Detrital zircon geochronology of the Fredericton Trough, New Brunswick, Canada: Constraints on the Silurian Closure of remnant Iapetus Ocean

The Fredericton Trough in the Appalachians of southwestern New Brunswick is filled by the Silurian Kingsclear Group, consisting mainly of turbidites, deposited during convergence of Laurentia with components of the peri-Gondwanan domain Ganderia. Its tectonic setting has been interpreted as a successor basin, trench, foredeep or foreland basin. We present new detrital zircon U-Pb data from four formations of the Kingsclear Group, collected north and south of the Fredericton Fault, which bisects the trough. South of the Fredericton Fault, detrital zircon ages from an early Silurian (Llandovery) unit show a late Neoproterozoic peak, typical of peri-Gondwanan provenance. Detrital zircons from a younger Silurian unit (Wenlock - Ludlow, intruded by the Pocomoonshine pluton, 422.7 ± 3 Ma) display a distinctive asymmetric peak at ∼1.0 Ga with a tail of older Proterozoic zircons, suggesting Laurentian provenance. North of the Fredericton Fault, a Llandovery sample also shows a signature consistent with Laurentian sources. In a mid-Silurian (Wenlock) unit zircon peaks indicate mixed Laurentian and peri-Gondwanan sources, consistent with exhumation of the Miramichi terrane to the north. The absence of Laurentian material in Llandovery strata south of the fault, contrasted with a strong Laurentian signature in rocks to the north, suggests that a remnant of the Iapetus Ocean, in which turbidites of the Kingsclear Group were deposited, persisted until at least the mid-Silurian. The timing of its closure is constrained by the arrival of Laurentian detritus south of the Fredericton Fault before 422.7 ± 3 Ma, and probably by the mid-Wenlock.