NEW U-PB ZIRCON EVIDENCE FOR PALEOZOIC (DEVONIAN) PROTOLITHS FOR METAIGNEOUS PORTIONS OF THE MAIDENS GNEISS, GOOCHLAND TERRANE, VIRGINIA

The Goochland Terrane in the central Piedmont Province of Virginia contains two documented units of Mesoproterozoic age, the State Farm Gneiss (~1023-1046 Ma) and the Montpelier anorthosite (~1045 Ma). The heterogeneous Maidens Gneiss, the most extensive unit in the terrane, has also long been assumed to be Mesoproterozoic. This inferred age is based primarily on the presence locally preserved granulite-facies assemblages (particularly in metapelites), with granulite conditions assumed to reflect Grenvillian metamorphism. However, we report new U-Pb zircon results for probable metaigneous varieties of Maidens Gneiss that indicate Paleozoic (Devonian) crystallization ages. Specifically, three mineralogically similar samples of plag + bio + qtz ± hbl ± cpx ± garnet gneiss located within a ~4 km radius of the town of Goochland yield concordant or nearly concordant zircon ages of 407 ± 2 Ma, 390 ± 3 Ma, and 371 ± 3 Ma, respectively. All zircons analyzed were clear, elongate prisms of apparent igneous origin, an interpretation supported by typical igneous Th/U values (0.45-0.78). Bulk compositions of these and additional samples suggest intermediate igneous protoliths of high-K, calc-alkaline affinity (55-64 wt.% SiO2; 4-7 wt.% CaO, 1.9-4.0 wt.% K2O), and MORB-normalized multi-element diagrams are virtually identical to those of high-K, calc-alkaline rocks from the Central Volcanic Zone of the Andes (for example). Our age determinations clearly demonstrate that the protoliths for these samples were not Mesoproterozoic, and could not have been affected by Grenvillian metamorphism. However, the Devonian ages reported here are remarkably similar to recently determined electron microprobe ages of monazite in other Maidens lithologies (Shirvell et al., this volume), which almost certainly indicate the timing of high-grade metamorphism. We suggest that the protoliths for these gneisses were plutonic igneous rocks, reflecting deep-seated emplacement of magmas approximately coincident with high-grade metamorphism in associated rocks. Whether our results can be applied to the rest of the Maidens Gneiss is uncertain, but if so, a large portion of the Goochland Terrane as currently defined is not Mesoproterozoic.