GARBENSCHIEFER IN THE CHOPAWAMSIC FORMATION, PIEDMONT PROVINCE, VIRGINIA

Garbenschiefer (“sheaf schist”) refers to metamorphic rocks that contain spectacular bundles or radiating sprays of coarse-grained amphibole in a finer-grained matrix. We have recognized more than a dozen occurrences of garbenschiefer from three distinct areas in the Ordovician Chopawamsic Formation, a predominantly metavolcanic unit of the Chopawamsic Terrane. All of the rocks are amphibolites, with protoliths ranging from basalt to andesite based on reconnaissance whole-rock geochemistry. The most common prograde assemblage is pargasitic hornblende (X_Mg=0.38-0.59) + plag + grt + qtz + ilm ± rutile. Other more distinctive samples contain additional amphiboles, specifically cummingtonite (X_Mg=0.54-0.63), or gedrite (X_Mg=0.51-0.63), and one sample contains all three (not necessarily in equilibrium). Kyanite and/or staurolite occur in a few samples. In many cases, plag and qtz comprise a fine-grained granoblastic matrix, in striking contrast to the larger amphiboles. We attribute the presence of kyanite or staurolite in some samples to reflect bulk composition, since samples that lack these minerals occur in the same outcrops. The more aluminous compositions may reflect slight hydrothermal alteration of the protoliths, given that these examples are found near prominent occurrences of kyanite quartzite (interpreted to reflect more intensely altered volcanic protoliths). Recent ideas on the origin of garbenschiefer texture in the Alps (e.g., Steffen et al., 2001, in The Nature and Tectonic Significance of Fault Zone Weakening, Geol. Soc. Lon. Spec. Pub. 186) suggest that it actually develops in high strain zones, where fluid-assisted diffusion creep enables high diffusion rates, leading to the rapid growth of amphibole porphyroblasts. Once formed, these porphyroblasts greatly increase rock strength, shifting deformation to adjacent weaker zones. The occurrences reported here are not obviously found in shear zones, although their presence may provide some cryptic evidence for prior shearing. Of possible significance is that all occurrences lie on the limbs of large map-scale anticlines. A better understanding of these rocks can be gained by further documentation of their distribution in the field (and we suspect there are many occurrences that have not been recorded in previous mapping).