British Caledonides Research Articles

First Irish occurrence of stibiopalladinite (PD5(SB,AS)2) in the Dooros Serpentinite, Connemara

A new platinum group mineral (PGM) occurrence is described from the Dooros serpentinite in Connemara, County Galway, Ireland. The ∼470Ma Dooros serpentinite intruded the Dalradian Cornamona Formation during the Grampian Orogeny. The mineral phase identified as stibiopalladinite has the stoichiometric formula: ƸPd+Pt+Cu4.99-5.08ƸSb+As1.92-2.02. This particular PGM occurrence in the Dooros serpentinite extends the range of platinum group element mineralisation in the Irish and British Caledonides.

First Irish Occurrence of Stibiopalladinite (PD5(SB,As)2) in the Dooros Serpentinite, Connemara

A new platinum group mineral (PGM) occurrence is described from the Dooros serpentinite in Connemara, County Galway, Ireland. The ~470Ma Dooros serpentinite intruded the Dalradian Cornamona Formation during the Grampian Orogeny. The mineral phase identified as stibiopalladinite has the stoichiometric formula: ƸPd+Pt+Cu4.99-5.08ƸSb+As1.92-2.02. This particular PGM occurrence in the Dooros serpentinite extends the range of platinum group element mineralisation in the Irish and British Caledonides.

Detrital zircon U-Pb-Hf isotope signatures of Old Red Sandstone strata constrain the Silurian to Devonian paleogeography, tectonics, and crustal evolution of the Svalbard Caledonides

Abstract Detrital zircon provenance studies of Mesoproterozoic basement and overlying Old Red Sandstone strata in northwestern Svalbard, Arctic Norway, were conducted to test competing models for Caledonian paleogeography and tectonics and constrain the magnitude of orogen-parallel, Silurian to Devonian strike-slip faulting following the Laurentia-Baltica collision. Mesoproterozoic basement strata, cut by earliest Tonian orthogneiss units, mostly yielded 1640–1050 Ma detrital zircon populations that are consistent with pre-Caledonian locations near northeast Greenland. Basal Old Red Sandstone deposits that filled pull-apart basins showed basement-derived signatures but also contained 530–450 Ma and 670–570 Ma populations with slightly subchondritic (ε Hf[t] = –4 to –2) Hf isotope compositions. These results are consistent with late Silurian–Early Devonian proximity to the northeast Greenland Caledonides and Pearya, which indicates limited (<200 km) strike-slip displacement of Svalbard’s Caledonian allochthons after the Laurentia-Baltica collision. Previously interpreted connections between the Svalbard Old Red Sandstone and British Caledonides are incompatible with these detrital zircon results. Lochkovian Old Red Sandstone strata were deposited after a second episode of strike-slip faulting and show recycled basement signatures. The lack of 530–450 Ma and 670–570 Ma populations suggests that the second deformation episode reorganized local drainages. Pragian–Givetian strata have provenance from local Old Red Sandstone sources that were uplifted during a third and final episode of strike-slip deformation. The results indicate that northern Caledonian (Svalbard, Pearya) crustal evolution was characterized by the reworking of Mesoproterozoic–Paleoproterozoic sources and mostly <600 m.y. crustal residence times, whereas the southern Caledonides (UK, Ireland) show evidence for the reworking of older basement and mostly >600 m.y. crustal residence times.

Volcanological and environmental controls on the Snowdon mineralization, North Wales, UK: A failed volcanogenic massive sulfide system in the Avalon Zone of the British Caledonides

The Snowdon caldera of North Wales is host to base metal sulfide-bearing veins and stockworks, mineralized breccias, disseminated sulfides, and localized zones of semi-massive to massive sulfide, with subordinate magnetite-rich veins. The late Ordovician host volcanic sequence accumulated in a shallow marine, back-arc environment in the Welsh Basin, which forms part of the Avalon Zone of the British and Irish Caledonides. New field evidence, sulfur isotopes, and U-Pb dating indicate that the Snowdon mineralization is genetically and temporally related to Late Ordovician magmatism and caldera formation. It is interpreted to represent volcanogenic pipe-style sulfide mineralization, resulting from focused hydrothermal fluids moving along caldera-related faults and simultaneous dispersal of fluids through the volcaniclastic pile. Sulfur isotope data suggest that, whilst a limited contribution of magmatic S cannot be ruled out, thermochemical reduction of contemporaneous Ordovician seawater sulfate was the dominant mechanism for sulfide production in the Snowdon system, resulting in a mean value of about 12‰ in both the host volcanic strata and the mineralized veins. Despite the tectonic setting being prospective for VMS deposits, strata-bound sulfide accumulations are absent in the caldera. This is attributed to the shallow water depths, which promoted boiling and the formation of sub-seafloor vein-type mineralization. Furthermore, the tectonic instability of the caldera and the high energy, shallow marine environment would have limited preservation of any seafloor deposits. The new U-Pb dates for the base (454.26±0.35Ma) and top (454.42±0.45Ma) of the host volcanic rocks, indicate that the Snowdon magmatic activity was short lived, which is likely to have limited the duration and areal extent of the ore-forming system. The absence of massive sulfide mineralization is consistent with the general paucity of economic VMS deposits in the Avalon Zone. Despite the highly prospective geological setting this study further illustrates the importance of volcanic facies mapping and associated paleo-environmental interpretations in VMS exploration.

Connemara: its position and role in the Grampian Orogeny

In the Irish and British Caledonides, the early Ordovician Grampian Orogeny was the result of collision between the Laurentian rifted margin and an oceanic island arc. The Connemara terrain in western Ireland differs in position and character from all other parts of the exposed Dalradian rocks of the Grampian Orogen in lying south of the collided arc and fore-arc, and in having north-verging fold nappes that developed synchronously with the intrusion of huge volumes of calc-alkaline magmas that provided the heat for regional Barrovian metamorphism. We have tested this hypothesis with a numerical model, which demonstrates its admissablity. Connemara is not a terrane, displaced with respect to the remainder of the Grampian Orogen but was overridden, northwards, by the arc and its fore-arc basin (South Mayo Trough), frontal ophiolite complex (Deer Park) and accretionary complex (Killadangan). Deposition in the South Mayo Trough occurred below sea level and above the evolving Grampian Orogen, which developed on a hyper-extended rifted margin bounded to the north by the Clew Bay Line.

The behavior of magnesium isotopes in low-grade metamorphosed mudrocks

Magnesium isotopic compositions of mudrocks metamorphosed at sub-greenschist facies from three lower Paleozoic basins (northern Lake District, southern Lake District, and Southern Uplands) in the British Caledonides were measured in order to understand the behavior of Mg isotopes during diagenesis and low-grade metamorphism. Carbonate-free mudrocks from the northern Lake District have heavy δ26Mg values varying from −0.17 to +0.25. By contrast, Mg isotopic compositions of carbonate-bearing mudrocks from the southern Lake District and Southern Uplands vary more widely, with δ26Mg ranging from −0.74 to −0.08. Acid leaching experiments on the latter show that the leachates have higher Ca/Al and Ca/K ratios than the residues due to the dissolution of leachable carbonates. The δ26Mg values of leachates (−1.54 to −0.21) are always lower than the corresponding residues (δ26Mg=−0.39 to +0.09), consistent with isotopically light Mg in carbonates. A rough, negative correlation between δ26Mg and Mg/Al for the residual silicate fraction of mudrocks suggests that their Mg isotopic compositions are controlled by the relative proportion of illite/muscovite and chlorite. Global clastic sediments display highly variable Mg isotopic compositions that are negatively correlated with CaO/Al2O3 and CaO/TiO2, implying that carbonates introduce light Mg isotopes to sediments, although the silicate end member itself has a wide range of δ26Mg, depending on its mineralogy. Magnesium isotopic compositions of mudrocks, as well as their silicate and carbonate fractions, do not vary systemically as metamorphism proceeds from diagenesis to low-grade metamorphism, suggesting limited Mg isotope fractionation during low-temperature metamorphic dehydration (<300°C). The general decrease of Mg fraction (by mass) contributed by carbonate with increasing metamorphic grade suggests that dissolution or decomposition of carbonates during metamorphism expelled light Mg isotopes. Thus, the Mg isotopic compositions of the silicate fractions in clastic sediments more faithfully reflect their provenance signatures. Our study shows that Mg isotopes can be used to study sedimentary diagenesis, and Mg isotopes may prove a useful tracer of sediments recycled into the mantle given their heterogeneous δ26Mg values.

Tectonic escape of a crustal fragment during the closure of the Rheic Ocean: U–Pb detrital zircon data from the Late Palaeozoic Pulo do Lobo and South Portuguese zones, southern Iberia

Abstract: The Pulo do Lobo Zone, which crops out immediately north of the allochthonous South Portuguese Zone in southern Iberia, is classically interpreted as a polydeformed accretionary complex developed along the southern margin of the Gondwanan parautochthon (Ossa–Morena Zone), during the late Palaeozoic closure of the Rheic Ocean. This closure was a major event during the amalgamation of Pangaea. U–Pb laser ablation inductively coupled mass spectrometry dating of detrital zircons from late Palaeozoic Devono-Carboniferous clastic units in the South Portuguese Zone and Pulo do Lobo Zone yield contrasting age populations and attest to the exotic nature of both zones. Detrital zircons from the South Portuguese Zone display populations typical of detritus derived from either Gondwana (Ossa–Morena Zone), or peri-Gondwanan terranes. In contrast, rocks from the Pulo do Lobo Zone contain populations consistent with derivation from Baltica, Laurentia or recycled early Silurian deposits along the Laurentian margin. An example of one such deposit is the Southern Uplands terrane of the British Caledonides. Taken together, these data can be reconciled by a model involving tectonic transport of a crustal fragment that was laterally equivalent to the Southern Uplands terrane between the allochthonous South Portuguese Zone and Gondwana as a result of an early Devonian collision between an Iberian indenter with Laurussia. Supplementary material: U–Pb data tables, concordia diagrams, methods and representative back-scattered electron images are available at http://www.geolsoc.org.uk/SUP18441 .

Li and δ 7Li in mudrocks from the British Caledonides: Metamorphism and source influences

Mudrocks from three lower Paleozoic basins in the British Caledonides (southern Lake District, northern Lake District and Southern Uplands) were investigated to determine the influence of sub-greenschist facies metamorphism on Li and the factors that control Li in fine-grained terrigenous sedimentary rocks. Metamorphic grade, as determined by KI (Kübler index) does not correlate with Li content ([Li]) and δ 7Li, indicating that sub-greenschist facies metamorphism has negligible effect on Li in these rocks. Collectively, the data for all three basins show a negative correlation between [Li] and δ 7Li and a positive correlation between [Li] and the Chemical Index of Alteration (CIA), suggesting that provenance exerts the greatest control on Li in mudrocks. Samples from the northern Lake District, which were deposited in an extensional basin, have homogeneous REE patterns, similar to shale composites (PAAS), the highest CIA, Th/U and [Li] and the lowest δ 7Li and ε Nd, consistent with their derivation from a highly weathered, ancient continental source. By contrast, mudrocks from the Southern Uplands range to the lowest CIA, Th/U and [Li] and have the highest δ 7Li and ε Nd. These samples were deposited in a forearc basin on the southern margin of the Laurentian craton and contain volcanic detritus. Their REE patterns are the most variable, ranging from average shale-like patterns to less LREE-enriched patterns. The compositional heterogeneity within the Southern Uplands mudrocks is consistent with a mixed provenance that includes juvenile crustal materials (lower [Li], ε Nd and Th/U, higher δ 7Li), likely derived from the arc, as well as more highly weathered continental detritus. Mudrocks from the southern Lake District were deposited in a foreland basin, and exhibit geochemical characteristics intermediate between the northern Lake District and the Southern Uplands mudrocks, indicating their derivation from a mixed source. Our study shows that Li concentrations and δ 7Li can provide additional information on the degree of weathering of the provenance of mudrocks.

Ancient Laurentian detrital zircon in the closing Iapetus Ocean, Southern Uplands terrane, Scotland

Early Paleozoic sandstones in the Southern Uplands terrane of Scotland were deposited during closure of the Iapetus Ocean between Laurentia and Avalonia. Their tectonic setting and sources are controversial, and different authors have supported subduction-accretion, extensional continental-margin development, or back-arc basin settings. We report new U-Pb detrital zircon ages from five Late Ordovician sandstones from the Northern Belt of the Southern Uplands and test models of their tectonic setting. The U-Pb zircon age distributions are dominated by peaks characteristic of sources in Laurentia and include grains as old as 3.6 Ga, older than any previously recorded in the British Caledonides SE of the Laurentian foreland. Discordant grains in one sample suggest derivation via erosion of metasedimentary rocks incorporated in the Grampian-Taconian orogen. Rare Neoproterozoic grains, previously interpreted as originating from a peri-Gondwanan terrane, may be derived from igneous rocks associated with Iapetan rifting. Only rare zircons are contemporary with the depositional ages. The results are difficult to reconcile with extensional continental-margin and back-arc models, but they support an active continental-margin subduction-accretion model. Close similarities with distributions from the Newfoundland Appalachians are consistent with sinistral transpression during closing of the Iapetus Ocean.

A regional geochemical assessment of some terrane relationships in the British Caledonides

Synopsis Regional geochemical data have been systematically collected across the British Caledonides and interpreted, using single and multielement image analysis, to provide geochemical constraints on terrane relationships. In the Hebridean Terrane, the Lewisian rocks show the characteristics of a large ion lithophile element (LILE) depleted lower crustal complex and seem unlikely to have been the source for the bulk of the overlying Torridonian clastic rocks. The psammite-dominated Moine Supergroup and the Dalradian rocks of the Grampian Group have closely similar evolved chemistries consistent with derivation from a higher crustal level than that of the currently exposed Lewisian complex. Across the Scottish Highland terranes the principal geochemical discontinuities coincide with the Moine Thrust Zone, the boundary between the Grampian Group and the Appin and Argyll Group Dalradian rocks, and the Highland Boundary Fault. The geochemical evidence provides little support for the Great Glen Fault as a significant terrane boundary. South of the Southern Upland Fault, which also coincides with a major geochemical discontinuity, a range of elements indicates division of the Southern Uplands terrane into northern and southern zones. These are transitional between the Orlock Bridge and Moffat Valley faults and reflect a change in sedimentary provenance during the early Silurian. Basic and ultrabasic elements, especially Cr, show exceptionally high levels over early Llandovery strata possibly indicating derivation from Iapetus Ocean crust obducted further NE as the Scandian Orogen was initiated. The geochemical characteristics of the southern part of the Southern Uplands Terrane continue into the Windermere Supergroup of the English Lake District, emphasising the continuity of mid- to late-Silurian deposition across the Laurentia–Avalonia (Iapetus) suture zone.

Regional geochemistry, terrane analysis and metallogeny in the British Caledonides

Abstract The Caledonides of northern Britain are commonly divided into an assemblage of terranes based on the interpretation of geology in relation to major fault structures. This paper incorporates metallogenic concepts into the terrane model and uses systematic regional geochemical data to constrain models of terrane configuration and evolution: analyses of one-, two- and three-component geochemical images are used to identify changes in the levels of single elements and their ratios across the orogen. The distribution of sedimentary exhalative (SEDEX) ore deposits and volcanogenic massive sulphide, and mesothermal and epithermal gold mineralization is shown to reflect the presence of long-lived metallogenic provinces which have distinctive geochemistry reflecting their origin as zones of crustal extension and lithospheric thinning. The geochemistry of the northern Caledonides reflects the predominantly lower crustal nature of the Laurentian continental margin, now exposed in the Lewisian foreland and Caledonized Lewisian inliers in the Moine succession. In contrast, the Avalonian Plate has a more evolved geochemistry, although the ratios of elements associated with basic rocks suggests that they were less fractionated during emplacement into the thinner crust of Avalonia and the Iapetus Suture Zone. The predominantly psammitic Moine Supergroup and the Grampian Group Dalradian have an evolved chemistry consistent with derivation from a higher crustal level than that of the exposed Lewisian complex; basin-scale variations, such as the proportion and composition of arkosic material, are also evident. The regional pattern was intermittently disrupted by the influx of mantle material during deposition of the mineralized Argyll and Southern Highland groups (Dalradian), and later as intrusions were emplaced from early Ordovician (Grampian Orogeny) to early Devonian times. South of the Southern Upland Fault, the geochemical patterns suggest a complex interplay of sedimentary provenance and depositional environments during the final stages of collision between Laurentia and Avalonia. Provenance changes in early Silurian times probably reflect the initiation of major tectonism in the Scandian Orogen to the northeast and, more locally, a change from a mineralized, extensional back-arc- to a foreland-basin environment as the Laurentian plate overrode Avalonia. Very similar geochemical trends extending from the highest Silurian strata in the Southern Uplands into the Windermere Supergroup of the English Lake District are consistent with sequential deposition in the same foreland basin. The principal crustal boundaries indicated by the geochemical data are the Moine Thrust Zone, the Grampian-Appin groups’ boundary, the Highland Boundary Fault and the Southern Upland Fault. The geochemical evidence provides little support for the Great Glen Fault as a significant terrane boundary, while the Iapetus Suture is shown to be a complex transition zone extending across the Southern Uplands and into the south of the Lake District. Within the former, the Moffat Valley Fault is identified as a major structure of regional significance. Throughout the orogen there is a direct relationship between geochemistry, tectonic setting and the presence of significant metalliferous mineralization.

Palaeomagnetism of (Palaeozoic) Peel Sandstones and Langness Conglomerate Formation, Isle of Man: implications for the age and regional diagenesis of Manx red beds

Abstract The Peel Sandstones are a sequence of fault-bounded fluvially deposited red beds of uncertain age exposed on the western coast of the Isle of Man. Palaeomagnetic study identifies partial preservation of an A component pre-dating deformation at four of eight sampled sites; the presence of both polarities in this remanence defines a positive class C reversal test. This component adjusted for tilt is D/I = 356.9/−47.1° (48 samples, α 95 = 5.6°) and is interpreted to be a post-depositional detrial remanence. The palaeolatitude (29° S) corresponds to the position of Britain during Late Silurian-Early Devonian times and implies that these sediments are correlative with Lower Old Red Sandstone molasse from elsewhere in the British Caledonides; a post-mid-Carboniferous age is excluded. Palaeomagnetic and geological evidence are used to constrain an age of c. 410–400 Ma for the Peel Sandstones, whilst a declination contrast of 40–50° is interpreted to reflect either: (a) tectonic emplacement of a unit including the Peel Sandstone outcrop; or (b) wider post-Early Devonian and pre-Late Carboniferous counterclockwise rotation between the Isle of Man and the mainland during final shaping of the Caledonides. A post-folding B component of uniform reversed polarity ( D/I = 195.6/−30.8°, 45 samples, α 95 = 4.8°) is present at all sites and was acquired during diagenesis at c. 250 Ma. Red sandstones within the Langness Conglomerate Formation, near the base of the Lower Carboniferous (Chadian-Arundian) succession exposed in the southeast of the Isle of Man, have a uniform reversed magnetization ( D/I = 209.7/−24.2°, 20 samples, α 95 = 6.8°). Like the B magnetization in the Peel Sandstones, this was acquired by diagenesis in Early Permian times ( c. 250 Ma) and during the Permo-Carboniferous Reversed Superchron. It is temporally related to early extensional tectonism in the Irish Sea region.

Window into the Caledonian orogen: Structure of the crust beneath the East Shetland platform, United Kingdom

Reprocessing and interpretation of commercial and deep seismic reflection data across the East Shetland platform and its North Sea margin provide a new view of crustal subbasement structure beneath a poorly known region of the British Caledonian orogen. The East Shetland platform, east of the Great Glen strike-slip fault system, is one of the few areas of the offshore British Caledonides that remained relatively insulated from the Mesozoic and later rifting that involved much of the area around the British Isles, thus providing an “acoustic window” into the deep structure of the orogen. Interpretation of the reflection data suggests that the crust beneath the platform retains a significant amount of its original Caledonian and older architecture. The upper to middle crust is typically poorly reflective except for individual prominent dipping reflectors with complex orientations that decrease in dip with depth and merge with a lower crustal layer of high reflectivity. The three-dimensional structural orientation of the reflectors beneath the East Shetland platform is at variance with Caledonian reflector trends observed elsewhere in the Caledonian orogen (e.g., north of the Scottish mainland), emphasizing the unique tectonic character of this part of the orogen. Upper to middle crustal reflectors are interpreted as Caledonian or older thrust surfaces that were possibly reactivated by Devonian extension associated with post-Caledonian orogenic collapse. The appearance of two levels of uneven and diffractive (i.e., corrugated) reflectivity in the lower crust, best developed on east-west–oriented profiles, is characteristic of the East Shetland platform. However, a north-south–oriented profile reveals an interpreted south-vergent folded and imbricated thrust structure in the lower crust that appears to be tied to the two levels of corrugated reflectivity on the east-west profiles. A thrust-belt origin for lower crustal reflectivity would explain its corrugated appearance. Regional seismic velocity models derived from refraction data suggest that this reflectivity correlates with a continuous lower crustal layer that has an intermediate seismic velocity. The lower crustal reflectivity is determined to be older than Mesozoic age by the bending down and truncation of the two reflectivity levels at the western margin of the North Sea Viking graben by a major mantle reflector inferred to be associated with Mesozoic rifting. The results of this study are thus in contrast with orthodox interpretations of the reflective layered lower crust as being caused by mantle-derived igneous intrusion or by deformation fabrics associated with stretching in response to continental rifting.

The Cambrian-Silurian tectonic evolution of the northern Appalachians and British Caledonides: history of a complex, west and southwest Pacific-type segment of Iapetus

This paper is included in the Special Publication entitled 'Lyell: the past is the key to the present', edited by D.J. Blundell and A.C. Scott. This paper presents new ideas on the Early Palaeozoic geography and tectonic history of the Iapetus Ocean involved in the formation of the northern Appalachian-British Caledonide Orogen. Based on an extensive compilation of data along the length of the orogen, particularly using well-preserved relationships in Newfoundland as a template, we show that this orogen may have experienced a very complicated tectonic evolution that resembles parts of the present west and southwest Pacific Ocean in its tectonic complexities. Closure of the west and southwest Pacific Ocean by forward modelling of the oblique collision between Australia and Asia shows that transpressional flattening and non-coaxial strain during terminal collision may impose a deceptively simple linearity and zonation to the resultant orogen and, hence, may produce a linear orogen like the Appalachian-Caledonian Belt. Oceanic elements may preserve along-strike coherency for up to several thousands of kilometres, but excision and strike-slip duplication, as a result of oblique convergence and terminal collisional processes, is expected to obscure elucidation of the intricacies of their accretion and collisional processes. Applying these lessons to the northern Appalachian-Caledonian belt, we rely principally on critical relationships preserved in different parts of the orogen to constrain tectonic models of kinematically-related rock assemblages. The rift-drift transition, and opening of the Iapetus Ocean took place between c.590-550 Ma. Opening of Iapetus was temporally and spatially related to final closure of the Brazilide Ocean and amalgamation of Gondwanaland. During the Early Ordovician, the Laurentian margin experienced obduction of young, supra-subduction-zone oceanic lithosphere along the length of the northern Appalachian-British Caledonian Belt. Remnants of this lithosphere are best preserved in western Newfoundland and are referred to as the Baie Verte Oceanic Tract. Convergence between Laurentia and the Baie Verte Oceanic Tract was probably dextrally oblique. Slab break-off and a subsequent subduction polarity reversal produced a continental magmatic arc, the Notre Dame Arc, on the edge of the composite Laurentian margin. The Notre Dame Arc was mainly active during the late Tremadoc-Caradoc interval and was flanked by a southeast- or south-facing accretionary complex, the Annieopsquotch Accretionary Tract. Southerly drift of Laurentia to intermediate latitudes of c.20-25°S was associated with the compressive (Andean) nature of the arc and the accompanying backthrusting of the already-accreted Baie Verte Oceanic Tract further onto the Laurentian foreland. Equivalents of the Notre Dame Arc and its forearc elements in the British Isles have been preserved as independent slices in the Midland Valley and possibly the Northern Belt of the Southern Uplands. During the late Tremadoc (c.485 Ma), the passive margin on the eastern side of Iapetus also experienced obduction of primitive oceanic arc lithosphere. This arc is referred to as the Penobscot Arc. The eastern passive margin was built upon a Gondwanan fragment (Ganderia) that rifted off Amazonia during the Early Ordovician and probably travelled together with the Avalonian terranes as one microcontinent. The departure of Ganderia and Avalonia from Gondwana opened the Rheic Ocean. Equivalents of the Penobscot Arc may be preserved in New Brunswick and Maine, Leinster in eastern Ireland, and Anglesey in Wales. An arc-polarity reversal along the Ganderian margin after the soft Penobscot collision produced a new arc: the west-facing Popelogan-Victoria Arc, which probably formed a continuous arc system with the Bronson Hill Arc in New England. The Popelogan-Victoria Arc transgressed from a continental to an oceanic substrate from southern to northeastern Newfoundland. Rapid roll-back rifted the Popelogan-Victoria Arc away from Ganderia during the late Arenig (c.473 Ma) and opened a wide back-arc basin; the Tetagouche-Exploits back-arc basin. The Popelogan-Victoria Arc was accreted sinistrally oblique to the Notre Dame Arc and, by implication, Laurentia during the Late Ordovician. After accretion, the northwestward-dipping subduction zone stepped eastwards into the Tetagouche-Exploits back-arc basin.

U-Pb zircon geochronological evidence for Neoproterozoic events in the Glenfinnan Group (Moine Supergroup): the formation of the Ardgour granite gneiss, north-west Scotland

The age and Precambrian history of the Moine Supergroup within the Caledonide belt of north-west Scotland have long been contentious issues. The Ardgour granite gneiss is essentially an in situ anatectic granite formed during deformation and regional high-grade metamorphism from Moine metasediments. High-precision TIMS and SHRIMP U-Pb zircon dating shows that the age of the anatectic Ardgour granite gneiss and its enclosed segregation pegmatites is 873 ± 7 Ma. This demonstrates the reality of a Neoproterozoic episode of high-grade metamorphism in the Glenfinnan Group Moine and, contrary to previous evidence, the absence of Grenvillian-aged metamorphism. This conclusion places constraints on Neoproterozoic palaeogeographic reconstructions of the North Atlantic region, indicating that the Moine rocks cannot be used as a link between the Grenvillian belt of North America and the Sveconorwegian orogen in Scandinavia. SHRIMP ages of between c. 1100 and 1900 Ma were obtained from detrital, inherited zircons and reflect the provenance of the Glenfinnan Group Moine sediments which must, therefore, have been deposited between c. 1100 and 870 Ma. Potential sources are found as relatively minor, tectonically bounded basement inliers within the British Caledonides, although more widespread source areas occur outside Britain in both Laurentia and Baltica. The most important feature of the provenance is the absence of detrital Archaean grains. This suggests that the Archaean Lewisian gneiss complex, which forms the basement component of the western foreland to the Caledonides in Britain, was not a major contributor to the Glenfinnan Group basin.

Tectonic evolution of the Quebec-Maine Appalachians; from oceanic spreading to obduction and collision in the Northern Appalachians

The Cambrian-Silurian tectonic evolution of the northern Appalachians and British Caledonides: history of a complex, west and southwest Pacific-type segment of Iapetus

Structure of a continental strike‐slip fault from deep seismic reflection: Walls Boundary fault, northern British Caledonides

Reprocessing of four offshore deep reflection profiles across the Walls Boundary strike‐slip fault, postulated as the northward continuation of the Great Glen fault over the Shetland platform, provides useful constraints on the lithospheric structure of a major continental transform and some insight into the manner in which strike‐slip faults deform. The reprocessing was aimed at delineating complex structure and reducing noise contamination and consisted of analyses of diffractions and amplitude variations and experimentation with seismic migration. The results demonstrate a highly variable crustal and uppermost mantle structure for the Walls Boundary fault along more than 150 km of its length. Six key observations can be made from this study: (1) as middle and lower crustal reflections approach the fault, they are truncated by, and do not continue across, the downward projection of the fault; (2) trends in reflection structure of the crust and/or uppermost mantle differ markedly from one side of the fault to the other; (3) diffractions typically are concentrated at the level of the Moho discontinuity directly beneath the fault; (4) after seismic migration, steeply dipping reflectors are imaged in places directly beneath the fault in the lower crust; (5) in one location, the Moho reflection is deflected across the fault into a narrow synform or “keel” structure which is exactly bisected by the downward projection of the fault; and (6) in a separate location, a prominent upper mantle reflector is truncated across the fault. A vertical or steeply dipping structure is deduced for the fault in much of the crust and perhaps parts of the uppermost mantle; however, more diverse reflecting structures are interpreted for the lower crust, such as the Moho deflection and steeply dipping reflectors directly beneath the fault that may have been related to a localized component of transpression and resultant crustal thickening. The presence of diffractions at the level of the Moho discontinuity where the downward projection of the fault intersects it suggests points of structural “roughness” that may be related to deformation of the Moho by late motion along the fault. Integrating the interpretation of the reflection data with previous geological studies implies that the age of the Moho deflection is post Early Cretaceous but that the dipping structure adjacent to and cut off by the fault is probably Caledonian (Silurian‐Early Devonian). Although the disparate structure of the crust on opposite sides of the fault supports geologic interpretations of large‐scale displacement, a major step in the Moho discontinuity directly beneath the Walls Boundary fault cannot be substantiated from the seismic data.

Geochemical comparison of the subvolcanic appinite suite of the British Caledonides and the durbachite suite of the Central European Hercynides: evidence for associated shoshonitic and Granitic Magmatism

Subvolcanic pyroxenite-hornblendite-kentallenite-diorite-granodiorite masses of the appinite suite that are spatially and temporally associated with the much more voluminous granitic plutons of the British Caledonides have major element proportions and REE patterns indicative of shoshonitic affinities. Hornblendite-monzonite-syenogabbro-pyroxene melasyenite-durbachite-biotite-rich syenite-biotite-rich granite masses of the plutonic durbachite suite of the Bohemian Massif of the Central European Hercynides, that also are spatially and temporally associated with much more voluminous granitic plutons, have geochemical characteristics that generally correspond with those of the appinite suite. Compositionally both suites resemble lamprophyres emplaced during the latter parts of the respective episodes. Both the appinite and durbachite suites show independence of K/Rb and SiO2 with the two suites having mainly different but somewhat overlapping K/Rb ratios. Other geochemical characters, as shown by fields and trends on K vs Rb, AFM and other plots, point to the durbachite suite representing generally more evolved products of shoshonitic magma than members of the appinite suite. However, there are different geochemical characteristics, including higher Cr/Ni ratios in the durbachite suite and Co present in lower proportions in the appinite suite. These differences are the result of different histories of freezing, remelting and partial separation and remixing of fractionation products and reflect the explosive subvolcanic vs plutonic regimes of the appinitic and durbachitic suites, respectively. Support for this petrogenesis is provided by mineral compositions and comparison of compositions of mineral phases and the rocks in which they occur.

Strike‐slip tectonics and granite petrogenesis

Transpression is a process that thickens the crust and therefore in obliquely convergent orogens as in normally convergent orogens there is the potential to generate granitic melts. Individual transcurrent shear zones may not only control the ascent paths, siting, and emplacement mechanisms of plutons, but they may also cause the genesis of the granitoids themselves. Two contrasting situations are examined. In the Hercynian shear zones of Iberia, thickening together with hydrous fluxing along shear zones created intracrustal wet melting of fertile Gondwanan sediments to produce syntectonic granites. In the northern part of the British Caledonides, the association of compositionally expanded granitoids with a major mantle component and transcurrent shear zones may be explained by melting of continental crust at the lower limits of crustal transpressional faults detaching into the Moho.

Discussion on Silurian and Early Devonian sinistral deformation of the Ratagain Granite, Scotland: constraints on the age of Caledonian movements on the Great Glen system

B. C. Lintern, R. P. Barnes &amp; P. Stone write: Hutton &amp; McErlean (1991) provide convincing evidence for two distinct phases of sinistral movement on the Strathconan Fault, the first during the mid-Silurian (Wenlock, c. 425 Ma) and the second during the Early Devonian. They conclude that the Early Devonian ( c. 400 Ma) low temperature shearing and brittle faulting event is well documented elsewhere in the British Caledonides and can be regarded as one of the consequences of the final collision between Laurentia and Avalonia. However, no correlatives of the earlier Wenlock ( c. 425 Ma) event were discussed and they considered that ‘...it remains to be seen how common a sinistral event of this age is in the rest of the British Caledonides’. Recently, we presented a summary of available tectono-stratigraphical and isotopic age data ( Barnes et al. 1989 , fig. 1) which constrained the timing of the culmination of deformation in the Southern Uplands thrust stack to within the M. riccartonensis biozone ( c. 424 Ma) of the Wenlock. Sinistral shear during the final stages of this diachronous event (e.g. Anderson 1987 ; Barnes et al. 1989 ) produced, at the thrust front, such variation in the plunge of first-phase folds that they are locally downward facing and form steeply-plunging sinistral fold pairs. Contemporaneously the older, previously deformed parts of the thrust belt hinterland were subjected to localized folding and shearing associated with sinistral wrench movements on and adjacent to steepened early thrusts such as the Orlock Bridge Fault ( Anderson &amp; Oliver 1986 )