DETERMINING THE EMPLACEMENT MECHANISM OF AN UNUSUAL RHYOLITE BODY IN THE CA. 1.1 GA MIDCONTINENT RIFT NEAR THE PORCUPINE MOUNTAINS IN THE UPPER PENINSULA OF MICHIGAN

The 1.1 Ga Midcontinent Rift of the Lake Superior region is one of the best exposed ancient rift systems in the world. Mafic rocks – basaltic subaerial lavas flows and diabasic, gabbroic, troctolitic and anorthositic intrusions – are by far the most dominant types in the Rift system. However, rhyolite bodies occur both within the North Shore Volcanic Sequence in Minnesota and in the Porcupine Mountains area in the Upper Peninsula of Michigan. Nd isotope analyses done by previous workers have demonstrated that these rhyolites were not products of fractional crystallization of the basaltic magmas but instead derived by partial melting of preexisting Archean continental crust. This suggests that the rhyolites may have been erupted at temperatures much higher than typical of rhyolites in convergent margin settings.

This study focuses on interpreting an unusual rhyolite occurrence, the “Potato Farm” rhyolite exposed southeast of the Porcupine Mountains near White Pine, MI. This body, up to 200 m thick, is one of the youngest volcanic units in the entire Rift, overlain by the Copper Harbor Conglomerate, the first of a thick sequence of post-rift sediments. The Potato Farm rhyolite, which is well exposed in a quarry, has features that could indicate emplacement either as a relatively low viscosity flow or as a superheated ashflow. Three zones can be recognized in the rhyolite within the quarry: a lower zone that is moderately foliated, with a fabric defined by planar zones of high porosity; a middle zone that is porphyritic and relatively massive; and an upper zone containing abundant lithic fragments, many of them intensely streaked and sheared. Understanding how this rhyolite was erupted can shed light not only on the waning stages of volcanism in the Midcontinent Rift but also provide insights into volcanic hazards in modern continental rift and hotspot settings.