P-T CONDITIONS OF SELECTED SAMPLES ACROSS THE BLUE RIDGE PROVINCE

The Blue Ridge province of the Appalachian Mountains has a complex geologic history characterized by more than one metamorphic and deformational event. Outcrops from Ducktown in the western Blue Ridge (WBR), Savannah Church and Sylva in the central Blue Ridge (CBR), and Beaucatcher Mountain in the eastern Blue Ridge (EBR) expose pelitic and psammitic rocks metamorphosed under amphibolite to granulite facies conditions. The Ducktown metapelites contain the assemblage garnet, biotite, staurolite, and plagioclase, whereas Savannah Church and Beaucatcher Mountain contain garnet, biotite, plagioclase, and hornblende. The Sylva outcrop in the middle of the CBR contains the assemblage garnet, biotite, plagioclase, sillimanite, gedrite, and anthophyllite. All samples from the CBR and EBR are characterized by almandine rich garnets (X_alm = 0.65 – 0.75, X_prp = 0.07 – 0.2, X_grs < 0.1), with the exception of samples from Sylva which have garnets with X_prp = 0.17 – 0.25. All garnets are unzoned with the exception of some retrograde rim re-equilibration. Feldspars from these samples are albite rich plagioclases (X_ab=0.50-0.70, X_an= 0.21-0.50, X_or=0.0-.02), with the exception of samples from Sylva which are more calcic with X_an=0.40-0.50).

Using the average P-T routine of Thermocalc 3.26 and conventional Gt-Bt and GASP thermobarometry, peak metamorphic conditions for the Ducktown samples (WBR) were calculated at 530°C, 6.5 kbar. Conditions for Savannah Church were calculated at 650-700°C, 8-9 kbar with Beaucatcher Mountain having conditions at 700°C, 7 kbar. On the other hand, P-T conditions for Sylva are estimated at 700°C, 14 kbar.

These results show that the P-T conditions across the Blue Ridge are only broadly consistent with the isograds mapped for the terrane. The most striking anomaly is the unusually high pressures calculated for the Sylva outcrop, which are inconsistent with the overall trend of decreasing temperatures and pressures from Winding Stair Gap in the south to outcrops in the north.