Age of zircon from volcanic rocks of the central North Carolina Piedmont and tectonic implications for the Carolina volcanic slate belt

摘要

U-Pb analyses of four fractions of zircon from felsic volcanic rocks in the upper part of the Uwharrie Formation plot on a concordia diagram along a chord between 586 and 340 m.y. These data indicate an Early Cambrian or late Precambrian age of 586 ± 10 m.y. for deposition of the upper part of the Uwharrie. The lower concordia intercept at 340 m.y. might represent a time of episodic lead loss, perhaps indicating Acadian metamorphism in the area. However, all data points cluster near the upper concordia intercept, and the lower intercept is not accurately defined. A continuous lead-loss model cannot be ruled out. The zircon ages from the Uwharrie show that in central North Carolina the conformable volcanic and sedimentary sequence ranges from rocks older than 586 m.y. up to beds about 552 m.y. old containing probable Middle Cambrian trilobites. This contrasts with relations reported from the Roxboro-Durham area to the northeast, where a similar volcanic and sedimentary sequence with a 620 ± 20–m.y. zircon age was folded and faulted by the Virgilina deformation before emplacement of a 575 ± 20–m.y.-old pluton. The events of the Roxboro-Durham area can be correlated with those of central North Carolina in three possible ways that are consistent with the radiometric and fossil data: (1) The stratigraphic sequences of the two areas are partly correlative. The Virgilina deformation was synchronous with deposition of the upper part of the central North Carolina sequence, but the deformation did not extend into the central North Carolina area. (2) The stratigraphic sequences of the two areas are correlative, and the Virgilina deformation was younger than the central North Carolina sequence but was weak or absent in that area. (3) The central North Carolina sequence is entirely younger than the Virgilina deformation, and the volcanic rocks may represent an extrusive phase of the plutonism of the Roxboro-Durham area. Interpretation 1 is preferred because it harmonizes best with both lithologic correlations and ages. Interpretation 2 offers the poorest agreements as to age, and interpretation 3 conflicts with the suggested lithologic correlation of the two stratigraphic sequences.