EVALUATING POSSIBLE HIGH-P AND T AMPHIBOLITES IN THE GOOCHLAND TERRANE, PIEDMONT PROVINCE, VIRGINIA

Two belts of pelitic gneiss in a small area in the northeastern portion of the Goochland Terrane contain the rare association kyanite + K-feldspar in an assemblage also containing quartz, plagioclase, garnet, biotite, rutile, and ilmenite. This assemblage (coupled with the paucity of muscovite) suggests that the kyanite in these rocks was produced via the “second Al-silicate” reaction at pressures exceeding about 8 kb where the muscovite breakdown curve intersects the kyanite-sillimanite boundary. Owens et al. (2012, GSA abstract) tested this hypothesis using conventional thermobarometry and pseudosection analysis, and reported temperatures ~730-760°C and pressures ~10 kb. Thus, this area appears to represent the deepest structural portion of this terrane. As a further evaluation of these metamorphic conditions, we investigated P and T using a different approach for four samples of associated amphibolites. We used the semi-quantitative thermobarometer of Ernst and Liu (1998, American Mineralogist), based on Al- and Ti-contents in Ca-amphibole in rocks of MORB composition. The common assemblage in all samples is amphibole + plagioclase + sphene ± clinopyroxene ± garnet. Amphiboles range from blue-green to dark green to pale brown, and compositions in all samples are pargasitic (TiO₂ = 0.9-1.9 wt%). Slight patchy zoning is common, but core and rim compositions are not markedly different. The amphiboles in two samples from an outcrop closest (~250 m) to the metapelite yield a T of ~820° and a P of ~7.5 kb. Samples further (but < 2.5 km) from the metapelite yield a T of ~740°C and a P of ~12 kb. The results clearly indicate high-grade conditions, though no individual amphibolite yields a P and T matching those of the metapelite. A possible interpretation is that the various samples record different points along an overall P-T-t path. Alternatively, the scatter may be due to: 1) bulk compositions that differ significantly from MORB; 2) imprecision of the thermobarometer; or 3) the effects of polymetamorphism. Nonetheless, an overall average based on all samples yields a T of 770 ± 50°C, and a P of 10 ± 2 kb (uncertainties based on one standard deviation of the determinations). These P and T estimates are broadly consistent with those determined for the metapelite, confirming the unique character of this portion of the terrane.