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Webster-Addie ultramafic ring dike, Webster-Balsam, Jackson Co., North Carolina, USA

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Location is approximate, estimate based on other nearby localities.
 
Latitude & Longitude (WGS84): 35° North , 83° West (est.)
Margin of Error:~9km


The Webster-Addie ultramafic body, one of the many alpine ultramafic bodies in the Appalachian belt (Larrabee, 1966), is located in the northwestern part of Jackson County, North Carolina near the town of Sylva. It is elliptical in shape (about 10 km long and 5 km wide), dips outward, and varies in outcrop width from a few meters to about 600 m.

Clinopyroxene-rich rocks, chiefly metawebsterite and olivine metaclinopyroxenite, are an important constituent of the Webster-Addie ultramafic complex, but are rare elsewhere in Blue Ridge metaultramafic bodies. They occur as layers and lenses in metadunite, ranging in thickness from a few millimeters to tens of meters. Compositional and structural (layering) characteristics are similar to, and suggest derivation from, the ultramafic cumulate portion of an ophiolite. Metamorphism of the clinopyroxene-rich rocks has produced the assemblage clinopyroxene (Cpx) + orthopyroxene (Opx) + olivine + calcic amphibole + Al-rich spinel, characteristic of the amphibolite-granulite facies transition. The absence of anthophyllite in associated metadunites indicates that pressure during metamorphism was at least 11 kb. A major feature of the metamorphism at Webster-Addie is the growth of calcic amphibole at the expense of pyroxene. Initially, this produced an aluminous hornblende, edenite. Later, edenite was, in part, retrograde metamorphosed to tremolite, resulting in composite crystals with edenite cores mantled by tremolite. Depending on the extent to which pyroxene was replaced by calcic amphibole, rocks grade from pyroxene-rich with minor amphibole to nearly pure amphibole rocks. Calcic amphiboles similar to those found replacing pyroxene at Webster-Addie are a minor, but widespread, constituent in Blue Ridge metaultramafic bodies, and may reflect
origin from a Cpx-bearing protolith.(http://www.southeasterngeology.org/Warner47-3.pdf)

Mineral List


18 valid minerals.

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References

Miller III, Roswell (1953), The Webster-Addie Ultramafic Ring, Jackson County, North Carolina, and Secondary Alteration of Its Chromite, American Mineralogist: 38: 1134-1147.

Johnson, Alvin C. (1958), A Magnetometer Survey of the Webster-Addie Ultabasic Ring, Jackson County, North Carolina (reprint), University of Michigan, Ann Arbor, 12 pp.

Larrabee (1966).

Condie, Kent C. and Madison, James A. (1969), Compositional and Volume Changes Accompanying Progresive Serpentinization of Dunites from the Webster-Addie Ultramafic Body, North Carolina, The American Mineralogist: 54 (July-August).

Wilson, W.F and McKenzie, B.J. (1978), Mineral Collecting Sites in North Carolina, Information Circular 24.

Snyder, Daniel R. (2011), Olivine weathering textures in serpentinized dunite, Webster-Addie ultramafic body, North Carolina, ProQuest, UMI Dissertation Publishing (September 30, 2011), 204 pp. [Paperback]

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