EVOLUTION OF CRUST AND MANTLE BENEATH THE BERING SEA REGION: EVIDENCE FROM XENOLITHS AND LATE CENOZOIC BASALTS

Abstract

Late Cenozoic volcanic centers throughout the Bering Sea region of Alaska consist of small-volume eruptions of flows and cones of tholeiitic to alkaline basalt. Peridotite (spinel lherzolite, websterite, wehrlite, and pyroxenite) and gabbro xenoliths are abundant in some of the most alkaline flows and cones. Spinel lherzolite xenoliths are characterized by granuloblastic-equant and coarse-equant textures typical of mantle peridotites and yield equilibration temperatures of 1000-1200 °C (1.5 GPa). Relative to other spinel lherzolites worldwide, these xenoliths have low MgO and high Fe2O3 T and are similar to some modeled compositions for fertile mantle. However, these xenoliths are relatively enriched in the light rare earth elements (REE), Th, U, K, Nb, and Ta. Rare amphibole peridotite xenoliths have textures and compositions that are consistent with an origin as pyroxenite veins in the mantle that were variably deformed and metasomatized with potassium-rich fluids. These xenoliths have concave-up light REE patterns similar to those of the host basalts, suggesting that the alkaline melts may have been produced by melting of amphibole-bearing mantle peridotites. Gabbroic xenoliths are variably deformed and recrystallized at relatively high pressure (0.4-0.6 GPa). Trace element ratios (e.g., Ba/Nb, La/Nb) indicate that these xenoliths crystallized from melts that were produced in a suprasubduction-zone setting and suggest that they represent cumulates that formed in the lower crust during Late Cretaceous subduction. We suggest that these cumulates are the source of the strong reflections that are observed in seismic reflection profile images of the Bering Sea region. Volcanic flows consist of tholeiite, olivine tholeiite, alkali-olivine basalt, and basanite, whereas cones consist mostly of alkali-olivine basalt and basanite. Geochemical variations within each volcanic field largely reflect the effects of fractionation and melting (depth and fraction of melting), whereas between-field geochemical differences are largely due to source composition differences. The enriched incompatible trace element compositions (light REE, Ba, Th, Nb, Ta) and ratios (Th/Ta, Rb/Zr, Nb/La, La/Yb) are similar to intraplate basalts from ocean island and continental extensional settings. In particular, the compositions of Bering Sea basalts are similar to those of the diffuse volcanic province of Southeast Asia, a region where late Cenozoic volcanism has been related to tectonic extrusion along reactivated strike-slip faults.