CHARACTERIZATION OF THE ULTRAMAFIC DIATREMES AND DIKES OF THE AVON MAGMATIC DISTRICT, SOUTHEASTERN MISSOURI, USA

The Avon Magmatic District of southeastern Missouri, is part of a broader region of long lived mid-to late Paleozoic alkalic ultramafic magmatism in the North American Mid-continent. The AMD consists of numerous (~80) known outcrops of diatremes and dikes over an area of ~ 195 km². Outcrops are poor to moderately well exposed, variably altered, and are very heterogeneous in composition and texture, commonly containing numerous xenoliths of Precambrian basement and lower Paleozoic cover rocks. All outcrops occur within Cambrian sedimentary rocks, and fossils from chert xenoliths, and published K/Ar dates (377-388 +/-20 Ma Zartman, 1977) indicate a mid-Paleozoic emplacement age. The current exposure level of the outcrops consists of the root zone and diatreme facies of Mitchell (1986). Olivine melilitite dikes and diatremes with pelletal lapilli are pervasive throughout the district. The olivine dikes primarily consist of hopper olivine and titaniferous augite set in a groundmass of melilite laths and perovskite. The diatreme facies exposures typically consist of phenocrysts of olivine and/or clinopyroxene commonly replaced by calcite in a groundmass that has altered to chlorite and carbonate minerals. Autoliths, xenocrysts, and xenoliths are also common and contribute to the heterogeneous nature of these rocks. Pelletal lapilli typically characterize diatreme facies however, they have been found in one dike exposure. One diatreme in particular, characterized as an alnöite by other workers, exhibits intriguing silicic-carbonatitic segregations suggestive of possible silica-carbonate immiscibility or incomplete magma mixing. Megacrysts of olivine and phlogopite along with lower crustal xenoliths indicates an upper mantle source. Paleozoic alkalic ultramafic magmatism, similar in character to the AMD is known to occur over a broad region throughout the middle Mississippi Valley (e.g., Dent Branch-Bee Creek District, Wauboukigou Alnöite District) with melitite being a common characteristic mineral and define the Middle Mississippi Alkaline Province.  The alkaline magmatism in southern Arkansas and Gulf coast states is younger (Cretaceous) and lacks melitite.  This suggests different mantle sources were sampled at different times to form these two distinct alkaline magmatic provinces.