NORTH BICHURA GABBRO-GRANITE ASSOCIATION OF WESTERN TRANSBAIKALIA: COMPOSITION, AGE, CORRELATION ISSUES AND GEODYNAMIC ASPECTS

This paper presents new data on the age and distribution features of the main rock-forming components and trace elements in the rocks of the North Bichura gabbro-granite association, which is a petrotype of the Bichura intrusive complex. The chemical composition of the rocks studied corresponds to the gabbro, monzogabbro, monzodiorite, monzonite, granosyenite, moderately alkaline granite and leucogranite families. The geological structure and composition of the North Bichura rocks provide important insights into the Early Mesozoic tectonomagmatic processes in the southern part of Western Transbaikalia during the closure of the Mongol-Okhotsk paleoocean. LA-ICP-MS U-Pb zircon geochronology allows us to distinguish two stages of magmatic activity: 247–245 and 228–226 Ma ago, corresponding to the Middle and Late Triassic periods. The early stage was characterized by the formation of gabbro-monzonite rocks, and the late stage – by the formation of granitoid rocks. More than 20 million-year gap between the gabbroids and granitoids of the North Bichura pluton contradicts the model of rock formation based on differentiation of the initial basic melt. Two petrographic types of gabbroids have been distinguished that show different geochemical characteristics – amphibole-pyroxene and pyroxene-biotite. The first type is comparable to rocks formed in intraplate settings, while the second is similar to subduction zone magmatism. The formation of the Middle Triassic basic melts with different geochemical parameters most likely indicates their origin from two different sources: suprasubduction lithospheric mantle and asthenospheric mantle. Analysis of geochemical and geochronological data indicates the Early Mesozoic tectonomagmatic-related convergence caused by the closure of the Mongol-Okhotsk paleoocean.

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