PELLETAL LAPILLI IN ULTRAMAFIC DIATREMES, AVON VOLCANIC DISTRICT, MISSOURI

The Avon Volcanic District (~275km²) in southeastern Missouri consists of more than 80 separate diatremes and dikes that were emplaced during the Devonian. Dikes and magmatic components of the diatremes are ultramafic and have been variously described as melilitite, alnoite, carbonatite, and kimberlite. The goal of our research is to locate and sample several of the sites described by Kidwell (1947) and study the magmatic components of these rocks within the modern framework recently developed for ultramafic rocks. Because these rocks are poorly exposed within the subdued topography of the Cambrian sandstones and carbonates, often deeply weathered, and commonly located on private property, only 4 diatremes have been sampled to date. The juvenile component of the diatremes occurs mostly as conspicuous pelletal lapilli whose primary mineralogy has been mostly replaced. Lapilli are typically smooth ovoids 0.5-2 mm in diameter, but can be up to 4 cm. Juvenile material also occurs in lapilli with rough surfaces or in irregular shaped clasts. Pelletal lapilli are usually cored by relict euhedral olivine megacrysts, and occasionally by euhedral magnetite and clinopyroxene; some are cored by lower crustal xenoliths as well. The lack of upper crustal xenoliths as lapilli cores suggests that lapilli formed at upper mantle or lower crustal depths. Lapilli cores are surrounded by a fine grained matrix of olivine melilitite consisting of relict aligned melilite laths, relict olivine microphenocrysts, and unaltered perovskite and oxides. One sample contained lapilli with a thinner outer rim of coarse interlocking calcite (carbonatite) surrounding the olivine melilitite. These lapilli were set in a matrix of coarse calcite as well. The texture and mineralogy of these dual composition lapilli possibly records the separation of a carbonatitic melt from the melilitic magma during ascent. In order to test this hypothesis, we intend to study the petrography of additional samples from the district, conduct mineral separations, and further characterize the primary mineralogy of these carbonatite-melilitite rocks.