Composition Characteristics and Model of Origin of Brecciated Porphyrites of the Shartym Massif (Balbuk Areal, Southern Urals): An Example of Paleovolcanic Analysis

The paper considers for the first time the composition of trachyandesite-basaltic porphyrites (including brecciated ones) of the Shartym massif and Pichugin intrusions, which were previously attributed to the diorites and granites of the Balbuk complex of the Southern Urals. Based on the results of petrographic, mineralogical and geochemical studies, 3 types of autoclasts and 1 type of xenoclasts were identified in the brecciated porphyrites. The autoclasts differ slightly from each other in composition, including the density of plagioclase phenocrysts (from 85 to 45 vol.%) and its anorthite content (An68–25–5), which generally corresponds to a single, regularly evolved volcanic system. The xenoclast corresponds to low-potassium trachyte and, according to its geochemical characteristics, it is not related to either brecciated porphyrites or younger rhyolite-porphyry of the Shartym massif. A model of the porphyrite origin related to the activity of a volcanic structure, where the studied rocks correspond to the vent facies, has been developed. Based on geochemical analysis, the time of their occurrence is associated with magmatism of the Balbuk monzonitoid complex (~350 Ma). The rocks have pronounced signs of subduction-related magmatism (negative Nb-Ta-Ti anomaly, positive Pb anomaly), which is quite typical of the Early Carboniferous magmatites of the Magnitogorsk megazone. Younger rhyolite-porphyry of the Shartym massif were intruded in the Late Carboniferous and probably led to strong deformation of the volcanic structure of basaltic trachyandesites.