The importance of geochemical data for geodynamic reconstruction: formation of the Olkhon metamorphic complex, Lake Baikal, Russia

Abstract

The Olkhon metamorphic complex consists of two geochemically distinct units—the Olkhon unit and the Anga formation. The basic crystalline schists of the Olkhon group are interpreted as marginal basin tholeiites. Garnet–biotite gneisses, quartzites and marbles have large contents of C, Cr, V, Mo and Ag, which is typical of black shales that are interpreted to be accumulating under reducing conditions. In the Anga unit, the amphibolites are interpreted as metaandesite-basalts and alkaline basalts with associated metalaterites. Anga unit gneisses, quartzites and marbles display an absence of graphite and low concentrations of Cr and V, and high Mn, P, F and Ba grading into gonditic compositions. These data indicate that these rocks were originally deposited under oxidizing conditions in shallow water or in an onshore environment. The geochemical features of both the metavolcanics and metasediments suggest that the Anga and Olkhon successions accumulated within a marginal basin to mature island arc tectonic setting. The structure of the Olkhon region is essentially nappe-thrust stack that has resulted from compressional thrust tectonics that was later transformed into strike-slip dominated tectonics. The structure and age (450–530 Ma) of the complex accord with a Caledonian collisional history. The entire volcanogenic-sedimentary sequence underwent folding and zonal metamorphism at 800–550±40°C and 8–5±1 kb due to the thrust tectonics. Metasomatic granitization and local melting caused the formation of K-leucogranites within the gneiss beds. These granites differ greatly geochemically from slab to slab, reflecting a heterogeneous parental composition, variable P– T conditions, and variable degrees of partial melting.