TWO PHLOGOPITE GENERATIONS IN AN ALNÖITE DIATREME: INSIGHTS INTO THE PETROGENESIS OF THE AVON ALKALINE IGNEOUS PROVINCE, MISSOURI

Alkaline Ultramafic Carbonatite complexes are windows into subcontinental mantle processes. We report on olivine and phlogopite from the 386 +/- 1Ma alnöite diatreme-facies of the Avon Alkaline Igneous Province Missouri. The alnöite is a mixture of domains of olivine pelletal lapilli (OPL) in a crystal-rich melilite matrix. Olivine is restricted to OPL occurring as subhedral, embayed, variably serpentinized, ovoid phenocrysts rimmed by oxides +/- phlogopite. While ~2 cm macrocrysts are conspicuous, a continuous size distribution to mm-size crystals is present. Olivine is fractured and serpentinized. Small islands of relict unaltered olivine are homogeneous, Mg-rich (Fo_86.9-Fo_89.9) with trace element (e.g., Ni, Cr, Co, Ti, P) variation consistent with fractional crystallization. Larger olivine exhibit embayment channels that contain euhedral phlogopite phenocrysts and melilitite matrix. Tabular, isolated mm-size phlogopite also occur along the margins of olivine. Phlogopite, texturally distinct from these phenocrysts, occur as anhedral, oxide-rich crystals in palmate porphyroblast clusters rimming serpentinized olivine.

We interpret these igneous textures, with respect to the evolution of the alnöite diatreme. Late-Devonian, collisional tectonics lead to partial melting of the mantle, forming a mafic magma and a carbonatitic melt (Shavers et. al, 2016). Euhedral olivine crystallized as the liquidus phase from the mafic magma. The first generation of phlogopite formed as a result of assimilation of hydrated altered felsic crust (2Mg₂SiO₄ +2Al₂O₃ + KAlSi₃O₈ + H₂O → KMg₃AlSi₃O₁₀(OH)₂ + MgAl₂O₄ + 2SiO₂ + 1.5 O₂). Crustal assimilation was likely facilitated by fragmentation during simultaneous intrusion of the carbonatitic and mafic magma. Increased H₂O activity promoted serpentinization of olivine (3Mg₂SiO₄ + SiO₂ + 4H₂O → 2Mg₃Si₂O₅(OH)₄). During decompression, exsolution of CO₂ leads to formation of an aerosol and pelletal lapilli. As XCO₂/XH₂O increases, serpentine was converted into palmate porphyroblastic phlogopite around serpentinized OPL (Mg₃Si₂O₅(OH)₄ + KAlSi₃O₈ → KMg₃AlSi₃O₁₀(OH)₂ + 2SiO₂ + H₂O) by lowering the activity of H₂O. The petrographic complexity of this rock demonstrates a pristine mantle geochemical signature is retained in the unaltered relict "islands" of olivine.