The Hillabee Greenstone: A mafic volcanic complex in the Appalachian Piedmont of Alabama
The Hillabee Greenstone is a distinctive horizon of low metamorphic grade rocks which occurs along the southeastern margin of the Talladega Slate Belt and crops out discontinuously for 170 km in the Appalachian Piedmont of eastern Alabama. It is bounded above and to the southeast by a major thrust fault, the Hollins Line fault, which has emplaced the high metamorphic grade, predominantly metasedimentary Ashland-Wedowee sequences, above the Hillabee. The fault cuts obliquely through the Hillabee along strike so that at some localities the Hillabee is as much as 2.6 km thick, while elsewhere it has been eliminated and the high-grade Ashland-Wedowee rocks are juxtaposed with the low-grade Talladega Group metasediments which occur to the northwest. The fault, which has a dip-slip movement on the order of tens of kilometres, has resulted in the tectonic removal of an unknown amount of the Hillabee. The lower Hillabee contact appears to be a gradational, conformable stratigraphic contact with the Talladega Group. Primary- and secondary-facing criteria imply that the southeastern portion of the Talladega Group, which is Early to Middle Devonian in its uppermost part, is upright and that the Hillabee occurs stratigraphically above it. The majority of the Hillabee lithologies are mafic phyllites and greenstones which have major- and trace-element characteristics of low-potassium tholeiite and basaltic andesite. Meta-quartz dacites compose as much as 15% of the sequence. Geochemical discrimination techniques in association with regional stratigraphic and structural relationships suggest that the Hillabee Greenstone represents an active continental margin (arc) volcanic sequence which spread northwestward during the Early to Middle Devonian. This volcanism probably occurred in response to the westward under-thrusting of an attenuated proto-North American continental margin by oceanic lithosphere, and resulted in closure of a back arc basin or small ocean basin. The volcanism was followed rapidly by regional dynamothermal events probably associated with Acadian orogenesis.