PETROLOGY OF THE MARION DUFRESNE BASALTIC SECTION IN THE SOUTH-CENTRAL PLATEAU CENTRAL OF THE KERGUELEN ARCHIPELAGO, SOUTHEAST INDIAN OCEAN

The Kerguelen Archipelago, the third largest oceanic island after Hawaii and Iceland, is located in the southeast Indian Ocean and is covered mainly by 29-24 Ma flood basalts. Previous studies on a series of stratigraphic reference sections have established a trend from older tholeiitic-transitional flood basalts in the north to younger, mildly alkalic basalts in the east and southeast parts of the archipelago. This transition has also recently been documented within a single section at Mont Tourmente, in the northern part of the Plateau Central region. Here we report petrologic and geochemical results for a 650 m succession of lavas from Mt Marion Dufresne, located in the south-central part of the Plateau Central, which is the southernmost section from the Plateau Central to be studied to date.

The lower 300 m of the Marion Dufresne section consists of a suite of highly fractionated (3-5.5 wt% MgO), mildly alkalic, aphyric basalts. A trend from alkalic to transitional basaltic volcanism (A.I.=1.7 to 0.1 with increasing elevation) is evident in the basal 100 meters of the section, which contrasts with widespread regional trends on the archipelago. Three strongly plagioclase-phyric (50-60%) basalts are also present in upper part of this unit. In contrast, the upper 350 meters of the section are dominated by olivine-phyric basalts (7-11.5 wt% MgO), which are relatively rare on the archipelago, that straddle the alkalic-tholeiitic boundary (A.I.=0.7 to -0.7).

This study demonstrates the existence of mildly alkalic basalts in the southern part of the Plateau Central, which significantly increases the distribution of alkalic volcanism on the Kerguelen Archipelago. Incompatible trace element abundance ratios suggest a common parental magma for the lower (aphyric) and upper (olivine-phyric) units. The presence of both aphyric and olivine-phyric basalts at Mt Marion Dufresne suggests that a change in magma flux occurred during flood basalt eruption. Future work on this section will focus on Sr-Nd-Pb-Hf isotope geochemistry and 40Ar/39Ar geochronology. These results will be used to compare Marion Dufresne to previously studied sections in the NW, north-central, east and SE parts of the archipelago to better understand the Cenozoic history of the Kerguelen mantle plume.