Current understanding of pre-Appalachian tectonic evolution in the southernmost Appalachians, Pine Mountain Window, Alabama and Georgia.

Steltenpohl, M., Heatherington, A., and Mueller, P., 2004, Geological Society of America Memoir 197, p. 633-646.

Abstract

The Pine Mountain window contains the Appalachians’ most southern Grenville basement massif. Granulite- and upper-amphibolite-facies granitic gneisses that form the basement complex are isotopically dated at 1.0-1.1 Ga. Locally, the gneisses contain rare mafic injections and supracrustal and plutonic xenoliths.            The Pine Mountain Group cover sequence nonconformably overlies Grenville basement and is interpreted to correlate with Blue Ridge units as follows: Halawaka/Sparks 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 were 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 extending southward beneath the Gulf Coastal Plain. In contrast to upper amphibolite and granulite facies metamorphism of the basement during the Grenville event, cover rocks contain staurolite and staurolite-kyanite zone assemblages reflecting Paleozoic, Appalachian metamorphism. SHRIMP and conventional single-grain U‑Pb dating of detrital zircons from the basal Hollis Quartzite document a distinct population of clear, subrounded zircons of ~1.09 Ga, which were most likely derived from underlying Grenville‑age gneiss.  An older, white/gray population found in the lowermost Hollis is ~2.3-2.4 Ga, an age restricted to Gondwanan continents and very limited occurences in northern Laurentia. 

            Tectonic reconstructions of Unrug (1997) and others depict SE Laurentia proximal to the Amazonia and Rio de la Plata cratons during the Neoproterozoic, offering the possibility that they may be the source for 2.3‑2.4 Ga zircons in Hollis sediments.  Alternatively, the AUSWUS reconstruction (Karlstrom et al., 2001) places East Antarctica and the Australian Gawler craton, both of which contain abundant 2.4 Ga granites, proximal to the southwestern US during this time.  Depending on the stream systems present during the Neoproterozoic, zircons from the Gawler may have been transported to the vicinity of the Pine Mountain window. In addition, three clear zircons yield ages of 1.4 Ga, and may have been derived from either the Laurentian mid‑continent granite‑rhyolite province or the Rondonian Province of South America.  AChilhowee Group sandstone contains a similar mixture of Grenville and mid-continent/Rondonian-age zircons, but none with ages of 2.3-2.4 Ga.

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