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Figure 3. Diagram plotting age against sample location along west-to-east transect for parts of the Westminster and Potomac terranes. Ages are based on 40Ar/39Ar data (Kunk and others, in press; Mulvey, 2003; and M.J. Kunk, unpub. data), U-Pb data (Aleinikoff and others, 2002), Rb-Sr data (Muth and others, 1979), K-Ar data (Reed and others, 1970), and zircon fission-track data (Kunk and others, in press). The five zircon fission-track ages from the Potomac terrane are statistically indistinguishable, with a mean of 282±13 Ma, suggesting that these rocks all cooled through the ~235°C zircon closure temperature (Brandon and others, 1998) in earliest Permian time. See text for additional discussion. Modified from Kunk and others (in press). ZFT, zircon fission track; Amph, amphibole; mu, muscovite; biot, biotite; Ord., Ordovician; Fm, Formation. |