40Ar/39Ar geochronology of flood basalts from the Kerguelen Archipelago, southern Indian Ocean: implications for Cenozoic eruption rates of the Kerguelen plume

The 6500 km(2) Kerguelen Archipelago formed on the northern Kerguelen Plateau (NKP) (4 X 10(5) km(2)) which is a shallow submarine plateau belonging to the Kerguelen large igneous province in the southern Indian Ocean. Flood basalts make up 85% of the archipelago and are interpreted as the most recent volcanism (< 40 Ma) from the Kerguelen hotspot which has erupted basalt for the last 115 million years. Based on 40Ar/39Ar incremental heating of acid-leached groundmass separates, we report isochron ages ranging from 29.26 +/- 0.87 Ma to 24.53 +/- 0.29 Ma for 15 basalts from five stratigraphic sections from diverse regions of the archipelago. The oldest dated basalt from the archipelago (similar to 29 Ma) is much younger than the similar to 40 Ma age of conjunction between the hotspot and the Southeast Indian Ridge. Basalt eruption seems to have ceased shortly after similar to 24 Ma although small volume, highly evolved lavas and plutons continued to form in the archipelago. The basalt age data suggest an average lava accumulation rate of similar to 1.6 +/- 0.9 km/my during the Oligocene. The archipelago's volumetric eruption rate (0.009 km(3)/yr) is lower than estimates made for the Cretaceous Kerguelen Plateau (1.7 km(3)/yr) and the Ninetyeast Ridge hotspot track (0.18 km(3)/yr), suggesting that the late Cenozoic extrusive activity of the Kerguelen plume is waning. Cenozoic volcanism attributed to the Kerguelen plume occurs over a diffuse area with Quaternary eruptions at Heard and McDonald Islands and within the Kerguelen Archipelago. The decreasing eruption rate and areally diffuse volcanism may be explained by the thick lithosphere of the Cretaceous Kerguelen Plateau overriding and insulating the plume. However, if the undated NKP, which underlies the archipelago, formed during the Cenozoic, then, the crustal production rate of the plume from 40 Ma to the present (similar to 0.25 km(3)/yr) would be similar to the crustal production rate (0.23 km(3)/my) previously estimated for the formation of the Ninetyeast Ridge (similar to 82-38 Ma). (C) 2000 Elsevier Science B.V, All rights reserved.