A bimodal alkalic shield volcano on skiff bank: Its place in the evolution of the Kerguelen Plateau

Abstract

A bimodal volcanic sequence of 230 m thickness on Skiff Bank, a western salient of the northern Kerguelen Plateau, was drilled during ODP Leg 183. The sequence comprises three main units: a mafic unit of trachybasalt flows sandwiched between two units of trachytic or rhyolitic flows and volcaniclastic rocks. Although interpretation is complicated by moderate to strong alteration of the rocks, their original chemical character can be established using the least mobile major and trace elements (Al, Th, high field strength elements and rare earth elements). High concentrations of alkalis and incompatible trace elements indicate that both mafic and felsic rocks are alkalic. The felsic rocks may have been derived by partial melting of mafic rocks, followed by fractionation of feldspar, clinopyroxene, Fe Ti oxides and apatite. The mafic and felsic rocks have similar Nd and Pb isotopic compositions; 204Pb/104Pb ratios are low (17.5 18.0) but, like the 111Nd/144Nd ratios (0.5125 0.5126), they are comparable with those basalts from the central and southern Kerguelen Plateau (e.g. Sites 747, 749, 750). The Sr isotopic system is perturbed by later alteration. There is no chemical or isotopic evidence for a continental crustal component. The bimodal alkalic character and the presence of quartz-phyric rhyolites is interpreted to indicate that the sequence forms part of a shield volcano built upon the volcanic plateau. The age of 68 Ma, obtained on Site 1139 rocks by Duncan (A time frame for construction of the Kerguelen Plateau and Broken Ridge, Journal of Petrology 43, 1109 1119, 2002), provides only a minimum age for the underlying flood volcanic rocks. The high age indicates none the less that Skiff Bank is not the present location of the Kerguelen plume.