Lamont-Doherty Earth Observatory, United States

Oceanic plateaus are giant volcanic features whose existence implies an extraordinary flux of magma from mantle to lithosphere. By understanding their formation, these large igneous provinces can be important indicators of fundamental processes of mantle convection and geodynamics. Although it is widely thought that oceanic plateaus arise from massive eruptions resulting from the arrival of a deep mantle plume head at the lithosphere, an alternative explanation is that plateau eruptions are related to decompression melting of unusually fusible mantle beneath fast-spreading ridges. Shatsky Rise was cored during Integrated Ocean Drilling Program Expedition 324 because it is a unique oceanic plateau, formed during the Late Jurassic and Early Cretaceous at a rapidly spreading triple junction, with characteristics that could be attributed to either model of formation. Shatsky Rise is also a monster volcanic construct whose formation style is poorly understood. The goal of Expedition 324 was to core the igneous rocks of Shatsky Rise and the sediments above to examine the age, physical volcanology, geochemistry, and tectonic evolution of the rise as well as the sedimentation history. Five sites were cored and four were logged, with one site (U1346) on the summit of Shirshov Massif and two sites each on Ori (Sites U1349 and U1350) and Tamu (Sites U1347 and U1348) massifs. Basaltic lava flows were recovered at four of these sites and complement previous Ocean Drilling Program Site 1213 (south flank of Tamu Massif) in providing a record of lava flow emplacement on Shatsky Rise. Instead of lava flows, cores from Site U1348 recovered a thick sequence ( approximately 120 m) of volcaniclastic sediments topped with shallow-water calcareous sandstones. Lavas recovered at Sites U1347 and U1350 are fresh enough to be suitable for high-quality radiometric dating and planned geochemical/isotopic studies. Although lavas from Site U1346 and U1349 were moderately to highly altered, it is expected that they will also provide important age information with suitable treatment and will be useful for most geochemical studies. Even though the volcaniclastic rocks of Site U1348 are highly altered, a single interval containing relatively fresh glass shards will provide valuable information on magma source characteristics. Shatsky Rise lava flows occur primarily as packages of pillow basalt and massive inflation units, frequently interbedded with volcaniclastic sediment. The thickest massive inflation flows, up to approximately 23 m thick, occur on Tamu Massif at Sites 1213 and U1347. They are similar to massive flows cored on the Ontong Java Plateau (OJP) and found in continental flood basalt provinces. At Site U1347, the relationship of pillows and massive flows suggests magmatic cycles that began with the emplacement of massive sheet flows and waned with pillow lavas. Moreover, paleomagnetic inclination trends at both Sites 1213 and U1347 imply that little time passed between the emplacement of individual flows. The implication is that Tamu Massif concluded with massive, high effusion-rate eruptions. Massive flows are also found at Sites U1349 and U1350, on Ori Massif's summit and flank, respectively, but only two larger cooling units (1.9 and 3.2 m thick) were recognized at Site U1346, on the Shirshov Massif summit, otherwise consisting of many smaller pillow units. The massive flow units are thinner at Ori Massif than most flows penetrated on Tamu Massif. Moreover, Site U1350 geochemical and paleomagnetic inclination trends imply greater time and more eruptive variability. The simplest conclusion from the observed trends in lava flow style across the three main edifices of Shatsky Rise is that the average eruptions become smaller and less effusive from Tamu to Ori to Shirshov massifs. Basement rocks recovered from the two summit sites (U1346 and U1349) on Shirshov and Ori massifs show the most severe alteration, apparently from both low and moderately high temperature fluidrock interaction, the latter especially for Site U1349. In contrast, lavas cored on the deeper flanks of the massifs show evidence of only light to moderate lowtemperature alteration. Although these highly altered sites represent only two individual cases, this dichotomy suggests that alteration and fluid-rock interaction was more intense on plateau summits. Apparently, the summits were a focus of heat and water circulation. Rocks at the flank sites (1213, U1347, and U1350) were apparently affected by lesser fluid flow and temperatures. Lavas at these sites seem to have been rapidly paved over by subsequent flows and thereby sealed from extensive seawater contact/circulation. Several sites attest that volcanic debris is more important on Shatsky Rise than anticipated. A significant portion ( approximately 40 m) of the Site U1349 section, located on a summit ridge on Ori Massif, consists of volcaniclastic breccia. Site U1348, which is situated on a buried volcanic high on Tamu Massif, yielded a succession consisting entirely of volcaniclastic material, mainly hyaloclastic sediment. In addition, volcaniclastic sediments were found at Sites U1346 and U1347. These observations indicate that volcaniclastic eruptions made an important contribution to the formation of Shatsky Rise, especially on the higher, shallower parts of the volcanoes. Shipboard geochemical data show that the lava flows consist of variably evolved tholeiitic basalt. Site U1347 and U1350 lavas, least affected by alteration, have broad similarities with Site 1213 basalts and display compositional ranges overlapping those of OJP basalts and mid-ocean-ridge basalt (MORB), albeit more with the latter. Samples from Site U1347 and many samples from Site U1350 resemble enriched-type MORB (E-MORB). A broad generalization is that Shatsky Rise basalts are slightly enriched in incompatible elements compared to normal MORB (N-MORB). This suggests a mantle source slightly richer in the more incompatible elements than NMORB source mantle and/or that Shatsky Rise magmas formed by slightly lower degrees of partial melting and possibly in the presence of residual garnet. Alteration-resistant element ratios indicate that basalts from Sites U1346, U1348, and U1349 are also tholeiites. Site U1349 basaltic flows appear to represent significantly less differentiated magmas than those recovered from other sites and have similarities to picritic Ontong Java Plateau basalts (i.e., the high-Mg Kroenke type). Multiple lines of evidence indicate that Shatsky Rise volcanoes had summits at or above sea level. Benthic foraminifers and/or sediment facies at all sites except Site U1350 (a lower flank site) show evidence of shallow-water deposition. Lavas at Site U1349 have alteration and flow structures consistent with subaerial eruption and weathering, as well as intercalations of shallow-water sediments (e.g., oolites). Most of the volcaniclastic deposition at Site U1348 occurred below sea level, but the succession was topped with shallow-water sediment. Two of the sites showing evidence of shallow-water deposition (Sites U1347 and U1348) are significantly downslope from today's highest points of basement level on Tamu Massif, implying that the summit was a large island. Taken together, evidence from Expedition 324 strongly implies that during Jurassic and Cretaceous times, Shatsky Rise was an archipelago of large volcanic islands.