Middle Proterozoic age for the Montpelier Anorthosite, Goochland terrane, eastern Piedmont, Virginia

摘要: Uranium-lead dating of zircons from the Montpelier Anorthosite confirms previous interpretations, based on equivocal evidence, that Goochland terrane in eastern Piedmont Virginia contains Grenvillian basement rocks Middle Proterozoic age. A very few prismatic, elongate, euhedral zircons, which contain 12–29 ppm uranium, are interpreted to be igneous origin. The vast majority more equant, subangular anhedral, 38–52 and metamorphic One fraction elongate zircon, four fragments a large zircon (occurring nelsonite segregation) yield an upper intercept age 1045 ± 10 Ma, as time anorthosite crystallization. Irregularly shaped dated at 1011 2 Ma (weighted average 207Pb/206Pb ages). U-Pb isotopic systematics titanite were reset during Alleghanian orogeny 297 5 Ma. These data provide minimum for gneisses intruded by anorthosite. may correlative with those Shenandoah massif Blue Ridge tectonic province, suggested similarities between Roseland Although areal extent basement-cover relations remain unresolved, results this investigation indicate is internal Laurentian crust rather than exotic accreted terrane.