GROWING EVIDENCE FOR THE PRESENCE OF LAURENTIAN MARGIN ORDOVICIAN BACK-ARC BASIN(S) IN THE SOUTHERN APPALACHIANS

In the southernmost Appalachian Blue Ridge, field, geochemical and geochronologic data indicate the presence of Middle Ordovician metavolcanic/metasedimentary basins, including the Hillabee Greenstone (AL-GA), Pumpkinvine Creek Fm. and correlative sequences in the Dahlonega Gold Belt (GA-NC), extending for >400 km at the western-eastern Blue Ridge boundary. These metamorphosed bimodal sequences consist of tholeiitic basalt and subordinate interstratified calc-akaline dacite/rhyolite, and contain primitive mafic rocks exhibiting intermediate geochemical characteristics between those of arc basalts and MORB, typical of suprasubduction setting back-arc rocks. Felsic metavolcanics have isotopic signatures (e.g. ΣHf in zircons) that indicate inheritance of evolved older crustal materials, and interstratified metasedimentary sequences (Canton Schist, Otto Fm., Emuckfaw/New Georgia Gps.) contain a preponderance of Mesoproterozoic 1.4-1.0 Ga. detrital zircons. Recent work on the Emuckfaw Gp. (AL) indicates that it also contains a significant population of ca. 474 Ma zircons (Pb²⁰⁶/U²³⁸), confirming the Ordovician or younger age of the associated metasediments. Although the ages of these metavolcanic complexes are consistent with the Taconic orogeny, they were not emplaced atop the western Blue Ridge-Talladega belt until at least latest Devonian or earliest Mississippian. Together, detrital zircons and geochemical constraints suggest a genetic connection between these back-arc rocks and Laurentian crust. Additionally, deposition of ca. 466-464 Ma sediments and K-bentonites in the Blount foreland basin (AL-GA-TN) is concurrent with and possibly genetically related to formation of these back-arc sequences. The collective geologic history of the region suggests a 480-460 Ma extensional accretionary orogen on the overriding Laurentian plate associated with a retreating subduction zone outboard of the Laurentian margin, with Neoproterozoic–Lower Cambrian(?) units of the Laurentian eastern Blue Ridge acting as basement to the younger Ordovician volcanic sequences and interstratified sedimentary rocks. A subsequent ca. 460 Ma contractional phase within this Laurentian plate back-arc may be responsible for Taconic-aged high P-T metamorphism recognized in the Central Blue Ridge.