HIGH-PRECISION U-PB ZIRCON GEOCHRONOLOGY OF THE ELBERTON GRANITE, NORTHEAST GEORGIA: PRELIMINARY CA-TIMS RESULTS

The Elberton Granite is one of the largest granite batholiths in Georgia and the hub of an extensive quarrying and stone cutting industry. Located in the Inner Piedmont, the batholith was intruded into polydeformed schists and gneisses of Paleozoic age during the end of the Alleghanian Orogeny. The batholith is remarkably homogeneous in major-element composition, modal mineralogy, and texture but displays cryptic gradations in trace-element and isotopic compositions. Previously reported crystallization ages for the granite were determined by thermal ion mass spectrometry (TIMS) on bulk zircon fractions (320 ± 20 Ma by discordia upper intercept; Ross and Bickford, 1980) and by ion microprobe (304 ± 14 weighted average; Heatherington and Mueller, 2004).

In this study, we present new high-precision U-Pb zircon age data, by chemical abrasion-TIMS, from three samples collected at different quarries spanning the geochemical gradients across the central part of the batholith. After heavy liquid and magnetic separation, the zircons were chemically abraded in an HF-HNO₃ solution for 16 hours at 220 °C, cleaned, and picked into 19 fractions, each consisting of <10 zircons of similar morphology. While preliminary, two concordant fractions and one nearly concordant fraction of elongate prisms from two samples yield an age of ca. 300 ± 4 Ma, which we interpret as the time of crystallization as it is consistent with previous age determinations. Most other analyzed fractions contained larger and more equant zircons and exhibit normal or reverse discordance (2%-30%) and older ²⁰⁶Pb/ ²³⁸U and ²⁰⁷Pb/²³⁵U ages. Discordant data do not define meaningful discordia but probably reflect variable inheritance. Reversely discordant fractions may be a result of isotopic fractionation during chemical abrasion, but further work is needed to fully evaluate this possibility.