EVIDENCE FOR TACONIAN AND ALLEGHANIAN OROGENESIS IN THE EASTERN BLUE RIDGE NEAR HIGHLANDS, NC

The headwaters region of the Cullasaja River near Highlands, NC is underlain by metasedimentary and metavolcanic rocks of the Neoproterozoic Ashe Metamorphic Suite (AMS), which were intruded by trondhjemite and tonalite of the Ordovician Whiteside pluton. The first deformation (D1) produced E-W-trending isoclinal folds in the AMS rocks and was accompanied by kyanite-zone metamorphism. D2 deformation produced NE-SW-trending tight to isoclinal folds and growth of sillimanite at the expense of kyanite. D2 folding was accompanied by the synkinematic intrusion of SE-dipping sheets of the Whiteside pluton, which truncated the F1 isoclines and generated migmatite along intrusive contacts with AMS country rock. A U-Pb zircon rim age of 465 Ma for the Whiteside is interpreted as magmatic (Miller and others, 2000), and implies that D1 and D2 were Taconian. D3 produced NE-trending, upright to NW-verging, outcrop- to map-scale folds of S2 foliation in migmatite and trondhjemite. These folds may have resulted from Alleghanian thrusting along the east-dipping, low-angle Chattahoochee fault, which surfaces less than 10 km west (Lamb, 2001). D4 produced late NE- and NW-trending crenulations and kink bands and is likely Alleghanian.

In an attempt to distinguish the multiple deformations experienced by Highlands-area rocks, 40Ar/39Ar closure ages were obtained for muscovite and hornblende from AMS rocks. Hornblende from D1 foliation in amphibolite yielded plateau ages of 319 Ma and 317 Ma, while muscovite from D2 foliation in schist and trondhjemite yielded plateau ages of 313 and 310 Ma, respectively. These Alleghanian ages contrast with the field evidence that the penetrative rock fabrics and most of the deformation are Taconian. The cooling rate of about 25oC/m.y. suggested by the comparison of hornblende and muscovite ages implies a period of rapid uplift and cooling from initial temperatures >500oC for the rocks during this time interval, perhaps as part of upper-plate motion along the underlying Chattahoochee thrust. The initial temperatures could have resulted from slow thermal decay from Taconian and/or Acadian loading, or alternatively, rapid Alleghanian overthrusting and thermal overprinting, followed by rapid uplift.