FIELD, GEOCHEMICAL, AND GEOCHRONOLOGICAL EVIDENCE FOR TWO PULSES OF NEOPROTEROZOIC MAGMATISM IN THE ROANOKE RAPIDS TERRANE, EASTERN PIEDMONT PROVINCE, VIRGINIA

A paucity of geochemical and geochronological data (as well as outcrop!) have made it challenging to understand the overall origin and tectonic significance of the Roanoke Rapids terrane of eastern Virginia and North Carolina. In addition, this lack has hindered the evaluation of possible correlations of this terrane with other peri-Gondwanan terranes of the southern Appalachians (e.g., Carolina terrane). Owens et al. (2010 GSA Annual Mtg.) reported a U-Pb zircon age of 613.6 ± 0.9 Ma for the trondhjemitic Three Creek pluton in the southern Virginia portion of the terrane. We also noted the presence of a large (0.5 x 1 km) screen or roof pendant of felsic metavolcanic rock (+ epidote amphibolite) in the interior of the pluton, and speculated that the plutonic and felsic volcanic rocks might be contemporaneous magmatic equivalents. To test this, we obtained whole-rock geochemical results for two samples of the felsic metavolcanic rock (and one amphibolite). The results for the felsic rocks suggest they are quartz keratophyres, and thus similar to trondhjemitic rocks of the pluton. However, they are higher in SiO₂ (75-77 wt%) and K₂O (1.4-1.9 wt%) than the plutonic rocks (62-69 wt% and 0.2-1.2 wt%, respectively). Additionally, new CA-ID-TIMS results for fine-grained igneous zircon reveal a significantly older age for the metavolcanic roof pendant, at ca. 673 Ma. Thus, the plutonic and volcanic rocks are demonstrably neither compositional or temporal equivalents. These two ages (~614 and ~673 Ma) are similar to the few previously reported ages from elsewhere in the terrane, and together indicate dual episodes of magmatism separated by about 60 Myr. The bimodal nature of rocks in the roof pendant mirrors that of other metavolcanic rocks in the terrane, and newly acquired whole-rock compositions of 17 regional samples shows that most are also quartz keratophyres, along with subordinate tholeiitic basalts. If all of these rocks are of a similar age, then a major portion of the Roanoke Rapids terrane consists of crust that is substantially older than that found in any of the other peri-Gondwanan terranes in the southern Appalachians.