Geochemical features of the Riphean fine-grained terrigenous rocks of the Southern Urals,Uchur-Maya region,and Yenisei Range: Estimation of the maturity of the pre-Riphean continental crust and its evolution within 1.65–0.6 Ga

The geochemical features of basal fine-grained terrigenous rocks from the Riphean sedimentary megasequences of the Southern Urals, Uchur-Maya region, and Yenisei Range were compared in order to estimate the maturity of the continental crust that was formed by the beginning of the Riphean. It was shown that initial shales from the base of the Riphean sequence of the Yenisei Range and fine-grained aluminosiliciclastic rocks from the base of the Riphean sections of the Southern Urals were formed by the erosion of a rather mature continental crust. In contrast, fine-grained terrigenous rocks from the base of the Riphean of the Uchur-Maya region were derived from immature Late Archean protoliths or their Early Proterozoic analogs. The fine-grained terrigenous rocks of the three sedimentary megasequences show different variations in the (La/Yb)_N ratio. In the Southern Urals, this ratio is high (12–15) in the Burzyan Group and decreases upsection to 6–10. In the shales of the Uchur-Maya region, the (La/Yb)_N ratio decreases upsection, and the La/Sc ratio shows a sympathetic behavior. This is due to a decrease in the proportion of “primitive” tonalite-trondhjemite associations of the Archean granite-greenstone terranes in the provenance area with time and the appearance of intra-plate (riftogenic?) granitoids and significant amounts of basic and ultrabasic rocks. The latter marks the onset of large rift-forming events in the Uchur-Maya region at the beginning of the Late Riphean. The (La/Yb)_N of the studied rocks from the Yenisei Range are mostly similar to the PAAS ratio, but higher values were found in the Upper Vorogovka and Chingasan groups, which was related to the contribution of strongly LREE-enriched granitoids and rift felsic and alkali basaltic volcanic associations to the formation of the terrigenous material. A comparison of Rb, Sr, Y, Zr, Ba, Hf, Th, U, Cr, and Ni contents and Zr/Y, (La/Yb)_N, Ni/Co, Cr/Th, Cr/Sc, and La/Th ratios in the fine-grained terrigenous rocks of the Riphean megasequences of the Southern Urals, Uchur-Maya region, and the Yenisei Range with those in the model geochemical objects (PAAS, UC_PR1, UC_AR2, and others) showed that, in terms of most of the parameters, the Riphean fine-grained terrigenous rocks from the three regions are similar to each other, PAAS, and Proterozoic cratonic shales. This indicates a fairly high general maturity of the protoliths that were eroded during the Riphean in the eastern East European craton and in the southeastern and southwestern parts of the Siberian craton.