Calderas in the Precambrian terrane of the St. Francois Mountains, southeastern Missouri

Abstract

Precambrian igneous rocks in the St. Francois Mountains of southeastern Missouri consist of silicic volcanic rocks, mostly ash flow tuff, which crop out mostly in the western and southwestern part of the region, and epizonal granitic plutons which are mostly exposed in the northeastern part of the area. These relations suggest that the exposures in the northeast are of a somewhat deeper part of the crust and that rocks there are now exposed because of west‐southwesterly tilting and beveling by erosion. In the eastern St. Francois Mountains the occurrence of a thick rhyolitic ash flow tuff, Grassy Mountain ignimbrite, and its distribution relative to a large, high‐silica pluton, Butler Hill granite, suggest that the ash flow is the principal eruptive material of a caldera, the Butler Hill caldera, and that Butler Hill granite is the frozen magma chamber that was originally beneath it. Structural relations indicate that Butler Hill granite was emplaced into the volcanic rocks of the caldera during resurgent doming. Four small low‐silica plutons, Silvermine, Stono, Knoblick, and Slabtown granites, are distributed peripherally to the Butler Hill granite and are interpreted as ring plutons emplaced along the ring fracture of the caldera or along arcuate intracaldera faults, after resurgent doming and emplacement of Butler Hill granite. More than 3 km of silicic volcanic rocks occupy a basinal structure in the western St. Francois Mountains. These rocks lie with angular unconformity upon up‐domed older rocks of the Butler Hill caldera along an arcuate zone of megabreccia and disturbed structure on the western margin of the older caldera. This feature is interpreted as the topographic wall of a second caldera which lies to the west. The occurrence of the thick Taum Sauk rhyolite stratigraphically about 3 km higher than the rocks involved in the megabreccias may indicate that there was yet another caldera formed in this western part of the St. Francois Mountains.

Proterozoic geology of the east-central Midcontinent basement

“…Sides et al (1981) interpreted this distribution to indicate that the whole region was tilted to the west-southwest such that deeper portions of the complex are exposed in the east. Sides et al (1981) and Nusbaum (1980) …”

“…ä V / // // // // / n n 1 / // < '///// / o r / // // // // /n il/ / eastern Missouri (e.g., Kisvarsanyi, 1981;Sides et al, 1981;Pratt et al, 1979). Here about 900 km2 is underlain by rhyolite, dacite, and related granitic plutons (Fig.…”

“…The only major outcrops of Precambrian rocks in this region are in the St. Francois Mountains of southeastern Mis souri (Bickford et al, 1981b;Sides et al, 1981;Kisvarsanyi, 1979Kisvarsanyi, ,1981 and the Arbuckle Mountains of southern Oklahoma (Denison, 1973;Denison et al, 1984); small outcrops also are present in the Spavinaw Creek region of northeastern Oklahoma (Denison, 1982). Because most Precambrian basement in the Midcontinent is buried beneath a few hundred meters to several kilometers of Phanerozoic platform cover, characterization of the basement requires several other sources of data, including (1) compilations of lithology using cuttings and cores from drill holes that penetrate the basement; (2) geochronologic and geo chemical data for subsurface samples, particularly U-Pb zircon ages, Sm-Nd model ages, and major and trace element abun dances; and (3) regional geophysical data, particularly mag netic anomaly maps that show features in the upper part of the crust, filtered gravity anomaly maps that show deeper-seated crustal features, and seismic surveys.…”

Detrital zircon evidence for non-Laurentian provenance, Mesoproterozoic (ca. 1490-1450 Ma) deposition and orogenesis in a reconstructed orogenic belt, northern New Mexico, USA: Defining the Picuris orogeny

“…Precambrian rocks in the St. Francois Mountains, ~1400 km to the east in Missouri, are interpreted to represent the remains of a ca. 1480-1400 Ma caldera complex (Sides et al, 1981;Van Schmus et al, 1996;Menuge et al, 2002) and could be a possible distal source for the meta-tuff.…”

Geochemistry of subsurface Proterozoic rocks in the eastern Midcontinent of the United States: Further evidence for a within-plate tectonic setting

“…Outcrops of Precambrian rocks occur in the St. Francois Mountains of southeastern Missouri (Sides, 1980;Bickford et al, 1981;Cullers et al, 1981;Sides et al, 1981), the Wolf River batholith of central Wisconsin (Anderson and Cullers, 1978), and the fel sic plutons of southern Manitoulin Island (Van Schmus et al, 1975). Inliers of 1.76 Ga rhyolite and granite in south-central Wisconsin, previously included in the Penokean orogen (Smith, 1978), have mainly A-type chemical affinities and are now known to postdate the Penokean orogeny (Smith, 1983(Smith, , 1993.…”