AGE AND TECTONIC ORIGIN OF THE MITCHELL DAM AMPHIBOLITE, ASHLAND SUPERGROUP, EASTERN BLUE RIDGE, SOUTHERN APPALACHIANS (USA)

The Mitchell Dam amphibolite (MDA) is one of the largest stratified orthoamphibolite units in the Ashland Supergroup of the southernmost Appalachian eastern Blue Ridge of Alabama (AL). The age and tectonic origin of the MDA and similar units (e.g. Ketchepedrakee amphibolite) have been subjects of interest for many decades since (meta)mafic rocks can provide important insights into petrotectonic settings. Rocks of the MDA, at the structural and stratigraphic base of the Higgins Ferry Group, are dominated by foliated amphibolite primarily composed of plagioclase and hornblende. The presence of relict augite, geochemical character, and concordant nature of the stratified MDA relative to primary layering in the surrounding metasedimentary rocks, led early workers to interpret their protoliths as mafic sills and/or flows. Much of the initial geochemical work (1970s and 1980s) focused on the MDA suggested formation of the subalkaline protoliths as ocean floor basalts. Coupled with the lithologic nature of the intercalated metasedimentary rocks, this led early workers to interpret the MDA and related amphibolites as originating along the Neoproterozoic, Iapetus-facing margin of Laurentia. However, modern geochemical analyses of the MDA revealed units with mixed mid-ocean ridge basalt and island arc tholeiite trace element characteristics. As a result of this new data, the MDA was reinterpreted as originating in a suprasubduction environment. Extensive work in the structurally higher and stratigraphically younger Wedowee and Emuckfaw Groups indicate their formation in an Ordovician-Silurian backarc basin (i.e. Wedowee-Emuckfaw-Dahlonega backarc: WEDB) on the Iapetus facing margin of Laurentia. As such, the entire Ashland Supergroup could represent the oldest portions of the WEDB. In an attempt to resolve these competing interpretations, we used an enhanced zircon recovery technique on a sample of MDA to isolate a population of >75 zircon grains, which are currently awaiting U-Pb isotopic analysis. Assuming these grains include primary igneous crystals, their age should resolve these competing tectonic models for the MDA and entire Lower Ashland Supergroup.