FORMATION, DOCKING, AND OROGEN-PARALLEL TRANSPORT OF A TACONIAN ARC AND BACK-ARC BASIN: THE DADEVILLE COMPLEX IN THE SOUTHERNMOST APPALACHIAN OROGEN

Abstract

The Taconian orogeny in the southern Appalachian orogen has not been well understood due to lack of recognized Taconian arc(s) and modern geochronology data. U-Pb LA-SF-ICPMS (laser ablation-sector field-inductively coupled plasma-mass spectrometry) dates of zircons from the arc-related meta-igneous units of the Dadeville Complex in the Alabama Inner Piedmont indicates peak magmatism at ca. 467‒457 Ma following an earlier phase at ca. 479 Ma. These meta-igneous rocks are interpreted to represent the remnant of a Taconian arc, the Dadeville arc. Metamorphic overgrowths on zircons from the meta-igneous rocks and a strongly sheared schist at the uppermost Emuckfaw Group, along with zircons from a felsic boudin of the metasedimentary cover of the Dadeville arc (the Agricola Schist), reveal a regional metamorphism at about 403‒398 Ma. The time of this metamorphism and the youngest detrital zircons of the Agricola Schist in the combined data from the current study and literature constrains that the Agricola Schist was likely deposited between ~440 Ma and ~400 Ma. Presence of dominant Grenville-aged zircons in the Agricola Schist suggests that the Dadeville arc possibly accreted onto Laurentia by ~440 Ma. The similarities of Early Devonian metamorphism and magmatism, common provenances of detrital zircons between rocks in the Dadeville Complex and those in the Carolina Appalachians, the distribution of the Taconian Blount clastic wedge, the location of Taconian volcano vents indicated by thickness distribution patterns of bentonites, and the channel flow structures in the Inner Piedmont, jointly suggest that the Dadeville arc was originally formed and docked in the Tennessee Embayment and was subsequently transported to the Alabama Promontory along orogen-parallel strike-slip shear zones likely associated with Acadian-Neoacadian transpressional convergence.