Petrology and regional significance of the Roxboro Metagranite, North Carolina
The pluton located at Roxboro, North Carolina, is predominantly a light gray to medium gray, microphaneritic metagranite. Phenocrysts of plagioclase, quartz, and perthite are accompanied by porphyroblasts of epidote. Relict igneous textural relationships suggest two possible fractional crystallization models, in both of which the order of crystallization was plagioclase, quartz, and then K-rich feldspar. A crude approximation of the composition of the original plagioclase phenocrysts is An 25 to An 40 . Based on the composition of locally present granophyre, the pluton was emplaced under almost dry conditions with a P total of about 350 bars and a temperature in the vicinity of 950 °C. The shallow depth of emplacement suggests that the Roxboro Metagranite represents the root of a volcanic sequence that once unconformably overlay the presently exposed sequence. During the middle Paleozoic, this granitic intrusive was metamorphosed at a minimum pressure of about 3 kb and a temperature of approximately 400 °C. A foliation, as shown by stringers of mainly biotite and epidote, was produced by the deformational phase accompanying regional metamorphism. All K-rich feldspar is now nearly pure microcline (∼Or 97 ), and all plagioclase is now nearly pure low albite (Ab 97–99 ). Such feldspar compositions accompanied by late growth of ferristilpnomelane indicate a re-equilibration under lower grade conditions than those realized during the peak of the major regional metamorphic event.