PLUTON AGES IN THE EASTERN BLUE RIDGE PROVINCE, NORTH CAROLINA: CONSTRAINTS ON TIMING OF TECTONICS AND METAMORPHISM IN DEEP LEVELS OF AN ORDOVICIAN ACCRETIONARY WEDGE COMPLEX

The Eastern Blue Ridge (EBR) of the southern Appalachians is the remnant of an accretionary wedge formed during Ordovician subduction of the Iapetus Ocean, prior to Taconian collision of the Piedmont arc with Laurentia. The EBR contains large bodies of high-T (>750°C) Ordovician eclogite, other high-pressure metamorphic rocks, and metapelite and amphibolite intruded by low-K plutons. Regional metamorphism reached amphibolite- to granulite-facies (650-800°C) synchronous with localized melting in metapelites.

Felsic plutons in the EBR are important time markers for regional-scale tectonics, deformation and metamorphism. Some plutons are strongly deformed and contain regional metamorphic fabrics, others are undeformed or cut by faults (e.g., Burnsville and Chattahoochee), or are deformed within major regional structures (e.g., Coweeta syncline). Plutons were thought to fall into two age categories; broadly "Taconian" (Ordovician) and broadly "Acadian" (Devonian-Silurian) based on ion-microprobe analyses (C.F. Miller et al., 1998; 2000). The ion-microprobe ages, however, are too imprecise to provide tight constraints on rates and time spans of tectonic and metamorphic processes in the deep parts of the accretionary wedge complex.

We dated zircon and monazite from the Rabun granite and Persimmon Creek gneiss to constrain better their ages and, thus, the timing of tectonic and metamorphic events. Our ID-TIMS analyses form slightly discordant arrays, easily recognizable as small degrees of Pb-loss, and yield ages of 335.1±2.8 Ma (Rabun) and 455.7±2.1 Ma (Persimmon Creek). Other analyses contain a clearly identifiable xenocrystic component. Single monazite grains from both samples yield ages identical to zircon and grains with disturbed systematics are identifiable. Our data show that carefully optimized conventional ID-TIMS dating can resolve Pb-loss, inheritance, and true concordance that are obscured by the low precision of ion-microprobe analyses. The new ages indicate that Persimmon Creek crystallized 3 m.y. after, not 20 m.y. before, 459 +1.5/-0.6 Ma eclogite metamorphism and the Rabun is the first known Alleghanian pluton in the EBR. The Chattahoochee fault, which was previously considered an Acadian structure, cuts the Rabun and thus, must have been active during the Alleghanian orogeny.