Synrift Sandstones and Mudstones: Bulk Chemical Composition and Position in Some Discriminant Paleogeodynamic Diagrams

Аннотация

The bulk chemical composition of synrift sandstones and associated mudstones is analyzed. Distribution of their fields in the discriminant paleogeodynamic diagrams SiO₂–K₂O/Na₂O (Roser and Korsch, 1986) and DF1–DF2 (Verma and Armstrong-Altrin, 2013) is examined. In terms of the bulk chemical composition, the sandstones correspond mainly to graywackes, litites, arkoses, and subarkoses. The samples also include sublitites and quartz arenites, as well as a significant part of psammites (with log(Na₂O/K₂O less than –1.0) that are lacking in Pettijohn’s classification diagram. This fact supports our conclusion based on the results of mineralogical-petrographic studies: the sedimentary infill of riftogenic structures combines immature sandstones, clastic framework of which is derived from local provenances represented by diverse igneous and sedimentary rocks. Synrift mudstones, relative to sandstones, are composed of a more mature clastic material. Distribution of data points of mudstones in diagram F1–F2 (Roser and Korsch, 1988) suggests that they were sourced mainly from sedimentary rocks. Contents of the majority of main rock-forming oxides in synrift sandstones are almost similar to those in silty–sandy rocks of the Upper Precambrian–Phanerozoic sedimentary megacomplex in the Russian Plate, but differ from contents in the average Proterozoic and Phanerozoic cratonic sandstones, as well as the average composition in the upper continental crust. It is shown that the distribution of synrift sandstones and mudstones in diagram SiO₂–K₂O/Na₂O lacks any prominent features, and their data points cluster in the fields of terrigenous rocks of passive and active continental margins. In diagram DF1–DF2, fields of the studied psammites and mudstones occur in zones of the riftogenic and collisional settings. We believe that the boundary between these zones in the diagram occupies a different position, and this suggestion needs further examination.