Synrift deposits of the Middle Riphean Mashak Formation, Southern Urals (a short lithochemical characterization)

Abstract

The article briefly describes the lithochemical features of sandstones and clayey rocks of Mashak Formation (Southern Urals), a second synrift volcano-sedimentary association in Riphean stratotype. It is shown that the sandstones of the Mashak Formation are characterised by wide variations in the bulk chemical composition (from greywacke to quartz arenite). The presence of the latter, as well as a relatively small proportion of arkoses and subarkoses, distinguishes the second synrift association from the first one (the Ai Formation, Lower Riphean). There is no overlap in between the Mashak’s and Ai’s psammites on the diagram log(SiO2/Al2O3)–log(Na2O/K2O) of F. J. Pettijon and co-authors. It indicates significant differences in provenances and common features of accumulation of the Mashak and Ai formations. On the basis of bulk chemical composition, the clayey rocks of the Mashak Formation are related to smectite (with kaolinite and illite admixture) clays and their chlorite-smectiteillite varieties. The clayey rocks of the Ai Formation differ by lower values of total normative alkalinity (0.27 ± 0.06 vs. 0.42 ± 0.07). It is established that sandstones and clayey rocks of the Mashak Formation contain a significant proportion of the “first-cycle material”, i. e. they are composed mainly by products that have passed one sedimentation cycle. As opposite, there is a significant proportion of “second-cycle/lithogenic material” of the first synrift sandstones association (the Ai Formation). The high K2O/Na2O values, which are characterised for a half studied samples of the Mashak Formation, indicate that they were influenced by processes of potassium metasomatism. The distribution of data points of terrigenous rock of the Mashak Formation on diagram F1—F2 (Roser and Korsch, 1988) displays that they are composed by products of erosion of acid, average or mafic igneous and/or metamorphic rocks. The individual data points of Mashak Formation terrigenous rocks, as well as sandstone composition midpoint, are localised in the areas of riftogenic and collisional settings on the DF1—DF2 diagram (Verma and Armstrong-Altrin, 2013). It does not allow us to define their paleogeodynamic settings. In such a manner, the a critical role, traditionally, remains on general geological data and petrogeochemical characteristics of igneous rocks.