AN ATTEMPT TO RESOLVE THE AGE OF THE SUB-MINERAL BLUFF UNCONFORMITY: EXTRACTING ZIRCON FROM THE MARBLE HILL HORNBLENDE SCHIST

The Marble Hill Hornblende Schist lies at the base of the Mineral Bluff Group in the western Blue Ridge of Georgia, unconformably above the Cambrian-Ordovician Murphy Marble. The age of the Marble Hill Hornblende Schist and the unconformity at the base of the Mineral Bluff Group has been a subject of debate amongst geologists for decades due to poor age constraints on the unconformity itself. Many geologists have associated the sub-Mineral Bluff Group unconformity with the Appalachian foreland, post-Knox Group unconformity, which is generally attributed to the onset of Ordovician Taconic orogenesis. Other geologists have argued that stratigraphic similarities to the sub-Lay Dam unconformity in the Talladega Slate belt of Alabama suggests the sub-Mineral Bluff unconformity is either Silurian or Devonian in age. Ultimately, a crystallization age for the metavolcanic Marble Hill Hornblende Schist would resolve the issue. Unfortunately, the unit has not been radiometrically dated due to the low silica concentration of the protolith and lack of abundant zircon commonly used to determine crystallization ages for metaigneous rocks. In an attempt to extract dateable minerals from the Marble Hill Hornblende Schist, we optimized the mineral separation process typically used for extracting zircon from granitoids by significantly reducing the granular feed rate on a water shake table, followed by standard magnetic separation using a Frantz isodynamic separator and heavy liquid separation in methylene iodide. Recovered heavy mineral grains were handpicked under a short-wave ultraviolet light. Using this low feed rate separation process, approximately 50 anhedral to subhedral zircons were recovered from 800 grams of Marble Hill Hornblende Schist, and have been submitted to the U.S. Geological Survey G3SC-Plasma Lab in Denver, CO for geochronologic analyses. If the recovered zircons yield a statistically meaningful radiometric age, competing theories regarding the age of the Marble Hill Hornblende Schist and the underlying unconformity at the base of the Mineral Bluff Group could be resolved. Ultimately, the age of the Marble Hill Hornblende Schist has significant implications for southern Appalachian orogenesis and Paleozoic tectonic models.