ACCRETION OF A VOLCANIC COMPLEX ALONG THE PALEOZOIC LAURENTIAN MARGIN: POTENTIAL LINKS BETWEEN THE HILLABEE GREENSTONE, EASTERN BLUE RIDGE OF ALABAMA, AND PALEOZOIC EVENTS IN THE FORELAND

The Hillabee greenstone (HG) of the southernmost Appalachians consists of interlayered low-K tholeiitic metabasalt and calc-alkaline metadacite/rhyolite. This metavolcanic complex lies at the structural top of the Cambrian to Mississippian(?) lower greenschist facies Talladega belt. Isotopic and geochemical data suggest formation of the HG ~460-470 Ma in an extensional environment on continental crust, possibly in a back-arc setting. Palinspastic reconstruction of the TB in the Ordovician places it outboard of the Pine Mountain Belt at the edge of the Laurentian margin. Emplacement of an Ordovician HG metavolcanic complex atop Laurentian shelf sequences of the Devonian-Mississippian(?) Talladega Group (TG) occurred between late Early Devonian (Emsian) and metamorphism of the TB (mid Mississipian?). This juxtaposition produced little or no deformation in the hanging wall HG and footwall TG stratigraphies, resulting in remarkable concordance across this cryptic thrust for >230km along strike and >20km across strike. Both the HG and TG were subsequently imbricated into map-scale horses by the post-metamorphic Hollins Line fault system, which separates TB rocks from those of the overlying Eastern Blue Ridge (EBR). The middle to upper amphibolite facies EBR is a composite terrane that likely includes both Laurentian Proterozoic slope-rise deposits and accreted components, intruded by a suite of pre-to syn-metamorphic Paleozoic plutons that include several~496 Ma to ~460 Ma I-type, pre-metamorphic bodies. The paleotectonic setting and palinspastic restoration of both the TB and EBR, when coupled with the structural-stratigraphic relationships along the HG-TG boundary and the isotopic ages of the HG and EBR plutons, suggest that the HG may have developed atop the outer, rifted margin of Laurentia. HG development in this setting suggests possible affinities with early Paleozoic plutonism in the EBR and widespread Ordovician K-bentonite deposits in the foreland. Additionally, emplacement of the HG atop the TB shelf may be coeval with the adjacent collision of the tectonically independent Ouachita volcanic arc along the southeastern Laurentian margin and initiation of clastic wedge propagation onto the adjacent Ouachita and southern Appalachian foreland.