GEOLOGIC AND GEOCHEMICAL ATTRIBUTES OF THE BEAVER RIVER DIABASE AND GREENSTONE FLOW: TESTING A POSSIBLE INTRUSIVE-VOLCANIC CORRELATION IN THE 1.1 GA MIDCONTINENT RIFT

Over the last century, numerous geological studies have fairly well constrained the overall tectonomagmatic evolution of the Middle Proterozoic Midcontinent Rift (MCR) in central North America. However, the correlation of the numerous flood basalts with their intrusive feeder systems remains elusive. One possible correlation is between the Beaver River Diabase (BRD), an extensive composite dike and sill complex in northeastern Minnesota, and the Greenstone Flow (GSF) an enormous (1,650 km³) flood basalt exposed on Isle Royale and the Keweenaw Peninsula in northern Michigan.

Evidence that the BRD and GSF are directly related as an intrusive-volcanic system include: 1) an overlap in U-Pb ages (1095.8 ± 1.2 Ma and 1094.0 ± 1.5 Ma, respectively); 2) similar composite lithologies (ophitic olivine gabbro to ferromonzodiorite); and 3) their enormous volumes. More circumstantial, although intriguing, evidence for a comagmatic link comes from the occurrence of numerous large (≤ 500m in diameter), lower crustal anorthosite xenoliths in the BRD. That these diabase feeder dikes were at one time wide enough to accommodate such large blocks within several kilometers of the Earth’s surface implies that such conduits would most certainly have reached the surface and resulted in enormous outpourings of lava such as those that would have created the GSF.

The extensive exposure of both the BRD and GSF in the Lake Superior region offers a unique opportunity to assess a possible genetic link between the intrusive and volcanic systems in the MCR. Here, we present preliminary findings of a detailed geological, petrographic, and geochemical study to test this link. Field exposures and petrographic analysis of both the BRD and GSF show lithological (ophitic olivine gabbro to ferromonzonite), mineralogical, and textural similarities between each unit. Furthermore, although the differentiated lithologies contained in the GSF have previously been thought to have formed through in situ processes, contact relationships suggest a composite emplacement similar to the intrusions in the BRD. Ongoing studies, including detailed field mapping, petrography, and lithogeochemical and isotopic (Sm-Nd) analyses, will be conducted to further test the validity of an intrusive-volcanic link between these two magmatic bodies.