Taconic suprasubduction zone magmatism in southern Laurentia: Evidence from the Dadeville Complex

摘要

The record of the Taconic orogeny in the southern Appalachian orogen lacks clear evidence for many attributes of this classic Ordovician orogeny in the Northern Appalachians, including subduction of Laurentian lithosphere, the presence of Taconic allochthons and associated metamorphism and deformation, obduction of arc terranes, and accretion of Gondwanan terranes. Consequently, identifying a Taconic arc fragment in the southern Appalachian orogen, constraining its origin (e.g., peri-Laurentian or peri-Gondwana or intra-oceanic), and determining the polarity of subduction it represents are critical factors needed to evaluate the character of Taconic geodynamics throughout the Appalachians and Caledonides, including the location of the Taconic suture(s). In the southern Appalachian orogen, the Dadeville Complex is an amphibolite-facies allochthon with prominent mafic and ultramafic units in the Inner Piedmont of Alabama and Georgia. It has been proposed to be an accreted, peri-Gondwanan arc fragment obducted onto the Laurentian (or peri-Laurentian) margin. In contrast, we interpret 450-440 Ma U-Pb ages of zircons from Dadeville Complex meta-igneous rocks, Paleozoic and Precambrian detrital zircons from Dadeville Complex metasedimentary rocks, initial epsilon(Nd) values of igneous protoliths (basalt-trondhjemite-granite), initial epsilon(Hf) values of igneous and detrital zircons, and normalized trace-element abundances of mafic rocks to indicate that the Dadeville Complex likely formed in an Ordovician suprasubduction environment on Laurentian crust. This conclusion is compatible with recent interpretations of west-directed subduction as the concluding event of the northern Appalachian Taconic orogeny, but it suggests that the southern Laurentian margin was open throughout the period of the Taconic orogeny.