Geology and geochemistry of midcontinent rift-related igneous rocks

Abstract

This study is focussed on the geology and geochemistry of Midcontinent Rift-related intrusive and extrusive rocks present in northern Ontario, Canada. The study focuses on three sections in order to elucidate the geochemical characteristics of Midcontinent Rift-related rocks and investigate geochemical evolutionary signatures through time. The Coubran Lake basalts, the rocks of the Logan Basin and the intrusive rocks of the Nipigon Embayment were all included in this study to provide a sound spatial representation of Midcontinent Rift (MCR) intrusive and extrusive rocks present within Canada. The Coubran Lake basalts within the Coldwell Complex, have provided new representation of Midcontinent Rift-related volcanism along the northeast shore of Lake Superior. Physical features, including proximity to surrounding syenites, as well as alteration features, suggest that the Coubran Lake basalts represent a pre-existing sequence into which the Coldwell Complex intruded. Geochemical evidence has linked the Coubran Lake basalt to the Two Duck Lake gabbro (TDLG) suggesting the basalts may represent the volcanic expression of the TDLG. Trace element and Nd isotope data suggest the Coubran Lake basalts are akin to the basalt type I composition, linking the unit to the Lower Siemens Creek volcanics, the basal units in the Ely’s Peak and Grand Portage areas of the North Shore Volcanics, and to the lower suite of the Osler Group. The geochemical data presented here is consistent with the available paleomagnetic data suggesting the basalts were erupted early in the history of the Midcontinent Rift.