40AR/39AR DATA IDENTIFY LATE EOCENE TO EARLY OLIGOCENE MAGMATISM ON AMATIGNAK AND ULAK ISLANDS, ALEUTIAN ARC, ALASKA

Amatignak and Ulak islands in the west central Aleutian arc near 179 degrees west are located closer (<175 km) to the trench than the active volcanoes of the modern Aleutian arc, and host remnants of older arc crust as magmatism has migrated northwards since inception. Nine new ⁴⁰Ar/³⁹Ar ages help establish a temporal framework for an investigation of the geochemical evolution of these islands. Pre-Miocene geochemical and geochronologic data for this region of the Aleutian arc are limited and our geochemical and isotope data are the first data available for Amatignak and Ulak Islands. In detail, the lavas and plutons from Amatignak Island have ⁴⁰Ar/³⁹Ar plateau ages ranging from 30.89 ± 0.22 to 36.83 ± 0.21 Ma, which overlap with the 29.26 ± 0.95 to 36.17 ± 0.46 Ma ages from Ulak. These ages coincide with those of lavas and plutons from other parts of the central and western Aleutian Islands supporting a major pulse of arc-wide magmatism in the Late Eocene to early Oligocene. Interestingly, the Amatignak and Ulak lavas have ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7031 to 0.7040, with the higher values being among the highest measured in the Aleutian Arc. The Epsilon Nd values in the same samples range from about +8 to +9 with the highest values generally correlated with the higher Sr isotopic ratios. The higher Sr isotopic values are best interpreted as reflecting hydrothermal alteration as the older of these lavas erupted into seawater. The Ulak lavas have more depleted REE patterns than those on Amatignak and display significant intra-island variability. Although the ⁴⁰Ar/³⁹Ar ages from both islands are similar, these islands preserve different evolutionary histories despite being only ~6km apart. Overall, the chemistry of these islands is transitional between the more arc-like pre-Miocene Aleutian rocks to the east and the less arc-like rocks to the west on Attu.