PETROLOGY OF THE TUMANSHET ZONAL METAMORPHIC COMPLEX, EASTERN SAYAN

The Tumanshet Complex fills a Proterozoic basin and rests with stratig raphic unconformity on the boundary between an Early Precambrian green stone belt and the migmatite-granulite complex of the Siberian craton. Petrographic study revealed metamorphic zonation in the Tumanshet Com plex. The thermobarometry of mineral equilibria based on the zoning of coexisting minerals locates temperature (up to 680 degrees C) and pre ssure (up to 7 kbar) maxima in the central part of the basin. On the g eological map, these maxima coincide with the gravity step which marks the deep-seated fault separating the greenstone belt and migmatite-gr anulite complex. The formation of the deep basin on this boundary is r elated to the influence of mantle plumes in the Proterozoic (similar t o 1750 Ma).