NEW U-PB ZIRCON AGES FROM THE ROANOKE RAPIDS TERRANE, EASTERN PIEDMONT PROVINCE, VIRGINIA: IMPLICATIONS FOR ASSEMBLING CAROLINIA

The Carolinia domain represents the southernmost tract of peri-Gondwanan crust in the Appalachian orogen. It is comprised of at least a dozen individual terranes, but many have only been characterized at the reconnaissance scale, particularly with respect to geochronology. Nonetheless, the available data indicate a three-stage magmatic history of Carolinia: 1) an oldest arc phase (~670 Ma), found only in the Roanoke Rapids terrane; 2) a younger juvenile arc phase ranging from 630 to 610 Ma; and 3) a youngest arc phase ranging from 575 to 532 Ma (Pollock et al., 2010 GSA Memoir 206). We previously reported U-Pb zircon (CA-TIMS) ages of ~673 Ma for a felsic metavolcanic rock and ~614 Ma for a tonalite (Owens et al. 2010 & 2011 GSA abstracts) in southern VA, which are similar to two ages reported earlier by Coler and Samson (2000 GSA abstract) elsewhere in the terrane. New ages reported here include 665 ± 3 Ma for a granodiorite pluton at Webbs Mill, VA; 630 ± 6 Ma for a second granodiorite near Adsit, VA; and 563 ± 4 Ma for a felsic metavolcanic rock in the northernmost part of the terrane. With the addition of the youngest age, Roanoke Rapids is thus far the only terrane in Carolinia with a record of all three magmatic stages. The oldest age bolsters the evidence for crust > 660 Ma in this terrane. The most precise new ages for granitic rocks in the terrane now span the range ca. 630-614 Ma, which mimics almost exactly the age range of oceanic arc rocks in the Virgilina sequence of the Carolina terrane (ca. 633-610 Ma). The latest Neoproterozoic age (563 Ma) is within the age range of the youngest arc phase elsewhere in Carolinia, well-documented in the Carolina, Charlotte, and Spring Hope terranes (less well-so in other terranes). We suggest several possible implications of these new age results, including: 1) the Roanoke Rapids terrane may have been a separate crustal block prior to ca. 660 Ma, since it is apparently the only terrane in Carolinia with rocks of this age or older; 2) the Roanoke Rapids terrane and the Virgilina sequence of the Carolina terrane may have been assembled sometime between 660 and 635 Ma, thereafter experiencing arc magmatism ca. 633-610 Ma; and 3) the Roanoke Rapids terrane shares a latest Neoproterozoic to Cambrian magmatic history with several other terranes, consistent with significant amalgamation of Carolinia by this time.