THE PERSIMMON CREEK GNEISS, EASTERN BLUE RIDGE, NC-GA: EVIDENCE FOR THE MISSING ARC?

Most models assume closure of a small ocean basin via subduction to accrete the Piedmont Terrane (Inner Piedmont + Eastern Blue Ridge) to Laurentia and thus appear to require subduction-related magmatism. However, published descriptions of plutonic rocks in NC and GA include few or no related mafic to intermediate units, suggesting that the Piedmont Terrane here lacks a typical arc assemblage. The purpose of this paper is to describe the Persimmon Creek Gneiss (PCG), a little-studied metamorphosed pluton in the Eastern Blue Ridge that seems to possess characteristics typical of subduction-related plutonic belts.

The PCG is dominantly tonalitic (~57-65 wt% SiO2, 2-3 wt% K2O) but ranges from gabbroic to granitic (53%-70 wt% SiO2). Coarse subhedral to fine-grained granoblastic plagioclase is the principal mineral; quartz, biotite, and epidote are also ubiquitous. Hornblende occurs in the more mafic rocks, and K-spar is present in the more felsic samples. Abundant (up to 5%) sub- to euhedral epidote with euhedral zoning is probably at least in part primary, suggesting deep crystallization from a wet, oxidized magma. Textures range from magmatic with modest recrystallization to thoroughly recrystallized. Field evidence indicates interaction of tonalitic magma with more mafic magma, as well as with screens of metasedimentary units.

Like other plutons of the Eastern Blue Ridge, the PCG is depleted in high field strength elements. In other elemental characteristics, however, it differs from the trondhjemitic plutons that characterize the region. The PCG displays moderate LREE enrichment (~50-150 x chondrite), flat HREE at ~10 x chondrite, negative Eu anomalies, and moderate Sr concentrations (~300-400 ppm). Other granitoids are richer in Sr, much poorer in HREE, lack negative anomalies, and are poorer in K2O. Zircon U-Pb data indicate that PCG crystallized ~480 Ma and carried abundant Grenville-age inherited cores. Based upon its geochemistry, mineral assemblage, and field relations, we suggest that the PCG represents a remnant of the deep roots of a subduction-related Taconian arc complex.