LIMITS ON TIMING OF DEFORMATION AND PLUTONISM IN THE EASTERN BLUE RIDGE, SOUTHWESTERN NORTH CAROLINA, FROM GEOLOGIC MAPPING AND U-PB ZIRCON GEOCHRONOLOGY

Multiple deformation events have affected the eastern Blue Ridge. Pluton intrusion and subsequent deformation delimit the timing of each event. New U-Pb zircon SHRIMP ages on several plutons have refined this timeline. The Pink Beds, Looking Glass, and the newly identified Round Mountain plutons were dated using U-Pb SHRIMP geochronology at the USGS-Stanford laboratory. All zircons have complicated zoning patterns. Most have distinct low-uranium inherited cores, with ages between 1.5-0.9 Ga. Magmatic oscillatory zoning encases the inherited cores, and the U-Pb magmatic age of the Pink Beds is 370.5 + 3.8 Ma. The Round Mountain is 342.5 + 2.4 Ma, and the Looking Glass is 337 + 11/-23. Each pluton is weakly to moderately foliated, but igneous textures are preserved.

These ages, previously obtained U-Pb zircon ion microprobe ages, and cross-cutting relationships observed in the field, indicate the timing of several observed deformation events. These observations include local truncation of dominant (S₂) foliation by the Whiteside pluton (466 + 10 Ma). A second event caused a pervasive regional foliation (D₂) to form and folded both the Whiteside and the surrounding Tallulah Falls Formation rocks. There are at least 2 generations of folding in the Whiteside, indicated by the presences of refolded folds. Surrounding rocks have at least two superposed sets of crenulation cleavage with NE axial orientations. A high temperature shear zone is truncated by the Round Mountain pluton at 342 Ma. The Cradle-of-Forestry-in-America fault (D₃) truncates the Looking Glass pluton and the D₂ foliation, limiting its movement to post 337 Ma (Alleghanian).

These relationships also delimit folding events. Early F₂ folds are present in the Whiteside and country rock; many of these folds have been refolded (F₃). Refolds in the Whiteside have the same SW axial orientation as crenulation cleavage (F₄?) observed in the surrounding rocks. Open recumbent folds are present in the Looking Glass pluton; these are probably fault-related (F₅ or later).