Abstract:Our study investigates a possibility of using isotope ratios of U-Th-Pb system in zircon analysis by the LA-ICP-MS technique and monitoring of precision, convergence and accuracy. New U-Pb isotope-geochronological data has been obtained for the salic and mafic members of the bimodal Late Paleozoic series of subparallel dykes in the central part of the Western Transbaikalia. It is shown that a series of subparallel dykes (290-280 Ma) gives evidence of extension of the continental crust at the final stage of the Late Paleozoic granitoid magmatism. Similar ages of zircons in the salic and mafic subvolcanic rocks confirm the geological indicators of the coexistence and interactions between the magmas of contrasting compositions.

Abstract:The alkaline mafic (lamprophyric) Gusinoozerskaya dyke in West Transbaikalia is composed of partially melted granite xenoliths. Among the xenoliths, two melted substrates are observed: (1) plagioclase and quartz, and (2) alkaline feldspar and quartz. Few millimeters thick microfelsite and microgranophiric rims are the products of melting around the granite xenoliths. Ultra-acid glass is observed in the inner parts of the xenoliths at the boundary of quartz and feldspars. A distinctive feature of the fresh melts (regardless of the composition of the protolithic substrate) is an increased content of potassium with K2O/Na2O≥2. Having compared the compositions of the products of contact melting with the experimental data, we conclude that melting took place in the presence of alkaline-chloride and/or alkaline-carbonic fluid released from the crystallizing host alkaline-basic magma. The probable geotectonic conditions for the occurrence of ultrapotassic acid magmas are estimated.

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