NEW CONSTRAINTS ON ORDOVICIAN MAGMATISM IN THE SOUTHERNMOST EXPOSURES OF THE EASTERN BLUE RIDGE IN ALABAMA

In the early 1990’s, one of the authors (Steltenpohl) suggested that the Opelika Complex in central Georgia and Alabama was erroneously assigned to the Inner Piedmont and more likely correlates with the Emuckfaw Group of the eastern Blue Ridge. Both the Opelika Complex and the Emuckfaw Group comprise mostly meta-arkosic schist and gneiss invaded by, respectively, the Farmville and the Kowaliga and Zana metagranites. The Emuckfaw Group occurs along the west limb of the major Tallassee synform whereas the Opelika Complex forms the east limb. The hinge of this NE-plunging synform is covered by Gulf Coastal Plain sediments rendering physical connection by surface mapping impossible. Our detailed, 1:24K geologic mapping, structural analysis, petrography, and geochemical work documents that the granitic intrusions on both limbs also are similar. Earlier geochronological studies had suggested similar Ordovician intrusive ages, but those data were considered suspect without confirmation by dating with more modern methods, mainly because pre-Devonian tectonism and/or magmatism in the southernmost exposed Appalachians of Alabama has been questioned. In 1997 authors Steltenpohl and Mueller reported U-Pb SHRIMP dates on zircons from the Farmville granite (previously dated at 369 Ma based on Rb/Sr whole-rock methods) with a large age range from ~477 to ~425 Ma. We report 36 additional analyses that are similarly spread but give a 440 +/- 7 Ma estimate for the time of igneous crystallization; three rim analyses with low (<0.1) Th/U (likely metamorphic) average 329 Ma. Total population zircon dates on the Kowaliga and the Zana granites reported in 1978 were, respectively, ~458 Ma and ~460 Ma. We report new U-Pb SIMS dates on zircons from the Kowaliga granite that yield an age of crystallization of ~450 Ma with no systematic or significant variations between the cores and rims. New whole-rock major-oxide, LOI, and REE-element analyses demonstrate petrogenetic relationships between the Kowaliga and Zana granites, supporting our working hypothesis that the Zana likewise is an Ordovician granite. Results strengthen our arguments that the Emuckfaw correlates with the Opelika Complex, and also that Ordovician magmatism is more extensive in the southernmost Appalachians than was previously thought.