Geochronology of the Mesoproterozoic State Farm gneiss and associated Neoproterozoic granitoids, Goochland terrane, Virginia

The Goochland terrane is an isolated block of Mesoproterozoic crust in the Piedmont Province of central Virginia. We report U-Pb zircon dates and whole-rock major and trace element data for the State Farm gneiss, one of the main units in the terrane, and additional results for a newly recognized suite of Neoproterozoic granitoids that intrude the gneiss. The State Farm gneiss ranges in bulk composition from quartz monzodiorite to granite, and three samples yield U-Pb zircon dates of 1046 +7/-6 Ma, 1039 +18/-11 Ma, and 1023 +/- 10 Ma. We interpret these dates as igneous crystallization ages, which indicate a maximum emplacement interval of ca. 1057-1013 Ma for these samples. Neoproterozoic granitoids (including the Fine Creek Mills and Flat Rock granites) are more alkaline than the State Farm gneiss and display all compositional characteristics of A-type granites (e.g., high Fe/Mg, Ga, Ga/Al, Nb, Zn, Y, Zr). U-Pb zircon analyses from five separate bodies indicate crystallization ages from ca. 654 to 588 Ma, but all results are complicated by Mesoproterozoic (ca. 1 Ga) inheritance, coupled, in some cases, with secondary Ph loss. We interpret the ages and compositions of the younger rocks to reflect Neoproterozoic rifting of the Goochland terrane, but its location during rifting is uncertain. The terrane may have been separated from Laurentia during the Neoproterozoic breakup of Rodinia and later reattached.