) Ma.
One of the most exciting discoveries of ODP Leg 192 was a thick succession of basaltic volcaniclastic rocks at Site 1184 on the eastern salient of the OJP (Fig. F1). Drilling at this site penetrated 337.7 m of tuff and lapilli tuff before the site had to be abandoned because of time restraints. A detailed volcanological study by Thordarson (2004) concludes that the volcaniclastic succession was the result of large phreatomagmatic eruptions in a subaerial setting. This setting contrasts strikingly with that of the lava flows sampled on the main plateau and in the Solomons, which were all erupted under deep water (Roberge et al., 2004; Petterson, 2004). Thordarson (2004) divides the succession into six subunits, or members, each representing a single massive eruptive event. Fossilized or carbonized wood fragments were found near the bottom of four of the eruptive members (Mahoney, Fitton, Wallace, et al., 2001). The volcaniclastic succession at Site 1184 provides the only evidence so far for significant amounts of subaerial volcanism on the OJP.
Three of the six eruptive members at Site 1184 contain blocky glass clasts with unaltered cores, and these cores allow the reliable determination of the composition of the erupted magma. White et al. (2004) used microbeam techniques to determine the major and trace element compositions of samples of the glass. The glasses are very similar in composition to the Kwaimbaita-type and Kroenke-type basalts sampled on the high plateau. Each member has a distinct glass composition and there is no intermixing of glass compositions between them, confirming Thordarson's (2004) conclusion that each is the result of one eruptive phase and that the volcaniclastic sequence has not been reworked. White et al.'s (2004) major and trace element data for the glass clasts suggest that the voluminous subaerially erupted volcaniclastic rocks at Site 1184 belong to the same magmatic event as that responsible for the construction of the main plateau. Thus, the OJP would have been responsible for volatile fluxes into the atmosphere in addition to chemical fluxes into the oceans. Both factors may have influenced the contemporaneous oceanic anoxic event (Sikora and Bergen, 2004; Erba and Tremolada, 2004).
The geochemical evidence (White et al., 2004; Fitton and Godard, 2004) linking the phreatomagmatic eruptions recorded at Site 1184 to the formation of the main plateau is supported by the Early Cretaceous age implied by the steep (–54°) magnetic inclination preserved in the volcaniclastic rocks (Riisager et al., 2004). However, this evidence appears to be contradicted by the presence of rare Eocene nannofossils at several levels within the succession (Bergen, 2004). In an attempt to resolve this paradox, Chambers et al. (2004) applied the 40Ar/39Ar dating method to feldspathic material separated from two basaltic clasts and to individual plagioclase crystals separated from the matrix of the volcaniclastic rocks. The clasts gave minimum age estimates of ~74 Ma, and the plagioclase crystals a mean age of 123.5 ± 1.8 (1 ) Ma.
Shafer et al. (2004) analyzed a suite of 14 basaltic clasts extracted from four of the volcaniclastic units and, despite their extensive alteration, showed that the clasts were derived from a source similar to that of the Kroenke- and Kwaimbaita-type basalt on the main plateau. Significantly, the composition of the clasts (Shafer et al., 2004) varies with the bulk composition of their host volcaniclastic units (Fitton and Godard, 2004), showing that they must be cognate. Chambers et al. (2004) conclude that both the clasts and the plagioclase crystals that they used in their 40Ar/39Ar dating belong to the same magmatic episode as the host volcaniclastic rocks and are not xenoliths or xenocrysts from older basement. Thus, the combined 40Ar/39Ar, geochemical, and paleomagnetic evidence favors an Early Cretaceous age for the volcaniclastic succession. Thordarson (2004) and Chambers et al. (2004) suggest that the Eocene nannofossils were introduced later, possibly along fractures.
Volcaniclastic rocks (recovered during previous ODP legs) are also found in thick, reworked, and redeposited successions overlying Early Cretaceous basalt in the Nauru and East Mariana basins east and north of the OJP, respectively. Much of the volcaniclastic material consists of hyaloclastite, and this, together with the presence of wood and shallow-water carbonate fragments, suggests that both the East Mariana Basin and the Nauru Basin volcaniclastic rocks were derived from once-emergent volcanic sources or from relatively shallow water. Castillo (2004) presents chemical and isotopic data for these volcaniclastic rocks and compares them with data for the OJP. The Nauru Basin volcaniclastic rocks have incompatible trace element and Nd isotope compositions typical of the Kwaimbaita-type tholeiitic lavas of the OJP, suggesting that these deposits were shed from the plateau itself. On the other hand, the East Mariana Basin volcaniclastic rocks have high concentrations of incompatible trace elements and Nd and Pb isotope ratios typical of alkalic ocean island basalts and are unrelated to the OJP.
