PRESSURE-TEMPERATURE (P-T) ESTIMATES FROM MINERALOGIC THERMOBAROMETRY IN AND NEAR THE WAYAH GRANULITE CORE, NC, INCLUDING NEW ZIRCONIUM-IN-RUTILE TEMPERATURES (ZRT): IMPLICATIONS FOR TECTONIC HISTORY, PRESERVATION OF PEAK T, AND POST-PEAK RE-EQUILIBRATION

The Wayah Granulite Core (WGC) in SW NC reached granulite facies conditions (GF) during the Ordovician Taconian orogeny (Eckert et al., 1989, GSA Bull. 101(11):1434; Eckert, 1988 unpub. PhD., and references therein). Mineral assemblages, textures, and major-element P-T indicate a continuous prograde transition from the St-Ky zone to the GF, crossing the pre-thermal-peak Hayesville fault without apparent disruption to this boundary between the Laurentian margin (western Blue Ridge), and the central Blue Ridge (Cartoogechaye terrane) that contains 1.1 Ga basement. P contours may correlate with known and potential F2 map-scale folds that are roughly synchronous with the thermal peak.

Zirconium-in-Rutile temperatures (ZRT) in Connecticut (CT) helped confirm ultrahigh-T (UHT) conditions, in addition to GF and Ky-zone re-equilibration (Ague et al., 2013, Geol. 41(2):271 and references therein). New ZRT for 7 samples in and near the WGC are compared to T from Grt-Bt at reference P. Many Rt grains are simple inclusions in Grt; some small acicular to prismatic grains may have precipitated from Grt upon cooling and/or P decrease, analogous to some Rt in CT. Most peak-estimate ZRT approximate or exceed Grt-Bt T, though non-peak ZRT range much lower. Peak ZRT occur in both apparent precipitates and larger inclusions. Lowest ZRT are either in or near the matrix and/or in exsolution clusters with Ilm. For the employed conditions (10 kV/150 nA/160 sec.), some lower Zr results fall below the detection limit (=n.d.) (0.006 wt% yielding T~530°C). Intragranular variation is small, though intergranular results range widely. Each sample displays a ~continuous or bimodal distribution of ZRT from higher to lower T; no UHT is indicated. Peak ZRT (within 3σ below maximum Zr in each sample) are {ZRT(Grt-Bt T/°C @ GASP P/GPa)}: {817 (925 @ 1.24)}, {843 (842 @ 0.98)}, {827 (784 @ 0.7)}, {838 (844 @ 1.0)}, {824 (673 @ 0.89)}, {678 (631 @ 0.7 Grt-Opx P; Grt-Opx T=774)}. Exsolved Rt at Winding Stair Gap yields sub-peak ZRT {568 (820 @ 0.79), 32% n.d.}.

GF likely extended farther east and west than confirmed previously. Some Rt clusters within apparent Grt growth domains may preserve near-peak prograde trends. Extensive lower-T re-setting of matrix ZRT below Grt-Bt T and mineral assemblage indicators may define points on cooling profiles from peak T.