A PROBABLE SILURO-DEVONIAN PROTOLITH AGE FOR THE RALEIGH GNEISS, SOUTHEASTERN VIRGINIA PIEDMONT

The Raleigh gneiss of the SE Virginia and NE North Carolina Piedmont Province includes a heterogeneous package of gneisses and schists that primarily reflect metamorphism under amphibolite facies conditions.  The Raleigh gneiss has been variously interpreted as:  1) a southern extension of the Goochland terrane in central VA;  2) a higher-grade equivalent of rocks in the Carolina terrane;  or 3) a separate, unique terrane.  Geochronological tests of these interpretations are limited by a paucity of precise U-Pb zircon age determinations on probable metaigneous rocks in the terrane.  The few previous age constraints are limited to reports of ²⁰⁷Pb/²⁰⁶Pb zircon ages or Nd model ages, all of which suggest Neoproterozoic to Cambrian protoliths.  We report here results from a single sample of Raleigh gneiss near South Hill, VA, which consists primarily of qtz, plag, and biotite, with additional pale green amphibole, epidote, sphene, and apatite.  The bulk composition of this sample (SiO₂=72%, CaO=3.1%, Na₂O+K₂O=3.9%) plots in the dacite field on a total alkalis vs. silica diagram, and we interpret the protolith to be a calc-alkaline igneous rock (see Cheek & Owens, this volume).  Ten zircon fractions display variable amounts of discordance, but yield a discordia line with intercepts at 424 ± 21 and 1293 ± 84 Ma (MSWD=27);  all fractions plot close to the lower intersection with concordia.  Analyzed zircons were clear, elongate prisms or rounded faceted grains, but apparently igneous based on Th/U values (0.14-0.41).  Despite the MSWD value, the results can be plausibly interpreted to reflect a crystallization age of ~424 Ma, with variable amounts of Mesoproterozoic inheritance.  Some of the scatter may be due to variability in the age of the inherited component.  If these interpretations are correct, this sample is younger than rocks of the Carolina terrane.  However, the ~424 Ma age is the same within error to one sample of Maidens gneiss (407 ± 2 Ma;  Owens et al. 2004 GSA abstract) in the Goochland terrane, but the Maidens zircons show no inheritance.  Correlation with the Goochland terrane is therefore permitted by these results, although hardly compelling.  At a minimum, the results require involvement of Mesoproterozoic crust in the petrogenesis of this sample, suggesting a continental arc setting on crust of broadly Grenvillian age.