PETROLOGY AND ISOTOPIC COMPOSITION OF QUATERNARY BASANITES DREDGED FROM THE BERING SEA CONTINENTAL-MARGIN NEAR NAVARIN BASIN

Quaternary basanites were recovered from shallow water depth from the continental margin of the Bering Sea (58-degrees-39.0'N, 177-degrees-1 2.9'W) near Navarin Basin. The basanites are highly vesicular flow roc k and hyaloclastites similar to other alkalic volcanic rocks erupted r epeatedly during the late Cenozoic on islands in the Bering Sea region and in mainland Alaska. K-Ar ages for the basanites indicate at least two episodes of volcanism at about 1.1 and 0.4 Ma. Similar alkalic vo lcanism occurred sporadically at geographically widely separated cente rs in the Bering Sea region for at least the past 6 Ma. Chemically, th ese alkalic lavas are intraplate basalts similar to those erupted from oceanic islands and in some continental settings. Trace-element data indicate these alkalic lavas have been generated by small, but variabl e, amounts of partial melting of a metasomatized lherzolite source. Th e relatively primitive compositions (MgO > 9%), presence of mantle-der ived xenoliths in some alkalic lavas, and presence of forsteritic oliv ine with low CaO and high NiO suggest that magma rose rapidly from gre ath depth without spending time in large, long-lived magma chambers. A lthough lavas from different volcanic centers in the Bering Sea region are similar with respect to major elements and many trace-element rat ios, isotopic compositions indicate heterogeneities in the source. The Navarin basanites have higher Sr-87/Sr-86 and lower Nd-143/Nd-144 val ues than any other lavas so far reported from this region. The Pb-207/ Pb-204 isotopic ratios indicate involvement of a crustal component, wh ich may have resulted from metasomatism associated with subduction-rel ated magmatic activity during the Early Eocene in this region. Althoug h some volcanic episodes appear to have occurred roughly synchronously at geographically widely separated centers, no large-scale regional e xtension nor presence of large mantle plumes are indicated. Instead, a lkalic volcanism apparently resulted from upwelling and decompressiona l melting of small isolated mantle diapirs in response to local lithos pheric attenuation associated with jostling of blocks during adjustmen t to regional stresses.