Title: Booming Plutons: Source of microearthquakes in South Carolina

Continuous low-level seismic activity was found to occur at Lake Jocassee after the impoundment of the reservoir in 1975. In addition to monitoring seismic activity we began (in January 1976) a series of geochemical measurements in an effort to search for precursors to M/sub 1/approx.2--2.5 events. Discrete water samples were collected from wells and a spring, and their radon content was determined by scintillation counting. These data were comparable to those obtained from a continuous Rn monitor in the spring using an ionization chamber. Soil Rn measurements in track etch cups were started in 1977. The results of the datamore » collected so far indicate (1) that long-term fluctuations occur in the radon concentrations in the spring water with a period of approx.44 weeks (the radon content being 50--100% lower in summer and fall), (2) that anomalous changes in the radon concentrations have occurred both before and after earthquakes, the timing of the anomaly beign dependent on the distance of the epicenter to the radon sample site, (3) that both high and low anomalous radon values occur, and (4) that the soil randon method using track etch cups is useful in determining areas of high and low Rn concentration but not for a study of short-term temporal changes.« less

High-resolution ion microprobe analysis of zircon has provided ages for previously undated plutons of the high-grade eastern Blue Ridge of northeastern Georgia and southwestern North Carolina. These data, together with backscattered electron imaging, reveal the presence of nearly ubiquitous inherited cores of highly variable age and magmatic rims that have experienced variable Pb loss, thus making interpretation of conventional U-Pb analyses very difficult. Ion probe rim analyses indicate that the plutons were emplaced during both the mod-Ordovician and mid-Devonian. Zircons from all intrusions have predominantly 1.0 to 1.25 Ga cores (Grenvillian). In addition, both Devonian and Ordovician plutons have smallermore » populations of Late Proterozoic-early Paleozoic (0.5--0.75 Ga), Middle Proterozoic (1.4 Ga), and Late Archean (2.6--2.9 Ga) cores. The ubiquitous, round cores and thick magmatic rims suggest significant resorption and then protracted growth within the melts. Zircon saturation temperatures based on whole-rock ({approximately}melt) Zr concentrations are lower than expected for magma generation (710--760 C). Zirconium concentrations may not reflect saturation at maximum temperature, if melting was very rapid (<{approximately}10{sup 5} yrs), or if zircon cores represent grains that were shielded from melt inside host grains for much of the magmatic history. Ages of magmatic and inherited zones of zircon from the plutons demonstrate that similar crust underlay the eastern Blue Ridge during both Taconian and Acadian orogenies, that there was no single episode of voluminous magmatism, and that metamorphism and deformation began before 470 Ma and continued after 370 Ma. These plutons do not constitute a significant convergence-related arc, though it is possible that they represent a displaced part of an arc that lies primarily to the east (in the Inner Piedmont?).« less

Use of Al content geobarometry on clinopyroxene inclusions in plagioclase from the Carboniferous Liberty Hill pluton, South Carolina, yields pressure estimates of 8-10 kbar. Amphibole crystallization pressures are 3.1-4.7 kbar; contact metamorphic pressure are 4.5 kbar. Clinopyroxenes could be early crystallization products from the melt, restite crystals from the source region, xenocrysts from the magma conduit walls, or xenocrysts from a more mafic magma. Compositional uniformity of granitoid clinopyroxenes but dissimilarity with those in contemporaneous gabbroids is evidence against a xenocrystic origin. The deeper depths are interpreted as either the source region or a pausing place important in the crystallizationmore » history of the magma during its ascent to the upper crust. Early crystallization assemblage of the granitoid, and possibly the assemblage of the source region, was amphibole + clinopyroxene +/- plagioclase-bearing. An amphibolitic lower crustal source is consistent with the Sr and O isotopic and rare-earth-element studies of this and many other similar-age granitoids in the southern Appalachians.« less