Na Metasomatism in the Island-Arc Mantle by Slab Melt—Peridotite Interaction: Evidence from Mantle Xenoliths in the North Kamchatka Arc

Abstract The Pliocene (7 Ma) Nb-enriched arc basalts of the Valovayam Volcanic Field (VVF) in the northern segment of the Kamchatka arc, Russia, host abundant mantle xenoliths, including spinel Iherzolites. Textural and microstructural evidence for high-temperature, multi-stage, creep-related deformations in spinel Iherzolites supports a sub-arc mantle derivation. Pyroxene chemistry indicates the existence of two compositional suites: (1) a Cr-diopside suite with low-Tt, moderate-Al clinopyroxene compositions, and (2) an Al-augite suite with high Al and Tt, and low Cr concentrations in clinopyroxene. Some spinel lherzolite xenoliths contain metasomatic Al-augite-type clinopyroxene, Al-Tt spinel, and felsic veins similar to trondhjemite melt. The Al-augite series xenoliths typically contain high-Na plagioclase, Cr-poor, Al-Fe-Mg and Al-Tt-Fe spinels, with occasional almandine-grossularite garnet and high-Al and -Na pargasitic amphibole. Pyroxene and spinel compositional trends suggest that the Crdiopside series xenoliths from the VVF Nb-enriched arc basalts represent an island-arc mantle affected by a metasomatic event. Occurrence of high-Na plagioclase and trondhjemitic veins favors the addition of a metasomatic component with high Na, Al and Si to the northern Kamchatka arc mantle. Trondhjemitic veins, representing siliceous slab melts, compositionally exemplify the metasomatic component. Na metasomatism by peridotite-slab melt interaction is an important mantle hybridization process responsible for arc-related alkaline magma generation from a veined sub-arc mantle.