Detrital zircon geochronology of the Smith River Allochthon and Lynchburg Group, southern Appalachians: Implications for Neoproterozoic-Early Cambrian paleogeography

Abstract

A detrital zircon geochronological study was undertaken to provide insight into the Neoproterozoic-Early Cambrian paleo-continental affinity of the Smith River Allochthon (SRA). The SRA has been depicted by some authors to represent a distal facies equivalent of the Lynchburg Group, a rift-related sequence that originated along the Laurentian margin during the Neoproterozoic-Early Cambrian opening of the Iapetus Ocean. Other workers, however, have interpreted the SRA as being exotic with respect to Laurentia and possibly of Gondwanan origin. The most predominant detrital age population in both terranes ranges from 0.9 to 1.25 Ga. Both terranes also contain minor populations ranging from 0.75 to 0.9 Ga and 1.3 to 1.4 Ga. The SRA also contains a minor population ranging from 1.4 to 1.5 Ga. The youngest detrital zircon ages in both terranes are <900 Ma. Uranium (U) concentrations in the zircons were also determined. The U concentration versus age plot of the SRA samples is similar to that of the Lynchburg Group. The majority of the grains in both terranes contain less than 500 ppm U, with the Lynchburg Group zircons containing a higher mean U concentration than the SRA (307 ppm versus 226 ppm). The striking similarities of the detrital zircon age and chemical data for the SRA and Lynchburg Group samples suggest that the SRA and Lynchburg Group share a common provenance along the Laurentian margin. These data conflict with electron microprobe (EPMA) monazite geochronological data that document an Early Cambrian tectonothermal event in the SRA. The eastern Laurentian margin was in the rift-to-drift transition by the Early Cambrian, so a tectonothermal event along the Laurentian margin at this time is difficult to envision. Based on the striking similarities of the detrital zircon data, we suggest that the SRA is most likely a distal facies equivalent of the Lynchburg Group, and thus of Laurentian origin, even though it records an Early Cambrian tectonothermal event. This interpretation is consistent with some interpretations of peri-Laurentian affinity for other terranes in the Piedmont zone of the southern Appalachians.