Carboniferous 40Ar/39Ar Age of Rare-Metal Enriched Rhyolites and Ignimbrites in the Sakmara Allochthon (Southern Urals)

Resumo

The Sakmara Allochthon (Southern Urals) comprises a widely distributed folded stratum of rhyolites and ignimbrites, which are characterized by Nb–Zr–REE geochemical specification. This stratum overlies unconformably folded Paleozoic complexes, including Devonian ones. Rhyolites contain potassium feldspar and quartz phenocrysts, as well as granite xenoliths. According to geochemical features, volcaniс rocks were formed under the intraplate setting; in terms of the chemical composition, they belong to the A-type granites. The ⁴⁰Ar/³⁹Ar age of the felsic rock groundmass of rhyolites is 303 ± 2 Ma. The feldspar phenocrysts yielded ages of 306 ± 3 and 337 ± 3 Ma. The first estimate, as well as the matrix age, is associated with the formation of the volcanic complex, while the second value is probably associated with the age of xenogenic material. The complex of rare-metal enriched rhyolites characterizes the stage of the terminal Late Carboniferous rift magmatism in the rear part of the active continental margin. This complex formed prior to collision and thrusting of ophiolitic allochthons.