A MAJOR PULSE OF LATE EOCENE/EARLY OLIGOCENE SUBMARINE AND SUBAERIAL MAGMATISM IN THE CENTRAL AND WESTERN ALEUTIAN ARC

Thirty four new ⁴⁰Ar/³⁹Ar and U/Pb zircon ages from eight Central and western Aleutian Islands supplement existing geochronologic data and provide new information on the history and evolution of the early central and western Aleutian arc. A clast from a volcanic sequence in southeastern Adak in the Finger Bay Volcanic Formation, interpreted to be the oldest unit on central Aleutian Islands, yields an age of 38.19 ± 0.53 Ma. The ⁴⁰Ar/³⁹Ar ages of the Finger Bay Volcanics are virtually indistinguishable from new ⁴⁰Ar/³⁹Ar and U/Pb zircon ages from the Finger Bay pluton. This age is similar to previous ages from this unit and to new 38-39 Ma lavas on Kiska and Amchitka islands. It is important to note that the Finger Bay Volcanics on Adak are not related to the initiation of the central Aleutian arc as these rocks have meteoric not sea water alteration and thus the arc must have been built above sea level when these lavas erupted. In contrast, the 39-31 Ma lavas on Kiska, Ulak, Amatignak, and Attu islands in the west central Aleutians, west of Adak have elevated ⁸⁷Sr/⁸⁶Sr ratios (up to 0.7045) in association with Epsilon Nd values of +7.5 to 9. We interpret this to reflect hydrothermal alteration of lavas that were submarine eruptions as the western Aleutian arc had not yet reached sea level. Northeast of Adak Island, lavas previously mapped as the Finger Bay Volcanics group on southern Great Sitkin Island range from 10.2 to 3.2 Ma in accord with northward migration of the arc front with time. A new ⁴⁰Ar/³⁹Ar age of 34.35 ± 0.05 Ma for a granodiorite in the calc-alkaline Hidden Bay pluton on Adak is in accord with new U/Pb zircon ages in this sample, which also importantly shows no evidence of older zircons. The central Adak Gannett Lake pluton, which was assumed to have an age like the ~14 Ma Kagalaska Island pluton to the east, yields an age of 31.68 ± 0.06 Ma. Thus, the new age determinations indicate that calc-alkaline plutonism lasted for more than 3 Ma on Adak Island and is part of a 38-29 Ma arc-wide period of volcanism and plutonism. The presence of continental like calc-alkaline plutons dominated by granodiorite after ~35 Ma and a lack of low-K island arc tholeiites and boninites paints a very different picture for the evolution of the central Aleutian arc than for western Pacific arcs.