A Bimodal Alkalic Shield Volcano on Skiff Bank: its Place in the Evolution of the Kerguelen Plateau

A bimodal volcanic sequence of 230 m thickness on Skiff Bank,a western salient of the northern Kerguelen Plateau, was drilledduring ODP Leg 183. The sequence comprises three main units:a mafic unit of trachybasalt flows sandwiched between two unitsof trachytic or rhyolitic flows and volcaniclastic rocks. Althoughinterpretation is complicated by moderate to strong alterationof the rocks, their original chemical character can be establishedusing the least mobile major and trace elements (Al, Th, highfield strength elements and rare earth elements). High concentrationsof alkalis and incompatible trace elements indicate that bothmafic and felsic rocks are alkalic. The felsic rocks may havebeen derived by partial melting of mafic rocks, followed byfractionation of feldspar, clinopyroxene, Fe–Ti oxidesand apatite. The mafic and felsic rocks have similar Nd andPb isotopic compositions; ²⁰⁶Pb/²⁰⁴Pb ratios are low (17·5–18·0)but, like the ¹⁴³Nd/¹⁴⁴Nd ratios (0·5125–0·5126),they are comparable with those of basalts from the central andsouthern Kerguelen Plateau (e.g. Sites 747, 749, 750). The Srisotopic system is perturbed by later alteration. There is nochemical or isotopic evidence for a continental crustal component.The bimodal alkalic character and the presence of quartz-phyricrhyolites is interpreted to indicate that the sequence formspart of a shield volcano built upon the volcanic plateau. Theage of 68 Ma, obtained on Site 1139 rocks by Duncan (A timeframe for construction of the Kerguelen Plateau and Broken Ridge,Journal of Petrology 43, 1109–1119, 2002), provides onlya minimum age for the underlying flood volcanic rocks. The highage indicates none the less that Skiff Bank is not the presentlocation of the Kerguelen plume. KEY WORDS: Ocean Drilling Program; Kerguelen Plateau; Skiff Bank