PETROLOGY OF THE SUBALKALINE VOLCANISM IN THE WESTERN SLOPE OF THE URAL MOUNTAINS - AN INDICATOR OF THE LATE VENDIAN RIFTING

Abstract

The subalkaline magmatism in the western slope of the Urals is an indicator of rifting processes, which accompanied the splitting of the Rhodinia supercontinent at the boundary of the Late Proterozoic and Early Paleozoic. The studied magmatic products of this age compose the Blagodat', Kus'ya, and Dvoretskii complexes and are generally represented by subalkaline basic and ultrabasic lavas and subvolcanic rocks. The rocks vary in concentrations of alkalis and incompatible trace elements. For example, trachybasalts of the Blagodat' complex are close in composition to the continental tholeiites with (La/Yb)n = 4-7. Trachybasalts of the Dvoretskii complex are similar in major- and trace-element chemistry to the rocks of the alkaline basaltic series; they are rich in incompatible elements and have (La/Yb)n = 15-25. The isotopic compositions of the rocks also vary. The Blagodat' trachybasalts are most depleted and have εNd values of 6.9. Trachybasalts of the Dvoretskii complex and subalkaline gabbroids have isotopic compositions (εNd from 4.4 to 5.0 and εSr from -15 to -20) similar to the rocks directly related to the activity of the modern mantle plumes. The Kus'ya alkaline picrites have a more enriched source with εNd = 2.9 and εSr = -10. We suggest that the studied rocks could be related to the activity of a single mantle plume. The compositional variability of volcanics of the trachybasalt-trachyandesite series of the Dvoretskii complex is determined by several petrogenetic processes. The dominating crystal fractionation of a parental magma was accompanied by its contamination (up to 5%) by crustal material with subsequent interaction of rocks with marine water.