Ultramafic Xenoliths from Adagdak Volcano, Adak, Aleutian Islands, Alaska: Deformed Igneous Cumulates from the Moho of an Island Arc

Deformed ultramafic xenoliths with basaltic salvages occur in a postglacial ejecta deposit from Adagdak volcano on Adak Island in the central Aleutian Island Arc. These dunites, wehrlites, clinopyroxenites, and amphibolites are interpreted to be cumulates from primitive arc magmas that formed in magma chambers beneath the crust-mantle boundary. The primitive arc magmas are similar in composition to the xenolith's host basalt (11.8% MgO). Trace element and Nd and Sr isotopic data are consistent with derivation of these xenoliths from the same parent as other Aleutian calc-alkaline volcanic rocks. The peridotite xenoliths contain olivine (F086.6 to 83.2), clinopyroxene (FS4.1En46.4Wo49.5 to FE5.2En46.6Wo48.2), Cr-Al spinel (19-30% Cr₂O₃ and 30-42% Al₂O₃), and secondary pargasitic amphibole that increase in FeO content as the percentage of clinopyroxene increases. These xenoliths form the necessary residuum to produce the high-Al basalt predicted in an open system mixing model for Aleutian calc-alkaline volcanism. The recrystallized textures of the peridotites, along with high equilibration temperatures (> 1000°C), suggest that the upper mantle asthenospheric flow regime reaches the crust-mantle boundary region.