The Precordillera mafic–ultramafic belt (PMUB), located in central-western Argentina, comprises mafic and ultramafic bodies interlayered and/or in tectonic contact with marine siliciclastic units. Whole-rock, mineral geochemistry and Nd–Sr isotope analyses performed in magmatic rocks suggest a relatively different spatial and temporal evolution along the belt. The southern PMUB (south of 32° S) evolved as an intra-continental rifted margin with an enriched mid-ocean-ridge basalt (E-MORB) tholeiitic to alkaline magmatism, to a proto-ocean basin (the Cuyano proto-ocean) with tholeiitic normal-MORB geochemical signature. Based on neodymium model ages (TDM), the magmatic activity started during the late Neoproterozoic Era and continued into the early Palaeozoic Era. Instead, the northern PMUB (28–32° S) evolved as an intra-continental rifted margin with dominant tholeiitic E-MORB to continental flood basalt (CFB) magmatism during the early Palaeozoic Era. ϵNd values (+3.4 to +8.4), rare earth element trends and high-field-strength element systematics, together with an estimated potential mantle temperature of c. 50–100°C above ambient mantle, suggest the PMUB magmatism derived from an enriched mantle source related to the effect of a rising plume linked to the Iapetus Ocean opening. In particular, TDM estimations of 600–550 Ma agree with reported magmatism in central to southern Appalachians. The magmatism in the PMUB, and those registered in the Neoproterozoic Catoctin Formation and in the Southern Oklahoma Aulacogen in the conjugated Laurentian margin, seem to be contemporaneous, sharing a similar plume-enriched mantle source. In this context, the E-MORB signature identified along the PMUB can be described as a plume-distal ridge tectonic setting over an extended margin.

Mafic lamprophyre dykes in northern England were emplaced soon after the final closure of the Iapetus Ocean at the end of the Caledonian orogeny. High Mg/(Mg+Fe) ratios and Cr and Ni contents are consistent with equilibrium with mantle peridotite. Incompatible trace element abundances suggest that the mantle sources were metasomatized prior to the melting events. Three components are recognized in the lamprophyre chemistry: (i) a depleted mantle source, taken to be that overlying the subducting lithosphere; (ii) a H2O-rich subduction zone component, related to the dehydration of the subducting oceanic crust; and (iii) a CO2-rich phase thought to result from the degassing of the mantle after ocean closure. This multi-component origin may be applicable to other lamprophyres of calc-alkaline affinity.

Radiolarian chert and associated siliceous claystone in the Southern Uplands of Scotland are examined, in order to study the Great Ordovician Biodiversification Event of benthic animals on the pelagic ocean bottom. Trace fossils which are uncommon, but convincing, are found in the grey chert and siliceous claystone of Gripps Cleuch. These observations constitute firm evidence that large benthic animals which could leave visible trace fossils had colonised the Iapetan Ocean by the late Middle Ordovician, confirming previous studies from Australia for Panthalassa, the other huge ocean. Red chert is, however, a poor recorder of trace fossils, probably because the highly oxidising environment breaks down organic matter, both inhibiting high-density activity of large benthic animals and removing clear traces of benthic animal life.

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.

Silurian and late Ordovician K-bentonites of the British Isles provide a record of prolonged volcanism during the convergence of terranes associated with closure of the Iapetus and Tornquist Oceans. In the Southern Uplands–Longford-Down and Midland Valley terranes, they range from late Caradoc to Telychian, withfurther early Homerian occurrences. South of the Iapetus suture, in Eastern Avalonia, the range is Hirnantianto early Ludlow in northern England and early Telychian to earliest Ludfordian in the Welsh Borderland and English Midlands. In both cases, the distributions indicate that volcanism was more long-lived and probably more extensive than is depicted in current plate tectonic reconstructions. Average intervals between K-bentonites are estimated, based on Harland et al. (1990), as: c. 65000 years at Dob's Linn (late Aeronian to early Telychian); c. 39000 years in the Cautley area (Telychian); c. 51000 years in a borehole at Walsall (late Llandovery to Sheinwoodian). Trace element geochemistry suggests mostly subalkaline dacitic to rhyolitic magmas in which LILE-enrichment accompanies variable enrichment in crustally derived elements (Ta, Nb). The geochemistry suggests comparison with continental arc volcanism of ‘withinplate, attenuated lithosphere’ character. Ta–Nb enrichment and an absence of Eu anomalies from REE profiles are consistently present northof the Iapetus suture, but trace element patterns are less consistent south of the suture where negative Eu anomalies are generally present. Discriminant function analysis successfully distinguishes Llandovery, Wenlockand Ludlow K-bentonites from south of the Iapetus suture, and Llandovery K-bentonites from north and south ofthe suture. Those from north of the Iapetus suture probably originated in volcanism along the southern marginof Laurentia before final closure of the Iapetus Ocean. Those from south of the suture may have been derived from volcanism associated with late destruction of Iapetus and Tornquist oceanic crust, although an alternative involving volcanism at the southern margin of Eastern Avalonia or Baltica may accord better with the distribution of K-bentonites and the geographical trend of the fragmentary outcrops of Silurian volcanic rocks from southern Ireland to Belgium.

A model for the early Palaeozoic metamorphic history of the Midland Valley and adjacent areas to the S in Scotland, England and Ireland is based on the results of new field mapping, thin section petrography, electron probe microanalysis, X-ray diffractometry, conodont and palynomorph colouration and graptolite reflectance measurement.

The oldest metamorphic rocks of the Midland Valley of Scotland, excluding xenoliths in post-Silurian lavas, are possibly the blueschist occurrences in the melange unit of the Ballantrae complex. These may be tectonised remnants of (?)pre-Arenig ocean-floor subducted during closure of the Iapetus Ocean. In the early Ordovician, the melange terrane was dynamothermally metamorphosed during obduction of newly-formed ocean crust. The obduction process piled up a thick sequence of various ocean-floor types such that burial metamorphism in parts reached pumpellyite-actinolite facies; elsewhere prehnite-pumpellyite and zeolite facies was attained.

Whilst the Midland Valley acted as an inter- or fore-arc basin during the Late Ordovician and Silurian and experienced burial metamorphism, an accretionary prism was formed to the S. Accretion, tectonic burial and metamorphism of ocean-floor and trench sediment was continuous in the Southern Uplands and the Longford-Down massif of Ireland through Late Ordovician to Late Silurian times. Rocks at the present-day surface vary from zeolite facies to prehnitepumpellyite facies. Silurian trench-slope basin sediments can be recognised in part by their lower grade of burial metamorphism. Greenschist facies rocks of the prism probably lie close to the surface.

The Lake District island-arc terrane of Northern England has an early Ordovician history of burial metamorphism up to prehnite-pumpellyite facies. The Late Ordovician and Silurian metamorphic history is one of sedimentary burial complicated by tectonism and intrusion of granite plutons to a relatively high level. The Iapetus suture is marked by a weak contrast in metamorphic grade.

The Ordovician and Silurian rocks of the Southern Uplands of Scotland have been interpreted as sediments deposited on the northern margin of the Iapetus Ocean. Trace fossils are abundant at many localities in ocean-floor turbidites and mudstones that usually lack all other evidence of life. Twelve ichnogenera are present, and they are mainly meandering locomotion and feeding trails and burrow networks: Dictyodora, Caridolites, Helminthoida, Neonereites, Nereites, Protovirgularia, Gordia, Megagrapton, Paleodictyon, Chondrites, Plano-lites and Skolithos. The trace fossils occur in at least five distinct assemblages and the composition of these was probably controlled by the frequency and nature of the turbidity currents, and possibly by the oxygen content of the mudstones. Where turbidity currents were weak, abundant Dictyodora, together with Caridolites, Neonereites, Nereites, Protovirgularia and Gordia occur in various combinations. Where currents were stronger, traces such as Gordia, Paleodictyon and Megagrapton may be exhumed and cast on turbidite soles, and the sand may contain Skolithos. The ‘deep-sea’ Nereites trace fossil facies is divisible into several assemblages, presumably environmentally controlled.

The lower Palaeozoic rocks of the Southern Uplands of Scotland have been interpreted as an accretionary prism of slivers of oceanic basalt and associated deep sea sediments situated on the northern margin of the lape tus Ocean. Electron microprobe analyses confirm the presence of albite, prehnite, pumpellyite, chlorite, phengite, calcite, hematite and quartz-bearing metamorphic assemblages in basalts and basic-clast greywackes in the prism. The metamorphism is prehnite-pumpellyite facies and inferred to be related to tectonic burial of the various slices of the prism as they were accreted during the closure of the lape tus Ocean.