The Midcontinent Rift System (MRS) in northern Minnesota consists of volcanic flows and intrusive rocks that were emplaced during a relatively short interval (approximately 23 Ma) during the Middle Proterozoic (1109-1086 Ma).   The largely mafic igneous rocks of the MRS were emplaced on older Archean granite-greenstone terranes and Early Proterozoic metasedimentary rocks (greywacke, slate and iron formation).   Early studies (e.g., Grout, 1918) suggested that the layered igneous rocks of the rift were part of a single lopolithic intrusion.   Recent studies (e.g., see Miller et al., 2002 and references therein) utilizing detailed mapping, geophysics, geochemistry, and high-resolution geochronology have established that the rift comprises multiple intrusions.

Briefly, the evolution of the rift can be divided into early, main, and late stages.  The earliest phases of MRS magmatism (1109-1107 Ma) included the emplacement of shallow mafic intrusions, now preserved as the diabase sills (Logan intrusions of northern Minnesota and southern Ontario) and layered gabbroic rocks (Poplar Lake Intrusion of northern Minnesota).   These early intrusions were accompanied by volcanic flows (lower North Shore Volcanic Group; reversed magnetic polarity) that are primarily mafic, but range from basalt to rhyolite.   Minor felsic intrusives (granophyres) were also emplaced at the base of the acumulating volcanic pile during the early stage of evolution.   After a brief period of diminished volcanic activity (latent stage; 1107-1102 Ma), the main magmatic stage (1102-1094 Ma) commenced with the eruption of voluminous volcanic flows (upper North Shore Volcanic Group; normal magnetic polarity) that are primarily basaltic, but also include minor icelandite and rhyolite.  During the main magmatic stage numerous intrusions, consisting of layered gabbroic rocks, anorthosite, or felsic granophyre, were emplaced along the base of (Duluth Complex), or within (Beaver Bay Complex) the thickening volcanic pile.   The late magmatic stage (1094-1086 Ma) was marked by decreased volcanic activity during continued subsidence and sedimentation.

The Sonju Lake Intrusion and associated Finland granophyre are part of the Beaver Bay complex that was intruded at a relatively shallow level within the volcanic sequence during the main magmatic stage.   A detailed map of the intrusion can be found here .