Precise U-Pb zircon constraints on the earliest magmatic history of the Carolina terrane

The early magmatic and tectonic history of the Carolina terrane and its pos sible affinities with other Neoproterozoic circum-Atlantic are terranes hav e been poorly understood, in large part because of a lack of reliable geoch ronological data. Precise U-Pb zircon dates for the Virgilina sequence, the oldest exposed part, constrain the timing of the earliest known stage of m agmatism in the terrane and of the Virgilina orogeny. A flow-banded rhyolit e sampled from a metavolcanic sequence near Chapel Hill, North Carolina, yi elded a U-Pb zircon date of 632.9 +2.6/-1.9 Ma. A granitic unit of the Chap el Hill pluton, which intrudes the metavolcanic sequence, yielded a nearly identical U-Pb zircon date of 633 +2/-1.5 Ma, interpreted as its crystalliz ation age. A felsic gneiss and a dacitic tuff from the Hyco Formation yield ed U-Pb zircon dates of 619.9 +4.5/-3 Ma and 615.7 +3.7/-1.9 Ma, respective ly. Diorite and granite of the Flat River complex have indistinguishable U- rb upper-intercept dates of 613.9 +1.6/-1.5 Ma and 613.4 +2.8/-2 Ma. The Os mond biotite-granite gneiss, which intruded the Hyco Formation before the V irgilina orogeny, crystallized at 612.4 +5.2/-1.7 Ma. Granite of the Roxbor o pluton, an intrusion that postdated the Virgilina orogeny, yielded a U-Pb upper intercept date of 546.5 +3.0/-2.4 Ma, interpreted as the time of its crystallization. These new dates both provide the first reliable estimates of the age of the Virgilina sequence and document that the earliest known stage of magmatism in the Carolina terrane had begun by 633 +2/-1.5 Ma and continued at least until 612.4 +5.2/-1.7 Ma, an interval of similar to 25 m .yr. Timing of the Virgilina orogeny is bracketed between 612.4 +5.2/-1.7 M a and 586 +/- 10 Ma (reported age of the upper Uwharrie Formation). The U-P b systematics of all units studied in the Virgilina sequence are simple and lack any evidence of an older xenocrystic zircon component, which would in dicate the presence of a continental-type basement. This observation, toget her with the juvenile Nd isotopic character of the Virgilina volcanic are s equence, suggests that the oldest part of the Carolina terrane was built on oceanic crust away from a continental crustal influence.