GEOLOGY OF THE MIDCONTINENT RIFT IN NORTHEASTERN MINNESOTA
The Midcontinent rift-related geology of northeastern Minnesota is generally composed of a thick (7 to 10 kilometers) sequence of plateau lavas and minor interflow sedimentary rocks into which numerous mafic and fewer felsic intrusions have been emplaced at various stratigraphic levels. The volcanic and sedimentary rocks, termed the North Shore Volcanic Group (NSVG), were erupted onto a peneplained crust composed of Early Proterozoic and Archean rocks. The NSVG is subdivided into five major stratigraphic sequences and numerous informal formational units. The lava flows are predominantly tholeiitic basalts that range in composition from high-Al olivine tholeiite to basaltic andesite. Intermediate compositions are rare, but felsic lavas (rhyolite, dacite, and icelandite) comprise 10 to 25 percent of the NSVG section.
Among the plutonic rocks, two major intrusive complexes are distinguished based on their general stratigraphic level of emplacement within the volcanic edifice. A high density of intrusions emplaced into the basal section of the NSVG lavas are collectively termed the Duluth Complex. Intrusions of the Duluth Complex are typically categorized into four major series, which are distinguished by general age, dominant lithology, internal structure, and extent of internal differentiation. In general order of younging, these are: 1. the felsic series, 2. the early gabbro series, 3. the anorthositic series, and 4. the layered series. A more dispersed collection of mafic to felsic dikes, sheets, and irregular-shaped intrusions comprise a generally younger suite of intrusions that were emplaced into the medial and upper parts of the volcanic pile. A particularly high concentration of these hypabyssal intrusions, which occur in the central part of the NSVG basin, are referred to as the Beaver Bay Complex.