PRESSURE-TEMPERATURE HISTORY OF THE BAKERSVILLE ECLOGITE: NEW CONSTRAINTS FROM NEW TECHNIQUES

The eclogites from the Bakersville, North Carolina area record critical information about the earliest events in the Appalachian Orogeny in the SE United States.

We apply modern thermobarometric approaches to these important and complicated rocks to test the existing P-T record. Bulk compositions from X-ray mapping and mineral compositions obtained by SEM-EDS were used as the source material for thermodynamic modeling of the P-T path using Equilibrium Assemblage Diagrams (EADs) calculated with Perple_X. Subsequent work will utilize quartz-in-garnet (QuiG) barometry to obtain separate P estimates to provide a point of comparison for the thermodynamic modeling. The peak metamorphic assemblage consists of garnet + omphacite (variably replaced by diopside + orthopyroxene + hornblende + ab-rich plagioclase) + quartz + rutile.

EADs for 3 samples of eclogite with garnet compositional isopleths yield peak conditions of 11-14 kbar at 550-650°C. EADs also indicate the presence of hornblende at peak P-T conditions. Retrograde metamorphism is evidenced by the presence of orthopyroxene, as well as intergrowths of plagioclase and pyroxene. The overprinted pyroxene has a composition of about 10% jadeite while undisturbed core pyroxene is up to 32% jadeite.

New EAD models yield prograde, peak, and retrograde conditions consistent with those of Page et al. (2003) based on classical thermobarometry. This includes the stabilization of orthopyroxene during decompression, and the formation of plagioclase-diopside symplectite. Furthermore, the EADs also predict the stability of hornblende at the peak conditions, which, coupled with the presence of prograde hornblende as garnet inclusions, points to amphibole stability throughout the evolution of these rocks.

The ability to establish P-T constraints for metamorphic rocks is critical for tectonic interpretation. With these new techniques, it is possible to confirm past results, as well as construct a richer and more nuanced understanding of these eclogites’ petrology, leading to a more comprehensive understanding of continental collisions, subduction zones, and the formation of mountain ranges.