INTRODUCTION
Volcanic oceanic plateaus are formed by immense volumes of magma emplaced in pre existing oceanic lithosphere or at spreading centers. Nearly all of the plateaus in the oceans today were formed in the Cretaceous period and may reflect a major mode of mass and energy transfer from the Earth's interior to its surface that was different from the ocean ridge-dominated mode of the Cenozoic (e.g., Stein and Hofmann, 1994; McNutt et al., 1996). Since the mid-1980s, oceanic 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 (LIPs) by 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 have been proposed (e.g., Smith, 1993; Anderson, 1996) but have thus far not received widespread support. The plume-head model predicts that LIPs are formed from ocean-island-like mantle in massive eruptive outpourings lasting only a few million years or less (e.g., Campbell, 1998). For many continental LIPs and at least some volcanic passive margins, eruption indeed probably occurred rapidly, but continental lithospheric contamination usually has overprinted the sublithospheric mantle-source signature. Most oceanic LIPs formed in locations remote from any continental lithosphere, but comparable data on eruption ages and source composition are lacking because very few basement sites have yet been sampled. Because of the thick sediments that blanket oceanic 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 appear to have important effects on subduction patterns, plate motions, continental growth, and crustal evolution. Large oceanic 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 in the western Pacific ( Fig. 1, Fig. 2) is the largest volcanic oceanic plateau in the world, with a crustal volume of ~5 x 107 km3 (Mahoney, 1987; Coffin and Eldholm, 1993). If the great bulk of this plateau 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 midocean-ridge system at the time. The plateau would then represent the largest igneous event of the last 200 m.y. (Tarduno et al., 1991; Mahoney et al., 1993).
The goal of Leg 192 was to sample the acoustic basement of the Ontong Java Plateau at four widely spaced sites, one of which was to be drilled at least 150 m and three at least 100 m into basement. Shipboard and shore-based studies of the rocks recovered would provide insights into the age and duration of magmatism, the compositional range of the mantle sources, and the processes of magmatic evolution. We also hoped to evaluate the environment and style of eruption and the association of plateau emplacement with changes in paleoceanographic and paleoclimatic conditions.