GEOCHEMISTRY OF THE RALEIGH GNEISS IN THE PIEDMONT PROVINCE OF SOUTHEASTERN VIRGINIA: IMPLICATIONS FOR THE NATURE OF THE PROTOLITH

A major unit in the Raleigh terrane of the SE Virginia and NE North Carolina Piedmont Province is the Raleigh gneiss, which consists of gneisses and schists derived from varying igneous and sedimentary protoliths.  A distinctive variety of the Raleigh gneiss occurs at numerous localities in the vicinity of South Hill, VA, and the purpose of this study was to:  1) use whole-rock chemical compositions to constrain the nature of the protolith; and 2) compare these rocks with the similar-appearing Maidens gneiss in the Goochland terrane in central Virginia.  The latter goal was motivated in part as a test of the idea that Raleigh terrane rocks represent a southern extension of the Goochland terrane.  All occurrences are fine- to medium-grained, thinly banded gray gneisses that have been pervasively intruded by pegmatite.  As such, these rocks resemble other occurrences of Raleigh gneiss described elsewhere.  Intriguingly, the Maidens gneiss also shares this feature.  Seven samples from three different localities of the gneiss in the South Hill area are all mineralogically similar, and consist primarily of quartz, biotite, plagioclase, pale green amphibole, epidote, and sphene, in variable proportions.  Major element compositions of 6 samples indicate intermediate to felsic protoliths, with SiO₂ ranging from 61-76 wt%, whereas one is more mafic at 48 wt%.  On a total alkalis vs. silica diagram, the more felsic samples plot primarily in the dacite field, but range from andesite to rhyolite (with CaO from 2.8-4.8 wt%).  Other classification schemes (AFM; Nb/Y vs. Zr/TiO₂) also indicate calc-alkaline protoliths of broadly dacitic composition.  The simplest interpretation of these results is that these rocks represent metamorphosed calc-alkaline igneous rocks, an interpretation supported by U-Pb zircon results reported by Owens et al. (this volume).  The bulk compositions may also be consistent with a greywacke protolith, but the rocks preserve no textural evidence for such an origin.  Metaigneous varieties of the Maidens gneiss are typically less felsic (mostly andesitic), but some samples do range to dacitic compositions.  The field, mineralogical, geochemical, and geochronological (see above) results reported here hint at the possibility of a Maidens-Raleigh correlation, but this is hardly a unique interpretation.