Flood Basalt from Mont Tourmente in the Central Kerguelen Archipelago: the Change from Transitional to Alkalic Basalt at ∼25 Ma
Abstract
The surface of the Cenozoic Kerguelen Archipelago, constructed on the Kerguelen Plateau in the southern Indian Ocean, is dominantly flood basalt. With the objective of understanding the Cenozoic history of the Kerguelen mantle plume, the age and geochemical characteristics of this flood basalt province are being determined by studying stratigraphic sections of basalt flows at several locations. Sections from the NW, north–central, east and SE parts of the archipelago have been studied. Here we report results for a 597 m succession of lavas from Mont Tourmente from the Plateau Central, a region of the archipelago that has not been studied in detail. Mont Tourmente lavas range from ∼26 Ma dominantly transitional basalts in the lower 80% of the section to ∼25·3 Ma dominantly alkalic basalts in the upper part of the section. The timing of this change from transitional to alkalic volcanism within the Mont Tourmente section is consistent with that defined by the older ∼28–29 Ma transitional basalts in the north and the ∼25 Ma alkalic lavas erupted in the east. This change in basalt composition may be related to migration of the archipelago away from the plume or to increasing lithosphere thickness over the ∼5 Myr of flood basalt volcanism. The alkalic and transitional Tourmente lavas are nearly homogeneous in isotopic ratios of Sr, Nd and Pb. They have lower 87 Sr/ 86 Sr and higher 143 Nd/ 144 Nd compared with lavas from the eastern sections, which have been proposed to be characteristic of the Kerguelen plume. Consequently, the Mont Tourmente isotopic data may reflect heterogeneity within the plume or a constant proportion of a depleted component mixed with the high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd plume. In contrast to many of the Cretaceous Kerguelen Plateau lavas, there is no evidence in trace element abundances or Sr and Nd isotopic ratios that the Cenozoic Kerguelen Archipelago lavas were influenced by continental lithosphere.