Bimodal volcanism as evidence for Paleozoic extensional accretionary tectonism in the southern Appalachians

Abstract

Detailed mapping, U-Pb ages of igneous and detrital zircons, and isotopic and geochemical characterization of a bimodal volcanic sequence from the Appalachian Blue Ridge province provide constraints for the timing and tectonic setting of volcanism along the Early Paleozoic southeastern Laurentian margin. These units, the Pumpkinvine Creek Formation and related rocks, are interpreted as having been deposited along an extended Laurentian margin in a backarc setting during the mid-Ordovician, and tectonically emplaced as late as the late Paleozoic Alleghanian orogeny. Two felsite layers from the Pumpkinvine Creek Formation, a bimodal metavolcanic sequence, have ages of 466 ± 5 Ma and 461 ± 3 Ma. Geochemical analyses of metabasalts indicate compositions derived from a suprasubduction zone, and more specifi cally a backarc. Furthermore, ε Nd(460 Ma) = (+)3.3-(+)7.7 and 87 Sr/ 86 Sr (initial) = 0.7044-0.7069 indicate a juvenile source. The protoliths of the felsic lithofacies were low-potassium rhyodacite, and have ε Nd(460 Ma) = (−)3.2-(+)4.65 and 87 Sr/ 86 Sr (initial) = 0.709-0.722, suggesting incorporation of an evolved crustal component, which we interpret as Laurentian continental lithosphere. Furthermore, detrital zircons derived from within the Pumpkinvine Creek Formation yielded a majority of ages between 1017 and 1190 Ma, indicating that the source is likely a component of the rifted eastern Laurentian margin. A backarc origin for the Pumpkinvine Creek Formation indicates that the geodynamic setting of the southeastern margin of Laurentia included a retreating subduction zone resulting in extensional accretionary tectonism. These results have implications for the evolution of the southeastern Laurentian margin during a time traditionally linked to collision of an exotic Taconic arc with Laurentia. Additionally, we suggest that there are signifi cant along-orogen disparities that may result from fundamental tectonic boundaries.

Early to Middle Ordovician back-arc basin in the southern Appalachian Blue Ridge: Characteristics, extent, and tectonic significance

“…German (1989) extended the unit as a very narrow belt from Canton to Dahlonega, Georgia, and recognized a felsic gneiss member (Barlow Gneiss metadacite) similar to the Galts Ferry Gneiss intercalated with the mafi c rocks. Northwest of Dahlonega, Settles (2002) on August 6, 2014 gsabulletin.gsapubs.org Downloaded from rentian continental lithosphere (Holm-Denoma and Das, 2010). This interpretation is supported by depleted mantle model ages of 1.2-2.0 Ga for the Cane Creek Gneiss (Bream, 2003).…”

“…The Villa Rica Gneiss is interpreted as a 458 ± 3 Ma (U/Pb zircon) dacitic subvolcanic intrusive (Thomas, 2001), providing an upper age limit to the Mud Creek Formation. Correlated with the Mud Creek Formation, the Pumpkinvine Creek Formation (McConnell, 1980;McConnell and Abrams, 1984;Holm-Denoma, 2006;Holm-Denoma and Das, 2010) consists of amphibolite (metabasalt) interlayered with metadacitic felsic gneiss (informal Galts Ferry Gneiss Member) and graphitic aluminous schist (Holm-Denoma, 2006). Zircons extracted from the Galts Ferry Gneiss yield U/Pb ages of 463 ± 3 Ma (Thomas, 2001) Southwest of the New Georgia Group units and northeast of the Alabama-Georgia state line, rocks of the "western Sandy Springs Group" have been correlated with rocks of the Wedowee and Emuckfaw Groups in Alabama by a number of workers Gilmer and Barineau, 2012;.…”

“…The Pumpkinvine Creek Formation near the base of the New Georgia Group, like the Hillabee Greenstone, is a bimodal volcanic sequence of similar geochemical character (McConnell, 1980;Holm-Denoma, 2006;Holm-Denoma and Das, 2010). It extends northeastward for >118 km along strike at least to Dahlonega, Georgia (Fig.…”

“…It extends northeastward for >118 km along strike at least to Dahlonega, Georgia (Fig. 2), and consists of amphibolite interlayered with felsic gneiss (metadacite, Galts Ferry Gneiss Member) and graphitic aluminous schist (Canton Schist;McConnell, 1980;German, 1989;Holm-Denoma, 2006;Holm-Denoma and Das, 2010). German (1989) extended the unit as a very narrow belt from Canton to Dahlonega, Georgia, and recognized a felsic gneiss member (Barlow Gneiss metadacite) similar to the Galts Ferry Gneiss intercalated with the mafi c rocks.…”

“…Recent geochronologic work has documented that the age of this sequence is Early to Middle Ordovician, potentially spanning a time interval of ~20 m.y. (480-460 Ma; Thomas, 2001;Settles, 2002;Bream, 2003;Tull et al, 2007McClellan et al, 2007;Holm-Denoma and Das, 2010). Rocks constituting the remnants of what was once a more extensive basin were metamorphosed, polydeformed, dismembered, and fragmented by multiple stages of late Paleozoic (Alleghanian) faulting.…”