EVOLUTION OF THE CHEMICALLY DIVERSE OCEANIC ALEUTIAN ISLAND ARC

Modern Aleutian arc volcanoes sit on a massive ridge that is generally considered to have built up rapidly in the Eocene after the arc initiated at ~55-51 Ma. The oldest dated rock is a Central Aleutian Kanaga gabbroic granulite xenolith (47.8±4.3 Ma, plag Ar/Ar) with an Alaskan type igneous complex affinity. Evidence for this part of the arc being above sea level and having a relatively thick crust by the Eocene comes from the 34.5-31 Ma calc-alkaline Hidden Bay pluton on Adak Island, which sits on a > 35 km thick modern arc crust. This pluton cuts the lower greenschist Finger Bay Fm. (Ar-Ar 38-32) that has meteoric oxygen isotopes and hosts the ~38 Ma tholeiitic Finger Bay pluton. The subsequent intrusion of the ~14 Ma calc-alkaline Kagalaska pluton to the north and the eruption of the ~11.7 Ma MORB-like Aleutian adakite are interpreted to reflect arc migration in response to forearc subduction erosion with adakite formation being due to melting of Cretaceous ocean crust subducted into the mantle wedge. The arc then stepped northward to where ~7-5 Ma eruptions occurred just south of the Plio-Pleistocene to Recent tholeiitic to calc-alkaline volcanic arc front.

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.