Guilin, M. (2019). Investigations of metabasites from the Ashe Metamorphic Suite, Eastern Blue Ridge Province, North Carolina. Are amphibolites near Boone retrogressed eclogites?. Unc Charlotte Electronic Theses And Dissertations.
The Ashe Metamorphic Suite in western North Carolina hosts metabasites ranging in grade from eclogite to amphibolite. To the west of the Grandfather Mountain Window (GMW) near Bakersville, retrograded eclogites, displaying primary eclogite-facies paragenesis are juxtaposed against lower-grade amphibolites. Across the GMW 65 km to the northeast near Boone, foliated amphibolites are in the same structural position have previously been interpreted to be retrograded eclogites. Their inference as retrogressed eclogites is questionable because they are not in direct contact with fresh eclogite, nor do they display any primary high-pressure relics like omphacite. In order to test the hypothesis that the foliated garnet amphibolites near Boone are retrograded eclogites, the amphibolites were investigated for high pressure features (e.g., omphacite inclusions in garnet) using quantitative major element mapping, thermobarometry, and petrography. Additionally, a series of samples from Bakersville displaying varying degrees of altered eclogite were similarly investigated to better understand their metamorphic evolution. Results from the Boone amphibolites show that garnet inclusions contain sodic plagioclase and lack omphacite which suggests that they are not eclogite relics. This finding, along with no observations of matrix omphacite relics suggests that these rocks are not retrogressed eclogites, which is in disagreement with previous studies. New late retrograde P-T estimates for the Boone metabasites straddle granulite and amphibolite facies ranging from 9-5 kbars and 750-600 °C. For the Bakersville samples, new P-T estimates based on symplectite formation adjacent to primary omphacite range from 700-550 °C and 16-6 kbar which is in general agreement with previous studies. New P-T estimates based on spongy CPX with the same assemblage of symplectites after omphacite suggest a hotter decompression path ranging from 800-565 °C and 16-4.5 kbar. Finally, new estimates of kelyphite formation range from 10-5.5 kbar and 825-625 °C and suggest an influx of heat along the decompression path.