OLIVINE AND PHLOGOPITE IN AN ALNÖITE DIATREME: INSIGHTS INTO THE MAGMATIC EVOLUTION OF THE AVON ALKALINE IGNEOUS PROVINCE, MISSOURI

Alkaline Ultramafic Carbonatite (AUC) complexes, although rare, are valued for diamonds and rare earth elements, and are useful for understanding subcontinental and mantle processes as recorded by mineral textures and rock fabrics. We report specifically on olivine and phlogopite relationships from an alnöite diatreme pipe of the Devonian (386 +/- 1Ma Freeman, 2016) Avon Alkaline Igneous Province, Ste. Genevieve County, Missouri. This pipe exhibits domains of crystal-rich ovoid “lapilli” set in a crystal rich matrix. Crustal xenoliths (e.g., granite) and xenocrysts (quartz, biotite) are common in these rocks. Olivine occurs as subhedral, partially embayed, variably serpentinized, phenocrysts. While macrocrysts ~2 cm in length are conspicuous, olivine exhibits a continuous size distribution with smaller crystals. Olivine phenocrysts are highly fractured and have a “shattered” appearance. Alteration occurs from the crystal margins inward and along fractures. Islands of unaltered olivine are homogeneous, Mg-rich (Fo_86.9-Fo_89.9), and exhibit variation in trace element (e.g., Ni, Cr, Co, Ti, P) abundances consistent with fractional crystallization. Olivine phenocrysts typically have dark oxide rims, are embayed, and exhibit “channels” filled with matrix and commonly contain phlogopite phenocrysts. Phlogopite phenocrysts occur along the margins of olivine and as tabular isolated mm size phenocrysts or in clusters. We interpret these textures to indicate early crystallized olivine became unstable and reacted with the melt to form phlogopite. This reaction may reflect decompression during ascent (Grant et al., 2014) which also could be responsible for the “fracturing” of olivine, or it could reflect crustal contamination of the melt (e.g., K, Al, Si, H₂O) as the magma drilled upward through the granite-rhyolite terrane and overly Paleozoic cover, or interaction between K-rich carbonatitic melt and a mafic mantle derived melt during ascent (Shavers et al., 2016).