Andesite magmatism and its place in geological history of the Ol'khon area(Western Baikal region)

The geochemistry of biotite-amphibole schists and amphibolites of the Anga Sequence and lower formation of the Ol'khon Group was studied. According to relies of porphyritic textures and morphology of geological bodies, these rocks were attributed to metavolcanics. The composition and geochemical fea tures of these rocks, including REE distribution, Nb minimum, and Sr maximu m, are similar to medium-potassium andesites of the calc-alkaline series of mature island arcs formed on a relatively thick crust. The metaandesites a re intruded by hypabyssal gabbroids of the Ozersk Complex and granodiorite- granite massifs of the Khaidai Complex. The latter also belong to the calc- alkaline series according to their geochemical features. The whole rock ass ociation reflects a temporal evolution of island-are magmatism. The geochem ical affinity of metavolcanic and metasedimentary rock associations of the Anga Sequence acid lower formation of the Ol'khon Group allows us to consid er both as tectonic plates of the same stratigraphic unit.