EVIDENCE FOR EXTENSIONAL TECTONICS AND ITS RELATIONSHIP TO THE ORIGIN OF THE PRECAMBRIAN ST. FRANCOIS MOUNTAINS, SOUTHEAST MISSOURI

Geological mapping (involving numerous students) in the older Precambrian rocks of the southeast portion of the St. Francois Mountains suggests an extensional origin along an ENE-WSW axis. Rock compositions are predominately high-silica dacites to low-silica rhyolites with plagioclase as the dominant phenocryst. Lavas are equal to or greater in volume than ash-flow tuffs, and there is seemingly a lack of caldera structures. One unit, the ‘black' rhyolite lava (68% SiO2 and 5% phenocrysts including 0.5% clinopyroxene), strikes along this trend of ENE-WSW and transited the crust with little fractionation to form voluminous flow domes.

The bulk of the St. Francois Mountain extrusives, younger Precambrian rocks to the north and west, are more evolved and originated from fractionating, high-level magma chambers. These rocks are medium- to high-silica rhyolites that formed large ash-flow sheets, considerably less abundant high-silica lavas and domes, and exhibit typical caldera structures.

Recent literature suggests that the origin of this terrain may involve extensional tectonics, possibly continental ‘back-arc' rifting, and the melting of juvenile crust formed by subduction at the Laurentian plate margin. As stretching occurred, crustal thinning and fracturing would lead to volcanism. If this is the origin of these older rocks in the southeast St. Francois Mountains, volcanism must have then moved north and west with an increase in melting, crustal thickening and ponding of magmas, with subsequent fractionation and caldera formation.

The field evidence from this work lends support to a local extensional event in the older SE portion of the St. Francois Mountains that may have regional implications. Rock compositions overall are less evolved, and the ‘black' rhyolite conforms to the composition of source magmas that have been hypothesized by several workers.