Petrogenesis of Mafic and Ultramafic Enclaves from the Central Aleutian Arc, and Implications for the Formation of New Crust

Abstract:

Mafic and ultramafic enclaves from the 2008 eruption of Kasatochi volcano, central Aleutians, provide insight into the sub-arc structure in this section of the subduction zone. Textural, mineralogical, and chemical similarities between these enclaves and those from neighboring Adak Island volcanoes suggest that sub-arc conditions are similar enough to form the same igneous "strata" in this part of the arc.

Kasatochi gabbroic enclaves are undeformed cumulates of 0.1–11 cm euhedral plagioclase and pargasitic hornblende crystals, with minor clinopyroxene and magnetite and cryptocrystalline interstitial glass. Adak gabbro inclusions also contain plagioclase, pargasitic hornblende, clinopyroxene, and magnetite. Gabbroic enclaves from both volcanic islands typically have elongate and aligned minerals, in contrast with the granular textures of the ultramafic suite. Kasatochi ultramafic samples include wehrlite, clinopyroxenite, and olivine clinopyroxenite with Fo_83–84 olivine, Mg- and Ca- rich clinopyroxene, and spinel, and pargasitic hornblende present only as a secondary, interstitial phase. Similarly, wehrlite and clinopyroxenite samples from Adak also contain forsteritic olivine, clinopyroxene, spinel, and interstitial pargasitic hornblende. The presence of hornblende and the lack of deformation textures in the cumulate gabbros from both islands suggest that these rocks were stored under similar pressure, temperature, and host-magma conditions prior to eruption.

Kasatochi gabbro enclaves are compositionally related to their host basaltic andesite, as suggested by fractionation trends. There is no apparent chemical relationship between the ultramafic enclaves and the 2008 basaltic andesite, nor are these samples ever found within their host rock. 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. Whole-rock REE analyses show moderate depletions in Nb and Ta, slight depletions Th, Sr, and Sm, and slight enrichments in Ba, Pb, and Pr.

Although more work is needed to further clarify their relations, similarities between Kasatochi and Adak volcanic enclaves suggest that new crust is forming under similar conditions below both islands.