Petrology of the Roxboro metagranite, North Carolina

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Virginia Polytechnic Institute and State University

The intrusion located at Roxboro, North Carolina is predominantly a light gray to medium gray, microphaneritic granite, now metamorphosed. Phenocrysts of plagioclase, quartz, and perthite are accompanied by porphyroblasts of epidote. Relict igneous textural relationships suggest two possible fractional crystallization models, in which the order of crystallization was plagioclase, quartz, and then K-rich feldspar. A crude approximation of the original plagioclase phenocryst An content ranges between An₂₅ to An₄₀. An estimate of the granophyric composition reveals that the pluton was emplaced under almost "dry" conditions with a Ptotal of about 350 bars and a temperature in the vicinity of 950°C. Some epidote porphyroblasts grew during minor hydrothermal activity on cooling. During the middle Paleozoic, this granitic intrusion was metamorphosed at a minimum pressure of about 3 kbar and a temperature of around 400°C. A foliation, as shown by stringers of mainly biotite and epidote, was produced by the deformational phase accompanying the regional metamorphism. All K-rich feldspar is nearly pure microcline (Or₉₇) and all plagioclase is nearly low albite (An₉₇₋₉₉). Such feldspar compositions accompanied by the late growth of ferristilpnomelane suggest that the Roxboro metagranite has reequilibrated under less intense conditions than those realized during the peak of the major regional metamorphic event.