THE FORMATION OF NEW CRUST ALONG THE CENTRAL ALEUTIAN ARC—INSIGHTS FROM KASATOCHI VOLCANO

The Aleutian arc forms the present-day southwestern margin of Alaska, and insight into its unexposed “roots” may reveal processes that are related to forming new crust in island arcs. The Aleutian island arc includes oceanic and continental portions, and understanding the sub-arc petrology and structure is essential for determining the stability of arc lower crust. Mafic and ultramafic enclaves from the 2008 eruption of Kasatochi volcano, central Aleutians, provide insight into the sub-arc structure in this part of the arc. Textural, mineralogical, and chemical similarities between these enclaves and those from neighboring volcanoes suggest that sub-arc conditions are similar enough to form the same igneous "strata" throughout the central section of the Aleutian arc. Kasatochi gabbroic enclaves are undeformed cumulates of euhedral plagioclase and pargasitic hornblende crystals, with minor clinopyroxene, magnetite, and cryptocrystalline interstitial glass. They typically have elongate and aligned minerals. The ultramafic enclaves, in contrast, are granular-textured wehrlite, clinopyroxenite, and olivine clinopyroxenite with Fo_83–84 olivine, Mg- and Ca- rich clinopyroxene, spinel, and pargasitic hornblende present only as a secondary, interstitial phase. Gabbro and ultramafic samples from neighboring Adak Island have similar textures and compositions. The presence of hornblende and lack of deformation textures in the mafic samples from both islands suggest that they were stored in the same pressure, temperature, and host-magma composition conditions prior to eruption. Kasatochi gabbros are likely cumulates formed during differentiation of their andesitic host lavas. Based on compositional similarities to ultramafic xenoliths from Adak Island, the Kasatochi ultramafic suite could have formed by the fractionation of spinel-lherzolite in the upper mantle. Recent seismic studies combined with petrologic modeling suggest that ultramafic cumulates, such as those sampled by the central Aleutian arc, may comprise up to 40% of the Aleutian crust. More work is needed to interpret the sub-arc petrology, clarify the similarities between enclaves from Kasatochi and other Aleutian volcanoes, and better constrain the composition of new crust forming beneath the arc.