FURTHER EVIDENCE FOR DEVONIAN MAGMATISM REVEALED IN THE MAIDENS GNEISS, GOOCHLAND TERRANE, VIRGINIA

The Maidens gneiss, a heterogeneous package of rocks derived from a variety of protoliths, is the largest unit of the Goochland Terrane in the Piedmont Province of Virginia. Although long thought to be Mesoproterozoic, Owens et al. (2010, GSA Memoir 206) reported Devonian U-Pb zircon dates (~384-403 Ma) for Maidens samples located near the town of Goochland, in the approximate center of the terrane. These rocks are mylonitic gneisses, but their whole-rock compositions are consistent with intermediate igneous protoliths of high-K, calc-alkaline affinity. Owens et al. (2010) further suggested that Paleozoic rocks might be much more widespread in the Maidens unit than previously thought. To test this idea, we investigated similar-appearing gneisses near the town of Amelia Court House, approximately 45 km further south in the terrane. These rocks are also strongly deformed, but are mineralogically and chemically similar to those near Goochland. The typical mineral assemblage is Pl + Kfs + Qtz + Bt + Grt ± Cpx ± Opx. SiO₂ ranges from 55 to 63 wt% in of six of eight samples (two are higher at ~70%), and most are high-K. Results from a variety of tectonic discriminant diagrams consistently indicate magmatism in an arc setting. We analyzed zircon from one sample by using high precision CA-TIMS methods. A cluster of three concordant grains yields a weighted mean ²⁰⁶Pb/²³⁸U date of 383.18 ± 0.17 Ma (MSWD = 0.90), which we interpret as the time of crystallization. This date is nearly identical within the uncertainties to the younger age population from samples near Goochland. Results for an additional ten grains, which are concordant to slightly discordant, yield a range of somewhat older dates up to ~400 Ma. The older dates may reflect some combination of inheritance and xenocrystic zircon from slightly older magmatic events. Regardless, the overall age range recorded by this sample is essentially the same as that found further north. Thus, we conclude that a large portion of the Maidens gneiss was derived from igneous protoliths that originated in an arc setting in the Devonian. The position of the Goochland terrane at this time is unknown, but the timing of this magmatism provides a plausible link to the Acadian Orogeny, consistent with a position much further to the northeast.