HARD TO JUDGE A ROCK BY ITS COVER: NEW PERSPECTIVES ON THE EASTERN RALEIGH TERRANE IN THE NORTH CAROLINA EASTERN PIEDMONT

The eastern Raleigh terrane (eRt) in the NC-VA eastern Piedmont lies between the Macon and Lake Gordon shear zone strands of the Eastern Piedmont fault system that formed during Alleghanian orogenesis. The eRt has been linked with the ca. 633–528 Ma peri-Gondwanan Carolinia or ca. 1100 and 385 Ma Laurentian Goochland domains. We report results of new geologic mapping coupled with LA-ICP-MS U-Pb zircon geochronology that provide new insights into the eRt.

The ca. 410–403 Ma Parktown dacite and Possumquarter monzodiorite gneisses yield unimodal age distributions typical of magmatic or single-source sedimentary protoliths. The ca. 414–413 Ma sillimanite-bearing Mill Branch schist and a garnet biotite schist yield multimodal ages typical of detrital rocks. Sillimanite-bearing schist also has low Th/U ca. 380–360 Ma metamict zircon that may record partial to complete metamorphic resetting. Liberia monzogranite gneiss intruded these units, yields multimodal age ranges, and bears low Th/U ca. 360–314 Ma (ca. 331 Ma weighted mean age), CL-dark zircon, interpreted as metamorphic. Field relationships and zircon ages suggest that Liberia gneiss may record ca. 331 Ma migmatization of the Mill Branch schist concurrent with high-grade sillimanite growth.

The eRt may have experienced lower to upper amphibolite facies metamorphism and migmatization as early as ca. 380–360 Ma. Its fabric elements are consistent with late Paleozoic dextral transpression. Lower amphibolite facies rocks typically lack indicators of intense dextral strain and appear to be contractionally dominated, while middle to upper amphibolite facies rocks record penetrative dextral shear, composite S-C-C’ foliations, and mineral stretching lineation. To account for metamorphic facies and strain intensity differences, we propose that late-stage contraction during Alleghanian transpression formed a positive half-flower structure. The Mill Branch schist and Liberia gneiss occupy a petal of dextrally deformed and migmatized eRt rock thrust eastward to higher structural levels over the lower grade rocks. Early Devonian magmatism and deposition suggests that protoliths for eRt rocks formed in an Early Devonian island-arc setting inconsistent with the history of Carolinia. Links between the eRt and Devonian rocks in Goochland need further testing.