P-T ESTIMATES IN METABASITES FROM THE EASTERN BLUE RIDGE, NORTH CAROLINA: A RE-EVALUATION

Metabasites from to the Ashe Metamorphic Suite in the Eastern Blue Ridge are usually interpreted as eclogites retrogressed into garnet-bearing amphibolites. Two outcrops, distant of 65 km, have been particularly studied: Boone and Bakersville. The previous PT estimates for Boone highlight a hot path, with peak eclogite at ~720°C and 16 kbar, derived from reintegrated omphacite, and a retrogression to amphibolite facies at 655°C and 8-9 kbar. The PT path of Bakersville retrogressed eclogites is insufficiently constrained: prograde path is estimated between 400 and 680°C and 6 and 16 kbars, while the peak metamorphism is 630-660°C and 10-13 kbars, and the retrogression into amphibolite occurred at 500-550°C and 9-12 kbars. Recent advances in microscale petrology, like XMapTools software, now permit to isolate local equilibrium, and likely have a better step-by-step definition of PT paths, particularly in partly retrogressed samples.

Boone (11 samples) and Bakersville (5 samples) have a basaltic composition, (SiO₂ = 47-53%), and a broadly similar mineralogy: major minerals are garnet, amphibole, clinopyroxene, and plagioclase, with epidote, quartz, and Ti-bearing phases as accessory minerals. Boone samples are foliated, with layers of garnet, plagioclase, and quartz, and layers of garnet, amphibole, plagioclase, and pyroxene. Additionally, there is no evidence of high-pressure features: clinopyroxene are Ca-rich with no relics of omphacite, and the Ti-minerals are mostly titanite ± ilmenite. Bakersville samples do not display any foliation and contain abundant rutile and rare relics of omphacite grain, likely indicating higher pressure.

The P-T estimates for Boone samples (P = 6-8 kbar, T = 590 – 730 °C) yield a single, low-P granulitic event, in agreement with the lack of high-pressure evidence in thin sections. These estimates also correlate with previous estimates for retrogression. However, the interpretation differs as Boone samples do not evidence any high-pressure event. Bakersville samples are indeed former eclogites (Pmin = 17 kbar) that underwent a hot decompression straddling amphibolite and granulite facies, with P-T estimates similar with those of Boone (P = 5-7 kbar, T = 695 – 705 °C).