17 Ultramafic and Mafic Magmatism

Tuesday, 20 April 2021
8:00 AM - 12:00 PM
Missouri State University- Geography and Geology MS Program Room

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

Paper number: 17-3
Program: Theme Sessions
Time: 8:25 AM – 8:45 AM

Authors

Abstract

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 (Fo86.9-Fo89.9) 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.

Geological Society of America Abstracts with Programs. Vol. 53, No. 3

doi: 10.1130/abs/2021NC-362860

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