ULTRAMAFIC XENOLITHS IN ISLAND-ARC VOLCANICS OF VARIOUS GEOCHEMICAL TYPE

Ultramafic xenoliths were studied in two geochemically different types of their host volcanic rocks (suprasubduction and within-plate volcan ics) using the example of the Kuril-Kamchatka-Koryak island are system and the adjacent continental margin (Chukot Peninsula). The ultramafi cs related to the suprasubduction volcanic rocks comprise two associat ions: (1) a dunite-harzburgite association, which is depleted in low-m elting components and lithophile elements, and (2) a pyroxenite-wehrli te-cortlandite association with widely occurring processes of Na metas omatism. Judging from the chemistry of minerals and mineral assemblage s in rocks of both associations, these rocks correspond to the plagioc lase depth facies. Xenoliths in the within-plate volcanics consist of dunite, Iherzolite, clinopyroxene olivinite, and ferrous harzburgite i n association with pyroxenite. A complex of features indicates that th ese rocks are intermediate between enclaves in within-plate volcanics (the depth facies of spinel Iherzolite) and xenoliths in suprasubducti on volcanics (plagioclase depth facies) and show evidence of active K- Na metasomatism. A systematic correlation was detected between the typ e of volcanics hosting the enclaves and the composition, type, and set ting of the abyssal xenoliths.