EVALUATION OF U-PB GEOCHRONOLOGIC EVIDENCE FOR THE NEOACADIAN IN THE SOUTHERN APPALACHIANS - A POSSIBLE CASE OF MISTAKEN IDENTITY?

The tectonic assembly of the southern Appalachians has traditionally been ascribed to three tectonothermal episodes: the Ordovician Taconian, Middle Devonian Acadian and Carboniferous-Permian Alleghanian orogenic events. Recent reports of pluton crystallization and metamorphic ages between 366-350 Ma have raised the possibility of late Devonian to early Carboniferous, or Neoacadian, events – younger than previously inferred.

A recently constructed comprehensive geochronology database for the southern Appalachians reveals a surprisingly small subset of U-Pb ages bearing on the timing of Acadian or Neoacadian events west of the Central Piedmont shear zone. Only 18 modern U-Pb zircon ages are reported in journals, conference abstracts, and other sources. Fourteen of these reported ages, evenly divided between Acadian and Neoacadian, await full documentation. Reported ages are a combination of ion-microprobe and isotope-dilution, thermal ionization mass spectrometry (ID-TIMS) analyses.

Existing ion-microprobe data commonly display a smearing along concordia of low-precision (3-5%), but concordant, data points. In the case of the Walker Top granite (Giorgis et al., 2002, C.G.S. field trip guidebook), 1-sigma data cover the span of 340-400 Ma, yielding a precise (but accurate only in the absence of systematic errors) weighted mean age of 366 ± 3 Ma. ID-TIMS data (2-sigma), from Spruce Pine bodies are an order of magnitude higher precision but are plagued by the effects of Mesoproterozoic inheritance and Pb-loss. The latter effect renders 206Pb/238U ages anomalously young, but regression to concordia can permit an accurate age. Available ion-microprobe data cannot resolve similar Pb-loss. Thus, “Neoacadian” ion-microprobe ages might actually be ca. 380 Ma “Acadian” ages, but with the weighted average 206Pb/238U ages biased to anomalously young values by the unrecognized effects of Pb-loss.

From published U-Pb data, the precise timing of Devonian to early Carboniferous events in the southern Appalachians remains unresolved. This evaluation clearly demonstrates that the best approach for deciphering the timing of (Neo)Acadian events in the southern Appalachians is by leveraging the strengths of both methods – the high temporal resolution of ID-TIMS and the high spatial resolution of ion-microprobe.