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°39.0′N, 177°12.9′W) near Navarin Basin. The basanites are highly vesicular flow rock and hyaloclastites similar to other alkalic volcanic rocks erupted repeatedly 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 volcanism occurred sporadically at geographically widely separated centers in the Bering Sea region for at least the past 6 Ma. Chemically, these 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 variable, amounts of partial melting of a meta-somatized lherzolite source. The relatively primitive compositions (MgO > 9%), presence of mantle-derived xenoliths in some alkalic lavas, and presence of forsteritic olivine with low CaO and high NiO suggest that magma rose rapidly from greath depth without spending time in large, long-lived magma chambers. Although lavas from different volcanic centers in the Bering Sea region are similar with respect to major elements and many trace-element ratios, isotopic compositions indicate heterogeneities in the source. The Navarin basanites have higher 87 Sr/ 86 Sr and lower 143 Nd/ 144 Nd values than any other lavas so far reported from this region. The 207 Pb/ 204 Pb isotopic ratios indicate involvement of a crustal component, which may have resulted from metasomatism associated with subduction-related magmatic activity during the Early Eocene in this region. Although some volcanic episodes appear to have occurred roughly synchronously at geographically widely separated centers, no large-scale regional extension nor presence of large mantle plumes are indicated. Instead, alkalic volcanism apparently resulted from upwelling and decompressional melting of small isolated mantle diapirs in response to local lithospheric attenuation associated with jostling of blocks during adjustment to regional stresses.