Vendian-Cambrian Rocks of the Upper Kalar Graben-Syncline of the Southern Part of Siberian Craton: Mineralogy and Major Elements Geochemistry

The features of the mineral and chemical composition (major elements) of the Vendian-Cambrian rocks of the Silimkun Formation (V-Є1sl), exposed on the western side of the Upper Kalar graben syncline in the southwest of the Aldan shield, are considered. The studied rocks – sandstones and siltstones, belong to arkoses and subarkoses. According to their chemotype, they correspond normosilites and myosilites. In terms of the alkaline modulus, the studied silites are hyperpotassium and serve as a fairly striking example of the products of Precambrian arid weathering. The standard mineral composition, its variations, the validity of calculations based on petrographic research data, the problems of using climate indices and lithochemical diagrams aimed at reconstructing climatic changes in the geological past are discussed. Chemical Index of Alteration (CIA) index values range from 49, which corresponds to a virtually unchanged rock, to 67. The average is about 58. Minor changes in the index value for this sample are associated primarily with variations in the content of illite cement and muscovite. Robust Weathering index (RW) values range from 37 to 68 (with an average of about 57).