GREENSCHIST-FACIES METAMORPHISM AND THE GENESIS OF AN ORTHOPHYLLITE – ROANOKE RAPIDS TERRANE, NORTH CAROLINA

The Roanoke Rapids terrane (RRt) of NC and VA contains some of the oldest rocks in the eastern Piedmont, making it vital to understanding the tectonic history of this province. Interpreted as a volcanic island arc suite, the RRt comprises metaplutonic, metavolcanic, and metavolcaniclastic rocks. Zircon U-Pb SHRIMP ages suggest arc magmatism between ~672-607 Ma. A penetrative greenschist-facies overprint is present, but the timing of this overprint is poorly constrained. Rocks of the RRt are moderately deformed with a NNE- to NNW-trending phyllitic foliation defined by chlorite and sericitic muscovite. The nearby, undeformed Butterwood Creek pluton yielded Rb-Sr biotite and whole-rock cooling ages of ~293-286 Ma, suggesting that regional Alleghanian metamorphism and foliation development predated the early Permian in the RRt. Questions remain as to the absolute timing of deformation and metamorphism in the RRt between island arc formation and Alleghanian orogenesis.

Just east of the Roanoke Rapids Dam, NC, the RRt crops out to expose variably metamorphosed and deformed volcaniclastic rocks, tonalite, leucogranite, and quartz diorite with local greenstone dikes. Four metatonalite samples were collected and analyzed via optical petrography, scanning electron microscopy, and electron microprobe analysis. Mineralogy is quartz + albite + muscovite + calcite + chlorite with minor epidote + K-feldspar + apatite ± titanite. Plagioclase is sericitized to varying degrees, with nearly complete replacement in one sample. A phyllitic foliation is present only in more sericitized samples; however dynamically recrystallized quartz is observed in all samples. This suggests that strain was relatively homogeneous, but heterogeneous sericitization altered some samples to orthophyllite. Additionally, titanite is partly to fully pseudomorphically replaced by rutile + calcite + quartz + K-feldspar + monazite ± xenotime. These reactions require metasomatic mass transfer of H₂O, CO₂, O₂^2-, K⁺, and PO₄^3-. Ongoing work aims to determine muscovite ⁴⁰Ar/³⁹Ar growth ages to constrain the timing of fabric development and U/Pb ages of monazite/xenotime replacing titanite to constrain the timing of metamorphism with potential implications for eastern Piedmont assembly.