Geology of the Wadley South Quadrangle and geochronology of the Dadeville Complex, southernmost Appalachians of east Alabama

Abstract

Geologic mapping results from rocks in the area of the Wadley South Quadrangle are reported to address basic research questions concerning the Paleozoic tectonometamorphic evolution of the southernmost Appalachians. Key findings of this mapping investigation include the following. (1) The Jacksons Gap Group (Brevard zone lithologies) is subdivided into three main lithofacies: a structurally lower section of predominantly of fine-grained garnetiferous-graphitic-quartz-biotite schist and phyllite and interlayered micaceous quartzite; a middle section of interlayered graphitic phyllite; and an upper section of graphitic and sericitic phyllite. (2) First generation, D1, structures accompanied Neoacadian lower-to middle-amphibolite-facies metamorphism of eastern Blue Ridge units, upper-greenschist to lower-amphibolite-facies metamorphism of the Jacksons Gap Group, and upper-amphibolite-facies metamorphism of rocks of the Dadeville Complex. (3) Early-syn D1 fabrics and lithologic contacts are truncated along the Katy Creek fault, implying juxtaposition of the Dadeville Complex and Jacksons Gap Group during syn- to late-stages of metamorphism. An inverted metamorphic gradient may be associated with the Katy Creek fault, suggesting formation during down heating associated with thrust emplacement of the overlying Dadeville Complex. (4) Crystal-Plastic reactivation of the Brevard shear zone under middle-greenschist facies conditions during Alleghanian movement is recorded in retrograde mylonites associated with the Abanda fault. Oblique-normal and dextral-strike-slip displacement along the Abanda fault apparently juxtaposed rocks of different metamorphic grade. (5) The presence of cataclasite along the northwest side of the Alexander City and Abanda faults marks the final translation of the eastern Blue Ridge and Jacksons Gap Group under supra-ductile-brittle conditions during the Mesozoic rifting of Pangea.

The precise age and tectonic affinity of rocks of the Dadeville Complex in Alabama and Georgia is not well defined, so a U-Pb geochronological investigation of magmatic and detrital zircons from the complex was performed. The Dadeville Complex comprises a thick (>6 km) klippe of amphibolite-facies sedimentary, volcanic, and plutonic tocks in the core of the shallow-northeast plunging Tallassee synform and cradled between Laurentian units of the eastern Blue Ridge and Pine Mountain window. Laser ablation sector field inductively coupled plasma mass spectrometry (LA-SF-ICP-MS) 206Pb/238U age dates indicate that this terrane is a Cambrian- to Early Ordovician-aged arc emplaced during the early Paleozoic Taconic orogeny. Detrital zircons from a metasiliciclastic unit (i.e., Agricola Schist) reveal populations at that are not typical of rocks found along the present-day southeastern Laurentian margin. Grenville-aged zircons are conspicuously sparse in the Dadeville Complex and suggest that the Inner Piedmont in Alabama and Georgia has an exotic tectonic affinity. A prominent zircon population in the detrital age spectrum at ~480 Ma contains a Th/U ratio of less than 0.1 that likely developed as result of Taconian emplacement of the Dadeville Complex arc atop the slope-rise facies of the eastern Blue Ridge. The ~480 Ma date overlaps with high-grade metamorphism reported in the eastern Blue Ridge (i.e., the Lick Ridge Eclogite and Winding Stair Gap granulite) in western North Carolina, and synorogenic clastic wedge deposition (Blount clastic wedge) reported in Alabama, Georgia, and Tennessee, documenting significant Taconian orogenic effects in the southernmost Appalachians where it had previously been considered absent.