PETROGENESIS OF MAFIC AND ULTRAMAFIC ENCLAVES FROM KASATOCHI VOLCANO AND IMPLICATIONS FOR THE FORMATION OF NEW CRUST ALONG THE CENTRAL ALEUTIAN ARC

The textural, mineralogical, and chemical characteristics of mafic and ultramafic igneous enclaves found in lavas from the 2008 Kasatochi volcanic eruption are similar to those from neighboring Adak Island volcanoes. This suggests that sub-arc conditions are similar enough to form the same igneous "strata" along strike of this central section of the Aleutian chain. Petrographically, the Kasatochi enclaves are undeformed cumulates of 0.1–11 cm euhedral plagioclase and pargasitic hornblende crystals, with minor clinopyroxene and magnetite and cryptocrystalline interstitial glass. The minerals in the gabbro suite are typically elongate and aligned, in contrast with the granular textures of the ultramafic suite. The ultramafic samples include wehrlite, clinopyroxenite, and olivine clinopyroxenite with Fo_83–84 olivine, Mg- and Ca- rich clinopyroxene, and spinel. Pargasitic hornblende is present only as a secondary, interstitial phase in the ultramafic enclaves. The presence of hornblende and the lack of deformation textures in both ultramafic and mafic sample suites suggests that the samples were stored in the same pressure, temperature, and host magma composition prior to eruption. Furthermore, major-element mineral compositions from both suites reflect a non-mantle, cumulate origin with Ni-in-olivine <1600 ppm and Cr# in spinel <50. Cumulate and cognate gabbro xenoliths found at the westward-adjacent Adak Island volcanoes also contain plagioclase, pargasitic hornblende, clinopyroxene, and magnetite. Wehrlite and clinopyroxenite samples from Adak are also similar to those from Kasatochi, containing clinopyroxene, spinel, and interstitial pargasitic hornblende, as well as forsteritic olivine with <1900 ppm Ni. Although more work is needed to further clarify their relations, these similarities suggest that new crust forming below Kasatochi volcano is under the same conditions as that under Adagdak and Moffett volcanoes on Adak Island.