Structure, composition, and formation conditions of metasedimentary-volcanogenic complexes of the Kan greenstone belt (northwestern Sayan region)

In the northwestern Sayan region, within the Kan block - a fragment of the granite-greenstone province on the southwestern margin of the Siberian crat on, - two greenstone belts, Idar and Kan, have been revealed. Study of the geologic sections and composition of the suite of the Kan greenstone belt ( GSB) and reconstruction of the protoliths of metamorphic rocks by the wide spectrum of trace, radioactive, and rare-earth elements have shown that roc ks of tholeiite-basalt and andesite-dacite-rhyodacite associations are pred ominant in the metasedimentary-volcanogenic section. The overlapping metate rrigenous sediments include graywackes and aluminous pelites. The revealed types of rock associations and specific features of the trace-element compo sition of metasedimentary-volcanogenic rocks are typical of ancient greenst one belts. The complexes of the Kan GSB might have formed in the setting of an ensialic island arc, whose evolution was complicated by a zone of back- arc spreading. Results of the first isotope U-Pb and Ar-Ar dating of minera ls show that the volcanic rocks of the Kan GSB accumulated in the Early Pro terozoic and underwent intense metamorphism in the Late Vendian. The trondj emites of the Upper Kan Massif also formed in the Late Vendian.