S. Andrew DuFrane

Ganderia–Laurentia collision in the Caledonides of Great Britain and Ireland

During terrane convergence, an influx of clastic sediment from an upper plate onto a lower plate is an early indication of terrane juxtaposition. In the Caledonides of Great Britain and Ireland, units accreted to Laurentia during the early Palaeozoic Era include peri-Gondwanan terrane assemblages that earlier separated from West Gondwana. However, the Southern Uplands Terrane contains detrital zircon populations apparently derived entirely from Laurentia, characterized by a large, asymmetric Mesoproterozoic peak and a scarcity of zircon at 600 Ma and 2.1 Ga. In contrast, Cambrian and Ordovician rocks from the Lake District and the Leinster Massif of Ireland show abundant grains with these ages, together with a range of Mesoproterozoic zircon. These characteristics are shared with the Monian terrane of Anglesey and with Ganderia in the Appalachians, indicating probable derivation from Amazonia in West Gondwana. Silurian sandstones from the Lake District show an influx of Laurentia-derived zircon, and lack the peri-Gondwanan signal. This indicates that in the Caledonides, Ganderia was not accreted to the Laurentian margin until c. 430 Ma, in contrast to the Ordovician accretion of Ganderian fragments recorded in the Appalachians, suggesting that the configuration of the closing Iapetus Ocean varied significantly along the strike of the orogen. Supplementary material: Details of sample coordinates, analytical procedure and U/Pb detrital zircon analytical data are available at www.geolsoc.org.uk/SUP18739.

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.