) = +2.5 mark the Late Triassic potassium granitoids. The geochronol ogical dates obtained and correlation of geological complexes in the Mounta inous Shoriya and adjacent regions suggest a conclusion that the Early Prec ambrian crystalline crust is missing in the study area.