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The 760 Ma Wilburn Rhyolite Member of the Mount Rogers Formation, in southwestern Virginia, is a mineralogically and compositionally zoned welded ash-flow sheet at least 660 m thick. Compositional zoning, preserved despite greenshist-facies metamorphism, developed in the pre-eruptive magma chamber and was inverted during eruption of the ash-flow sheet. Microphenocrysts of aegerine and riebeckite occur at the base of the sheet, riebeckite alone or with biotite at higher levels, and Fe-rich biotite at the top. Alkali feldspar phenocrysts are more potassic, and riebeckite and biotite exhibit decreasing Mg/Fe toward the top of the ash-flow sheet; F content of biotite increases...

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