SEDIMENTATION, GRANULITE-FACIES METAMORPHISM, DEFORMATION, AND IGNEOUS INTRUSIONS DEFINE MULTIPLE TECTONIC EVENTS ASSOCIATED WITH THE OTTAWAN-RIGOLET OROGENIES IN PROTEROZOIC ROCKS OF THE APPALACHIAN BLUE RIDGE

Granulite-facies paragneiss suites occur within a complex of orthogneisses (~1.20-1.11 Ga, n=58) and metagranitoids (~1.08-1.00 Ga, n=35) in the Appalachian Blue Ridge (BR). Exposed in the Shenandoah (SM, north) and French Broad (FBM, south) massifs, the paragneiss crops out at 45 localities in NW-striking belts.

Metasedimentary rocks are quartz-plagioclase-garnet-graphite-biotite-sillimanite-pyroxene gneisses, quartzite, and rare marble, that are often associated with amphibole-orthopyroxene-plagioclase gneiss. Detrital zircon from 18 samples were dated by SHRIMP. Samples from 6 localities include U-Pb ages of detrital zircon, monazite, and titanite, U-Pb ages of zircon from cross-cutting igneous rocks, and Lu-Hf and Sm-Nd ages of garnet. The presence or absence of foliation constrains the relative times of deformation.

The paragneiss was derived from erosion of local igneous rocks but also contain ~1.8-1.7 Ga zircon. Unlike in the Adirondacks and the massifs from VT to NJ, BR paragneiss was not the ~1.3-1.2 Ga country rock into which ~1.2-1.0 Ga igneous rocks intruded. These results suggest that the BR contains several tectonic blocks defined by depositional and metamorphic events that occurred within time intervals of ~10-20 m.y. from ~1.10-0.90 Ga. The ages of the events vary across the NW-trend. The oldest (~1.10-1.09 Ga) and youngest (~1.00-0.99 Ga) events were in the NE and SW portions of FBM, respectively. The youngest rocks were previously interpreted as accreted Amazonian crust. Six blocks in the SM record events decreasing northeastward from ~1.08-1.07 to ~1.03-1.00 Ga. Ti-in-Qtz and Gt Gd/Dy thermobarometry of 4 paragneiss samples in the SM constrain metamorphic conditions to 770-900°C and 0.58-0.83 GPa which differ from some blocks.

Multiple depositional and metamorphic events from ~1.10-0.90 Ga demonstrate complexities within the timeframe of the Ottawan (~1.09-1.03 Ga) and Rigolet (~1.0-0.98 Ga) orogenies as defined in Canada and the Adirondacks. BR data support accretionary/thickened crust during amalgamation of Rodinia. Tectono-metamorphic events were out-of-sequence in a collision zone of continental crust that was oriented NW-SE. The Late Neoproterozoic rifted margin of Laurentia transected this framework complicating reconstructions of Rodinia.