A STORY OF BLANTON BRANCH ULTRAMAFICS: HOW AN OUTCROP EXPOSES THE HISTORY OF THE MANTLE

This study sought to investigate ultramafic rocks in the Webster-Addie Ultramafic Ring in order to learn about the early geologic history of this feature and the Southern Appalachian Mountains. Few studies have examined the ring in detail at an outcrop-scale. Our field site was an outcrop along Blanton Branch road, located in Jackson County, NC. At the outcrop, we saw diversity in grain size, foliation, mineralogy, and extent of serpentinization. We hypothesized that the variations we observed were due to compositional differences at a small scale in the mantle followed by hydrothermal alteration and deformation. We collected samples from three sub-outcrops, which were made into thin sections. Petrographic analyses yielded the modal mineralogy and evidence of deformation and hydrothermal alteration. Samples were also analyzed for bulk geochemistry (major, minor, and trace elements) using ICP-AES, and ICP-MS. Petrographic and geochemical results complimented our initial outcrop-scale observations by confirming variation in classification (modal mineralogy), deformation textures, hydrothermal alteration, and bulk geochemistry. The classifications included olivine websterite, websterite, orthopyroxenite, harzburgite, serpentinite, and chlorite (talc) schist. The ultramafic rocks contained ultramylonite fabrics and triple junctions or blurry grain boundaries, showing evidence of high-temperature recrystallization with shearing. This suggested that the rocks originated at a spreading center and were deformed in a lithosphere-asthenosphere shear zone. The rocks showed heavy serpentinization coinciding with microfaulting and fracturing, which suggested hydration during a later, colder stage of deformation in the rocks’ history, likely in an accretionary wedge. Further research at the outcrop-scale both at our field site and throughout the ring may provide a clearer picture of the lithologic diversity and geologic history of the ring. The structural history of the outcrop could be illuminated by further mapping and examination of thin section and outcrop-scale deformation features. Further geochemical and geothermetric studies could be conducted to place constraints on hydrothermal alteration, deformation and origin of the lithologies in the outcrop.