Tectonism and metamorphism along a southern Appalachian transect across the Blue Ridge and Piedmont, USA

Abstract

The Appalachian Mountains expose one of the most-studied orogenic belts in the world. However, metamorphic pressure-temperature-time (P-T-t) paths for reconstructing the tectonic history are largely lacking for the southernmost end of the orogen. In this contribution, we describe select field locations in a rough transect across the orogen from Ducktown, Tennessee, to Goldville, Alabama. Metamorphic rocks from nine locations are described and analyzed in order to construct quantitative P-T-t paths, utilizing isochemical phase diagram sections and garnet Sm-Nd ages. P-T-t paths and garnet Sm-Nd ages for migmatitic garnet sillimanite schist document high-grade 460–411 Ma metamorphism extending south from Winding Stair Gap to Standing Indian in the Blue Ridge of North Carolina. In the Alabama Blue Ridge, Wedowee Group rocks were metamorphosed at biotite to staurolite zone, with only local areas of higher-temperature metamorphism. The Wedowee Group is flanked by higher-grade rocks of the Ashland Supergroup and Emuckfaw Group to the northwest and southeast, respectively. Garnet ages between ca. 357 and 319 Ma indicate that garnet growth was Neoacadian to early Alleghanian in the Blue Ridge of Alabama. The P-T-t paths for these rocks are compatible with crustal thickening during garnet growth.

Defining the Timing, Extent, and Conditions of Paleozoic Metamorphism in the Southern Appalachian Blue Ridge Terranes of Tennessee, North Carolina, and Northern Georgia

“…In all samples, ages obtained from monazite grains included in metamorphic phases such as garnet and staurolite yield ages that are essentially indistinguishable from ages derived from matrix monazite grains, which highlights the punctuated and pervasive nature of metamorphism across the southern Appalachian Blue Ridge. The monazite ages reported here are also in good agreement with garnet Sm-Nd ages of Dickson et al (2019) and Stowell et al (2021) for the WBR (Burra Burra Mine, ∼449 Ma) and 441 ± 4 and 411 ± 12 Ma for Winding Stair Gap (CBR) and sample 12NC03 (CBR), respectively.…”

“…(2019) and Stowell et al. (2021) for the WBR (Burra Burra Mine, ∼449 Ma) and 441 ± 4 and 411 ± 12 Ma for Winding Stair Gap (CBR) and sample 12NC03 (CBR), respectively.…”

“…Sample 12NC03, which is from the Shooting Creek Schist in the Cartoogechaye terrane, was originally examined in detail by Stowell et al. (2021). It contains the mineral assemblage Grt + Ky + Sil + St + Bt + Ms + Pl + Qz + Ilm + Rt.…”

“…Corresponding P-T and deformational conditions in the different thrust sheets are enumerated 1-7 (circles). For the central Blue Ridge Cartoogechaye terrane, prograde and peak Taconic metamorphism constraints derived from 12NC03 (solid pink line, Stowell et al, 2021;this study) and previous studies at Winding Stair Gap (dashed pink line, Absher & McSween, 1985;Eckert et al, 1989;Moecher et al, 2004). Exhumation path of eastern Blue Ridge Lick Ridge eclogite derived from P-T estimates of Anderson and Moecher (2007).…”

“…Microprobe transects across representative garnets from these samples yield increasing proportions almandine (Alm; Fe) and pyrope (Pyr; Mg) and decreasing spessartine (Spss; Mn) and grossular (Grs; Ca) from core to rim, likely indicative of single phase prograde growth (Figures 9a-9c). Garnet from 12NC03 shows zoning of all four major elements, with an abrupt break in zoning across the core-rim boundary (Figure 9d), which was recognized by Stowell et al (2021) to coincide with the inclusion of sillimanite in garnet rims. 12NC03 garnet.…”