RECRYSTALLIZED OLIVINE GRAIN SIZE SHOWING TECTONIC EMPLACEMENT HISTORY OF ULTRAMAFIC ROCKS IN THE BLUE RIDGE MOUNTAINS OF WESTERN NORTH CAROLINA

The peridotites in the Blue Ridge metamorphic suite were formed from the collision of an island arc and North America during the Taconic orogeny. The dominant theory on the formation of the Ashe Metamorphic suite is that it was previously an ophiolite suite (Abbott and Raymond, 1984). This model explains that the basalt from the sequence metamorphosed into the amphibolite surrounding the ultramafics which were part of the oceanic lithosphere as peridotites. The emplacement history from the oceanic lithosphere to the crust on North Carolina however, is not well understood. Ophiolites generally are emplaced by scraping the bottom of the oceanic crust from the subducting material onto the continental crust it is subducting under (Frost and Frost, 2014).

In order to obtain a more detailed emplacement history, we sampled several of these bodies exposed in western North Carolina for studies of their microscopic textures. The grain sizes of the peridotites vary between bodies and grains show annealed textures. The two bodies studied used samples obtained from tailing piles outside of the mines near the towns of Daybook and Frank. Preliminary studies showed the average recrystallized grain size of the Daybook body is 234±103µm while the average grade size of the Frank body is 407±186µm.

Using the average recrystallized grain size in each, we calculated the emplacement paleo-stress conditions using the recrystallized olivine grain size piezometer (Van der Wal, et al., 1993) and found the Daybook body to average at around 13MPa while and the Frank body at 20MPa. Different paleo-stress and textures observed in nearby bodies indicate they could have different emplacement conditions.