INTRODUCTIONVolcanic oceanic plateaus are formed by immense volumes of magma emplaced in the oceanic lithosphere. Nearly all of the plateaus in the present oceans were formed during the Cretaceous period and may reflect a major mode of mass and energy transfer from the Earth's interior to its surface that is different from the mid-ocean ridge-dominated model of the Cenozoic (e.g., Stein and Hofmann, 1994; McNutt et al., 1996). Since the mid-1980s, plateaus have been recognized as the counterparts of continental flood basalt provinces and associated thick volcanic sequences at many passive continental margins, collectively termed large igneous provinces, or LIPs (e.g., Coffin and Eldholm, 1994). In the last decade, these features have been ascribed by many workers to the initial plume-head stage of hot-spot development (e.g., Richards et al., 1991; Saunders et al., 1992). Alternative, nonplume models exist (e.g., Smith, 1993; Anderson, 1996) but have thus far not received widespread support. The plume-head model, in particular, predicts that such provinces are formed from ocean-island-like mantle in massive eruptive outpourings lasting only a few million years or less. For many continental flood basalts and at least some volcanic passive margins, eruption appears to have occurred rapidly, but melting in the continental lithosphere has often overprinted the signature of the sublithospheric mantle source. Many plateaus appear to have been formed in intraoceanic locations far from any continental lithosphere; however, comparable data on eruption ages and source geochemistry are lacking because very few crustal basement sites have yet been sampled. Because of the thick sediment blankets that cover plateaus, drilling is generally the only way to sample basement crust effectively.
The climatic, oceanographic, and associated biospheric effects of plateau emplacement are poorly known but appear to have been very significant in some cases (e.g., Jones et al., 1995; Kerr, 1998; Tarduno et al., 1998). After emplacement, plateaus also appear to have important effects on subduction patterns, plate motions, continental growth, and crustal evolution; large plateaus, in particular, tend to resist subduction and thus may form an important early stage in the growth of continents (e.g., Kroenke, 1974; Cloos, 1993; Tejada et al., 1996; Albarède, 1998; Wessel and Kroenke, 1999; Polat et al., 1999). The Ontong Java Plateau (OJP) in the western Pacific (Fig. 1) is the largest plateau, indeed the largest existing LIP, in the world, with a crustal volume of approximately 5 x 107 km3 (e.g., Mahoney, 1987; Coffin and Eldholm, 1993). If the great bulk of the OJP was formed in a single geologically brief magmatic episode, then the rate at which it was emplaced would have rivaled the entire magma production rate of the global mid-ocean ridge system at the time; if so, the OJP represents the largest igneous event of the last 200 m.y. (Tarduno et al., 1991; Mahoney et al., 1993).
The goal of Leg 192 is to sample the basement of the OJP to minimum depths of 100-150 m at four widely distributed sites. The rocks recovered will be used, together with data for previously studied sites, to determine the age and duration of magmatism, the composition of the mantle source(s), and the characteristics of magmatic evolution. They will also be used to evaluate the environment and style of eruption with special reference to the association of OJP emplacement with changes in paleoceanographic and paleoclimatic conditions.
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