MINERALOGY, GEOCHEMISTRY, AND TIMING OF NEOPROTEROZOIC MAFIC MAGMATISM IN THE ROANOKE RAPIDS TERRANE, PIEDMONT PROVINCE, VIRGINIA

A distinctive, elongate mafic/ultramafic body occurs in the Roanoke Rapids terrane (RRT) in the Piedmont Province of Virginia. On the 1963 state geologic map, this unit is labeled “hgb,” applied to a diverse set of rocks including hornblende gabbro and gneiss, amphibolite, and various ultramafic rocks. On the 1993 state geologic map, this unit is classified as “adg,” indicating altered diorite and gabbro. Thus, the basic character of this unit (igneous or metamorphic?) has remained somewhat unclear. This study characterizes the unit in more detail, with the goals of better understanding its overall petrogenesis and significance within the RRT, and determining a primary age for this magmatism. Eight samples (all massive) from the southern portion of the unit are mineralogically variable, but all contain abundant amphibole (tremolite to hornblende). Many samples contain additional relict clinopyroxene. For the sample set as a whole, SiO₂ ranges from 47 to 54 wt%. The samples can be categorized into three groups based primarily on chemical composition. The first is a low Al₂O₃ (3-5 wt%) group, interpreted as metapyroxenites (X_Mg=70-76; 1122-1410 ppm Cr). The second, with intermediate Al₂O₃ (8-12%; X_Mg ~70; 438-684 ppm Cr), are hornblendites, dominated by coarse-grained hornblende and minor interstitial plagioclase. The third group, with the highest levels of Al₂O₃ (~18 wt%; X_Mg=51-65; 62-349 ppm Cr) contains more abundant plagioclase; these are metagabbros. The metapyroxenites and hornblendites clearly represent original cumulate rocks, whereas the metagabbros may approach the composition of liquids. One metagabbro contains high-quality, doubly terminated, elongate (up to 3:1) zircon euhedra – concordant single-grain analyses yield a ²⁰⁶Pb/²³⁸U (CA-TIMS) age of 609.5 ± 0.8 Ma (MSWD=0.29). This age is only slightly younger than those reported for several tonalitic to granitic plutons in the RRT (613-628 Ma). The overall evidence suggests that these previously enigmatic rocks represent contemporaneous deep roots of a magmatic arc constructed in a peri-Gondwanan setting. The timing is essentially coincident with juvenile arc magmatism elsewhere in Carolinia (Virgilina sequence of the Carolina terrane), but previously reported Nd-isotopic results suggest that the RRT was built in part on evolved crust.