OUR CURRENT UNDERSTANDING OF THE GRENVILLE EVENT IN THE SOUTHERNMOST APPALACHIANS, PINE MOUNTAIN WINDOW, ALABAMA

The Pine Mountain window (PMW) contains the Appalachians' most southern Grenville basement massif. Granulite and upper-amphibolite facies granitic gneisses that form the basement complex are isotopically dated at 1.1-1.2 Ga. Locally, the gneisses contain rare mafic injections and supracrustal and plutonic xenoliths. The Pine Mountain Group cover sequence nonconformably overlies the Grenville basement, and is interpreted to correlate with Blue Ridge units as follows: Halawaka Schist=Ocoee Supergroup (late Proterozoic, rift), Hollis Quartzite=Chilhowee Group (late Proterozoic-Cambrian, rift-to-drift), and Chewacla Marble=Shady Dolomite (Cambro-Ordovician, drift). Facies variations within the sedimentary cover units have been cited as evidence for a southward decrease in the extent of the Ocoee rift basins, but new mapping documents continuity of thick packages of Halawaka (i.e., Ocoee) rocks southward beneath the Gulf Coastal Plain. SHRIMP and conventional single-grain U-Pb dating of detrital zircons from the basal Hollis Quartzite can be used to characterize possible source areas for the PMW's cover sequence. A distinct population of clear to subrounded zircons is ~1.1 Ga and most likely derived from the underlying Grenville-age gneiss. An older, white/gray population is ~2.2-2.4 Ga, an age restricted to Gondwanan continents of the Atlantic realm. Tectonic reconstructions of Hoffman (1991) and others depict SE Laurentia proximal to the Kalahari and Congo cratons during the Neoproterozoic, offering the possibility that they may be the source for the 2.2-2.4 Ga zircons in the Hollis sediments. In addition, three clear zircons are 1.37, 1.35 and 1.39 Ga, and may have been derived from either the mid-continent granite-rhyolite province or the Rondonian province of South America. Similar analyses performed on zircons from a Chilhowee Group sandstone, a quartzite from the Piedmont terrane, and xenoliths from the PMW basement gneiss appear to contain a similar mixture of Grenville and mid-continent/Rondonian-age zircons, but lack the older ~2.2-2.4 Ga population. The combination of apparent Gondwanan and probable Laurentian zircons in these rocks suggests that deposition of the Hollis may have begun prior to, or coincident with, the separation of the PMW from Gondwana.