The Goochland granulite terrane: Remobilized Grenville basement in the eastern Virginia Piedmont

The Goochland terrane is defined here by the areal extent of a granulite-facies metamorphic sequence in the Piedmont of eastern Virginia. The sequence of units in this terrane includes the State Farm Gneiss of Grenville age, and the overlying Sabot amphibolite and Maidens gneiss. Also included is the Montpelier metanorthosite which intrudes the other units. The granulite-facies event (M g) produced orthopyroxene + plagioclase and clinopyroxene + garnet + plagioclase assemblages in intermediate to mafic rocks, and K-feldspar + sillimanite in interlayered pelitic gneisses. An amphibolite-facies event (M 1) produced biotite + garnet and biotite + hornblende assemblages from the intermediate to mafic granulite gneisses, and muscovite + quartz ± kyanite ± staurolite schists from the pelitic gneisses. Because the entire Goochland terrane was subjected to the same granulite-facies event as the Grenville-age State Farm Gneiss, and because of the similarity of the Montpelier metanorthosite to the Grenville-age Roseland metanorthosite of the Blue Ridge, the entire Goochland terrane is interpreted to be Grenville in age, or older. A Paleozoic tectonic event, at least in part Alleghanian, remobilized this terrane under amphibolite-facies conditions. The Goochland terrane is the largest and easternmost internal basement massif of the southern Appalachians.