DISPARATE PRESSURE CONDITIONS FOR KY-GRADE METAPELITES AND HIGH-PRESSURE AMPHIBOLITE (NOT RETROGRADE ECLOGITE) NEAR THE EBR-WBR TERRANE BOUNDARY, DELLWOOD, NC

Pressures and temperatures were calculated to further our understanding of tectonism and polyphase deformation near the WBR (western Blue Ridge) – EBR (eastern Blue Ridge) boundary in western NC. Electron probe point analyses and Grt X-ray maps were obtained from samples collected near Maggie Valley and Dellwood, NC. Two lithologies were examined: (1) Ky-St-Grt-Ms-Bt-Pl-Qtz-Ilm-Ru pelitic schist (Great Smoky Group, GSG; WBR ‘cover’); and (2) Grt-Hbl-Cpx-Pl-Qtz-Ttn-Ru amphibolite (interlayered with migmatitic orthogneiss and paragneiss of the EBR ‘basement’). Pelitic schists exposed along the Blue Ridge Parkway (0.4 km N of Big Witch Tunnel) were selected for analysis due to the absence of a late, penetrative slip cleavage associated with regional F3 folding. D3 fabrics were avoided as they indicate retrogression of Grt, Ky, and St-bearing assemblages and resetting of peak Taconian mica and feldspar compositions. Amphibolite (U.S. 276, 1.9 km N of Dellwood) exhibits textural features (Pl + Cpx symplectite) and mineral compositions that are similar to retrograded EBR eclogites in the Grandfather Mt. Window vicinity. Pl in the Dellwood amphibolite is considerably more sodic (up to Ab 83) relative to retrograded EBR eclogites. Sodic Pl occurs as inclusions in late, post-kinematic Grt, is a stable phase within the prograde mineral assemblage, indicates that eclogite facies conditions were not attained, and precludes the rationalization to re-integrate Pl-Cpx symplectites. Cpx-Grt-Pl assemblages (EBR amphibolite) yield P of 11-13 kbar at 650-700 C. GSG metapelites yield lower pressures (7-8 kbar) at similar temperatures (650-700 C). Calculated T is consistent with the preservation of growth-related compositional zoning of Mn-Ca-Mg-Fe in garnets of both lithologies. Calculated P is not continuous across the EBR-WBR boundary, suggesting that an 11-13 kbar metamorphic event occurred in the EBR prior to its docking onto the Laurentian margin. The Dellwood metabasites are more appropriately considered ‘high-pressure amphibolites’, similar to the Grt-Cpx metabasites once suspected to be retrograde eclogites from the Lake Chatuge (GA) mafic-ultramafic complex.