Abstract

Over a 100 new major-element analyses and 48 analyses for a restricted suite of minor and trace elements have been made on samples from 31 localities in the Kenora–Kabetogama (KK) dyke swarm of Minnesota and northwestern Ontario. A subset of 14 analyses, selected to minimize the aberrations of secondary hydrous alteration, flow layering, and oversampling bias, characterizes the range of dyke compositions. The dykes are interpreted as having been derived from a single magma type; their compositions span a smooth continuum of MgO and TiO2 variations. The magmatic system was tapped most vigorously at two stages: at a relatively early stage when the magma contained about 7.6% MgO and 0.9% TiO2, and again at a later stage when the magma contained about 5.5% MgO and 2.2% TiO2. Large master dykes (~100–190 km long) and many smaller dykes are of these two compositions. Dykes throughout the compositional spectrum are indistinguishable paleomagnetically. Available radiometric data suggest that KK magmatism may have been prolonged (> 100 Ma?), but the paleomagnetic results suggest a much shorter probable life-span.The KK swarm is related to tectonic activity in the Penokean orogen (Early Proterozoic). It may have developed as a manifestation of peripheral foreland bulging initiated by thrust loading of the crust in the hinterland of the Animikie basin.

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