EVOLUTION AND GEOCHRONOLOGY OF THE YOUNGEST ERUPTIVE CENTER IN THE BERING SEA BASALT PROVINCE: ST. PAUL ISLAND, PRIBILOF ISLANDS, ALASKA

The evolution of a Pleistocene to Holocene basaltic volcanic field is investigated at St. Paul Island, Alaska. St. Paul Island, the youngest eruptive center in the Bering Sea basalt province, is located in the Aleutian back-arc region in a transtensional tectonic environment on the southern edge of the Bering Sea shelf. New ⁴⁰Ar/³⁹Ar and ¹⁴C age determinations indicate that the oldest subaerial lava flows on St. Paul occurred as early as 450 Ka and the most recent eruption occurred at Fox Hill on southwest St. Paul approximately 3,230 yr B.P. Because of the young age of its most recent volcanic activity, future eruptions at St. Paul are probable. Magmas erupted are basaltic with MgO contents ranging from 14 to 4 wt% and phenocryst assemblages of olivine +/- clinopyroxene and plagioclase; all are alkalic. Based on geochronology, stratigraphy, and geochemistry, eruptive styles on St. Paul have evolved from early, mostly effusive eruptions of primitive lavas that form the base of the island, to more explosive monogenetic cinder cones, to polygenetic centers that are forming shields from repeated eruptions of evolved low viscosity lavas. Layered lava flows that form the faulted base or platform of St. Paul are interpreted as the products of eruptions from fissures and inferred low shields typical of plains style volcanism. Locations of monogenetic cinder cones that overlie the platform lavas appear to be controlled by east-west and northeast-southwest trending fissure systems. Polygenetic centers are located at intersections of volcanic and structural lineaments where Hawaiian style eruptions are forming growing tube-fed shields. St. Paul’s volcanic system as a whole is trending toward the progressive development of shallow crustal magma chambers where cooling and differentiation are occurring.