Multiple generations of phlogopite in an alnöite diatreme: Insights into the petrogenesis of the Avon Alkaline Igneous Province, Missouri

“Alkaline Ultramafic Carbonatite (AUC) complexes, although rare, are valued for diamonds and REEs, and as windows into subcontinental and mantle processes as recorded by rock fabrics, mineral spatial relationships, and mineral compositions. We report specifically on olivine and phlogopite petrographic relationships and compositions from the Devonian alnoite diatreme-facies of the Avon Alkaline Igneous Province, Missouri. The diatreme alnoite is a mixture of domains of olivine magmaclasts (OM) set in a crystal-rich melilite matrix. OM are highly fractured, serpentinized olivine pseudomorphs with variable amounts of pristine olivine fragments. Larger olivine domains typically exhibit clear crystal faces and irregular “channels” indicative of a skeletal morphology due to rapid growth. Islands of unaltered olivine are homogeneous, Mg-rich (Fo86.9-Fo89.9) and exhibit variation in trace element (e.g., Ni, Cr, Co, Ti, P) abundances consistent with limited fractional crystallization and origin from a metasomatized fertile peridotite partial melt. Three textural varieties of phlogopite are recognized. Tabular, euhedral phlogopite (phl-Ia) crystals occur along the margins of OM and within the melilite matrix. Small (mm-size) euhedral to subhedral phlogopite (phl-Ib) phenocrysts occur inside the “channels” of skeletal olivine. Phlogopite (phl-II) crystals occur as anhedral, oxide-rich, palmate porphyroblast clusters, rimming serpentinized olivine and scattered throughout the matrix. These phlogopite populations are geochemically distinct, showing ion substitution enrichments (i.e. Ba, Al, Ti). These mineral and geochemical relationships enable the development of a comprehensive petrogenetic model of the Avon Alkaline Igneous Province AUC diatreme complex”--Abstract, page iv