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Collisions between oceanic island-arc terranes and passive continental margins are thought to have been important in the formation of continental crust throughout much of Earth's history. Magmatic evolution during this stage of the plate-tectonic cycle is evident in several areas of the Ordovician Grampian-Taconic orogen, as we demonstrate in the first detailed geochemical study of the Tyrone Igneous Complex, Ireland. New U--Pb zircon dating yields ages of 493 [+ or -] 2 Ma from a primitive mafic intrusion, indicating intra-oceanic subduction in Tremadoc time, and 475 [+ or -] 10 Ma from a light rare earth element (LREE)-enriched tonalite intrusion that incorporated Laurentian continental material by early Arenig time (Early Ordovician, Stage 2) during arc--continent collision. Notably, LREE enrichment in volcanism and silicic intrusions of the Tyrone Igneous Complex exceeds that of average Dalradian (Laurentian) continental material that would have been thrust under the colliding forearc and potentially recycled into arc magmatism. This implies that crystal fractionation, in addition to magmatic mixing and assimilation, was important to the formation of new crust in the Grampian--Taconic orogeny. Because similar super-enrichment of orogenic melts occurred elsewhere in the Caledonides in the British Isles and Newfoundland, the addition of new, highly enriched melt to this accreted arc terrane was apparently widespread spatially and temporally. Such super-enrichment of magmatism, especially if accompanied by loss of corresponding lower crustal residues, supports the theory that arc--continent collision plays an important role in altering bulk crustal composition toward typical values for ancient continental crust.

Dalradian meta-sediments of the Laurentian margin and mafic intrusions thereof in SW Connemara, Ireland, tectonically overlie meta-rhyolites of the Delaney Dome Formation. The two units are separated by the Mannin Thrust. A new U--Pb age of 474.6 [+ or -] 5.5 Ma shows that the Delaney Dome Formation is a temporal equivalent of arc volcanic rocks preserved in the adjacent South Mayo Trough: the Tourmakeady Volcanic Group, erupted during the collision of an oceanic island arc with the Laurentian margin in the Grampian Orogeny. New rare earth and high field strength element data show that the Delaney Dome Formation and Tourmakeady Volcanic Group are chemically similar and arc-like in character. This suggests that the Delaney Dome Formation is an along-strike equivalent of the Tourmakeady Group, strike-slip faulted south of the South Mayo Trough during or after the Grampian Orogeny. Further correlation of these units with northern Appalachian rhyolites is also possible. The Delaney Dome Formation is an extrusive temporal equivalent of intrusions that penetrate the Connemara Dalradian. Thus, movement along the Mannin Thrust brought mid-crustal plutons and Dalradian country rocks tectonically above the extrusive volcanic sequence. The Mannin Thrust is identified as a major imbricating structure within a continental arc, but not a terrane boundary. Keywords: Ireland, Grampian Orogeny, U--Pb, geochemistry, subduction, plate collision.

Late Cretaceous-Early Tertiary terrigenous and calcareous turbidites exposed in the Argolis Peninsula of southern Greece represent an accretionary complex assembled during the closure of a Late Cretaceous Neotethyan ocean. Massive sulphide bodies, thrust slices of cherts and serpentinized ultramafics are found imbricated within a tectonic stack of approximately 8 km thickness. Basalts with overlying Cenomanian-Maastrichtian pelagic limestones and metalliferous sediments are found distributed throughout the complex. The basalts give trace-element chemical signatures consistent with an origin at a mid-ocean ridge or back-arc basin. Folding and shearing is locally intense in fault zones 10-20 m across, resulting in a block-in-matrix fabric. Metamorphism within the complex is of low-to-high diagenetic grade, with grade generally increasing structurally up-section. Palaeo-slope indicators suggest that the ocean basin lay to the present SE. The SSW to NNE emplacement is at variance with the SW-vergent Early Tertiary thrusting of the Hellenides, and may be explicable in terms of transpressional tectonics along the southeastern termination of the Pelagonian micro-continent. The processes which assembled the Ermioni Complex appear to have operated in a similar fashion to those seen along the margins of major ocean basins, although active over much shorter periods of time. Keywords: Tethys, accretion, plate collision, metamorphism, illite.

A transect of drill sites across the East Greenland margin made by ODP Leg 152 provides important new data on the subsidence history of this margin, complementing earlier ODP and DSDP drill sites on the conjugate Hatton Bank, as well as the Voring Plateau. Fluvial sediments recovered from under the basalts of the dipping reflector series indicate uplift and subaerial exposure of the margin prior to basalt eruption. The basaltic basement at Site 918, located oceanward of the continent-ocean transition, shows anomalously slow subsidence during the first 15-20 Ma after continental break-up, compared to normal ocean crust. Similar anomalous subsidence characterizes the early subsidence at DSDP Sites 552-554 on the Hatton Bank. Here however the anomaly is of lower magnitude and of shorter duration. At the Voring Plateau dynamic support is apparent on the landward side of the continent-ocean transition but offshore the evidence for strong, long duration support is lacking. We interpret the subsidence results to reflect dynamic support of the North Atlantic rifted margins by the Iceland plume, with the greater support at the East Greenland sites reflecting their position closer to the plume centre during the initial post-rift period than those on the Hatton Bank or Voring Plateau. The subsidence data are in accord with models of a large plume head of 1000 km radius but cannot resolve between models proposing arrival of a pre-existing plume into the area during the late Palaeocene, rather than the incubation of a new plume under the area at that time. The low degree of dynamic support seen at the Voting Plateau and the marked difference between the support at the East Greenland drill sites versus those at Hatton Bank does not support the plume centre being at Kangerdlugssuaq at 55 Ma, but instead at a location further west, under the Greenland craton. Keywords: North Atlantic, plumes, uplifts, subsidence.