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*Corresponding author: dwrankin@usgs.gov

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 toward the base. The ash-flow tuff is a high-silica rhyolite (SiO2 = 76.6 wt%); the basal one-sixth of the sheet is interpreted as originally peralkaline, whereas the remainder is metaluminous. Major- and trace-element zoning within the sheet is similar to other well-documented ash-flow sheets: SiO2, Na2O, and F increase toward the base of the sheet, whereas Al2O3, MgO, CaO, K2O, and TiO2 decrease. Concentrations of Be, Rb, Zr, Nb, Sn, Hf, Ta, Th, U, Tb, and Yb increase toward the base; elements more abundant toward the top include Sc, Sr, Ba, La, Ce, Nd, and Eu. These gradients developed in a high-level silicic magma chamber in which peralkaline high-silica rhyolitic magma overlay metaluminous high-silica rhyolite. The 765–740 Ma Crossnore Complex, which includes the Mount Rogers Formation, in the Grenvillian French Broad massif includes A-type granitoids and is interpreted to reflect aborted rifting of Laurentian continental crust. The locus of aborted rifting migrated northeastward to the Shenandoah massif, where A-type magmatism occurred at 735–680 Ma. Continental breakup and opening of the Iapetus Ocean followed Late Neoproterozoic (ca. 572–554 Ma) rifting. Formation of a voluminous high-silica, partly peralkaline magma chamber indicates that the initial pulse of rifting took place in relatively thick continental crust, perhaps explaining why this pulse did not culminate in continental breakup until ∼200 m.y. later.

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