Leg 183 snthesis: Kerguelen Plateau-Broken Ridge-a large igneous province

The Kerguelen Plateau and Broken Ridge form a large igneous province (LIP) in the southern Indian Ocean. The main objectives of Ocean Drilling Program Leg 183 were to understand the origin and evolution of this LIP and the impact of its formation on the environment. Igneous basement (33 to 233 m of penetration) has been recovered from 11 drill sites on the LIP, and 7 are Leg 183 sites. Studies of the basement and sediment cores lead to the following conclusions. 1. Formation of the LIP postdated breakup between India and Antarctica, with eruption ages (40Ar/39Ar) ranging from ~119 Ma in the southern Kerguelen Plateau (SKP) to ~34 Ma in the northern Kerguelen Plateau. Apparently, peaks in magmatic output (~0.9 km3/yr) occurred in the intervals of 119–110 and 105–95 Ma. Although an important caveat is that we have access only to uppermost basement of a thick (~20 km) igneous crust, these results are inconsistent with massive volcanism associated with a single plume head and continental breakup. 2. The uppermost igneous basement is dominantly tholeiitic basalt. Based on the physical characteristics of the lava flows, which indicate subaerial eruption, and the occurrence of overlying terrestrially derived sediments containing wood fragments, fern remains, and terrestrial palynoflora, much of the LIP was above sea level when magmatic output was high. 3. The geochemical characteristics of basalt forming the LIP are unlike mid-ocean-ridge basalt (MORB). There are, however significant geochemical differences between tholeiitic basalt erupted at each site. These differences are attributed to varying proportions of components derived from the Kerguelen plume, depleted MORB-related asthenosphere, and continental lithosphere. 4. Based on radiogenic isotopic ratios and incompatible element abundances, tholeiitic basalt of Cretaceous age from spatially diverse locations (Site 738 in the SKP, Site 1137 on Elan Bank, Site 747 in the Central Kerguelen Plateau (CKP), and Site 1142 on Broken Ridge) contains up to ~5% of a component derived from continental crust. The Proterozoic-age zircon and monazite in clasts of garnet-biotite gneiss in a conglomerate intercalated with basalt at Site 1137 demonstrate that continental crust fragments are present in the Indian Ocean lithosphere. 5. Surprising new results for oceanic LIPs are the dominance of alkalic lavas, trachybasalt to trachyte and rhyolite, at Skiff Bank and the pyroclastic deposits present on Skiff Bank, Elan Bank, and the CKP. Explosive subaerial volcanism at high latitudes can contribute volatiles to the stratosphere; therefore, LIP volcanism may have had a significant environmental impact. 6. Flora and fauna preserved in Kerguelen Plateau sediments provide a long-term record of the plateau’s environment, beginning with terrestrial and shallow-marine deposition followed by equable Cretaceous paleoceanographic conditions, the abrupt Cretaceous/Tertiary boundary, the Late Paleocene Thermal Maximum, and cooling of the Southern Ocean through Tertiary time.