UNIQUE LITHOLOGY PERMITS TRACING SHEAR ZONE IN POORLY EXPOSED GOOCHLAND TERRANE ANORTHOSITE, VA

Hess (1910) described two rutile prospects northwest of Richmond, VA. One of these, the Gouldin (now Montpelier), developed first for titanium and later for feldspar, is now exposed in a large quarry. The Nuckols prospect, 13 km to the south, has never been developed. Hess (1910) and Watson and Taber (1913) described these bodies as “very feldspar-rich gneiss” intruded by rutile-bearing, quartz-feldspar-rich, “pegmatite dikes”. Then, as now, the deeply weathered gneiss and pegmatite were most easily traced by knots and individual grains of resistant rutile, ilmenite, blue quartz, and chunks of pyroxenite in the saprolite and soil. Both articles proposed that the two bodies are connected by the feldspar-rich gneiss.

Numerous studies of the now well exposed Montpelier body reinterpret that the “pegmatite dikes” of Hess (1910) comprise primary, very coarse, antiperthitic anorthosite, and the “feldspar-rich gneiss” is derived from the coarse anorthosite by shearing and recrystallization, at upper amphibolite or granulite –facies conditions, producing refolded isoclinal folds and pencil gneiss with near horizontal lineation parallel to the shear zone. Large clots to individual grains of rutile and ilmenite characterize the coarse anorthosite, whereas thin lenses and small grains of rutile and ilmenite characterize the sheared, gneissic anorthosite. Traced by these resistant minerals in the soil, the sheared anorthositic gneiss extends not only the 13 km southward from the Montpelier body, along the eastern limb of the State Farm dome, to the Nuckols prospect (which retains some large slabs of the original coarse anorthosite), but at least another 2 ½ km south from Nuckols, perhaps cut off in the ultramylonite of the Late Paleozoic Hylas zone. Yet to be determined is whether the shearing of this Grenville anorthosite occurred during Grenville metamorphism or during a later, Paleozoic, metamorphic event. It does clearly predate the mylonitization and brittle deformation of the Hylas and subsidiary mylonite zones of the northern Goochland terrane.