Petrogeochemical and petrogenetic characteristics of the Devonian volcanic rocks of south-eastern part of Chebakovo-Balakhtino depression of Minusinsk intermountain trough (Northern Khakassiya)

The paper considers peculiarities in the distribution of petrogenic oxides and trace elements within the lower Devonian volcanic rocks of Northern Khakassiya. Dominating rocks, namely basaltoids, belong to the subalka-line series and are assigned to medium-aluminiferous, medium-Ti, K-Na varieties. On plots of normalized distributions of rare-earth elements, they are characterized by steep slopes of the geochemical spectra at the account of high concentrations of light lantanoids (mainly La and Ce). The spider diagrams demonstrated the proximity of microelement composition to the model standard of subalkaline intraplate basalts (OIB), from which the studied basits differ by the reduced Th, Ti, Ta, Nb, Hf concentrations and higher Rb, Ba, Sr concentrations. Geochemical spectra of basaltoid differentiates (trachyrhyolites and trachytes) vary noticeably in the behaviour of europium minimum, the discordance between negative and positive anomalies, microelements content. This results in the absence of the direct succession between the normalized concentrations of admixed elements in volcanites of acidic and medium composition. On the binary diagrams ZREE-ZR и ZREE - Z(Sr+Ba), two autonomous basite fields were established (B-1 and B-2), which are noticeably distinguished by a number of characters: by a mean content of the norm nepheline - 2.3% in field B-1 and 4.8 % in field B-2; by different concentrations of rare and rare-earth elements; by the content of Z (Sr+Ba). This enables considering the basites of the established fields as self-dependent genetic objects connected with different igneous sources, which work simultaneously but differ in the alkaline levels and in the quantitative sets of element-admixtures. The late basaltoid differentiates form two spatially segregated geochemical trends connected closely with field B-1 (trachyrhyolites) and with field B-2 (trachytes). Each of the trends was found to possess individual features, namely: different levels of accumulation of rare and rare-earth elements in allied rocks; distinct geochemical habits of final differentiates; the principally distinctive behaviour of the Sr - Ba sum in the process of the geochemical evolution. This suggests that the late differentiates are formed owing to dissimilar basaltoid melts. The evolution of these latter might have resulted in the formation of two coincident bimodel associations of the basalt-trachyrhyolitic and trachybasalt-trachyte compositions.

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