Geochemical characteristic of rocks and isotope dating of zircon from the andesite dyke of the rudyanskaya sequence (D2 rd) of the Middle Urals Eastern zone

Abstract

Relevance. In the basin of the r. Pishma subvolcanic formations, localized in the fields of spreading of the Middle Devonian Rudyanskaya sequence, are presented by different on composition small bodies, sills and dykes, forming with volcanogenic rocks of the Rudyanskaya sequence a basalt-andesite-rhyolite complex, the formation of which is associated with volcanic activity, that took place in Middle Devonian in the environments close to the modern island arcs. Their isotope dating and geochemical characteristic are rather relevant for specification of the composition, volume and paleogeodynamic environment of the vulcanite formation. In the paper new data are presented on geochemistry of andesites of the dyke from the Diviy Kamen rock, considered as part of the Rudyanskaya sequence, as well as the age of zircon from them. Methods. Chemical analyses of vulcanites have been performed in the Centre of collective usage “Geoanalyst” of the Institute of Geology and Geochemistry of the UB of RAS (Ekaterinburg) by X-ray fluorescent method and ICP–MS method. Zircons were studied with scanning electron microscope JSM-6390L of the Jeol firm and with electrone-probe microanalyser Cameca SX100. The data on the U and Pb isotopes, as well as on elements-impurities in zircon grains were obtained by mass-spectrometry method with inductively coupled plasma with laser ablation (LA–ICP–MS) at the quadrupole ISP–MS NexION 300S with the attachment for laser ablation LA NWR 213. Purpose of researches. Getting new data on geochemical composition and age of the andesites of dyke in the section along the r. Pishma. Results. The andesite dyke intersects medium clastic unstratified tuffs and dipply occurring fragment of the lava flow of porphyry dacite. Andesites present themselves porphyry rocks with 20–30% of plagioclase phenocrysts. At the normalized many-component diagram the Ta–Nb minimum is displayed. For zircon grains are characteristic differentiated spectra, demonstrating the growth of compositions from light to heavy REE, positive Ce and negative Eu anomalies, typical of the zircon of magmatic genesis. For the first time geochronological study of zircons from andesites has been maded by method of LA–ICP–MS. 4 age groups of zircons, Ma: 390–387, 369–362, 337–345, 327–329 were revealed. Conclusions. The data obtained testify that andesites were formed under suprasubduction environments, and magmatic stage of rock formation corresponds to the upper Devonian varieties of zircons. Early Carboniferous datings can be associated with the thermal impact caused by the intrusion of a later dolerite dyke.