ABSTRACT
With a surface area of 1.6 x 106 km2 and a volume of 4-5 x 107 km3, the Ontong Java Plateau is the world's largest volcanic oceanic plateau and may represent the largest magmatic event on Earth in the last 200 m.y. During Ocean Drilling Program (ODP) Leg 192 we recovered igneous basement and sediment cores in five widely separated sites in previously unsampled areas across the plateau. Primary objectives of the leg were to determine (1) the age and duration of emplacement of the plateau, (2) the compositional range of magmatism, and (3) the environment and style of eruption.Acoustic basement at the four sites on the main or high plateau consists of pillow and/or massive basalt flows with rare, thin sedimentary interbeds. Biostratigraphic evidence indicates that basement ages at Sites 1183, 1186, and 1187 are Aptian. At Site 1185, two groups of basalt are present; the lower group is Aptian, whereas the age of the upper group is estimated only loosely as latest Cenomanian to Albian. These results, together with data from Deep Sea Drilling Project (DSDP) Site 289 and ODP Site 807, demonstrate that the great bulk of the high plateau formed in a single episode in the early Aptian. Later volcanic events, including the ~90-Ma event recorded at Site 803 and in the eastern Solomon Islands, appear to have been volumetrically minor on the high plateau and mainly confined to its margins. One of these late-stage events is recorded in our fifth site, Site 1184, on the plateau's eastern lobe or salient, where we cored 338 m of a middle Eocene basaltic volcaniclastic sequence.
The basalt at Sites 1183 and 1186 and that making up the lower group of lava flows at Site 1185 are closely similar in composition and belong to the remarkably homogeneous Kwaimbaita magma type found at Site 807 and in the eastern Solomons. Thus, much of the high plateau's upper crust seems to consist of Kwaimbaita-type basalt. The Eocene volcaniclastic rocks of Site 1184 also have a Kwaimbaita-like bulk composition. No flows of Singgalo-type basalt, which overlies Kwaimbaita-type lavas at Site 807 and on the island of Malaita, were encountered. An exciting discovery of Leg 192 was that basement at Site 1187 and the upper group of flows at Site 1185 are composed of a high-MgO (8-10 wt%), incompatible element-poor (e.g., TiO2 = 0.72-0.77 wt%; Zr = 36-43 ppm) type of basalt not found previously on the plateau. These rocks appear to represent very high total fractions of partial melting of their mantle source, and their presence in >100-m-thick lava piles at two sites 146 km apart suggests that such basalt is voluminous on the eastern edge of the high plateau.
Emplacement of lavas at all four high-plateau sites was entirely submarine. The shallowest estimated Aptian water depth for basement is several hundred meters, at Site 1183 on the broad dome of the plateau. Together with previous evidence, our results indicate that most of the Ontong Java Plateau formed well below sea level. The only evidence that a portion of the high plateau was ever at shallow depth is two thin intervals of Aptian vitric tuff above basement at Site 1183 and possibly a vitric tuff just above basement at DSDP Site 289. The mainly submarine emplacement of the plateau probably accounts for its apparently limited paleoenvironmental effects. However, ferruginous claystone layers above basement at Sites 1183 and 1187 provide evidence for at least local Aptian "dead zones."