THE EOCENE MAGMATIC HISTORY OF THE EARLY CENTRAL ALEUTIAN ARC

The massive Aleutian Ridge has long been argued to have initiated at ~55-51 Ma and then been rapidly built along a fracture zone south of the dying Beringian margin arc The oldest age of the ridge in the central Aleutians has remained elusive with Rubenstone (1984) reporting an Ar/Ar plagioclase plateau age of 34.7±0.6 Ma with a high temperature tail suggesting an age of up to ~ 50 Ma on a Finger Bay Fm. flow from Adak Island in the Andreanof block (~76°W lat). The oldest reliable age on Adak is a new Ar/Ar groundmass age of 38.19±0.53 from a southern Adak mafic dike cutting the Finger Bay Fm., while the oldest existing U/Pb zircon age is a new 36.83±0.8 Ma age from the tholeiitic Finger Bay pluton that intrudes the Finger Bay Formation in northern Adak. The Finger Bay pluton has also yielded a biotite Ar/Ar age of 37.89±0.34 (Jicha et al. 2006). Chemical signatures of most Finger Bay volcanic rocks show them to be tholeiitic basalts to mafic andesites with light REE enrichment (La/Yb=3-8) and other features, including Nd isotopic ratios (eNd = 7.5-8.5), which overlap the characteristics of modern central Aleutian arc rocks. Notable differences with the younger lavas, like lower Th/La ratios (0.09 to 0.2), can be explained by a distinct subducted sediment component. The lack of LREE depleted Eocene island arc tholeiites or boninites is consistent with the oldest part of the central Aleutian arc having geochemical similarities to the modern arc and being chemically distinct from a Marianas style arc. The chemistry of arc-like gabbroic pyroxene granulite xenoliths in a ~ 7 Ma basalt on neighboring Kanaga Island, one of which has yielded an imprecise 47.8 ± 4.3 Ma Ar/Ar plagioclase age (oldest age yet reported in the arc) show a well-developed arc basement existed in the central Aleutians by the Eocene. The last Eocene magmatic event on Adak is characterized by the ~35-31 Ma calc-alkaline amphibole-bearing granitoid Hidden Bay pluton, whose mineralogical and chemical features are best explained as developing in a crust whose thickness might approach the modern estimate of ~37 km (Janiszewski et al., 2013). The magmatic contrast with the Finger Bay Formation rocks requires a late Eocene crustal thickening event that could coincide with an episode of forearc subduction erosion after which the magmatic front was displaced to the north.