Geological Overview of the Southern St. Francois Mountains, Missouri

Abstract The exposed igneous rocks in the st.Francois Mountains are part of the Eastern Granite-Rhyolite Province formed between 1480 and 1440 Ma (Bickford and Mose, 1975; Bickford and others, 1981; Thomas and others, 1984; Nelson and DePaolo, 1985). This Precambrian window in the midcontinent consists of granitic plutons in the northeast and predominantly rhyolitic volcanics (with minor intermediate volcanics and mafic dikes and sills) to the west and south (Fig. 1; Tolman and Robertson, 1969). These rocks are intrepreted as a volcanic pile that was intruded by comagmatic plutons, and later tilted to the southwest and beveled by erosion; hence rocks are generally younger to the west (Allingham, 1960; Snyder and Wagner, 1961; Sides, 1978; this study). These exposures exhibit variously dissected levels from tops of thick ash-flow sheets through subcaldera plutons. Ring intrusions and central plutons adjacent to this exposed terrane are also known from the subsurface (Kisvarsanyi, 1981).