The New England Appalachians contain two north-south–trending sets of gneiss domes. The western belt, which includes the Chester dome, contains 13 domes that expose either 1 Ga Laurentian basement rocks or ca. 475 Ma rocks of the Shelburne Falls arc. The eastern belt contains 21 gneiss domes cored by either 600 Ma crust of possible Gondwanan affinity or ca. 450 Ma rocks of the Bronson Hill arc. Domes in both belts are surrounded by Silurian and Early Devonian metasedimentary rocks, which were deposited in two north-south–trending basins before the Acadian orogeny. The Chester dome in southeastern Vermont, the main focus of this study, is an intensively studied, classic example of a mantled gneiss dome. Lower Paleozoic units around the Chester dome are dramatically thinner than they are elsewhere in southern Vermont, and are locally absent. A strong spatial correlation between the highly attenuated mantling units and highly strained, mylonitic rocks suggests the presence of a ductile, normal-sense shear zone. Garnet-bearing rocks in the core of the dome record metamorphism during decompression of 2–3 kbar, whereas rocks above the high-strain zone were metamorphosed during nearly isobaric conditions. Strain markers and kinematic indicators suggest that extension occurred during northward extrusion of lower- to middle-crustal wedges of Proterozoic and Ordovician quartz-feldspar–rich gneisses below and up into a thick tectonic cover of Silurian mica-rich metasediments that had been transported westward in large-scale nappes. If the ductile, normal-sense shear zone was responsible for synmetamorphic decompression, as we propose, extrusion occurred at ca. 380 Ma.

Full article available in PDF version.

Full article available in PDF version

Terminal Neoproterozoic (Ediacaran) granitoid rocks underlie most of the Southeastern New England Avalon Zone. Major- and trace-element analyses on representative samples from Massachusetts and Rhode Island corroborate earlier interpretations that these rocks were formed in a subduction-related setting. New crystallization ages from the same suite are 609.5 ± 1.1 Ma, 609.1 ± 1.1 Ma, and 608.9 ± 1.2 Ma for units of the Dedham Granite; 606.3 ± 1.2 Ma for the Milford Granite; 604.4 ± 1.2 Ma for the Fall River Granite; and 599 ± 2 Ma for the Esmond Granite (2σ errors, including internal and external uncertainties). The Avalonian magmatic interval defined by these and other reliable dates is ca. 610–590 Ma, which is considerably shorter and younger than previously thought. These dates provisionally link the southeastern New England Avalon zone with the Antigonish and Cobequid Highlands in northern mainland Nova Scotia as distinctive blocks in the Northern Appalachian Avalonian collage.

The Goochland terrane in the central Piedmont Province of Virginia contains two documented units of Mesoproterozoic age, the State Farm Gneiss (ca. 1023–1046 Ma) and the Montpelier Anorthosite (ca. 1045 Ma). The heterogeneous Maidens gneiss, the most extensive unit in the terrane, has previously been assumed to be Mesoproterozoic. This inferred age was based primarily on the presence of locally preserved granulite-facies assemblages (particularly in metapelites), with granulite conditions assumed to reflect metamorphism accompanying the Grenville orogeny. However, we report new U-Pb zircon results for meta-igneous varieties of Maidens gneiss that indicate Paleozoic (Devonian) crystallization ages. Whole-rock major- and trace-element compositions of these and additional samples suggest intermediate igneous protoliths of high-K, calc-alkaline affinity. Our age determinations clearly demonstrate that the protoliths for these samples were not Mesoproterozoic, and could not have been affected by Grenvillian metamorphism. As an alternative, we suggest that the protoliths were plutonic igneous rocks, reflecting deep-seated emplacement of magmas approximately coincident with high-grade metamorphism in associated rocks during the Acadian orogeny. Whether our age results can be applied to the entirety of the Maidens gneiss is uncertain, but if so, a large portion of the Goochland terrane as currently defined is not Mesoproterozoic.