NEOPROTEROZOIC AGE OF OLDEST ROCKS FROM THE TOM' INLIER (MOUNTAINOUS SHORIYA): IMPLICATION OF U-PB, SM-ND, RB-SR, AND AR-AR DATING

Abstract

Age of metamorphic and magmatic rocks from the Mountainous Shoriya is measured 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 (2σ) Ma, εNd(T) = 7.1 ± 0.1 (2σ), MSWD = 2.0), whose isotopic composition corresponds to that of abyssal and ocean island basalts. Geochronological dates obtained show that the crystalline schists, and gneissoid to normal granites of the Tom' inlier, which have been attributed earlier to the deep Precambrian, range in age from the Carboniferous to Late Triassic. None of the geochronological methods revealed dates older than the Late Riphean. The same age constraint for oldest rocks from the Mountainous Shoriya is inferred from the Sm-Nd model ages TNd(DM-2st) = 590-920 Ma calculated for protoliths of granitoids. Maximum values εNd(T) = +6.0 are shown to be characteristic of the Early Cambrian plagiogranites of the ophiolite association, whereas low parameters εNd(T) = +2.5 mark the Late Triassic potassium granitoids. The geochronological dates obtained and correlation of geological complexes in the Mountainous Shoriya and adjacent regions suggest a conclusion that the Early Precambrian crystalline crust is missing in the study area.