HETEROGENEOUS STRAIN IN GRANITIC MYLONITES FROM THE ROCKFISH VALLEY FAULT ZONE, CENTRAL VIRGINIA

Grenvillian leucogranite and biotite granitoid are deformed in the Roc kfish Valley fault zone in the central Virginia Blue Ridge province. U ndeformed leucogranite is characterized by high silica content (>69% S iO$-2$/) and <10% mafic materials, whereas biotite granitoid contains 62-66% SiO2 and up to 25% modal biotite. The Paleozoic Rockfish Valley fault zone is a belt of anastomosing high strain zones that records N W-SE directed contractional deformation under greenschist facies condi tions. In the Rockfish Valley fault zone, leucogranite occurs as moder ately-foliated protomylonite with widely spaced zones of ultramylonite . Biotite granitoid occurs as well-foliated porphyroclastic mylonite a nd commonly envelopes boudins of deformed leucogranite. Strain measure ments based on feldspar microstructures indicate that deformed biotite granitoid records higher strains than deformed leucogranite. Mineralo gical and bulk-rock geochemical data indicate that leucogranitic proto mylonite experienced approx. 40% volume loss during deformation, where as biotite mylonite experienced approx. 25%. Although significant volu me losses occurred during deformation, mylonitic rock from the Rockfis h Valley fault zone generally records true tectonic flattening strains .