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The geochemistry,geochronology and paleomagnetism of dikes and sills associated with the Mesoproterozoic Midcontinent Rift near Thunder Bay,Ontario, Canada
Authors:Pete Hollings  Mark Smyk  Larry M Heaman  Henry Halls
Institution:1. Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON P7B 5E1, Canada;2. Ontario Geological Survey, 435 James St. South, Thunder Bay, ON P7E 6S7, Canada;3. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada;4. Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, ON L5L 1C6, Canada
Abstract:The Midcontinent Rift (MCR) of North America comprises a series of basaltic sheets, flows and intrusive rocks emplaced in the Lake Superior region during the Mesoproterozoic. The mafic rocks preserved on the northern flank of Lake Superior represent the older portions of the rift sequence and offer insights into the early development of the rift. New geochronological, geochemical and paleomagnetic data are presented for the dikes and sills located in and south of Thunder Bay, Ontario. Three sill suites are recognized within the study area; an earlier, spatially restricted ultramafic unit termed the Riverdale sill, the predominant Logan sills and Nipigon sills in the north of the study area. In addition three dike sets are recognized, the north-east trending Pigeon River swarm, the north-west trending Cloud River dikes and the Mt. Mollie dike. The geochemical data demonstrate that the majority of sills south of Thunder Bay are of Logan affinity and distinct from those of broadly similar age in the Nipigon Embayment to the north. The Pigeon River dikes that intrude the sills are geochemically coherent but distinct from the Logan sills and could not be feeders to the sills. The new age of 1109.2 ± 4.2 Ma for the Cloud River dike and its R polarity are consistent with published magnetostratigraphy. The Mt. Mollie dike age (1109.3 ± 6.3 Ma) indicates that it is not coeval with the spatially associated Crystal Lake gabbro as previously thought. The complexity of the dike and sill suites on the northern flank of suggests that the early phases of rifting occurred in distinct and changing stress fields prior to the main extensional rifting preserved in younger rocks to the south. The geochemistry and geochronology of the intrusions suggest a long-lived and complex magmatic history for the Midcontinent Rift.
Keywords:Midcontinent Rift  Geochemistry  Geochronology  Paleomagnetism  Dikes  Sills
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