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The Beaver Bay Complex (BBC) is a hypabyssal, multiple-intrusive igneous complex exposed over a 600 km2 area in northeastern Minnesota. It was emplaced into comagmatic volcanic rocks at 1.1 Ga during the development of the Midcontinent Rift System. Thirteen intrusive units have been identified that represent a minimum of six major intrusive events. With the exception of a body of granophyre, most intrusions were formed from gabbroic to dioritic parental magmas with successive intrusions generally involving less evolved compositions. Compositional variations within intrusive units developed as a result of in situ magmatic differentiation, assimilation of footwall rocks, and composite intrusions of evolved magma from deeper staging chambers. Over the exposed extent of the BBC, intrusion shapes appear to have been controlled by a shallow crustal ridge that trends northwesterly across the BBC. The focus of emplacement appears to have migrated toward the rift axis and toward higher stratigraphic levels with time perhaps reflecting plate drift and thickening of the volcanic pile. In the larger context of the Midcontinent Rift System, geologic, geochronologic, geophysical, and geochemical data consistently indicate that the BBC, particularly the youngest Beaver River diabase dike and sheet network, acted as a magma conduit and structural boundary to the development and infilling of the western end of the Portage Lake Volcanic basin during the main stage of rift volcanism and graben formation. Miller, J. D., Jr., and Chandler, V. W., 1997, Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota, in Ojakangas, R. W., Dickas, A. B., and Green, J. C., eds., Middle Proterozoic to Cambrian Rifting, Central North America: Boulder, Colorado, Geological Society of America Special Paper 312. |
