LAURENTIA-GONDWANA INTERACTIONS IN THE DETRITAL ZIRCON RECORD OF THE BRITISH CALEDONIDES

The southern part of the British Caledonides comprises terranes that formed adjacent to Gondwana whereas much of Scotland and the northern half of Ireland has a Laurentian affinity. The boundary between the two domains is traditionally drawn at the Solway line, marking the position of the Iapetus suture. The domain to the south has been assigned to 'East Avalonia' in many reconstructions. North of the Solway line, a southward-migrating wedge of sandstone shows clear indications in its detrital zircon populations of derivation from Laurentia. Most distinctive are a dominant peak at 1.0 Ga, with a tail of older zircon as far back as 2.0 Ga. Grains between 2.0 and 2.5 Ga are scarce, but an Archean population (2.5 - 3.6 Ga) is present. South of the Solway line, samples from the Ordovician of the English Lake district also show diverse Mesoproterozoic zircon, but Paleoproterozoic grains span 1.6 - 2.3 Ga. Ages close to 1.0 Ga are rare. Abundant Neoproterozoic grains around 0.6 Ga attest to the sources in 'Panafrican' or 'Avalonian' Gondwana. The distribution is strikingly similar to that recorded in the Cambrian Monian Supergroup of Anglesey, and we ascribe both regions to Ganderia. A sand influx in the Wenlock records the first Laurentian detritus, suggesting that this part of Ganderia did not contact Laurentia until the mid-Silurian. Farther south, a Cambrian population from the Harlech Dome of North Wales contrasts with those of both Ganderia and 'East Avalonia' in containing almost no Mesoproterozoic zircon, but resembles Cadomia in showing a large peak between 1.95 and 2.3 Ga, corresponding to the Eburnean orogen of west Africa. Because of close similarities with the Meguma Terrane of southern Nova Scotia, we include both terranes in a new domain termed 'Megumia'. These results, combined with Sm-Nd data, suggest that 'East Avalonia' is a composite of at least four terranes, which were independent at least until the Early Ordovician Monian/Penobscot event.