PETROLOGIC AND STRUCTURAL ANALYSIS OF THE COLUMBIA PLUTON, CENTRAL VIRGINIA PIEDMONT

The Columbia pluton intrudes metavolcanic rocks of the Chopawamsic terrane in the central Virginia Piedmont. This 200 km² pluton is composed predominantly of granodiorite with lesser amounts of granite. Texturally, the pluton consists of porphyritic granite with feldspar megacrysts, medium-grained equigranular granodiorite, and medium- to fine-grained granodioritic gneiss. Our mapping indicates that granitic rocks northwest of Columbia, near Carysbrook, formerly mapped as a separate pluton, are part of the Columbia pluton. There is, however, a distinct transition in both metamorphic grade and deformation intensity from NW to SSE. In the NW part of the pluton feldspars with primary igneous microstructures are preserved, but have been extensively saussuritized. In this region quartz occurs as polycrystalline aggregates with interlobate grain boundaries and undulose extinction. Some quartz grains preserve core/mantle texture. In the SSE part of the pluton, no igneous microstructures are preserved as quartz and feldspar both display polygonal grain boundaries and straight extinction. Collectively, microstructural evidence suggests that the NW part of the Columbia pluton experienced middle to upper greenschist facies, whereas the SSE part of the pluton experienced middle amphibolite facies metamorphism. The fabric intensity (D-value) as defined by elongate quartz aggregates is weakest (D ~ 0.3) in the NW and generally increases towards the SSE (D >1). LS-tectonites, with K-values of 0.6-2.0 occur throughout the pluton. However, strong prolate fabrics (K >8) are localized near the nose of the regionally extensive Quantico/Columbia synform and oblate fabrics (K< 0.5) occur on the NW limb. Foliations have been folded into asymmetric structures that plunge moderately to gently to the NE. Penetrative elongation lineations consistently plunge subparallel to fold axes.