25 Terranes of the Southern Appalachian Blue Ridge and Piedmont: Insights into Their Tectonic Heritage and Incorporation into the Orogen from Recent Geochronologic, Isotopic, Provenance, and Field Studies II

Monday, 2 April 2012
1:30 PM - 3:30 PM
Windsor Ballroom

NUCKOLS ANORTHOSITE: REDISCOVERY- REDEFINITION OF A GOOCHLAND TERRANE PLUTON

Paper number: 25-5
Program: Theme Sessions
Time: 2:50 PM – 3:10 PM

Authors

Abstract

Hess (1910) described two new rutile prospects northwest of Richmond, VA. One of these (the Montpelier Anorthosite) has been developed, first for titanium and later for feldspar, and is well exposed in a large quarry. The second, 13 km to the south on the Nuckols farm, has been prospected over a period of about 200 years, but has not 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. Highly weathered feldspar was misidentified as Kfeldspar – it is antiperthite in both bodies. They appear to have been correct that these two bodies are connected over the intervening 13 km by the feldspar-rich gneiss. Saprolite of this gneiss was better exposed in the early 20th Century than it is now. Then, as now, it was most easily traced by knots of resistant rutile, ilmenite, blue quartz, and chunks of pyroxenite in the saprolite.

Numerous studies of the now well exposed Montpelier body confirm that the “pegmatite dikes” of Hess (1910) comprise a very coarse antiperthitic anorthosite, and the “feldspar-rich gneiss” is derived from the coarse anorthosite by recrystallization and shearing, probably under granulite–facies conditions. This sheared anorthositic gneiss extends 13 km horizontally to the south from the Montpelier along the eastern limb of the State Farm dome, apparently ending at the Nuckols body which retains some large slabs of the original coarse anorthosite. This 13+ km shear zone shows none of the late transition to ultramylonitic and cataclastic textures that dominate the Hylas zone (the eastern boundary of the Goochland terrane) just 1.5 km east of the Nuckols body.