MONAZITE EMPA DATES FROM THE AMPHIBOLITE-GRANULITE METAMORPHIC PROGRESSION TO THE WAYAH GRANULITE CORE, WESTERN NORTH CAROLINA: CONFIRMATION OF TACONIAN PEAK METAMORPHISM

The amphibolite-granulite metamorphic progression to the Wayah Granulite Core, NW of Franklin, NC, is primarily continuous and prograde. Petrologic, stratigraphic, and structural evidence indicates this progression crosses the premetamorphic-peak Hayesville fault (HF) without significant disruption, from the western Blue Ridge (WBR) to the central Blue Ridge (CBR). Previous geochronology in the WBR and CBR is limited; ion-probe zircon dates from Winding Stair Gap (WSG) in the CBR (Miller et al., 1998, Geol.) and several previous studies demonstrated evidence for an Ordovician metamorphic event. In contrast, recent U-Th-Pb Monazite (Mnz) dates by electron microprobe (EMPA) yielded only Acadian dates from WBR Great Smoky Group (GSG) rocks; results were taken to refute Taconian terrane accretion along this Laurentian margin.

New EMPA data from Mnz in five samples, which range in metamorphic grade from Sil+Msc to granulite facies (GF), require a Taconian metamorphic peak in both the WBR and CBR. Samples include two GSG schists (Sil+Ms+Kfs) ~140 m NW of the HF, two CBR schistose gneisses (Sil+Msc) ~100 and 470 m SE of the HF, and one CBR gneiss (GF) ~1800 m from WSG. Complete, high quality Mnz analyses (99-102 wt%) maximize accuracy of matrix corrections. Potential interferences were identified carefully, especially of Th on U and Pb, as calculated dates are very sensitive to small U-Pb variations. From 144 calculated Mnz dates, five outliers were excluded. Remaining dates range from 349-575 Ma, with a global average of 470 Ma ±8 (2sigma standard error, n=139, 1sigma std dev=50). GSG dates (487±7) are older than CBR dates (459±7); the GF date is youngest (449±9). These data confirm Taconian metamorphism of both the WBR and CBR. Younger, excluded dates imply local (Acadian?) Pb loss. Additional data should test these and other apparent trends. Clearly, the southeastern Laurentian margin experienced Taconian metamorphism, likely in response to terrane accretion.