REFINING THE AGE, PETROLOGY AND TECTONIC SETTING OF PALEOZOIC PLUTONS IN THE EASTERN BLUE RIDGE OF ALABAMA

Granitoids of the eastern Blue Ridge (Ashland-Wedowee-Emuckfaw-Dahlonega gold belt: EBR) and Inner Piedmont (Opelika Complex: IP) in the southern Appalachians of Alabama and Georgia (Almond pluton, Blakes Ferry pluton, Bluff Springs granite, Elkahatchee quartz diorite, Farmville metagranite, Kowaliga gneiss, Mulberry Rock gneiss, Rockford granite, and Zana granite) have the potential to provide important timing and tectonic constraints for Paleozoic orogenesis in the southern Appalachians. Many of these plutons have been the subject of repeated attempts to constrain their age and tectonic setting via geochemical analysis. Whole rock and multi-grain zircon analyses from rocks of the EBR in Alabama during the 1980’s suggested a latest Cambrian crystallization age for the Elkahatchee, a Middle Ordovician age for intrusion of the Zana and Kowaliga, and a Late Devonian crystallization age for the Rockford. Recent studies using spatially resolved, single grain analyses of zircon (ICP and ion probe) not available when the original analytical work was conducted suggest these ages were complicated by Pb-loss, xenocrystic components, and/or higher uranium content in zircon overgrowths. This growing body of work indicates that the Elkahatchee batholith is Late Devonian (not Cambrian) in age and provides more detailed chronologic information on the Elkahatchee and other Ordovician through Mississippian pre- and syn-metamorphic plutons. SHRIMP-RG data on the Mulberry gneiss in the EBR of northeastern Georgia suggest a Late Ordovician to early Silurian crystallization age, while work on the Farmville in the IP suggests intrusion during the early Silurian. These new data should provide high precision ages for many EBR Late Devonian-Mississippian pre- and syn-metamorphic granitoids which had never been subjected to radiometric analysis. Field observations coupled with new age constraints suggest that some granitoids mapped as single intrusive phases (e.g. Rockford and Elkakatchee) may have a multi-stage intrusive history. This growing body of work from a number of different researchers is important for understanding the structure and petrogenetic settings of plutonic rocks. This new data has already begun to shed light on a poorly understood Silurian orogenic event in the southern Appalachians.