CONSTRAINING THE ORIGIN AND EVOLUTION OF GRENVILLE-AGED BASEMENT ROCKS IN THE BLUE RIDGE PROVINCE

The Grenville Orogen represents one of the largest geological events in the history of North America. Understanding this major event, therefore, is critically important for understanding the evolution of North American lithosphere. Geochronology plays an important role in this by providing constraints on the timing and duration of the Grenville orogeny and, ultimately, the assembly of Rodinia. To date, geochronology of Grenville-aged meta-igneous basement rocks in the Blue Ridge Province of the Appalachians has focused on how different suites of magmatic rocks relate to specific orogenic phases in the Mesoproterozoic. Previous studies have shown that the meta-igneous rocks were emplaced during two main phases of the Grenville orogeny: the Shawinigan, from 1.18 to 1.14 Ga, and the Ottawan, from 1.08-1.02 Ga. Ages of most zircon overgrowths are similar to the crystallization ages of both Shawinigan and Ottawan plutons. Because some overgrowths are as young as 0.96 Ga, however, the time of metamorphism during the assembly of Rodinia has remained uncertain.

Here we report Lu-Hf garnet dates of leucogneisses from the Blue Ridge Province of the Appalachians. These dates provide direct constraints on the timing of garnet growth, and thereby of metamorphic processes during the Grenville orogeny. Metamorphic garnets from two samples of foliated gneiss that crystallized ~1140 Ma yield Lu-Hf garnet ages of ~1.03-0.97 Ga. These data support the interpretation that the metamorphic and deformation event in these samples was Ottawan. Petrographic analysis of these samples, in conjunction with their ages, clearly demonstrates that these are metamorphic and not magmatic garnets. These data demonstrate that garnet geochronology provides important complementary information to zircon and monazite U-Pb geochronology that allows us to more fully understand the magmatic and metamorphic aspects of tectonic events.