A SILURIAN AGE FOR THE MARBLE HILL HORNBLENDE SCHIST: IMPLICATIONS FOR IGNEOUS ACTIVITY AND METAMORPHISM IN THE SOUTHERN APPALACHIANS

The Marble Hill Hornblende Schist (MHHS), at the stratigraphic base of the Mineral Bluff Group, lies unconformably above the Cambrian Murphy Marble and Brasstown Formation in the core of the western Blue Ridge (WBR) Murphy synclinorium in Georgia (USA). A lack of restrictive age constraints has led some workers to associate the MHHS and sub-Mineral Bluff unconformity with the Ordovician Taconic orogeny, while others have proposed a Silurian or Devonian age for these features. In an attempt to determine the age of the MHHS and sub-Mineral Bluff unconformity, we optimized the zircon separation process typically used for silicic igneous rocks in order to recover a population of zircons from the metamafic MHHS. 50 zircons recovered from 800 g of MHHS yielded predominantly Mesoproterozoic ages, but 4 grain analyses (ICP-LA-MS) yielded a concordant age of 430.3 ± 7.5 Ma. Th/U ratios from the Paleozoic grains were >0.1, consistent with other geochemical, textural, and map constraints suggesting a metavolcanic protolith for the MHHS. Subsequently, >150 additional zircon grains from ~1.4 kg of MHHS were submitted to the U.S. Geological Survey G3SC-Plasma Lab in Denver, CO for geochronologic analysis. A Silurian age for the MHHS is inconsistent with interpretations of the sub-Mineral Bluff unconformity and Mineral Bluff Group as the Taconic unconformity and Taconic clastic wedge, respectively. Instead, a ca. 430 Ma MHHS suggests the WBR sub-Mineral Bluff unconformity is the equivalent of the Talladega Slate belt sub-Lay Dam unconformity, and a similar structure in the WBR Foothills Belt, all of which can be attributed to Early Paleozoic back-arc extensional tectonics along the Laurentian plate margin. If correct, the MHHS represents a distal, back-arc, mafic alkaline volcanic suite recording the youngest volcanism associated with a more extensive back-arc basin (Wedowee-Emuckfaw-Dahlonega) primarily recognized from rocks in the eastern Blue Ridge. A Silurian age for the MHHS also calls into question the nature and extent of Taconic (i.e. Ordovician) metamorphism in the WBR. Our preliminary data is consistent with models that recognize other post-Ordovician successor basins across the WBR and attribute peak dynamothermal metamorphism in the southern Appalachian WBR to the Late Paleozoic, Neoacadian/Alleghanian orogeny.