Mantle Sources and the Highly Variable Role of Continental Lithosphere in Basalt Petrogenesis of the Kerguelen Plateau and Broken Ridge LIP: Results from ODP Leg 183

Abstract

Ocean Drilling Program (ODP) Leg 183 was designed to investigate the origin and evolution of the large igneous province composed of the Kerguelen Plateau and Broken Ridge. Of the eight sites drilled, basalt was recovered from seven, five on the plateau and two on Broken Ridge. We present results from four of these sites, 1136, 1138, 1141 and 1142. Although this large igneous province is interpreted as being derived from the Kerguelen mantle plume, the geochemical characteristics of basalt from some parts of the province indicate a role for continental lithosphere. The 118–119 Ma basalt flows recovered in the Southern Kerguelen Plateau (Site 1136) have a more subtle continental signature than shown by basalt at Leg 119 Site 738. A continental signature is absent in the 100–101 Ma tholeiitic basalts at Site 1138 in the Central Kerguelen Plateau (CKP); their age-corrected Nd–Sr–Pb isotopic values and incompatible element ratios are similar to those estimated for primitive mantle. These flows may represent a major mantle source in the Kerguelen starting-plume head. The 20 basalt units identified are a product of magma chamber replenishment, fractional crystallization, and resorption of crystallizing phases. The topmost unit, Unit 1, is a dacite that evolved from a basalt magma similar to those represented by Units 3–22; unlike the basalts the dacite magma was probably influenced by continental material. Middle Cretaceous (∼95 Ma) lavas of Sites 1141 and 1142 on Broken Ridge (originally part of the CKP) are alkalic, with one exception (a tholeiite at the base of Site 1142). The alkalic lavas may represent a late-stage cap or carapace of relatively low-degree partial melts that overlies a thick tholeiitic lava pile. The tholeiite and pebbles from the top of a probable talus deposit (Unit 2) at Site 1142 have geochemical signatures consistent with a minor contribution from continental material. This signature is absent in the other units from these two sites, which have ocean-island-like incompatible element ratios and age-corrected isotopic characteristics similar (but not identical) to those proposed for the post-30 Ma Kerguelen plume. These alkalic basalts may be the purest representatives of the Cretaceous plume tail composition yet found.