THE DEVONIAN AVON VOLCANIC DISTRICT, SOUTHEASTERN MISSOURI: INSIGHTS INTO THE NORTH AMERICAN MIDCONTINENT LITHOSPHERE

In southeast Missouri, a region of unusual and enigmatic ultramafic diatremes and dykes are exposed in the Avon Volcanic District (AVD). Their ultrabasic nature was initially recognized and described in 1927. The fieldwork of subsequent workers increased the number of outcrops to over 80 making the district comparable to the Urach district in Germany in which 350 diatremes of a similar lithologic character outcrop. The AVD outcrops consist of dikes and deeper exhumed sections of diatremes that intruded during Middle-Late Devonian time based on stratigraphic relations, xenoliths containing Devonian-age fossils, and K-Ar dating. Petrologically, the outcrops of the AVD form a melilitite-alnöite-carbonatite series. Samples studied thus far exhibit a petrological gradation from one type to another, sometimes on a single outcrop scale. Melilitites commonly intrude as dykes in contrast to the alnöites that are typically emplaced as diatremes. Primary carbonatite appears to be restricted to isolated segregations in the melilitite dikes and larger pods in the alnöites. The intrusions of the AVD require a unique mantle source to produce these unusual carbonate rich mafic magmas and a unique event that triggered mantle melting beneath the Midcontinent during the Devonian. One possible trigger is a suggested change in the continental stress field in the late Early Devonian. The Ste. Genevieve Fault system was reactivated at this time possibly due to the increasing subsidence in the Illinois basin and the structural high of the Ozark Dome creating regional tension. A study of the basement and mantle xenoliths will elucidate lithospheric and/or possibly the asthenospheric composition at and prior to the time that the AVD activated. A complementary passive seismic experiment will be conducted to determine crustal thickness, Vp/Vs ratio, Poisson's ratio, Moho “sharpness”, crustal structure, and anisotropy (shear wave splitting) of the lithosphere beneath the AVD. Xenoliths from the basement will further constrain the geophysical data to obtain an accurate 1-D crustal model of the North American Midcontinent. The AVD xenoliths, complemented with geophysical data, provides an opportunity to study basement rocks beneath the veneer of the Eastern Rhyolite & Granite Province and the formation of Proterozoic continental crust.