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Unformatted text preview: American Mineralogist, Volume 64, pages 736-741, 1979 Zinc-rich hercynite in high-grade metamorphic rocks: a product of the dehydration of staurolite Eowano F. Sroooeno Department of Geosciences, North Carolina State University Raleigh, North Carolina 27650 Abstract Ternary spinels in the system (Fe,Zn,Mg)Alroo are apparently relatively common and widespread constituents of some metamorphic rocks in the amphibolite and granulite facies. Two new occurrences, one from granulite facies metapelitic gneiss, and one from cordierite- orthoamphibole (metavolcanic?) rock, illustrate the two major lithologies in which such spin- els occur. Compositions of associated phases in both parag€neses are consistent with spinel formation as a result of the dehydration of Zn-enriched staurolite. Introduction The origin of natural spinels in the series (Fe,Mg,Zn)AlrOo has commonly been ascribed to high-fllow-P metamorphism of silica-poor rocks such as emery, impure carbonates, and ultramafics, or to zinc ore bodies or pegmatites enriched rn Zn (see, e.9., Rumble, 1976). However, such spinels also have been reported from two other parageneses: (l) pelitic metasedimen- tary rocks which have undergone high-grade regional metamorphism (Gable and Sirns, 1969, p. 19; Frost, 1973; Wagner and Crawford, 1975) or contact meta- morphism (Loomis, 1972; Atkra,1978); and (2) mafic metavolcanic or metasedimentary rocks rich in cor- dierite and gedrite (Robinson and Jaffe, 1969; Gable and Sims, 1969,p.30; James et al.,1978). Although rocks in both parageneses collmonly contain qvuttz, the spinel is not in direct contact with qaaftz and, at least in some cases, it is restricted to portions of the rock with anomalously high AllSi (e.g., Robinson and Jaffe, 1969). Nevertheless, some workers con- sider these spinels to be in equilibrium with quartz (e.9., Wagner and Crawford, 1975). All such spinels reported in the literature (Table I, analyses 4-8) con- tain appreciable ZnO, and it appears probable that this stabilized them in the presence of quartz. New occurrences Two new occurrences of zincian hercynite are de- scribed here. The first is in granulite facies metape- litic gneisses from the northwest Adirondacks. New York, the second in amphibolite facies cordierite + amphibole rocks from the Old Woman Mountains, southeastern California. Northwest Adirondacks Small green translucent crystals of spinel are pres- ent in pelitic Grenville paragneiss interlayered with other metasediments and with mafic metavolcanics in the West Parishville synform, about five km north- east of the town of Colton, New York (Stoddard, 1976a). Spinel-bearing rocks contain the assemblage: qtrartz + plagioclase + perthitic K-feldspar + garnet * biotite * sillimanite * spinel + ilmenite + rutile + cordierite. The spinel is not in direct contact with quaftz; it occurs most commonly in the cores of seg- regations of coarse prismatic sillimanite with ilmenite-r rutile. These segregations are surrounded by zones rich in plagioclase and biotite or garnet (Fig. l).rich in plagioclase and biotite or garnet (Fig....
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- Stoddard, Old Woman Mountains