THE TASTE OF CROW: A REVISED AGE FOR A METAIGNEOUS VARIETY OF THE RALEIGH GNEISS, SOUTHEASTERN VIRGINIA PIEDMONT

Two years ago (SE GSA 2007) we reported a probable Siluro-Devonian U-Pb zircon age for an example of the Raleigh gneiss (metadacite) in the southeastern Piedmont near South Hill, VA. This age interpretation was based on analyses of ten zircon fractions (a combination of single and multi-grain) that displayed variable amounts of discordance, but which yielded a discordia line with intercepts at 424 ± 21 and 1293 ± 84 Ma. All fractions plotted close to the lower concordia intersection. Despite a high MSWD value of 27, we suggested that the lower intercept age represents a crystallization age for this sample. Although we considered this a plausible interpretation at the time, the high MSWD left considerable lingering uncertainty regarding the true age of this rock. The original analyses were performed on zircons prepared by conventional air-abrasion techniques, which can lead to problematic results for complex zircon populations. Therefore, we re-evaluated the age of this sample by obtaining new analyses of zircons that were pretreated using the “chemical abrasion” technique (“CA-TIMS”). This method has been shown to be very successful in removing portions of grains that have lost Pb, typically yielding more concordant results. In the present case, analyses of five single grains yield a discordia line with intercepts at 560 ± 2 and 1644 ± 14 Ma (MSWD = 0.61), and the array is anchored by an analysis that is only ~5% discordant. Inclusion of two additional, but slightly less well-aligned analyses yields identical results within uncertainties and only slightly higher MSWD (560 ± 2 and 1665 ± 27 Ma; MSWD = 1.7). As in the previous study, all analyses plot near the lower intersection with concordia. Thus, our revised age for this sample is 560 ± 2 Ma, interpreted as a crystallization age. This age is similar to the few previously reported ages elsewhere in the Raleigh terrane, which suggest Neoproterozoic to Cambrian protoliths. Although we attach no strong significance to the upper intercept age, the results are nonetheless consistent with Mesoproterozoic inheritance and imply the involvement of older continental crust in the petrogenesis of this sample.