Syn-rift Clayey Rocks: Bulk Chemical Composition and Position on Discriminant Paleogeodynamic Diagrams

Abstract

The paper analyzes the bulk chemical composition and distribution of the fields of syn-rift clayey rocks on a number of discriminant paleogeodynamic diagrams. It is shown that the syn-rift clayey rocks, in general, demonstrate significant variations in the bulk chemical composition. For example, the average SiO₂ content varies from 44.74 to 66.42 wt %, the average Al₂O₃ content varies from 16.62 to 29.92 wt %, and the K₂O_aver is in the range 0.24 … 5.77 wt %. Based on the distribution of the data points of syn-rift clayey rocks from various objects in the F1–F2 diagram, it can be assumed that the sources of fine aluminosiliciclastic material were magmatic and sedimentary rocks of a wide composition range. The substantial overlap of the fields of various objects in the classification diagrams [(Na₂O + K₂O)/Al₂O₃]–[(Fe₂O_3tot + MgO)/SiO₂] and K/Al–Mg/Al indicates, in general, the similar composition of syn-rift fine-grained clastic rocks of various types of riftogenic structures. The localization of the composition fields of the clayey rocks of different riftogenic structures in such discriminant paleogeodynamic diagrams as K₂O/Na₂O–SiO₂/Al₂O₃ and SiO₂–K₂O/Na₂O suggests that they cannot be used to discriminate reliably between syn-rift clayey rocks and fine-grained clastic rocks of other geodynamic environments. The position of the syn-rift clayey rocks fields from our database in the diagram DF1–DF2 has its own characteristics. In most cases, they occupy a particular position in the areas characterizing collision and rifting environments, and a number of fields are located in all three classification areas of this diagram. A significant part of the midpoints of the syn-rift clayey rocks is localized in the collision field in the DF1–DF2 diagram. All these facts indicate that the DF1–DF2 diagram also cannot determine the geodynamic nature of terrigenous associations.