THE LOWER RIPHEAN FINE-GRAINED ALUMINOSILICATE CLASTIC ROCKS OF THE BASHKIR ANTICLINORIUM IN THE SOUTHERN URALS: COMPOSITION AND EVOLUTION OF THEIR PROVENANCE

Abstract

The major and trace element geochemistry of the Burzyan fine-grained aluminosilicate clastic rocks of the Bashkir Anticlinorium was used to estimate the composition of provenance and its evolution from 1650-1640 to 1350 Ma. Total REE contents are higher than 200 ppm in all the shale samples analyzed, and approximately 3 times lower in the clayey rocks of the upper unit of the Ai Formation. The (La/Yb)N and (Gd/Yb N in the shales of this level are also much lower. The Eu/Eu value of Burzyan shales is 0.6-0.7, which is typical of most post-Archean clayey rocks. The REE patterns of shales from all the lithostratigraphic levels analyzed are rather similar and close to the PAAS pattern. The evolution of the Early Riphean sedimentary basin that occupied the territory of the present-day Bashkir Anticlinorium is interpreted, as follows. The beginning of the Early Riphean was characterized by (1) the formation of a shallow sedimentary basin, (2) the erosion of large blocks of the mature continental crust composed of felsic rocks impregnated by basic dikes and other minor intrusions, and (3) the predominance of proximal provenances. In late Ai time, the aforementioned blocks were completely graded; and distal sources of aluminosilicate elastic materials became prevalent. Felsic rocks were eroded in the source areas. Basic rocks contributed to the provenance in early Kusa time, but subsequently they have not been involved in erosion until the end of the Early Riphean. Clastic material was mainly removed during that time period from the west and northwest, i.e., from the eastern parts of the East European Platform. An abrupt and relatively short-term rearrangement of the migration paths of terrigenous material is supposed for late Satka time, when a great amount of magnesium was supplied to the basin. As follows from the model Nd ages of shales throughout the Burzyan section, the source of magnesium had a pre-Riphean age and was situated north or northeast of the present-day Bashkir Anticlinorium.