Stability of zircon u-pb systematics in a greenschist-grade mylonite - an example from the rockfish valley fault zone, central Virginia, USA

The mid-Paleozoic, greenschist-grade Rockfish Valley Fault Zone (RVFZ) of central Virginia cuts the Grenville-aged Pedlar River Charnockite Suite (PRCS) and contains zircons that underwent brittle failure during ductile deformation. Electron microprobe analyses and scanning electron microscope (SEM) backscattered electron (BSE) imaging show that zircons from the protolith PRCS are concentrically zoned (with alternating U-Hf-rich and U-Hf-poor bands), and contain numerous radial microcracks. Zircons from the RVFZ mylonite are unzoned, fragmented, show no internal microfractures, and have low U and Hf concentrations relative to the PRCS zircons. U-Pb isotopic studies of zircons from the mylonites and from the charnockitic protolith demonstrate that no preferential Pb loss occurred in the zircons from the mylonite, and that the Pb-207/Pb-206 ages of the mylonite zircons are identical to those of the protolith zircons. The loss of primary zoning from the zircons of the RVFZ ultramylonites can be explained by the physical removal of microfractured, U-rich, alpha-damaged zircon domains as the result of brittle failure and disaggregation during mylonitization. Mechanically resistant (low-U) portions of zircon grains tended to remain intact in the mylonite. Thus, it may not always be possible to predict whether or not the zircon U-Pb system has been disturbed by mylonitization by using physical criteria (e.g., grain size reduction, obliteration of primary zoning textures) alone. Evidently, fluids present during mylonitization accomplished the hydration of primary mineral assemblages, but did not chemically interact with zircons, and their primary U-Pb and Pb-Pb ages were preserved.