PROTEROZOIC TECTONIC EVENTS RECORDED IN BLUE RIDGE ROCKS, VIRGINIA-NORTH CAROLINA

Circa 100 samples of Mesoproterozoic orthogneiss, paragneiss, pegmatite, and metagranitoid exposed in the core of the allochthonous Late Paleozoic Blue Ridge anticlinorium, VA-NC, were dated to define Proterozoic tectonic events as they relate to the framework of the Grenville Province. Proterozoic orogenies defined in eastern Canada are the ~1.3-1.2 Ga Elzevirian, ~1.19-1.14 Ga Shawinigan, ~1.09-1.03 Ga Ottawan, and the 1.0-0.98 Ga Rigolet.

Compilation of the data reveals: (1) Ages of magmatic suites are ~1.20-1.11 (n=58) and ~1.08-1.00 (n=35). Xenocrystic zircon ages are as old as 1.76 Ga, whereas whole-rock T_DM ages range from 1.67-1.37 Ga, consistent with variable involvement of pre-Grenvillian lithosphere. (2) Sedimentary rocks were deposited unconformably on the magmatic rocks locally at ~1.10-1.00 Ga. They contain detrital zircon eroded from crust as old as 1.75-1.35 Ga and as young as 1.00 Ga. (3) Metamorphic rims on igneous zircon in orthogneiss and metagranitoid formed between 1.17-0.95 Ga. Metamorphic monazite, titanite, and zircon grains formed at 1.06-0.95, 1.03-0.91, and 0.94 Ga, respectively. Metamorphic rims on detrital zircon formed at 1.05-0.95 Ga. (4) Amphibole from 1.18-1.04 Ga orthogneiss, marble, and metagranitoid yields ⁴⁰Ar/³⁹Ar cooling ages of ~1.00-0.92 Ga. Orthogneiss as old as ~1.20-1.14 Ga exhibits strong deformational foliations containing partial-melt leucosomes, whereas ~1.11-1.01 Ga metagranitoid and pegmatite are weakly- or non-foliated. The paragneisses were metamorphosed between ~1.06-0.98 Ga. They exhibit strong foliations that locally developed concordant with the structural grain of foliated orthogneiss. Age of non-foliated pegmatite that intruded foliated paragneiss constrains deformation at ~1.06, ~1.01, and 0.98 Ga at various locales.

In summary, most of the Blue Ridge rocks are associated with the Shawinigan orogeny, which persisted ~50 my longer than in eastern Canada. Sediments were locally deposited during a hiatus between the Shawinigan-Ottawan, but also from 1.06-1.00 Ga during the contractional Ottawan and Rigolet orogenies. Blue Ridge crust adds to the complexity of amalgamation of Rodinia, as the rocks are most similar in age to those along the Grenville Front, rather than the Appalachian inliers and Adirondack Mountains to the north.