VERY COARSE-GRAINED PLAGIOCLASE-CPX-HORNBLENDE ORTHOCUMULATE XENOLITHS FROM EAST TANAGA VOLCANO, WESTERN ALUETIAN ISLANDS, ALASKA

Major element concentrations in plagioclase, CPX, and amphibole from olivine and Cr-spinel-free orthocumulate xenoliths and host vesicular basalt scoria from East Tanaga Volcano are presented here and serve to enhance our understanding of the lower crust beneath the western Aleutian arc. Xenoliths are up to 20 cm in diameter and display two main textures: medium-grained equigranular orthocumulate texture and very coarse-grained porphyritic texture, although some xenoliths display both. Plagioclase is the dominant mineral phase in both xenoliths (65-75%) and host scoria. Whereas xenolith plagioclase is typically euhedral, 1-5 mm in diameter, and unzoned (An 90.4-95.3) with rare 50-100 micron diameter melt inclusions, plagioclase in host basaltic scoria occurs as oscillatory zoned and dusty sieved plagioclase phenocrysts (An 55.4-78.8) as well as skeletal and swallow-tail microlites (An 60.8-70.9). Plagioclase in basaltic scoria have more FeO and less Sr compared to xenolith plagioclase. CPX is the second most common mineral phase in xenoliths (20-25 modal %), where it occurs almost exclusively as anhedral crystals situated within interstices of cumulate plagioclase (Mg# 74.2-83.5) and opaque minerals. CPX also exists as rare phenocrysts in the host scoria (Mg# 82.0-84.6). Despite differences in Mg# between xenolith and scoria CPX, all CPX crystals are high aluminum diopside (~7 wt% Al₂O₃, ~0.2 wt % Na₂O) with an average composition of En 40, Fs 15, Wo 45. All amphibole (5-15%) are unzoned magnesiohastinsite (Mg# 85.7-91.4). Amphibole typically occurs as large (up to 12 cm long) elongate phenocrysts in very coarse-grained porphyritic xenoliths, although some 2-5 mm equant amphibole phenocrysts and plagioclase-hosted inclusions were also observed. No amphibole was observed in basaltic scoria. Estimates for crystallization conditions from a variety of amphibole and amphibole-plagioclase geothermobarometry models yield temperatures of 1020-1045°C and pressures that correspond to depths of 25-31 km. These depths generally coincide with proposed Moho depths based on geophysical techniques, supporting the idea that the lower crust beneath the western Aleutian volcanic arc may indeed be comprised of mafic residua from high-pressure and hydrous crystal-liquid fractionation.