AGE AND HF ISOTOPIC COMPOSITIONS OF DETRITAL ZIRCONS REVEAL CONTRASTS IN OROGENIC STYLE AND CRUSTAL GROWTH IN THE S. APPALACHIANS

Paired U-Pb and Lu-Hf isotopic compositions of individual detrital zircons from a single Pleistocene sand deposit in northern Florida reveal that Mesoproterozoic (Grenville sensu lato) lithosphere in the southernmost Appalachian orogen evolved almost continuously from ~1200 to ~900 Ma. The evolution of the Lu-Hf system in this crust is characterized by a limited range of 176Hf/177Hf that correlates strongly with measured Lu/Hf ratios and produces a linear array intermediate between the evolution curves of the depleted mantle and average crust. This pattern is most consistent with a continuously evolving mixing scenario between relatively juvenile, mantle-derived magmas and older crustal components (e.g., MASH zone of Hildreth and Moorbath) to create a new Mesoproterozoic lithospheric reservoir. In contrast, Paleozoic zircons (ca. 450 to 250 Ma) define the temporal range of the Appalachian orogenies, but do not display a correlation of Hf isotopic composition and age. Consequently, extrapolation of the Lu-Hf systematics of the reservoir defined by Mesoproterozoic zircons using typical crustal values for Lu/Hf cannot explain the Paleozoic pattern because the range of Hf isotopic compositions extends to much higher values (near the contemporaneous depleted mantle) and the lower limit lies well below reasonable extrapolations of the Mesoproterozoic array using the Lu/Hf of average crust. Appalachian Paleozoic crust, therefore, is characterized by a wider range of Hf isotopic compositions for a given age-interval compared to Mesoproterozoic crust. These contrasting patterns suggest: 1) Mesoproterozoic crustal growth in the southern Appalachians was largely by magmatic additions that were produced from a mixture of older crust and juvenile mantle and 2) Paleozoic crust reflects a largely accretionary orogen that involved the addition of few, if any, true juvenile terranes and resulted in limited crustal homogenization. This study demonstrates the efficacy of using age-epsilon Hf patterns in detrital zircons to define the temporal limits of orogeny and to constrain orogenic style and patterns of crustal growth.