THE GEOCHEMISTRY, PETROGRAPHY, AND IMPLICATIONS OF THE SUBAERIAL RHYOLITES OF MT. HOUGHTON AND MT. GRATIOT, KEWEENAW PENINSULA, MI

The Keweenaw Peninsula in Michigan’s Upper Peninsula is uniquely situated on the Eastern limb of the Lake Superior Syncline, exposing the Portage Lake Volcanic series, which hosts the largest native copper deposit in the world. The Portage Lake Volcanic series (PLVs) is a massive group of subaerial flood basalts that were extruded roughly 1.1 Ga in relation to the attempted breakup of Laurentia via the failed Midcontinental (Keweenawan) Rift (MCR). The exposed portion of the PLVs is roughly 5 km thick and is composed of individual flows ranging from 1-250 m thick, broken up by multiple interflow conglomerate layers (Nicholson, 1991; Davis & Paces, 1990; Bornhorst, 1975). Felsic volcanic rocks comprise roughly 10% of the MCR-related rocks exposed in the Lake Superior region; however, less than 1% of the PLVs is represented by felsic volcanic material, and most of these rhyolites are restricted to a roughly 0.5 km thick interval near the Keweenaw Fault (Nicholson, 1991; Bornhorst, 1975). These rhyolites occur as dome structures that typically overlie a thin, felsic pyroclastic unit. They exhibit strong flow foliation and have in areas retained delicate hydration and devitrification textures such as perlitic fractures and spherulites (Nicholson, 1991). The samples collected display evidence of hydrothermal metamorphism, which is estimated to have occurred between 1.060 and 1.047 Ga (Bornhorst & Paces, 1988). There is a suite of over 100 minerals found in the Keweenaw (Butler & Burbank, 1929; Bodden, 2019), and the peninsula can be subdivided into several metamorphogenic zones based on the temperature of the intruding hydrothermal fluid (Bodden, 2019; Bodden & Bornhorst, 2022). The location of the rhyolite domes places them squarely in the epidote zone, however only one set of those samples shows epidote mineralization. The purpose of this study is to present the petrochemistry and petrography of rhyolites collected from Mt. Houghton and Mt. Gratiot to determine the variation in secondary hydrothermal mineralization with respect to protolith composition and geographical location. Analysis of microstructures (Befus et al., 2014), will allow the determination of flow direction and emplacement conditions.