METAMORPHIC HISTORY AND ASSEMBLAGES OF MATRIX ROCKS IN THE CULLOWHEE OLISTOSTROMAL TERRANE

Rocks from roadcut outcrops along NC 107 near Western Carolina University in Cullowhee, NC are mapped as part of the Cullowhee olistostromal terrane by Raymond et al (1989), and as part of the Cartoogechaye terrane by Hatcher et al (2005). Raymond et al (1989) suggests these rocks may be part of a mélange complex, and they are suggested to have reached granulite facies metamorphic conditions in Hatcher et al (2005). Monazite and zircon ages from other Cartoogechaye terrane exposures correlate either with Ordovician metamorphism associated with the Taconic orogeny (Moecher et al., 2004), or Neoacadian (360-350 Ma) metamorphism related to the subduction of the Carolina superterrane (Bream, 2003).

To clarify metamorphic events in the Cullowhee area, we have conducted electron microprobe (EMP) mineral chemistry studies of selected gneissic samples representing the matrix of this block-in-matrix unit. As well, we have attempted Th-U-Pb dating of sample monazites, following methods outlined in Montel et al (1996) and Williams and Jercinovic (2002). Microprobe measurements on major and minor minerals were conducted from USF via remote operation technologies, using the JEOL 8900R Superprobe, housed at the Florida Center for Analytical Electron Microscopy at Florida International University. Data were corrected following ZAF protocols, and a range of mineral standards were used for WDS calibration. Anhedral and euhedral monazites from 10 to 100 µm were analyzed separately for U, Th, Pb, La, Ce, Pr, and Nd using uraninite, monazite, and crocoite as EMP standards. U data were calculated to U²³⁸ and U²³⁵ based on continental rock abundances. Decay iterations were run using Excel to estimate ages and conduct error propagations.

The mineral assemblages in the matrix gneisses include quartz, plagioclase feldspar, muscovite, almandine garnet and biotite, with minor sillimanite chalcopyrite, pyrite, zircon and monazite. This assemblage indicates a likely pelitic protolith, and metamorphic conditions in the sillimanite zone of the upper amphibolite facies. No relict granulite assemblages are evident. Preliminary monazite dating results suggest these minerals reflect both Taconic and Alleghenian/Appalachian orogenic events.