TECTONIC SIGNIFICANCE OF THE WESTERN BLUE RIDGE MARBLE HILL HORNBLENDE SCHIST, SOUTHERN APPALACHIANS (USA)

The Marble Hill Hornblende Schist (MHHS) in the southern Appalachian, western Blue Ridge of Georgia (GA) is a metamafic volcanic unit that has been alternatively attributed to Ordovician (i.e. Taconic) or Silurian tectonism. A lack of restrictive age constraints has led some geologists to interpret the MHHS and associated Mineral Bluff Group as part of the Taconic clastic wedge, while other geologists have interpreted these units as the equivalent of the late Silurian or lowermost Devonian Lay Dam Formation in the Talladega belt of Alabama (AL). Improvements and modifications to standard zircon separation techniques allowed for recovery of >900 zircon grains from a sample of the metamafic MHHS, with ~200 grains isotopically analyzed by LA-ICP-MS. While the majority of U-Pb analyses yielded Mesoproterozoic ages (i.e. xenocrysts), a population of primary igneous zircon yielded a preliminary ca. 430 crystallization age for the MHHS protolith, consistent with stratigraphic interpretations that correlate the Mineral Bluff Group in GA with the Lay Dam Formation of AL. Th/U ratios from analyzed zircon are consistent with a volcanic protolith for the MHHS. A middle Silurian age for the MHHS calls into question models that attribute peak dynamothermal metamorphism in this segment of the WBR to the Taconic orogeny. Instead, stratigraphic, geochemical, and tectonic constraints suggest the MHHS may represent the youngest mafic volcanic unit associated with the extensional, Laurentian plate, Wedowee-Emuckfaw-Dahlonega back-arc (WEDB) basin. The WEDB, represented by metamorphosed sedimentary-volcanic rocks and silicic plutons in the western Blue Ridge-equivalent Talladega belt and eastern Blue Ridge of AL-GA, formed in a marginal basin between the Lower Paleozoic, Laurentian platform and Iapetus-facing Dadeville complex arc terrane. Metavolcanic alkaline rocks of the Silurian MHHS, which were erupted unconformably atop the Ordovician Murphy Marble (i.e. Laurentian shelf), suggest a renewed period of extension on the overriding Laurentian plate at the distal margin of the back-arc, similar to alkaline mafic rocks erupted into Cenozoic back-arc systems.