Granulite-grade rocks exposed on US 64 at Winding Stair Gap, N.C., provide an excellent opportunity to study high-grade metamorphism in the southern Appalachians. These rocks, exposed in a 370 m-long roadcut, represent some of the highest grade Paleozoic metamorphic rocks found in the southern Appalachians (Absher and McSween Jr., 1986). The purpose of this stop is to observe these rocks and determine the metamorphic grade and metamorphic/structural relationships preserved in the outcrop. With these observations, you should have a better understanding of the metamorphic and deformational conditions rocks in this region of the southern Appalachians have enjoyed.
The Winding Stair Gap roadcut lies within the Eastern Blue Ridge Belt of the southern Appalachians. The southern Appalachian Blue Ridge is separated from the Valley and Ridge to the west by the Great Smoky fault (or Blue Ridge thrust), and from the Piedmont to the east by the Brevard fault zone (fig. E1). The Western Blue Ridge and Eastern Blue Ridge are separated by the Hayesville fault in the central and southern portion of the southern Appalachians, and the Fries fault in the northern portion of the southern Appalachians. The Hayesville fault is probably early Taconic in age (Eckert et al., 1989). The majority of the rocks in the Blue Ridge are supracrustals that are Late Precambrian to early Paleozoic in age, with a minor amount of Precambrian Grenvillian aged (1.0-1.2 Ga) basement exposed. In the North Carolina region, supracrustals in the western Blue Ridge belong to the Great Smoky Group of the Ocoee Supergroup, while the supracrustals in the eastern Blue Ridge have been correlated with the Tallulah Falls Formation and Coweeta Group (Absher and McSween Jr., 1985, and Eckert Jr. et al., 1989). The North Carolina Blue Ridge has undergone regional metamorphism up to granulite facies, and the resulting NE-SW trending isograds are shown in figure E1. Metamorphic isograds cut across the Hayesville fault, which is therefore interpreted to be pre-metamorphic (Eckert Jr. et al., 1989). The Hayesville fault itself is folded by regional F2 folds (Eckert Jr. et al., 1989).
Figure E1. Regional geological map showing metamorphic isograds and structural features of the Blue Ridge in western North Carolina and surrounding regions. HF=Hayesville Fault, BRT=Blue Ridge Thrust, WSG=Winding Stair Gap. Isograds are marked by colored regions. Isograd abbreviations: bio=biotite isograd, gar=garnet isograd, st=staurolite isograd, ky=kyanite isograd, sill-sillimanite isograd, hyp=hypersthene isograd. After Eckert Jr. et al., 1989.
Lithologies
Six different lithologies are exposed at the Winding Stair Gag roadcut. Rock descriptions, taken from Absher and McSween Jr. (1986), are given below.
1) biotite-hornblende granulite: a dark-green, fine- to-medium grained, granoblastic rock with hornblende, plagioclase, biotite, ilmenite, and quartz, with minor garnet, orthopyroxene ± cummingtonite.
2) orthopyroxenite: a dark reddish-brown weathering rock composed of orthopyroxene. This lithological unit contains abundant plagioclase+quartz veins.
3) calc-silicate: This lithology is composed of garnet, hornblende, diopside, and clinozoisite, and can usually be distinguished from the quartzofeldspathic gneiss by its greenish color.
4) biotite-garnet granulite: This unit is dark-gray, and has been subdivided into two different lithologies: a) hornblende, biotite, and orthopyroxene, and b) biotite, garnet (up to 3 cm in diameter), and hornblende.
5) sillimanite schist: biotite, garnet, sillimanite, plagioclase, k-feldspar, and quartz.
6) quartzofeldspathic gneiss: quartz, plagioclase, k-feldspar, biotite, and garnet.
Portions of the outcrop are migmatitic, noticed by the presence of leucosome material. Some leucosomes contain kyanite.
Structure
Rocks in the Winding Stair Gap area have undergone several deformational phases, as shown by multiple foliations and multiple fold generations. Multiple foliations (S0, S1(?), and S2) and two main generations of folding (F1 and F2) are visible in the Winding Stair Gap roadcut (fig. E3; Absher and McSween Jr., 1986). S0 is represented by lithological layering in the biotite-garnet granulite and boudinaged layers of calc-silicate. The biotite-garnet granulite layers are seen to be repeated, and are interpreted to be isoclinally folded (F1). No metamorphic minerals have been identified as forming a foliation/lineation during this folding, so F1 is interpreted to be premetamorphic (Absher and McSween Jr., 1986). Limbs of small intrafolial folds in migmatite lead to foliation which may be S1 or S0. F2 folds are near-recumbent isoclinal folds which refold F1 folds. The dominant schistosity in outcrop consists of gneissic-banding and biotite-hornblende-sillimanite foliation associated with F2, and therefore is considered S2. Because sillimanite is part of the S2 foliation, the F2 deformational event is considered to have occurred at near-peak metamorphic conditions (Absher and McSween Jr., 1986). S3 represents weak crenulations of S2 fabric. For more information on structural fabric in the eastern Blue Ridge, see the paper on overprinting structural fabric in the eastern Blue Ridge (1997, this volume).
Significance
This outcrop is very significant because it represents one of the best exposures of the highest-grade metamorphic rocks in the eastern Blue Ridge of the southern Appalachians. It allows observations relating to the metamorphic and deformational history, and the relationship between the two. It has also been the major source for debate (outlined below) regarding the timing and number of metamorphic episodes in this area, as well as the overall metamorphic history.
One of the main controversies in this region of the eastern Blue Ridge concerns the timing of metamorphism and number of metamorphic episodes. Absher and McSween Jr. (1985 and 1986) suggested that there was only one main metamorphic event (approximately Taconic in age, 450 Ma) in the Winding Stair Gap area, and calculated peak metamorphic temperatures and pressures of 750-775°C and 6.5-7.0 kbar. They suggest that sillimanite represents the stable peak metamorphic aluminum silicate, and that the kyanite found in migmatite leucosomes was the stable aluminum silicate during retrograde metamorphism. They further conclude that cooling from peak conditions was isobaric (at constant pressure).
However, McLellan and others (1989) suggest polymetamorphism, claiming there are two distinct packages of rocks in the Winding Stair Gap area (Corbin and Winding Stair Gap lithologies) which have experienced different metamorphic histories (one package has experienced two metamorphic events, and the other only one). They conclude that the Winding Stair Gap lithologies are much older (Grenvillian aged rocks) than the Corbin lithologies, and have experienced more deformation and more metamorphic episodes. Winding Stair Gap lithologies contain two metamorphic mineral fabrics (M1 and M2), whereas the Corbin lithologies only possess M2 fabrics. They calculated peak pressures and temperatures for M1 of 700-800°C and >10 kbar, and peak M2 conditions in the Corbin lithologies of 550-720°C and 5-10 kbar. They suggest that granulite-grade metamorphism is Grenvillian in age (only affected Winding Stair Gap lithologies) and that the Corbin lithologies (representing cover-rocks to the Winding Stair Gap lithologies) that only experienced a slightly lower-grade (M2) metamorphism. They claim that sillimanite was the stable aluminum silicate during M1 metamorphism, and that kyanite was the stable aluminum silicate during M2 metamorphism.
Another interpretation of metamorphism at Winding Stair Gap was presented by Moecher (1997), based on petrographic studies. Moecher (1997) claims that kyanite is the stable peak-metamorphic aluminum silicate, and that sillimanite is the retrograde aluminum silicate. They further suggest that there may be more than one generation of sillimanite growth.
Both structural and metamorphic observations should be made at this outcrop. First, observe the major folds, which include F1 and F2 isoclinal folds. Try to identify lithological layering (S0) in the biotite-garnet granulite near the middle of the outcrop at ground-level. Also, identify the different foliations, and observe their relationship to the major folds in this outcrop. See if you can identify sillimanite and kyanite in hand samples (these might only be clearly visible in thin section). Observe the nature of migmatite leucosomes and calc-silicate boudins. See if you can find the late-stage crenulations. Can we identify the pyroxene?
The WINDING STAIR GAP roadcut is exposed on U.S. Hwy. 64, in Macon County, North Carolina. The outcrop is approximately 10.5 miles south of the U.S. 64 and U.S.441 intersection near Franklin (fig. E2). Parking can be found at the top of the Winding Stair Gap next to the Appalachian Trail, or to the north of the outcrop at a scenic overlook (Absher and McSween Jr., 1986). The Global Positioning System gave us a location of N35° 07.207' and W 83° 32.638'.
Figure E2. Location map. Taken from the northeast portion of the Rainbow Springs quadrangle topographic map, North Carolina.
Absher, S.B., & McSween, H.Y., Jr., 1986. Winding Stair Gap granulites: The thermal peak of Paleozoic metamorphism: in Geological Society of America Centennial Field GuideÑSoutheastern Section, p. 257-260.
Eckert, J.O., Jr., Hatcher R.D., Jr., & Mohr, D.W., 1989. The Wayah granulite-facies metamorphic core, southwestern North Carolina: High-grade culmination of Taconic metamorphism in the southern Blue Ridge: Geological Society of America Bulletin, 10l, p.1434-1447.
McLellan, E., Linder, D., & Thomas, J., 1989. Multiple granulite-facies events in the southern Appalachians, USA: in Evolution of Metamorphic Belts, eds. Daly, J.S., Cliff, R.A., and Yardley, B.W., Geological Society Special Publication No. 43, pp. 309-314.
Moecher, D.P., 1997. Kyanite-sillimanite relationships in high grade gneisses at Winding Stair Gap, NC: An alternative view: in Geological Society of America Abstracts with Programs, p. 59-60.
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