Petrology of calc-alkaline volcanic rocks of the Pavda complex, Tagil depression, based on a study of zonal clinopyroxene: Evidence from materials recovered by the Ural Superdeep Hole

Rock-forming minerals of the pyroxene group provide important genetic infor mation, which is "recorded" in certain subtle compositional and structural features of the minerals. The crystallization trends or the character of co mpositional variations from the cores to margins of solid-solution crystals reflect variations in the temperature, pressure, composition and the dynam ics of the fluid phase, cooling rate of the melt, and the magma composition , i.e., the general concentric zoning of crystals is indicative of the crys tallization conditions of the magma. Detailed data are presented on the com position of clinopyroxene from the Silurian basaltic andesites and andesite s of the Pavda magmatic complex in the fossil Tagil island are, which was p enetrated by the Ural Superdeep Hole. The crystallization trends of the cli nopyroxene are discussed and an attempt is made to decipher the genetic inf ormation recorded in them with the purpose of determining the evolution of the melts parental for the basaltic andesites and andesites. Our studies ma de it possible to determine that the basaltic andesites and andesites had a common source of their parental melts, whose evolution proceeded similarly until a certain time. The parental melt of the basaltic andesite horizon w as mixed with a newly-injected melt of similar composition, and this proces s was responsible for the crystallization of clinopyroxene phenocrysts with reversed zoning. The zoning of clinopyroxene can be used as an indicator o f the latent acid-to-basic or basic-to-acid evolution of magmatism in islan d arcs. A correlation was revealed between the crust thickness of the Ural fossil are and the crystallization trends of clinopyroxene in the calc-alka line volcanic rocks: the clinopyroxene of volcanics derived within a thicke r crust exhibits longer crystallization trends and is richer in the ferrosi lite end-member.