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

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. The olivine and clinopyroxene in both cumulate and composite xenoliths show the largest and the most complete variation trends for Ni, Cr, and FeO/MgO ratio yet reported in igneous xenoliths from island arc volcanic rocks. Variation of Ni in olivine indicates that the parent magmas for the xenoliths had minimum MgO contents of 9 wt. per cent. Variation of Cr in clinopyroxene indicates that the magmas were basaltic rather than picritic, probably in equilibrium with spinel lherzolite at near Moho depths. Successive injections of batches of primary melt into a magma chamber fractionating olivine and clinopyroxene can reproduce observed compatible element depletion trends. A steady-state process of cotectic crystallization in a magma chamber continually replenished with basaltic magma is a possible mechanism for producing large accumulations of olivine and clinopyroxene, suggesting that Alaskan-type ultramafic complexes are related to hydrous basaltic magmas in island arcs. This steady-state open-system crystallization process can also yield the abundant high-alumina basalt type in the Aleutian arc. Continued crystallization of high-alumina basalt in lower crustal magma chambers, recorded in a mineralogically coherent series of pyroxenite to hornblende gabbro xenoliths, can yield basaltic to andesitic magmas of the calc-alkaline series. No xenoliths with a sedimentary protolith have been found at Mt Moffett, evidence that the arc crust is igneous in origin, with the lower crust formed of gabbro crystallized from mantle-derived melts. Ultramafic cumulates may reside in both the lower crust or upper mantle beneath the arc. A model is proposed whereby the cumulate crystallization products of hydrous, mantle beneath the arc. A model is define the upper mantle and lower crust of the arc over time.The net composition added to the crust of the arc is that of high-alumina basalt.