Volcano-sedimentary complexes of Riphean standard section (Ai and Mashak formations) in the west slope of the South Urals (Bashkirian meganticlinorium) suppose to have riftogenous-platform nature. Middle Riphean Mashak formation (1348+/-30 Ma) (Krasnobaev et al., 1985) is one of the well known example of such "graben formations" (Ivanov, 1977; etc.). It consists of conglomerates, sandstones and shales with metabasalts and metarhyolites. Total thickness of these deposits is more than 2000 m. More than half of it is represented by metabasalts. There are different geodynamic reconstructions for this level of the Middle Riphean. Some researchers suppose that Mashak formation was formed under conditions similar to the Basin and Range Province, another compare them with the Red Sea etc. So these geological ideas needs to be checked by modern geochemical data. Metabasalts of Mashak formation are metamorphosed under green schist facies conditions. It consists of epidote, actinolite and chlorite (in various proportions), there are also relicts of primary magmatic minerals and textures. Chemical composition of basalts is rather variable. According to the main element contents these basalts correspond to tholeiites. According to TiO2 and P2O5 contents, considered as being conservative during green schist metamorphism (Pearce et al., 1973), there are typical "continental","transitional" and "oceanic" basalts. In the P2O5-TiO2-MnO diagram (Mullen, 1985) Mashak metabasalts are located basically in the fields of MORB and IAT. Minor element contents in Mashak basalts are similar to tholeiitic basalts of the ocean floor, but the Zr, Y and REE distribution pattern in the former are rather similar to continental riftogenous volcanics. Thus, primary chemical composition of Mashak metabasalts were essentially transformed under low grade metamorphism: K, Rb, Sr, Ba and some other elements were partly removed. Subalkaline and alkaline basalts with similar REE pattern are known in the continental part of East African Rift Zone (Hart et al., 1989). Total content of REE in Mashak metabasalts is considerably lower than in Mesozoic and Cenozoic volcanic rocks of continental rifts, but general distribution (predominance of LREE, absence Eu anomaly etc.) are analogous. Chemical composition of Mashak metabasalts and especially REE contents are rather similar to basalts of such well known pre-Phanerozoic riftogenous volcanic assoziations as Keweenawan (1,1 Ga), and Catoctin and Tibbit Hills (Latest Precambrian) of Appalachian orogen.
This study was supported by INTAS grant 94-1857.