PROGRESSIVE ENRICHMENT OF ISLAND-ARC MANTLE BY MELT-PERIDOTITE INTERACTION INFERRED FROM KAMCHATKA XENOLITHS

The Pliocene (7 Ma) Nb-enriched are basalts of the Valovayam Volcanic Field (VVF) in the northern segment of Kamchatka are (Russia) host abu ndant xenoliths of spinel peridotites and pyroxenites. Textural and mi crostructural evidence for the high-temperature, multistage creep-rela ted deformations in spinel peridotites supports a sub-are mantle deriv ation. Pyroxenites show re-equilibrated mosaic textures, indicating re crystallization during cooling under the ambient thermal conditions. T hree textural groups of clinopyroxenes exhibit progressive enrichment in Na, Al, Sr, La, and Ce accompanied by increase in Sr/Y, La/Yb, and Zr/Sm. Trace elements in various mineral phases and from felsic veins obtained through ion microprobe analysis suggest that the xenoliths ha ve interacted with a siliceous (dacitic) melt completely unlike the ho st basalt. The suite of xenoliths grade from examples that display lit tle evidence of metasomatic reaction to those containing an assemblage of minerals that have been reproduced experimentally from the reactio n of a felsic melt with ultramafic rock, e.g., pargasitic amphibole, a lbite-rich plagioclase, Al-rich augite, and garnet. The dacitic veins within spinel Iherzolite display a strong enrichment in Sr and depleti on in Y and the heavy rare earth elements (e.g., Yb). The dacites are comparable to adakites (melts derived from subducted metabasalt), and not typical are melts. We believe that these potential slab melts were introduced into the mantle beneath this portion of Kamchatka subseque nt to partial melting of a relatively young (and hot) subducted crust. Island are metasomatism by peridotite-slab melt interaction is an imp ortant mantle hybridization process responsible for are-related alkali ne magma generation from a veined sub-are mantle.