Paradoxes in the deformational and metamorphic history of the eastern Blue Ridge: Evidence from the Lake Toxaway and eastern Big Ridge quadrangles, North Carolina

The Tugaloo terrane in the eastern Blue Ridge, located in the high-grade southern Appalachian crystalline core,contains small internal basement massifs, the Neoproterozoic Tallulah Falls Formation, and Paleozoic granitoid plutons. Detailed geologic mapping in the Lake Toxaway and eastern Big Ridge quadrangles was done to better understand the regional tectonic history.

Whole-rock geochemistry was used to determine similarities between the augen phase of the 1.15 Ga Toxaway Gneiss and the 1.15 Ga Wiley Gneiss of northeastern Georgia. The study found that all eastern Blue Ridge orthogneisses are similar and probably share a source.

The previously identified Whiteside, Looking Glass, and Pink Beds plutons, and the newly identified Horseshoe Rock and Round Mountain plutons were also characterized. All plutons are low-K, catazonal granodiorites and trondhjemites that plot as volcanic arc or syncollisional granites on tectonic discrimination diagrams. The Looking Glass, Pink Beds, and Round Mountain plutons were dated using U-Pb SHRIMP zircon geochronology, and their ages are 333 + 16 Ma, 371.3 + 4.2 Ma, and 342.5 + 2.4 Ma, respectively. Zircon saturation temperature estimates for these plutons, and a Whitney and Stormer two-feldspar estimate for the Round Mountain pluton, indicate that they intruded at 700-800° C.

Whole-rock geochemistry was used to constrain the origin of amphibolites and hornblende gneisses around the Toxaway dome. One sample was a metabasalt with MORB composition, like other eastern Blue Ridge samples. Two other samples have a metasedimentary protolith . Migmatitic aureoles found in the amphibolite facies rocks around the Whiteside, Looking Glass, and Horseshoe Rock plutons are syn-intrusional and represent a zone of contact metasomatism.

The new pluton ages constrain the regional deformation history. At least 6 deformations are recognized in the eastern Blue Ridge. Dominant regional foliation is traditionally attributed to the second event (~466 Ma). However, foliations measured within all plutons are identical to foliations measured in the surrounding rock, indicating that foliations had to form after the youngest pluton intruded (~333 Ma), and that Alleghanian deformation was dominant in this region. These observations do not explain cross-cutting relationships observed around older plutons and raise new questions about southern Appalachian tectonics.