LEATHERWOOD CREEK SHUT-INS, MISSOURI: A MESOPROTEROZOIC LAVA DOME

The Leatherwood Creek Shut-ins (LCSI) is located in the southern part of the St. Francois Mountains, which host the only sizable exposures of the vast Mesoproterozoic Granite-Rhyolite Province of the North American Midcontinent. Although eight separate units have been identified in the LCSI, two dominate: the Tile Red and Crane Pond rhyolites, both initially recognized by Brown (1983). Outcrop and petrographic characteristics of these two rhyolites clearly indicate they were effusively emplaced. The Crane Pond rhyolite is easily distinguished by its porphyritic texture. The nearly aphyric Tile Red rhyolite is commonly flow-banded and its basal portion is entirely composed of what are tentatively identified as megaspherulites, although they lack a microscopic radiating habit. In numerous areas the two units are complexly intermingled and in one location the Tile Red intrudes the Crane Pond. The LCSI also contains three distinct ash-flow tuffs, two volcanic breccias, and an epiclastic unit. New major and trace element data suggest that the Tile Red and Crane Pond rhyolites are likely genetically related and may have been erupted from a zoned magma chamber as previously proposed by Brown and Kumar (1986). All in all, the LCSI appears to be the site of a significant Mesoproterozoic lava dome that perhaps developed on the perimeter of one of the calderas of the southern St. Francois Mountains (e.g., Brown, 1983).