PRIMITIVE LAVAS, CHEMOSTRATIGRAPHIC VARIATION AND MAGMATIC EVOLUTION IN THE KEWEENAWAN MID-CONTINENT RIFT

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The Keweenawan rocks of the Lake Superior region afford an excellent opportunity to study contemporaneous volcanic and plutonic activity associated with continental rifting in the Proterozoic. By comparing genetically related volcanic sequences and plutons on a regional basis, a range of basic problems can be addressed. In a pilot study the PI has discovered very primitive basaltic and picritic lavas near the base o the Mamainse Point Formation (MPF) in Ontario. These lavas display spectacular quench-olivine textures and systematic chemical variations with stratigraphy. This project will sample the remaining four-fifths of the MPF and analyze for major and trace elements by XRF and DC plasma spectrometry. Mineral chemistry will be determined by electron microprobe, and geochemical modeling will be attempted in order to delineate the processes responsible for the origin and compositional evolution of this important volcanic sequence. Results will be used to evaluate the hypotheses derived from previous work, and to determine the relationship between the plutonic and volcanic rocks. An attempt will be made to characterize the nature of the subcontinental mantle source and to evaluate possible relationships between these lavas and plutonic anorthosite complexes.