THE IMPORTANCE OF GEOCHEMICAL DATA FOR GEODYNAMIC RECONSTRUCTION - FORMATION OF THE OLKHON METAMORPHIC COMPLEX, LAKE BAIKAL, RUSSIA

The Olkhon metamorphic complex consists of two geochemically distinct units-the Olkhon unit and the Anga formation. The basic crystalline sc hists 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 inte rpreted to be accumulating under reducing conditions. In the Anga unit , the amphibolites are interpreted as metaandesite-basalts and alkalin e basalts with associated metalaterites. Anga unit gneisses, quartzite s 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. T he geochemical features of both the metavolcanics and metasediments su ggest that the Anga and Olkhon successions accumulated within a margin al basin to mature island are tectonic setting. The structure of the O lkhon 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 co mplex accord with a Caledonian collisional history. The entire volcano genic-sedimentary sequence underwent folding and zonal metamorphism at 800-550 +/- 40 degrees 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 c omposition, variable P-T conditions, and variable degrees of partial m elting. (C) 1998 Elsevier Science B.V. All rights reserved.