Petrology of the Mount Airy granite

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Virginia Polytechnic Institute and State University


Modal analysis on slabs, thin section petrography, microprobe analyses, and geological mapping have been performed on the Mount Airy granite and enclosing country rock. The unzoned, simply intruded, and composed of a medium-grained, white granite, containing 33-41% plagioclase (An₁₀₋₂₀), 27-35% quartz, 20-28% alkali feldspar (Or₉₀), and 3-13% color index minerals, primarily biotite, muscovite, and epidote. A more granodiorite aplite phase intrudes the granite and is composed of An₉₋₃ plagioclase, Or₉₅ alkali feldspar, quartz, apatite, muscovite, biotite, and traces of epidote and zircon, and garnet. Aplite is a late segregation of granite and is associated with pegmatite.

The enclosing country rock is amphibolite, pelitic schist, and pin-striped gneiss of the Alligator Back Formation. The pelites in contact with the granite on the southeast side have the assemblage garnet + biotite + staurolite consistent with the albite-epidote amphibolite facies metamorphism of the region. The granite has deformed the prior Ordovician foliation of the country rock and is itself cut by shears and folds close to the Brevard Zone. These are the result of Devonian-Mississippian movement on the Brevard.

Some of the muscovite is primary in the granite, and epidote is not. Most of the muscovite, sphene, and low-An plagioclase are subsolidus. Alteration of biotite and plagioclase show an early, water-rich fluid evolving into a more CO₂-rich fluid as the rocks cooled.

An injection temperature of 700-750°C and pressure of at least 5.5-7 Kbar is consistent with the mineral assemblages seen in the granite, xenoliths, and contact country rocks. The notable lack of hydrothermal activity at the granite margins is attributed to low H₂O content of the original magma and hydration reactions within the granite below the solidus.