Ultramafic and Mafic Inclusions from Adak Island: Crystallization History, and Implications for the Nature of Primary Magmas and Crustal Evolution in the Aleutian Arc

Abstract

Xenolithic inclusions in calc-alkaline andesite from Mt. Moffett volcano, Adak Island, Aleutian arc, reveal a nearly continuous record of crystallization of basaltic magmas in the crust, and possibly upper mantle, of the arc. The record is more detailed and continuous than that obtained from study of calc-alkaline volcanic rocks in the arc. Cumulate xenoliths form a progressive series in modal mineralogy from ultramafic, hornblende-bearing olivine clinopyroxenite to both hornblende-bearing and hornblende-free gabbros. The cumulate hornblende gabbro xenoliths are typical of those found in island arc andesites worldwide. Xenolithic inclusions without cumulate textures, here termed composite xenoliths, are characterized by forsteritic olivine, zoned Cr-diopside and hornblende, and are interpreted to have resulted from reaction and chilling upon magma mixing at depth.