INVESTIGATING CLASTIC SEDIMENTS DEPOSITED DURING RIFTING: EBSD AND MINERALOGICAL ANALYSIS OF THE LATE PROTEROZOIC GRANDFATHER MOUNTAIN METACONGLOMERATE OF WESTERN NORTH CAROLINA

The roughly half kilometer thick Grandfather Mountain Formation (GMF) polymictic paraconglomerate, exposed within the Grandfather Mountain window, was deposited during the late Proterozoic rifting of Laurentia and was subsequently metamorphosed to greenschist facies during the Alleghenian orogeny. We used optical and scanning electron microscopy to investigate 15 samples of the GMF from northwest North Carolina for the purpose of determining deformation temperatures, strain intensity, and mineralogy. We collected samples from four different outcrops of the GMF. They are composed of mainly quartz, feldspar, muscovite, chlorite, and opaques, and can include up to boulder-sized clasts of felsite, rhyolite, basalt, crystalline basement, and sedimentary rocks. In order to determine crystallographic preferred orientation (CPO) in quartz, several grain maps of clasts and matrix were created using Electron Backscatter Diffraction and were subsequently employed for Rf-phi and Fry strain analysis. We distinguished two distinct recrystallization textures in microscopically analyzed quartz clasts. Rough, irregular boundaries between quartz clasts consistent with bulging recrystallization are common throughout the studied samples. Additionally, we observed larger polygonal recrystallized grains with smooth, straight borders, similar in size and shape to adjacent subgrains, indicative of subgrain rotation. Bulging and subgrain rotation recrystallization occur at greenschist facies conditions, consistent with previous suggestions. Quartz c-axis pole figures from matrix grains are broadly consistent with subgrain rotation and bulging recrystallization; pole figures from the clasts lack a strong CPO. Differing orientations and strain intensities between the clasts and matrix were found through analysis of grain maps. Variations in pole figures between clasts and matrix indicate that the clasts experienced a previous metamorphic event at greater than greenschist facies conditions.