A COMPARISON OF RETROGRADE ECLOGITES ALONG THE EBR-WBR BOUNDARY, WESTERN N.C.: PLAG-BEARING GRT-CPX AMPHIBOLITES PRESERVE A RECORD OF HIGH-P TECTONICS IN THE EASTERN GREAT SMOKY MOUNTAINS

The presence of eclogites near the Eastern Blue Ridge (EBR)-Western Blue Ridge (WBR) boundary are crucial for understanding Taconian tectonics in the southern Appalachians. P-T estimates from retrograded Grt+Omp+Qtz+Rt eclogites discovered in the 1990's (Lick Ridge/Bakersville NC eclogite) demonstrate active subduction prior to accretion of the EBR/Tugaloo terrane with Laurentia (WBR). New P-T estimates are used to compare high-pressure metabasites from Dellwood N.C. (U.S. 276, 1.9 km N of Dellwood, NC) to the Lick Ridge eclogite. Unlike the Lick Ridge eclogite, the Dellwood metabasites contain sodic Pl (Ab75-Ab86) that is interpreted as part of the peak metamorphic assemblage. Although they are not eclogites sensu stricto, Grt-Cpx-Pl-Qtz thermobarometry applied to Dellwood Grt-Cpx-Pl-Qtz-Hbl-Ttn-Rt-Ilm metabasites suggest P-T conditions of 15-18 kbars at 650-775C. Sodic, non-symplectic Cpx (Jd 15-25), Pl cores (Ab ~84), and Grt compositions (Grs 31; Pyp 14; Alm 55; Sps 1) inside retrograde rims were used to estimate eclogite-facies conditions that are similar to P-T estimates of the Lick Ridge eclogite 90 km to the NE (i.e. minimum P-T of 13-17 kbars at 625 – 730C). Other similarities include: (1) Tectonic setting: both lithologies occur near the base of the EBR terrane at the EBR-WBR boundary, and occur as tectonic blocks partly enveloped by regional Ky-grade pelitic paragneisses; (2) Mineral textures: both lithologies contain abundant Cpx-Pl-Hbl-Qtz symplectites after omphacite, as well as Pl rims on Grt; (3) Spatial variations in Grt composition: although Dellwood Grt is Ca-rich and Mg-poor relative to Lick Ridge Grt, interior compositions are relatively homogeneous, with Fe-rich, Mg-poor compositions in the outer 50 microns; (4) Inclusion suites within Grt: Grt contains Cpx, Hbl, Qtz, Rt, Ep, Ttn, and Bt. These similarities suggest similar geologic histories along the EBR-WBR boundary in western NC: prograde high P/T facies series metamorphism, followed by amphibolite facies reactions during retrograde deformation, uplift and cooling. Although geochronologic constraints are lacking, Dellwood metabasites may have formed concurrently with eclogites from Lick Ridge (Taconic, ~ 459 Ma) and offer evidence of extensive subduction along the western margin of the Tugaloo terrane in western NC.