ND-ISOTOPIC CONSTRAINTS ON THE MAGMATIC HISTORY OF THE GOOCHLAND TERRANE, EASTERNMOST GRENVILLE CRUST IN THE SOUTHERN APPALACHIANS

The Goochland Terrane is an isolated block of Mesoproterozoic crust exposed in the Piedmont Province of Virginia. It may be a displaced fragment of Laurentia, but an exotic (peri-Gondwanan?) origin has been suggested. However, a lack of comprehensive geochronological and geochemical data has precluded evaluation of these competing hypotheses, and hindered understanding of possible petrogenetic relationships among various rock types. We report the first Nd-isotopic compositions of rocks from this terrane, including the State Farm gneiss (~1026-1046 Ma), the Montpelier anorthosite (~1045 Ma), and several Neoproterozoic granites (~600-629 Ma). The State Farm gneiss, which ranges chemically from qtz-monzodiorite to granite, has e_Nd values of -0.3 to +1.3 (n=4), and T_DM ages ~1.4 Ga. The highly potassic (3.5 wt% K₂O; 1300 ppm Ba) Montpelier anorthosite has e_Nd of +1 (n=2), and also has T_DM ~1.4 Ga. Neoproterozoic A-type granites, which intrude the State Farm gneiss and are probably rift-related, have e_Nd from -0.5 to +0.5 (n=3). Proximity in the field, overlapping crystallization ages, and similarity in Nd-isotopic composition and T_DM ages all suggest a close petrogenetic relationship between the State Farm gneiss and the Montpelier anorthosite, and derivation from a slightly depleted source. The State Farm gneiss lacks pyroxene, perhaps due to retrograde metamorphism, but is similar chemically to charnockites of the Blue Ridge Province and elsewhere. Thus, these units are analogous to coeval anorthosite and charnockite plutons that characterize other portions of the Grenville Province. Results of U-Pb zircon geochronology on the Neoproterozoic rocks show a large component of Mesoproterozoic (~1 Ga) inheritance, consistent with derivation from, or significant contamination by the State Farm gneiss. At 600 Ma, e_Nd for the State Farm gneiss ranges from -5.6 to -3.1. These results preclude an origin for the granites by complete anatexis of the gneiss, but they permit a significant contribution from Grenville-age crust in their petrogenesis. Collectively, our results provide no compelling evidence for an exotic origin for the Goochland Terrane, despite its current allochthonous position. It may, however, have been separated from Laurentia in the Neoproterozoic, and then re-attached in the Paleozoic.