EVOLUTION OF THE CHEMICALLY DIVERSE OCEANIC ALEUTIAN ISLAND ARC
This history contrasts with the western Aleutians where the oldest age is on a non-arc-like Murray Canyon dredge sample (46.31±0.91 Ma). The oldest ages on Attu Island to the west are on an amphibolite (K-Ar 42.5 Ma) and volcanic rocks (Ar/Ar plag 40.28±0.12 Ma) that host 36-29.5 Ma gabbros. All have depleted LREE tholeiitic Marianas-like arc chemistry. To the west, a NE-trending band of more MORB-like mafic volcanics (Ar/Ar 35.6-29.3 Ma) is east of MORB-like rhyolite/albite granites (~19 Ma). An E-W 8-6 Ma line of hornblende-bearing andesites then erupted at a time of calc-alkaline volcanism along much of the arc. Older Attu rocks have higher eNd values than in the central Aleutians (to +9-10.8) and 87Sr/86Sr ratios fit with seawater hydrothermal interaction. Seismic data allow a ~ 28 km thick crust. Models to explain Attu’s setting range from westward translation and clockwise rotation to formation behind an extensional basin. The absence of adakites on the relatively intact Attu block contrasts with widespread adakites on more disrupted parts of the western ridge and is consistent with adakite formation in response to forearc subduction erosion rather than slab-melting.