ANIAKCHAK: A CASE STUDY OF ALEUTIAN VOLCANISM

Aniakchak Volcano is located at 158^o10’ W in the Aleutian volcanic arc, east of the 165^o W suture between continental and oceanic crust. The Holocene caldera is 10 km in diameter and 1 km deep. There were several post-caldera eruptions, the youngest in 1931. Samples were collected from both pre-caldera and post-caldera (including the 1931 flow) flows, domes, and plugs within the caldera. Three samples collected from the caldera walls or floor are tholeiitic basaltic andesites and andesites (SiO₂=53.5 – 61.2%; Al₂O₃=15.8 – 17.1%, CaO=5.6 – 8.5%; MgO=4.2 – 2.4%; K₂O=1.6=2.0%) and represent pre-caldera lavas. They contain plagioclase, augite, +/- orthopyroxene, +/- olivine. Four samples are post-caldera calc-alkaline two-pyroxene dacites (SiO₂=64.7 – 67.3%; Al₂O₃=15.3 – 15.7%, CaO=3.6 – 4.2%; MgO=1.3 – 1.6%; K₂O=2.3 – 2.4%). Pre-caldera lavas have lower SiO₂ and K₂Oand higher Al₂O₃ and CaO than post-caldera lavas. One sample is a granitic xenolith from a pre-caldera flow exposed in the caldera wall. Mineralogically, the granitic xenolith contains stressed quartz, orthoclase, plagioclase, augite, biotite, and epidote. Pre-caldera and post-caldera samples fall on trends of different slopes on TiO₂-SiO₂, MnO-SiO₂, and Al₂O₃-SiO₂ plots. This suggests different differentiation mechanisms during pre- and post-caldera building stages, as observed by Nye et al. (1995: GSA Cordilleran Abstracts, p. 69). The granitic xenolith falls off major and trace element trends, with significantly higher Nb (xenolith=16 ppm; lavas=2 – 6 ppm) and lower Zr (xenolith=111 ppm; lavas=157-222 ppm) and Y (xenolith=14 ppm; lavas=38-43 ppm) than Aniakchak lavas. These differences, the stressed quartz, and the presence of hydrothermal epidote together suggest that the xenolith is a sample of pre-Aniakchak plutonic crust. The continental volcanoes in the Aleutian region (including Aniakchak) contain higher K₂O, Na₂O, and SiO₂, and lower CaO, MgO, and FeO than the oceanic volcanoes. In addition, the continental volcanoes are calc-alkaline, in contrast to the tholeiitic nature of the oceanic volcanoes. Aniakchak is unusual because it erupted both tholeiitic and calc-alkaline lavas.