Formation and orogen-parallel transport of the Dadeville Complex, Alabama, USA: Implications for the Taconian orogeny in the southern Appalachians

Abstract

The Taconian orogeny in the southern Appalachians has not been fully understood due to a lack of connections among the various Ordovician lithotectonic units. Zircon U-Pb ages obtained by laser ablation-sector field-inductively coupled plasma-mass spectrometry from arc-related meta-igneous rocks comprising the Dadeville Complex in the Alabama Inner Piedmont (southeastern USA) indicate continuous arc magmatism at ca. 465 to 454 Ma following an earlier phase at ca. 480 Ma. These meta-igneous rocks, in conjunction with literature data, are interpreted to represent remnants of the Taconian-aged Dadeville arc, which formed on a rifted Laurentian crustal block. Metamorphic overgrowths on zircons from the meta-igneous rocks of the Dadeville Complex and a schist of the uppermost Emuckfaw Group, along with zircons from a felsic melt in the Agricola Schist (metasedimentary cover of the Dadeville arc), reveal a regional metamorphic event at ca. 408 to 394 Ma. Dominant 1300 to 900 Ma zircons in the Agricola Schist suggest that the Dadeville Complex sedimentary rocks were primarily sourced from Laurentia. The depocenters of the Taconian bentonites and clastic wedge and the location of peak Taconian metamorphism, based on literature data, suggest that the locus of the Taconian orogeny in the southern Appalachians was located at the Tennessee embayment. Early Devonian metamorphism/magmatism, Taconian arc rocks, and detrital zircon provenances are shared by the Dadeville Complex at the Alabama promontory and Taconian assemblages at the Tennessee embayment, which, in conjunction with the southwest-directed extrusion/shear structures found in the Inner Piedmont and uppermost eastern Blue Ridge from North Carolina to Alabama, suggest that the Dadeville arc and its companion back-arc basin originally formed in the Tennessee embayment and were subsequently transported to the Alabama promontory along orogen-parallel shear zones likely associated with the Acadian-Neoacadian transpressional convergence. This study provides a palinspastic view of the original Taconian orogeny in the southern Appalachians.