Possible influence of CaO admixture in the Pre-Ural Foredeep terrigenous rocks on the determination of the source rocks composition based on geochemical data

Research subject. The article discusses the analysis results of correlations between the calcium oxide content and a number of other rock-forming oxides in the Asselian sandstones and in the Middle–Upper Permian clay rocks of the Pre-Ural trough with some of their inherent ratios of rare and trace elements (La/Sc, Th/Cr, Th/Co, Ce/Cr, etc.), which act as traditional indicators of source rocks composition.

Methods. Our main method is to analyze the correlation coefficients values between different pairs of oxide-indicator ratios. During the discussion, we proceeded from the fact that a statistically significant (both positive and negative) correlation between calcium oxide, ignition loss, or magnesium oxide, on the one hand, and the values of indicator ratios of rare and trace elements, on the other hand, suggest the dependence of the noted ratios on the features of the aluminosilicate matrix replacement with carbonate minerals.

Results. We have established, there is a statistically significant positive or negative correlation between the calcium oxide content in Permian terrigenous rocks (different sedimentary intervals of the Pre-Ural Foredeep) and some indicator ratios of rare and trace elements (Asselian sandstones – Th/Cr, Ce/Cr and Eu/Eu*, Urzhum–Viatsk clay rocks – La/Sc, Th/Co, (La/Yb)_N, etc.). As a result, the values of these ratios depend to some extent on the carbonate minerals content (mainly calcite) in terrigenous rocks. These carbonate minerals corrode the aluminosilicate matrix and in one way or another change the primary contents and ratios of impurity elements. On the contrary, there is no significant correlation between the CaO content in the Asselian sandstones and the values of La/Sc, Th/Co, etc. There is no correlation between the CaO content and the Ce/Cr ratio in the Urzhum–Viatsk clay rocks.

Conclusions. We suggest, the correlation (not statistically significant at 5% confidence level) between calcium oxide content (in sandstones and clay rocks) and some indicator ratios of rare and trace elements can be considered as an indication that such ratios can be used to reconstruct the source rocks composition.

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