Ordovician–Silurian back-arc silicic magmatism in the southernmost Appalachians

Abstract

Abstract Ordovician and Silurian siliciclastic, volcanic, and plutonic rocks in the southern Appalachian eastern Blue Ridge and western Inner Piedmont, formerly interpreted as components of an obducted Taconic arc terrane, are recognized as parts of an Ordovician paired arc/back-arc system that formed on the seaward edge of the Laurentian plate. New crystallization ages from metamorphosed plutonic-volcanic components intruding and intercalated with metasedimentary back-arc basin lithologies of the Wedowee-Emuckfaw-Dahlonega back-arc basin confirm rapid deposition (hundreds of meters per million years) in an Ordovician–Silurian extensional basin that received input from continentally derived Mesoproterozoic crust and bimodal volcanic components. U-Pb (206Pb/238U) zircon ages from 12 samples of Zana Granite yield minimum crystallization ages between 459 Ma and 430 Ma, while zircon grains from six samples of Kowaliga Gneiss have minimum crystallization ages between 452 Ma and 435 Ma. Zircons from a probable metavolcanic unit within the Wedowee Group yield a 206Pb/238U age of 454 ± 3 Ma. Mesoproterozoic depleted mantle model ages and negative initial εhf values of zircons from the Ordovician–Silurian metaigneous rocks suggest a significant Mesoproterozoic component in their genesis. Formation of these silicic plutons was likely associated with the intrusion of mantle-derived mafic melts and decompression melting of Mesoproterozoic lower crust as part of a tectonically thickened crustal section with a contribution from sub-continental mantle lithosphere during backarc extension. Silicic magma intruding thick successions of bimodal volcanic rocks and intercalated sedimentary rocks shortly after deposition is typical of back-arc extension and contraction phases above B-type subduction zones (i.e., Benioff oceanic subduction) in Lachlan-style orogens, which is consistent with the accretionary orogenic nature of the southern Appalachian Taconic orogen.