Crustal Evolution in the Southern Appalachian Orogen: Evidence from Hf Isotopes in Detrital Zircons

摘要

Paired U-Pb and Lu-Hf isotopic compositions of individual detrital zircons reveal that Mesoproterozoic lithosphere in the southernmost Appalachian orogen evolved almost continuously from 1200 to 900 Ma, with a noticeable concentration only at 1050 Ma. The evolution of the Lu-Hf system in this crust is characterized by a systematic, linear evolution pattern of 176Hf/177Hf versus age over this interval, which suggests a limited range of mixing ratios between juvenile and older crustal components to create a new Mesoproterozoic lithospheric reservoir. Extrapolation of the Lu-Hf systematics of this reservoir from the Mesoproterozoic to the Paleozoic using typical crustal values shows general coincidence with the lower part of the Hf isotopic array defined by Paleozoic detrital zircons. Because the Paleozoic array extends to both much higher and much lower Hf values than are likely to develop from extrapolations of the Mesoproterozoic array, the Paleozoic array is most compatible with an orogen formed largely from the accretion of numerous discrete crustal blocks that may or may not have derived partly from the southern Appalachian Mesoproterozoic lithosphere. These data are most compatible with existing models for Appalachian orogenesis that invoke accretion of exotic terranes, few, if any, of which were true juvenile terranes.