Classic tectonic models for the Caledonian orogeny in Scotland involve Ordovician collision of Laurentia–Midland Valley arc (Grampian orogeny), followed by middle Silurian collision of Laurentia–Baltica (Scandian orogeny) and 500–700 km of sinistral displacement along the Great Glen fault separating the Northern Highlands (Moine Supergroup) from the Grampian Highlands (Dalradian Supergroup). A review of the timing of magmatic and metamorphic rocks across Scotland allows a simpler explanation that fits with a classic Himalayan-style continent–island arc–continent collision. Late Cambrian – Early Ordovician NW-directed ophiolite obduction (Highland Border complex) coincided with the ending of stable continental shelf sedimentation along the eastern margin of Laurentia. Following collision between Laurentia and the Midland Valley arc–microcontinent in Early Ordovician time, crustal thickening and shortening led to almost continuous regional metamorphism from c. 470 to 420 Ma, rather than two discrete ‘orogenies’ (Grampian, Scandian). U–Pb monazite and garnet growth ages indicating prograde metamorphism, and S-type granites related to melting of crustal protoliths are coeval in the Grampian and Northern Highlands terranes. There is no evidence that the Great Glen fault was a terrane boundary, and strike-slip shearing post-dated emplacement of Silurian – Early Devonian granites. Late orogenic alkaline granites (c. 430–405 Ma) in both Moine and Dalradian terranes are not associated with subduction. They are instead closely related to regional alkaline appinite–lamprophyric magmatism resulting from simultaneous melting of lower crust and enriched lithospheric mantle. Caledonian deformation and metamorphism in northern Scotland, with continuous SE-directed subduction, show geometry and time scales that are comparable to the Cenozoic India–Kohistan arc–Asia collisional Himalayan orogeny.

Zircon separates from the contact aureole of the syn-tectonic Dawros–Currywongaun–Doughruagh Complex, western Ireland, are studied to constrain the nature and timing of magmatism associated with the early stages of the Grampian Orogeny. The samples analysed come from the uppermost part of the Dalradian Supergroup in northern Connemara (the Ben Levy Grit Formation), where a laterally extensive (>10 km) package of metamorphosed siliciclastic sedimentary rocks containing heavy mineral seams crops out. The seams mainly comprise magnetite, but zircon is also present in greater than accessory quantities. The seams have been locally reworked at granulite-facies metamorphic conditions during intrusion of the Dawros–Currywongaun–Doughruagh Complex magmas. Here we combine in situ mineral chemical and U–Pb geochronological analyses of zircons from samples of these heavy mineral seams collected at different locations in the Dawros–Currywongaun–Doughruagh Complex thermal aureole. An important finding is that the zircons studied have magmatic trace-element compositions, interpreted here as a function of their growth during contact metamorphic-induced partial melting. The zircons yield a range of U–Pb spot ages whose uncertainties suggest a maximum duration of zircon growth of ˜11 Ma, between 477.1 and 466.1 Ma, though it is likely that zircon growth occurred much more quickly than this. The age constraints revealed here match well with the range of 475 to 463 Ma previously proposed for the Grampian Orogeny overall in Connemara and lend useful support to models that argue for high-intensity, relatively short-lived Grampian orogenesis in the Connemara Caledonides.

Two regional-scale, recumbent folds control the structure of the Beinn Udlaidh area, Tyndrum, Perthshire. They reached their maximum development during D2, following the regional metamorphic peak, and are part of a stack of larger SE-facing recumbent folds formed during the ∼470 Ma Grampian Orogeny. The rocks belong to the Neoproterozoic–Lower Ordovician Dalradian Supergroup, and preserve a sedimentary transition between the Grampian Group and the overlying Appin Group. The latter occupies the core of the S-facing, recumbent Beinn Udlaidh Syncline (D2) which, with the underlying complementary Glen Lochy Anticline, is gently folded by a regional-scale structure, the Orchy Dome. The recumbent folds postdate an early fabric (S1), which is generally obliterated by the D2 imprint, but preserved as inclusion trails in regional metamorphic garnets, that are highly oblique to, and wrapped by, S2. It is concluded that the Dalradian rocks described here from below the Iltay Boundary Slide are in structural continuity with those of the Tay Nappe above, and that the Slide represents a structurally-modified disconformity between the Leven Schist (Appin Group) and the overlying Ben Lui Schist (Argyll Group). The Orchy Dome probably influenced the spatial distribution of minor intrusions and explosion vents of the lamprophyre suite.

Because magmatism associated with subduction is thought to be the principal source for continental crust generation, assessing the relative contribution of pre-existing (subducted and assimilated) continental material to arc magmatism in accreted arcs is important to understanding the origin of continental crust. We present a detailed Nd isotopic stratigraphy for volcanic and volcaniclastic formations from the South Mayo Trough, an accreted oceanic arc exposed in the western Irish Caledonides. These units span an arc–continent collision event, the Grampian (Taconic) Orogeny, in which an intra-oceanic island arc was accreted onto the passive continental margin of Laurentia starting at ∼ 475 Ma (Arenig). The stratigraphy corresponding to pre-, syn- and post-collisional volcanism reveals a progression of εNd(t) from strongly positive values, consistent with melt derivation almost exclusively from oceanic mantle beneath the arc, to strongly negative values, indicating incorporation of continental material into the melt. Using εNd(t) values of meta-sediments that represent the Laurentian passive margin and accretionary prism, we are able to quantify the relative proportions of continent-derived melt at various stages of arc formation and accretion. Mass balance calculations show that mantle-derived magmatism contributes substantially to melt production during all stages of arc–continent collision, never accounting for less than 21% of the total. This implies that a significant addition of new, rather than recycled, continental crust can accompany arc–continent collision and continental arc magmatism.

Heavy minerals in Ordovician successions in western Ireland record, in the Upper Arenig Sheeffry Formation, the erosion of an ophiolite/island arc complex. The appearance of staurolite and garnet at a basin-wide horizon in the Lower Llanvirn Upper Derrylea Formation signals the unroofing of the Dalradian metamorphic complex. Parts of the Ordovician sequence on Inishturk and in two small inliers are correlated with the standard sequence with unexpected results. The garnet-, sillimanite-, and staurolite-bearing Letter Formation correlates with the Upper Derrylea Formation and, on Inishturk, heavy minerals in south-younging turbidites reveal a sinistral ramp zone, that places the Sheeffry Formation structurally above and to the south of the younger Derrylea Formation.

The first section of the paper is concerned with a detailed account of the structure of the classic Schiehallion area. The account is extended to the flat belt via the Glen Lyon-Ben Lawers area. It is demonstrated that major D1 folds involve the whole of the Appin and Argyll groups and now face SE. However, the original D1 folds were probably open upward-facing folds which have been refolded and modified by the dominant D2 deformation. D2 is represented by an antiformal complex which culminates downwards in the Boundary Slide, a zone of strong deformation associated with NW translation of Dalradian over the ‘young Moine’ rocks. D3 has been responsible for the rotation of the originally flat D2 structures into their present ‘steep’ attitudes N of Glen Lyon. D4 is represented by the strike-swing of the lower Dalradian formations, possibly related to early movements on the two major faults that bound the area. The concept of a D1 Tay Nappe which roots in the ‘young Moine’ to the N is retained, although the scale and flat attitude of that fold is regarded as in part a product of D2.