RAPID BUILDUP OF THE CHEMICALLY DIVERSE ALEUTIAN RIDGE IN THE EOCENE

The modern Aleutian arc volcanoes sit on the massive Aleutian Ridge that has long been recognized to have been built rapidly in the early history of the arc in the Eocene. The leading model for arc initiation is that subduction started along a fracture zone after arc volcanism ceased on the Beringian Margin to the north at ~55-51 Ma. Attempts to find the oldest Aleutian rocks have been elusive with the oldest age being a plagioclase Ar/Ar date of 47.8±4.3 Ma from a central Aleutian gabbroic granulite xenolith from Kanaga Island, whose characteristics are like those of Alaskan type arc basement complexes. The lack of older zircons found to date is consistent with there being no pre-Eocene basement except the pre-existing Cretaceous oceanic crust upon which the arc was built. Evidence that the central Aleutian crust was above sea level and relatively thick by the end of the Eocene comes from the ~34.6-30.9 Ma calc-alkaline Hidden Bay plutonic rocks on Adak Island, which have many similarities to continental margin plutonic rocks. The pluton cuts the voluminous lower green-schist facies volcanic and sedimentary rocks of the Finger Bay Formation, which host the ~38 Ma tholeiitic Finger Bay pluton. The metamorphism of these rocks is considered to be associated with meteoric water (Rubenstone, 1984) with most rocks assigned to the Finger Bay Formation yielding Ar/Ar ages from 38 to 33 Ma. Seismic observations (Janiszeski et al., 2013) allow the modern Adak crust to be ~37 km thick with ~7 km of that likely being the underlying oceanic crust. The oldest ages in the western Aleutians include an Ar/Ar plagioclase date of 40.28±0.12 from the volcanic host rocks of 35-29 Ma oceanic-ridge-like gabbroic plutons on Attu Island, which together with pre-29 Ma MORB-like pillow lavas and dikes make up much of the island. Seismic data suggust a ~28 km thick modern crust. A dredge sample lacking an arc chemical signature from Murray Canyon to the east has an age of 46.31±0.91 Ma (Jicha et al., 2006). The heterogeneous chemistry of the Eocene and earliest Oligocene magmatic units along the arc reveals the complex early history of the rapid construction of the Aleutian Ridge.