GANDERIA-LAURENTIA COLLISION IN THE CALEDONIDES OF GREAT BRITAIN AND IRELAND

During terrane convergence, an influx of clastic sediment derived from an upper plate onto a lower plate is an early indication of terrane juxtaposition. In the Caledonides of Britain and Ireland, units accreted to Laurentia during the early Paleozoic include peri-Gondwanan terrane assemblages that earlier separated from from West Gondwana. However, Ordovician and Silurian sandstones in 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 English Lake District and the Leisnter Massif of SE 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 northern Appalachians, and indicate probable derivation from Amazonia in West Gondwana. In contrast, 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 ca. 430 Ma, in contrast to the Ordovician accretion of Ganderian fragments recorded in Atlantic Canada, suggesting that closure of the Iapetus Ocean was highly non-cylindrical.