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 (SiO 2 = 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: SiO 2 , Na 2 O, and F increase toward the base of the sheet, whereas Al 2 O 3 , MgO, CaO, K 2 O, and TiO 2 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.

The Central and Feather River peridotite belts of the northern Sierra Nevada metamorphic belt constitute a major suture zone between Paleozoic–early Mesozoic continental-margin rocks (Shoo Fly Complex and superjacent strata) of the Eastern belt and Jurassic arc and ophiolitic rocks (Smartville Complex) of the Western belt. This suture zone is structurally complex and has previously been described as mélange. Our data suggest that six major fault-bounded rock assemblages are present across this zone. The faults are isoclinally folded and transposed along steep hinge planes, but have shallowly dipping enveloping surfaces. Rocks of the Eastern belt occupy the highest of five east-dipping thrust sheets which are technically overlain by a sixth, west-dipping thrust sheet. The Western belt rocks are built into a basement composed of the last thrust sheet and postdate the thrust faults. All these rocks and structures are cut by faults of the Late Jurassic Foothills fault system which bound the lithotectonic “belts” (Eastern belt, etc.). The Feather River peridotite belt consists of the Feather River peridotite and the Red Ant Schist, and is extended to include the newly recognized Devils Gate ophiolite. The Feather River peridotite is correlative with or intruded by the Devils Gate ophiolite, and together they constitute a nearly complete cogenetic or polygenetic Paleozoic ophiolite. The Red Ant Schist contains metasedimentary and metavolcanic rocks with local blueschist parageneses; blueschist facies metamorphism is early Mesozoic or older. Several small outliers of Shoo Fly (Eastern belt) sandstone are present in the Feather River peridotite belt. The Central belt consists of the Calaveras Complex, the Fiddle Creek Complex, and the Slate Creek Complex. The late Paleozoic–early Mesozoic Calaveras Complex is an assemblage of phyllite-diamictite, chert, and minor volcanic rocks and marble in the eastern part of the Central belt. The Fiddle Creek Complex lies west of the Calaveras Complex and contains an intact stratigraphic succession, which includes, in ascending stratigraphic order, late Paleozoic ophiolitic mélange, pillow basalt with minor felsic tuff, Middle Triassic–Early Jurassic volcaniclastic and hemipelagic sedimentary rocks, and Middle–Late Jurassic(?) quartzose clastic rocks. The Slate Creek Complex is an Early Jurassic pseudostratigraphic sequence that contains a basal serpentinite-matrix mélange overlain by plutonic and volcanic rocks. The Western belt consists of the Middle–Late Jurassic (160 Ma) volcanic and intrusive rocks of the Smartville Complex and, at one locality, older tonalitic basement equivalent to the Slate Creek Complex. The Slate Creek Complex was juxtaposed against the other rock assemblages before formation of the Smartville Complex, so the Smartville Complex formed in situ. Crosscutting relations define three generations of macroscopic structures. The earliest structures include major mapped and cryptic, west-vergent (east-dipping) thrust faults that juxtapose, in descending structural order, the Shoo Fly Complex (Eastern belt), Feather River–Devils Gate Ophiolite, Red Ant Schist, Calaveras Complex, and Fiddle Creek Complex. These faults predate the east-vergent “Slate Creek thrust,” which carries the Slate Creek Complex over the Fiddle Creek Complex, Calaveras Complex, and Red Ant Schist. The Slate Creek thrust is cut by a pluton dated at about 165 Ma, which places an upper age constraint on assembly of the nappe pile. This nappe pile is cut and overprinted by the steep throughgoing faults (Foothills fault system), folds, and cleavages that dominate the structure of the area; this youngest set of structures makes up the Late Jurassic Nevadan orogeny. Eastward overthrusting of the Slate Complex along the Slate Creek thrust occurred after amalgamation of the other units. Eastward overthrusting was followed by rifting and arc magmatism represented by the Smartville Complex, then by Nevadan faulting, folding, and penetrative deformation. These data preclude the existence of a Late Jurassic suture or collision, but the Fiddle Creek and Slate Creek Complexes, both interpreted as remnants of Early Jurassic arcs, could have collided in the late Early Jurassic or early Middle Jurassic.