Precise time and conditions of peak Taconian granulite facies metamorphism in the southern Appalachian Orogen, U.S.A, with implications for zircon behavior during crustal melting events

The time of peak thermal conditions in the granulite facies zone of the southern Appalachian orogen corresponds to the time of formation of garnet-bearing leucosomes generated by biotite dehydration melting in garnet-sillimanite gneisses. Leucosomes contain unusually abundant zircon occurring as inclusions in euhedral plagioclase phenocrysts and garnet. Isotope dilution thermal-ionization mass spectrometric analysis of 13 elongate, euhedral zircon crystals from the leucosome yields a concordant U-Pb age of 458 ± 1.0 Ma (MSWD = 0.095). Ion microprobe U-Pb analysis of 11 euhedral zircons yields a weighted age of 460 ± 12 Ma (2σ); 458 Ma is taken as the time of partial melting and peak thermal conditions attending melting. Only two of 39 leucosome zircons exhibit Proterozoic inheritance typical of most zircons in metapelitic rocks and granitic intrusions in the southern Blue Ridge. Mineralogic thermobarometry, oxygen isotope thermometry, and experimental studies constrain melting to have occurred at ∼850°C and 8 kbar. The garnet-rich leucosomes are unfoliated, crosscut the dominant regional fabric in sillimanite gneisses, and contain kyanite as a late or subsolidus phase. Leucosome textures (euhedral plagioclase phenocrysts with interstitial quartz) and major and trace element concentrations (low SiO₂, and K₂O, high Zr) suggest that the leucosomes are not primary melts; they appear to be plagioclase-rich cumulates remaining after fractionation of a more felsic melt component. The Zr concentration of the leucosome is excessively high (1500-4000 ppm) for melts with compositions equivalent to the leucosome. The Zr enrichment in the leucosome resulted primarily from physical concentration of zircon by inclusion in plagioclase and garnet that remained after loss of the felsic component of the melt.