AGE BIASES FROM THE CHOICE OF MINERAL PHASES IN DETRITAL GEOCHRONOLOGY: CASE STUDIES OF MUSCOVITE 40AR/39AR AGES IN COMPARISON WITH MONAZITE AND ZIRCON U/PB AGES IN TECTONIC STUDIES OF THE SOUTHERN APPALACHIANS

Detrital mineral ages are functions of source characteristics, sedimentation history, and the specific mineral growth history and physical properties. Muscovite 40Ar/39Ar ages are well suited to record metamorphism and deformation in the middle to upper crust. Once formed, zircon is very refractory and commonly does not record superimposed metamorphic events. Thus, detrital zircon from the Carboniferous Pottsville formation of the Appalachians tends to yield U/Pb ages that are biased to the Grenville event (e.g., Hietpas et al., 2010, and references therein) whereas detrital muscovite from the Pottsville formation yields Paleozoic 40Ar/39Ar ages with prominent and distinct age modes that are biased to Paleozoic Appalachian tectonic events (Peavy, 2008; Uddin et al., in prep). Hietpas et al. (2010) sampled modern stream sediments derived from the Western Blue Ridge (WBR) province and demonstrated a greater fidelity for U/Pb ages of monazite (as compared to zircon) to detect the Grenville and Appalachian tectonic events. However, the relative proportion of Alleghanian monazite ages in the Hietpas et al. data set appears relatively minor in consideration with the substantial metamorphic and deformation effects of Alleghanian tectonics in the southern Appalachians. We sampled sediment from tributaries of the WBR province in order to evaluate the 40Ar/39Ar record of detrital muscovite and compare it to the record of Appalachian tectonic events in exposures of the WBR and also in the Carboniferous strata of the greater Black Warrior basin. Muscovite crystals sampled from modern sediment of the Etowah river near Cartersville, Georgia generally yield laser single crystal 40Ar/39Ar ages of 320-330 Ma (n~110), with no Precambrian ages detected. The age distribution for muscovite in this stream sediment is the same as determined for single crystal ages of muscovite from exposed lithologies of the Etowah river catchment, based on results for ~ 20 samples of phyllite and schists. On the basis of these results and similar studies, we suggest that muscovite in modern sediment as well as the Pottsville formation yields 40Ar/39Ar age distributions that are more representative than U/Pb zircon and monazite age distributions in regard to sediment contributions from Taconian, Acadian, and Alleghanian metamorphic source rocks.