1. Leg 192 Summary¹

Shipboard Scientific Party²

ABSTRACT

With a surface area of 1.6 × 10⁶ km² and a crustal volume of 4-5 × 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 rock 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 magmas, 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 suggests that basement ages at Sites 1183, 1186, and 1187 are Aptian. At Site 1185, two groups of basalt are present; the lower group appears to be Aptian, whereas the age of the upper group is estimated only loosely as latest Cenomanian to Albian. These preliminary results, together with data from Deep Sea Drilling Project (DSDP) Site 289 and ODP Site 807, suggest that the great bulk of the high plateau formed in a single episode in the early Aptian. More recent 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 confined mainly to its margins. One of these late-stage events may have been recorded in our fifth site, Site 1184, on the plateau's eastern lobe or salient, where we cored 338 m of a basaltic volcaniclastic sequence that yielded a rare and poorly preserved middle Eocene nannofossil assemblage. However, such a young age is difficult to reconcile with the steep paleomagnetic inclination, which implies that the sequence is much older.

The basalts at Sites 1183 and 1186 and those making up the lower group of lava flows at Site 1185 are moderately evolved, low-K tholeiites with closely similar compositions. They 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.

Although the volcaniclastic sequence cored in Hole 1184A was emplaced in a shallow-marine or even subaerial environment, emplacement of lavas at all four high-plateau sites was entirely submarine. The shallowest estimated Aptian water depth for basement is at least 800 m at Site 1183 on the broad dome of the plateau; estimated paleodepths for Sites 1185, 1186, and 1187 are much greater. Together with previous evidence, our results suggest that most of the Ontong Java Plateau formed well below sea level. The only evidence that a small portion of the high plateau was ever at shallow depth is two thin intervals of Aptian vitric tuff above basement in Hole 1183A and possibly a vitric tuff just above basement at DSDP Site 289. The submarine emplacement of most of the plateau probably limited its paleoenvironmental impacts; to date, no major extinction events can be correlated with massive Aptian volcanism on the plateau.