Late Ordovician Volcanism of the Northern Part of Altai–Sayan Area and Its Geodynamic Nature

Resumo

The results of geological–geochemical studies of some Late Ordovician associations in the frame of the Minusinsk Trough with available geological and U–Pb, Rb–Sr, and K–Ar age dates are presented. The Late Ordovician volcanic rocks form a continuous igneous series, the basalts of which are different from Devonian basalts of the Minusinsk Trough in the lower TiO₂ content (≥ 1.7 wt %) and more fractionated REE pattern. These features should be considered the index characteristics of Late Ordovician rocks. Their compositions reflect processes of fractionation crystallization and mixing of trachibasaltic magmas with crustal melts. When taking into account the regional geological data, it is shown that magmatic activity at the Late Ordovician endogenic evolution stage in the northern part of the Altai–Sayan fold area was caused by the interaction of a mantle plume and the lithospheric mantle, which was metasomatically reworked and enriched in water during former subduction processes.