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Major- and trace-element data and Nd isotope compositions for granitoid samples from the Grampian Highlands in Scotland show a systematic evolution in the composition of their sources in the course of the Caledonian Orogeny. Granitoids of 511–451 Ma, related to the collision of the Midland Valley island arc with the Grampian terrane, show S-type affinity and fractionated REE patterns with minor Eu anomalies and low initial ϵ Nd values of −14.1 to −11.2 suggesting melting of predominantly Dalradian metasediments. Subsequently formed granitoids of 425–406 Ma derived from an assumed Andean plate margin comprise a wide spectrum of rock types including I-type granite–granodiorite, and S-type granitoids, monzonites and alkali granites. The trace-element patterns of these rocks and the range of initial ϵ Nd values of −2.1 to −6.9 are consistent with melting of variably rejuvenated crust as found in continental margin settings. We conclude that the Grampian Highlands were affected by two major crust-modifying events during the Caledonian Orogeny: predominantly recycling of older crust during docking of the Midland Valley arc and addition of juvenile, mantle-derived material to the crust during the convergence of Avalonia with Laurentia.

A total of 200 detrital zircon grains from three stratigraphically distinct samples of the Neoproterozoic Moine Supergroup, NW Scotland were analysed by ion microprobe. Samples were taken from the Lower and Upper Psammite Formations of the oldest Moine unit, the Morar Group, and one from the tectonostratigraphically overlying, and presumed younger, Glenfinnan Group. The Morar Group is dominated by Palaeo- and Mesoproterozoic crystals and minimal Archaean grains whereas the Glenfinnan Group has broadly the same overall age range but a greater proportion of Grenville (late Mesoproterozoic) detritus. These data indicate an increase up-section in Grenville-age detritus and a Laurentian provenance, consistent with initiation of Moine deposition during the final stages of development and exhumation of the Grenvillian Orogen. The youngest concordant detrital zircons provide maximum depositional ages for the Morar Group at 1022 ± 24 Ma and for the Glenfinnan Group at 1009 ± 22 Ma. Sedimentation must have ceased prior to 842 ± 20 Ma, the age of high temperature conditions recorded by magmatic zircon overgrowths in one of the Morar Group samples; this is compatible with mid-Neoproterozoic Knoydartian orogenic events documented elsewhere within the Moine Supergroup. Robust statistical comparison of the Moine detrital zircon age distributions to those from Neoproterozoic successions elsewhere enable definition of three main lithotectonic units: (1) late Mesoproterozic to early Neoproterozoic basins with a range of late Grenville intrusives; (2) mid- to late Neoproterozoic basins in extension-related intra-Rodinia settings or fringing Rodinia's oceanic margin; and (3) late Neoproterozoic to early Palaeozoic basins formed during the main phase of Rodinia break-up. Statistical comparisons also indicate that: (i) in Scotland, the Morar Group rocks are similar to the sub-Grampian Group rocks (Dava–Glen Banchor succession) and permit linkage with the Torridonian of the Caledonian foreland (Applecross and Ault Bea formations), but differ significantly from the Moinian Glenfinnan and Loch Eil groups, as well as the Dalradian rocks; and (ii) the provenance signal and depositional timing of the Moine compares closely to the Soroy Succession of Arctic Norway and the lower Eleonore Bay Supergroup of East Greenland. All of these have a Laurentian affinity and thus tectonic models invoking a geologically exotic status (i.e., a terrane) for the Moine appear unnecessary.

The Tyrone Central Inlier is a metamorphic terrane of uncertain affinity situated outboard of the main Dalradian outcrop (south of the Fair Head–Clew Bay Line) and could represent sub-arc basement to part of the enigmatic Midland Valley Terrane. Using a combination of isotopic, structural and petrographic evidence, the tectonothermal evolution of the Tyrone Central Inlier was investigated. Sillimanite-bearing metamorphic assemblages ( c . 670 °C, 6.8 kbar) and leucosomes in paragneisses are cut by granite pegmatites, which post-date two deformation fabrics. The leucosomes yield a weighted average 207 Pb/ 206 Pb zircon age of 467 ± 12 Ma whereas the main fabric yields a 40 Ar– 39 Ar biotite cooling age of 468 ± 1.4 Ma. The pegmatites yield 457 ± 7 Ma and 458 ± 7 Ma Rb–Sr muscovite–feldspar ages and 40 Ar– 39 Ar step-heating plateaux of 466 ± 1 Ma and 468 ± 1 Ma, respectively. The metasedimentary rocks yield Palaeoproterozoic Sm–Nd model ages and laser ablation inductively coupled plasma mass spectrometry detrital zircon U–Pb analyses from a psammitic gneiss yield age populations at 1.05–1.2, 1.5, 1.8, 2.7 and 3.1 Ga. Combined, these data permit correlation of the Tyrone Central Inlier with either the Argyll or the Southern Highland Group of the Dalradian Supergroup. The inlier was thus part of Laurentia onto which the Tyrone ophiolite was obducted.

Within the Scottish Caledonides, the Glen Scaddle Metagabbro was intruded into the Moine Supergroup of the Northern Highland Terrane after Grampian D2 folding and prior to regional D3 and D4 upright folding and amphibolite-facies metamorphism. A U–Pb zircon age of 426 ± 3 Ma obtained from the metagabbro is interpreted to date emplacement. D3–D4 folding is constrained to have occurred during the Scandian orogenic event. In contrast, polyphase folding and regional metamorphism of the Dalradian Supergroup southeast of the Great Glen Fault is entirely Grampian. These differences are consistent with published tectonic models that invoke a minimum of 700 km of post-Scandian sinistral displacements across the Great Glen Fault to juxtapose the Grampian and Northern Highland terranes.

The regionally metamorphosed, Riphean–Cambrian Argyll Group Dalradian rocks of NW Achill Island, western Ireland are disposed in a large-scale, regionally west-facing, tight, recumbent F2 curvilinear fold, with which two ductile shear zones are associated. Clasts in conglomerates within the Dalradian sequence that are deformed by the shear zones preserve evidence for a constrictional overprint of earlier plane strain as the fold became curvilinear, while stretched clasts maintained a constant orientation as the hinge curvilinearity developed. During the constrictional overprint a crenulation fabric, S2b, overprinted a penetrative foliation, S2a, in the shear zones. The S2b has an orientation that varies systematically with that of the fold hinge. It is inferred that, although the S2b surfaces initiated as a dip-slip fabric, there was an increasing degree of strike slip on these surfaces as the fold hinge approached parallelism with the direction of tectonic transport. It is possible that many curvilinear folds have an early history involving plane strain, but that increasing constrictional strain is intrinsic to the later stages of their development.

Synopsis The Dalradian succession in Shetland is shown to have experienced an extensional tectonic and metamorphic episode prior to the Caledonian orogeny. Sedimentary deposition ended with a major basic and ultrabasic volcanic episode following a long period of extensional basin deposition involving both basic volcanics and turbidites. The ending of deposition was marked by the emplacement of a spinifex-textured komatiite indicating instantaneous rifting of the crust and the underlying Laurentian plate, and the formation of a Laurentian passive continental margin, probably during the opening of Iapetus. The entire contents of the Dalradian depositional basin were rotated bodily 90° to face the rift and to form a 12 km wide vertical belt in the middle of Shetland. A continued rotation in the same sense formed a 25 km wide recumbent inverted limb to form a mega-monocline. The formation of this monocline is considered to have occurred during the extensional phase accompanying rifting because the absence of subsidiary buckle folds of any and every size precludes compressional folding. The metamorphism of the rocks shows a gradual increase from bottom to top of the stratigraphic succession, predating the formation of the monocline, and probably owes its existence to crustal thinning and heating from below the deposition basin during extension. Much of the tectonic and metamorphic history shown by the Dalradian of Shetland may have been overprinted and masked in Scotland by the Caledonian orogeny. In the Dalradian of Shetland the Caledonian orogeny is represented only by the emplacement of the Shetland Ophiolite.

Dark limestones in the old quarries at Leny, Perthshire contain sparse beds with tiny fossils. They are poorly preserved and, though barely affected by the Ordovician Grampian Event tectonism, there is some taphonomic distortion and many are corroded along stylolitised horizons. The fauna mainly comprises trilobites of two types, open-ocean miomerids and polymerid shelf dwellers. MiomeridsCondylopygecf.eliandKiskinella cristataindicate a stratigraphical position equivalent to the base of the paradoxidid Amgan Stage of Siberia; traditionally regarded as ‘Middle Cambrian’. However, the bulk of the Leny miomerids, notably species ofPagetides, are forms described from the outer edge of Laurentia, within theBonnia–OlenellusZone, where it is considered to be ‘Lower Cambrian’. The Leny polymerids were likely transported off-shelf and some are conspecific with taxa in the Laurentian allochthonous Quebec and New York successions of the Early–Middle Ordovician (Taconic) Appalachian Orogen. The Leny Limestone and Shale Member of the Keltie Water Grit Formation is part of the Dalradian Supergroup deposited in an off-shelf Caledonide Grampian Terrane of the Humber Tectonostratigraphical Zone, midway between the North American successions and the Greenland Caledonides.Additional to the trilobites, brachiopods, sponges, hyoliths, bradoriids and a selection of indeterminable organic fragments occur; none of which has any particular age significance.

Seven phases of deformation are recognised in Dalradian metasediments within the NW aureole of the Main Donegal Granite. Major NW facing F2 folds (the Aghla Anticline and Errigal Syncline) refold an originally NW facing slaty cleavage, which is usually parallel to bedding. D3 structures cross-cut the major F2 folds and verge and face to the SE on their normal limbs. A fourth phase of deformation intensifies towards the granite contact and is shown to be broadly coeval with intrusion and responsible for a major structure in the aureole. Three later phases are variably developed throughout the aureole. The kinematics of F4 folds in relation to the granite intrusion are briefly discussed.

The structure of the Dalradian rocks of the Banffshire coast has been re-examined. It is shown that D1 structures face upwards along the entire section and the major Boyndie Syncline (Sutton and Watson 1956) must also be of D1, age and not D3 as proposed by Johnson (1962). D2 structures are developed in the west, most strongly in the Portsoy Group and Cowhythe Gneiss where they may be related to the Portsoy Thrust (Elles 1931). East of the Cowhythe Gneiss D2 structures are not developed but D3 structures are of local importance. Porphyroblast growth is considered to be post-D1, pre-D3 related to the ‘active’ D2 deformation in the west (Barrovian) and to the ‘static’ D2 interval in the east (Buchan). The wider implications of these conclusions for Dalradian structure are briefly considered.

The Moine-like rocks are shown to have major, southward-leaning, isoclinal F2 folds which may be synchronous with the nappe-like Ballybofey Antiform of the overlying Dalradian rocks. The deformation of the immediately adjacent Dalradian rocks is consistent with a previous suggestion that they are in contact with the Moine-like rocks along a major tectonic slide.

Pre-Caledonian basement is exposed in three areas within the Irish orthotectonic Caledonides: 1. In northwest County Mayo the Erris Complex comprises the Annagh Division, which is largely Grenvillian but includes older gneisses, and the Inishkea Division, which is probably equivalent to the older Moines of Scotland. 2. In the northeast Ox Mountains, Rosses Point and Lough Derg inliers there is a granulite fades, dominantly metasedimentary basement which is probably late Grenville in age. 3. Laxfordian and probably older gneisses are seen in the Inishtrahull Platform northeast of Malin Head, County Donegal. In addition, some gneisses within the belt of ‘Connemara migmatites’ in south Connemara may be fragments of allochthonous basement and some high grade metamorphic rocks in the Tyrone Central inlier have been referred to as pre-Caledonian basement, but there is at present no conclusive support for these suggestions. The Ox Mountains succession seen in the Clew Bay region and the southwest and central Ox Mountains is probably not pre-Caledonian basement. Basement in the paratectonic Caledonides is confined to the Cadomian and possibly pre-Cadomian Rosslare Complex (older than 625 Ma) and the nearby late Precambrian (to early Cambrian ?) Cuilenstown Formation in southeast County Wexford. In addition, an unseen block of basement defined by magnetics immediately to the north of the Dingle peninsula of County Kerry is probably of Rosslare Complex affinity. Granulite facies xenoiiths recovered from volcanic rocks of Carboniferous age about 70 km to the west of Dublin, indicate the character of basement there, though its age is not proven. All contacts between basement and Lower Palaeozoic Caledonian cover in the orthotectonic Caledonides are synmetamorphic slide zones and faults and in the paratectonic Caledonides the contacts are either faults, which are often rooted in older mylonites, or are unexposed and presumed to be major structural breaks.

The lithologies and structures of the Dalradian rocks of the Ardsheal Peninsula, Argyllshire are described. The structures associated with the main synformal fold (part of the Appin Syncline) are shown to be F1 in age. F2 structures cross the Syncline and show progressive development towards the east. The Syncline is correlated and compared with other major F1 folds in the region. This interpretation is contrasted with other interpretations of the Syncline as a third-phase structure which can be correlated with late-phase folds in the Dalradian.

The structural relationships of several intrusive sheets belonging to the extensive metadolerite suite of the Appin (lower) Dalradian rocks of the Creeslough area, northwest Donegal, Ireland have been examined. It is concluded that, relative to the regional chronology of deformation of the enclosing metasediments, sheet emplacement has occurred: pre-D2; syn-D2; between D2 and D3; and syn-D3. The height of activity came in the post-D2, pre-D3 interval and coincided with the peak of regional metamorphism in the area. These relationships in Donegal are compared with those of the pretectonic basic rocks of the southwest Highlands and models are discussed.

A tight early fold, the Ballybofey anticline, trends south-eastwards across central Donegal. South-west of its axial trace the Dalradian strata are inverted so that the rocks high in the Series occur at the lowest structural level, next to, and facing downwards, the supposedly Moinian psammites in the core of the Lough Derg antiform. Rapid facies changes occur in the Dalradian rocks across the Caledonian strike.

A composite tectonic profile, from Clew Bay to central Murrisk is constructed and the sequence of structural events from the Upper Cambrian to post-Wenlock—pre-Visean (C2 S2) times is outlined. Four main structural episodes affected the Dalradian rocks in pre-Arenig times. ‘Taconic’ thrusting and concentric folding were important, and finally a complex sequence of structures were variably imprinted on the Dalradian, Ordovician and Silurian rocks during post-Wenlock, pre Visean (C2 S2) times.

Komatiite lavas occur in the Dunrossness Spilitic Group in southeast Shetland in the Cunningsburgh area. This metavolcanic group is the easternmost member of the East Mainland Succession of Shetland and probably the youngest. It overlies a thick succession of deep-water metasediments and metavolcanic rocks probably forming part of an extensional basin and has been correlated with a part of the upper Dalradian of Scotland. Komatiites are not common in rock sequences as young as this. Their eruption is thought to require tectonic and thermal conditions which have been unusual since Archean times. Such conditions could arise when the crust beneath an extensional basin splits to form a constructive margin.

A Rb-Sr whole-rock isochron study indicates that the entire Donegal granite suite was emplaced into orthotectonic Caledonian (Dalradian) rocks over a short interval during mid-Silurian to earliest-Devonian times. The Thorr pluton, probably the earliest member of the suite, yields an age of 418 ± 26 Myr and initial 87Sr/86Sr ratio of 0·7055 ± 4, while the latest member, the Main Donegal pluton has an age of 407 ± 23 Myr and initial 87Sr/86Sr ratio of 0·7063 ± 5 (Λ87Rb = 1·42 ± 10−11 yr−1). Errors on both the age and initial Sr isotope ratios incorporate both a priori and geological scatter components and are quoted at the 2-sigma level. The low and restricted range of initial Sr isotope ratios suggests small but significant differences in the composition of the parental granitic magmas which were derived from a low Rb/Sr, low 87Sr/86Sr source.

Two phases of deformation are described from the area. D1 is related to the first phase movements widely recognised in the Dalradian; D2 is related to more localised modifications of the D1 fold pattern. Four major D2 folds are recognised from the evidence of minor structures. Three of these, the Beinn Chuirn, the Ben Lui and the Ra Chreag Folds, had previously been considered as D1 structures, whilst the fourth, the Dalmally Fold, was unrecognised. These folds provide continuity between the southeast facing of the D1 folds of the Moine and Ballachulish Succession of Glen Orchy (Thomas and Treagus 1968) and the southeast facing of the D1 Jay Nappe (Shacketon 1958) in the Iltay Succession above. The D2 deformation reaches a climax at the junction between the two Successions, the Iltay Boundary Slide, which is now regarded as a D2 structure. The regional implications of these results are briefly considered.

Marine turbidites, tuffs, black mudstones and conglomerates of the Cambro-Ordovician Clew Bay Group, were deposited in the E–W elongate transtensional Clew Bay Graben that is centred on Clew Bay, NW Ireland. The group is characterized by extensive sedimentary deformation and mass movement on slides; olistostromes, autoclastic breccias and course proximal turbidites are interbedded with apparently less disturbed but often overturned sediments. The Clew Bay Group lies structurally above serpentinized dunite/harzburgite breccias, schistose carbonate peridotites, and other basic and ultrabasic igneous rocks that have ophiolitic geochemical affinities; the sediments may have been in part deposited upon oceanic crust. Ophiolites and sediments that now rest on the Clew Bay Thrust abut Silurian shallow water strata in which the main tectonothermal history, associated with sinistral transcurrent faulting along the thrust zone, is dated at about 410 Ma. The sole thrust dips northward and coalesces with a major deep structure along the Fair Head-Clew Bay Line (FCL) that is the western continuation of the Highland Border Fault of Scotland. Blueschist relics in the Dalradian immediately to the north of the FCL indicate that subduction was active early in the history of the late Cambrian–early Ordovician Grampian orogeny. The Clew Bay Thrust was a sinistral, transpressional shear zone late in its history, but it probably originated as an obduction complex. The Clew Bay Group cannot be traced into sedimentary, metamorphic or structural continuity with the adjacent Dalradian to the north or Ordovician and Silurian rocks in the South Mayo Trough to the south. It should be considered as a distinct terrane (Clew bay Terrane) or a subterrane of Highland Border-type rocks along the southern margin of the Grampian Terrane.

An unusual matrix-supported microbreccia of Caledonian age is described from the Dalradian rocks of Donegal. Field and textural relationships indicate that the breccia formed ‘in situ’ by a non-dilational mechanism during regional deformation. The presence of breccia veins of low competancy during this deformation caused multi-decollement folds to be formed adjacent to the breccias. A cyclical mechanism of formation of these breccias is proposed in which fluctuations in the overpressuring of pore fluids escaping from an adjacent crystallizing appinite mass caused periodic excursions from ductile to brittle deformation during the regional folding.

The Monadhliath and Leven Schists are separated north of Glen Roy by some 500m of psammitic rock. Junctions are gradational and conformable, and evidence from sedimentary structures indicates that all three face upwards. The two schists should not be equated, nor should the Monadhliath Schist be included in the Dalradian Supergroup unless the entire Grampian Division is also included.

Within the Dalradian Series of Donegal, the Creeslough, Maas and Fintown successions, which have hitherto been regarded as stratigraphically distinct, are correlated. A standard terminology for this sequence is proposed and a detailed correlation is made with the Ballachulish Succession, which thus forms all the country around the Donegal Granites, northwest of the faults and thrusts which separate it from the Islay Succession.

Major element composition of the available analysed pelitic rocks of the Dalradian of Scotland is examined in relation to geographical position, mineralogy, metamorphic grade and stratigraphy. Some of the large variations found are due to geographical position, possibly reflecting different provenance. A major compositional difference exists between the two lower Barrovian metamorphic zones and the four higher zones. This is considered to be a reflection of the original sedimentary composition and large scale metasomatism is not considered likely.

Field and petrological studies of the Highland Border Ophiolite demonstrate that it was obducted onto the Neoproterozoic–Ordovician rocks of the Dalradian block, and not emplaced against them by post-orogenic strike-slip movement. It was welded onto the upper, southward-younging limb of the already recumbent Tay Nappe (D1), and deformed by the Downbend Antiform (D4). However, its emplacement was not accompanied by significant internal deformation of the Dalradian block. As the ophiolite is correlated with those at Clew Bay, Tyrone and Shetland, this result will necessitate complete revision of the current model for basin closure on the southeastern margin of Laurentia.

Previous work on early Paleozoic mafic–ultramafic intrusions in northeast Scotland identified two targets for Ni–Cu–PGE exploration. The first prospect is at Arthrath, near the town of Ellon, and the second, in the southeastern corner of the Knock intrusion, near the town of Huntly, around the farms of Littlemill and Auchencrieve. Both prospects occur within a group of mafic–ultramafic rocks known as the Younger Basic intrusions, emplaced broadly synchronously with the later stages of Ordovician amphibolite-facies metamorphism of metasedimentary units of the Dalradian Supergroup. We have re-examined the available samples from the Ni–Cu exploration program in terms of their PGE potential, placing particular emphasis on the less-well-known Arthrath prospect. PGE abundances of up to 418 ppb Pt (Littlemill) and 458 ppb Pd (Arthrath) associated with zones of Cu–Ni–Fe sulfide mineralization have been identified. At Arthrath, the Pd is hosted primarily in merenskyite enclosed in base-metal sulfide; the occurrences of platinum-group minerals define a bimodal distribution. At Littlemill, the occurrences of platinum-group minerals involve mobilization and redeposition of precious metals in sheared rocks enclosed within the base-metal sulfide zones. When the mineral compositions of all the rocks associated with the zones of Cu–Ni–PGE mineralization are considered relative to the known differentiation series present within these intrusions, a model involving the mixing and mingling of relatively late, primitive magma and pre-existing differentiates is favored for both prospects. In particular, a cumulate orthopyroxenite unit at Arthrath closely associated with a zone of Cu–Ni–PGE mineralization is directly interpreted as the result of the influx into a magma chamber of a late, primitive magma. Previous work had suggested crustal contamination as the mechanism of sulfide immiscibility. Although evidence of crustal contamination is present, it is not favored as the direct mechanism for the formation of the sulfide-rich zones that comprise the principal targets of Ni–Cu–PGE exploration.

Scotland occupied a unique position within the Neoproterozoic supercontinent Rodinia, lying close to the junction of Laurentian, Amazonian and Baltican continental blocks. During the attempted break-up of the supercontinent, a series of intracratonic rift basins were created along the Scottish sector of the future Laurentian margin. These basins were filled with the thick, siliciclasitc Torridonian, Moine and Dalradian Supergroups. Combined U–Pb detrital zircon geochronology and Nd-isotope analyses of whole rocks are used to compare the provenance of two contemporaneous lower Dalradian rift basin-fills. The main control on their provenance signature is sedimentary transport direction, which allows the relative palaeogeographical positions of surrounding basement terranes to be assessed. Westerly derived sedimentary systems supplied voluminous ca. 1.8 Ga aged detritus to the western Corrieyairack Basin. By contrast, sediment supplied from the east within the eastern Strathtummel Basin was dominated by ca. 0.9–1.0 Ga sources. Distal and further travelled basin-axial depositional systems supplied the basins with a greater variety of detritus, dominated by early Mesoproterozoic (1.4–1.6 Ga) and late Palaeoproterozoic (1.6–1.8 Ga) ages. The dataset is consistent with the lower Dalradian Supergroup being sourced from the eastern Laurentian (Labrador region) and/or possibly western Baltica blocks. These data indicate that the basins were flanked to the west by ca. 1.8 Ga Makkovikian-Ketilidian (Rhinnian) igneous basement and to the east by 1.0 Ga Grenvillian-Sveconorwegian basement. This interpretation, combined with the similarities with other penecontemporaneous Laurentian rift-basin fills, indicates that the Dalradian Supergroup is not allochthonous or exotic to the Laurentian continental block.

Various hypotheses have tried to explain the relationships between the metamorphic blocks of Scotland and the craton edge of Laurentia. Whilst the recent dating of the basins, which preceded the metamorphism, is exciting, it overlooks the point that the blocks now sit where a passive margin to the Laurentian continent should be. Given that there should be c . 300 km or more of passive margin sediment and some distance of oceanic crust seaward of it implies that the blocks are allochthonous, having shifted in from some area along the Laurentian craton. The timing of the interaction between the blocks and the Laurentian craton is equivocal. The Moine, deposited between c . 900 Ma and 870 Ma may overlap with the deposition of the Torridonian, but, between 470 Ma and 420 Ma, underwent metamorphism and should have delivered sediment to the Ordovician ( c . 460 Ma) rocks on the craton. Moreover the relationship between the blocks is also questionable. The Dalradian sequence was deposited pre-720 Ma and terminated c . 780–730 Ma. So far, no Moine zircons have been recorded from the Dalradian Supergroup and this is explained by the Moine metamorphism being insufficiently high grade in order to produce zircon. It may also be that the Dalradian and Moine were not united at this time.

A previously inexplicable difference between the Highland Border Complex, Scotland, and its correlative in Ireland, the Clew Bay Complex, is that rocks of Caradoc–Ashgill age occur only in the former. We reject evidence from supposed chitinozoa for this dichotomy: no sedimentary rocks of proven age younger than Arenig occur in the Highland Border Complex. Consequentially, the stratigraphy must be totally recast, and the ‘exotic terrane model’ replaced by one in which a largely autochthonous Highland Border Complex in stratigraphical continuity with the Dalradian (Grampian terrane), was overridden by the Highland Border ophiolite (Midland Valley terrane). Tectonic models for SE Laurentia are thereby considerably simplified.

Mobility of Au and related metals during metamorphism has been suggested to be the source of metals enriched in orogenic Au deposits. This study investigates the mobility of Au, As, and Sb during metamorphism of the Dalradian metasedimentary rocks of Scotland. The metamorphic processes in the Dalradian of Scotland are extremely well studied, and the terrane is an ideal area to investigate mobility of these metals. Our results show that of the 25 major and trace elements analysed, only Au, As, Sb, S and volatile contents as shown by loss on ignition (LOI) values show systematic variation with the metamorphic grade of the samples. Average Au concentrations decrease from 1.1 ± 0.55 ppb and 0.72 ± 0.34 ppb in chlorite and biotite zone rocks down to 0.4 ± 0.22 ppb and 0.34 ± 0.13 ppb in kyanite and sillimanite zone rocks. Average As concentrations decrease from 4.8 ppm (range 0.5 to 17.8 ppm) and 1.96 ± 1.9 ppm in chlorite and biotite zone rocks down to 0.24 ± 0.15 ppm and 0.2 ± 0.12 ppm in kyanite and sillimanite zone rocks. Average Sb concentrations decrease from 0.18 ± 0.15 ppm and 0.11 ± 0.10 ppm in chlorite and biotite zone rocks down to 0.04 ± 0.02 ppm in both kyanite and sillimanite zone rocks. Sulphur and LOI concentrations also show significant decreases. Mass balance calculations indicate that compared to chlorite and biotite zone samples, sillimanite zone samples have an average mass loss of 62 ± 14%, 94 ± 4% and 74 ± 14% for Au, As, and Sb respectively. Every 1 km3 of chlorite–biotite zone mixed psammitic–pelitic protolith rock that is metamorphosed to sillimanite zone conditions would release 1.5 t Au, 8613 t As, 270 t Sb, and 1.02 Mt S. The mobility of these elements is strongly controlled by the paragenesis of sulphide minerals. Pyrite, sphalerite, galena and cobaltite (as well as gersdorffite) decrease in abundance with increasing metamorphic grade in the Dalradian metasedimentary rocks. A critical aspect of the sulphide paragenesis is the transition of pyrite to pyrrhotite. This transition is complete by mid greenschist facies in the Loch Lomond samples but is more gradual at Glen Esk occurring between biotite and sillimanite zones. The Au, As, and Sb content of the sulphide assemblage also decreases with increasing metamorphic grade, and we suggest that this is a controlling factor on the mobility of these metals from the Dalradian metasedimentary rocks during metamorphism. Chlorite may be an important host mineral for As in the greenschist facies rocks. Breakdown of chlorite indirectly drives the mobility of Au, As, and Sb, as this produces the bulk of metamorphic fluid that drives transition between pyrite and pyrrhotite. We suggest that there is potential for significant undiscovered mineralisation in the Central and SW Highlands of Scotland. However, as the total mass of gold mobilised is lower than observed in other metasedimentary terranes such as the Otago and Alpine Schist's, New Zealand, very efficient fluid focussing and trapping mechanisms would be required to form large deposits in the Dalradian of Scotland.

This work was in part supported by Natural Environment Research Council grant NE/J021822/1, National Science Foundation grant EAR-0087650, and funds from our respective institutions. Neoproterozoic siliciclastic-dominated sequences are widespread along the eastern margin of Laurentia and are related to rifting associated with the breakout of Laurentia from the supercontinent Rodinia. Detrital zircons from the Moine Supergroup, NW Scotland, yield Archean to early Neoproterozoic U-Pb ages, consistent with derivation from the Grenville-Sveconorwegian orogen and environs and accumulation post–1000 Ma. U-Pb zircon ages for felsic and associated mafic intrusions confirm a widespread pulse of extension-related magmatism at around 870 Ma. Pegmatites yielding U-Pb zircon ages between 830 Ma and 745 Ma constrain a series of deformation and metamorphic pulses related to Knoydartian orogenesis of the host Moine rocks. Additional U-Pb zircon and monazite data, and 40Ar/39Ar ages for pegmatites and host gneisses indicate high-grade metamorphic events at ca. 458–446 Ma and ca. 426 Ma during the Caledonian orogenic cycle.The presence of early Neoproterozoic siliciclastic sedimentation and deformation in the Moine and equivalent successions around the North Atlantic and their absence along strike in eastern North America reflect contrasting Laurentian paleogeography during the breakup of Rodinia. The North Atlantic realm occupied an external location on the margin of Laurentia, and this region acted as a locus for accumulation of detritus (Moine Supergroup and equivalents) derived from the Grenville-Sveconorwegian orogenic welt, which developed as a consequence of collisional assembly of Rodinia. Neoproterozoic orogenic activity corresponds with the inferred development of convergent plate-margin activity along the periphery of the supercontinent. In contrast in eastern North America, which lay within the internal parts of Rodinia, sedimentation did not commence until the mid-Neoproterozoic (ca. 760 Ma) during initial stages of supercontinent fragmentation. In the North Atlantic region, this time frame corresponds to a second pulse of extension represented by units such as the Dalradian Supergroup, which unconformably overlies the predeformed Moine succession. Publisher PDF

The Trawenagh Bay Granite (TBG) is shown to be a tabular pluton with gently inclined contacts that, from anisotropy of magnetic susceptibility (AMS) studies, was emplaced as a series of flow lobes whose geometries indicate that it flowed horizontally towards the W out of late stage adjacent steeply inclined monzogranite sheets of the Main Donegal Granite (MDG). We thus confirm in detail the central broad idea of the Pitcher & Read (1959) model that the Main Donegal Granite fed the Trawenagh Bay Granite. Early TBG flow lobes cut and are cut by deformation associated with the sinistral shear zone in which the MDG lies, thus demonstrating synchronicity of shearing and magmatism. The TBG magma leaked out of the shear zone and emplaced into undeformed country rocks and was probably guided by shear zone splays that die out along its northern and southern margins. At a late stage in the development of MDG, the splays developed from the NNE-trending SW boundary of the shear zone and caused a gap in this structure through which TBG magma was channelled out of the MDG. A review is presented of the last twenty-five years of published and unpublished work on the batholith, showing that the MDG shear zone was a long-lived structure almost certainly in existence before the emplacement of that body, and that four of the contiguous granitiods (Thorr, Ardara, and Rosses, as well as Trawenagh Bay) were all sourced within the shear zone. A new model is presented for the development of the batholith. The pre-existing crustal structure was a deep-seated N12°E fault in the basement to the Dalradian wall rocks of the granites, that was coupled to up to six other more minor WNW–ESE basement faults in the W. A NE–SW-trending sinistral shear zone was initiated at the end of the Caledonian orogeny, as calc-alkaline and deep-seated appinites were generated in the area. This shearing activated the pre-existing structures at the current crustal level, and the N12°E structure acted as a continental transform fault which allowed the dilation needed to facilitate the wedging space requirements of the MDG and the other units in the shear zone, as well as transferring regional sinistral shear through the system. The Thorr and Ardara plutons were emplaced first into the shear zone and then those magmas leaked out into the adjacent wall rocks: one to form a large laccolith, the other to form a balloon. Steep early MDG complex sheets (granodiorites and tonalities) were emplaced in the shear zone between the Thorr and Ardara emplacement sites. Dilation continued until late stage extensive monzogranite sheets were intruded in the NW and SE of the pluton. One of these probably leaked material westward to form the Rosses laccolith and southwestwards to form the TBG in the final stages of shear zone movement.

The Late Caledonian granites of Donegal are all intruded into metasediments of the Dalradian Supergroup of Neoproterozoic age, which were metamorphosed and deformed during the Grampian Phase of the Caledonian orogeny at c. 470–460 m.y. They were intruded in a singular pulse well after the main tectonic event, apparently peaking at 407–402 m.y.; importantly after the strong collision of Laurentia with Baltica on closure of the Iapetus Ocean. The plutons are mainly made up of granodiorite and granite, and are all 'I' type, but different to Cordilleran ‘I’ types of the eastern Pacific margin. Major element chemistry indicates they are high-K calc-alkaline rocks with a large range in SiO2 content. However three of the plutons (Fanad, Thorr, Ardara), have very high Ba and Sr contents, even higher than Mainland Scotland counterparts; they are high Ba–Sr plutons. Three plutons (Barnesmore, Rosses, Trawenagh Bay) are evolved and are low-Ba–Sr types, while one (Main Donegal) has atypical, intermediate characteristics. The origin of the magmas is still much debated; here we suggest slab breakoffon Iapetus Ocean closure accounts for the special compositions of these magmas and the other major features of Late Caledonian granitic magmatism, including the singular intrusion peak and the associated appinite–lamprophyre suite.

Synopsis The Ben Vuirich intrusion is a small, elongate body of monzogranite that occurs in the Tummel Steep Belt, Perthshire. It was emplaced into Dalradian rocks (Appin Group) at 590 Ma, prior to the D1 phase of the Grampian Event (Caledonian Orogeny), and was strongly deformed during D2. Locally preserved cordierite- and andalusite-bearing hornfelses were altered to garnet ± kyanite-bearing assemblages during post-D2 regional metamorphism. A new study of the lower-grade hornfelses shows that the protolith was an undeformed, fine-grained, parallel-laminated sedimentary unit, confirming that the pluton is pre-orogenic with respect to the Grampian Event. Whole-rock and trace element analyses of 33 samples of the intrusion, together with rare earth elements, Rb–Sr and O-isotope data, show that it is an A 2 -group monzogranite. This finding supports the hypothesis that the granite, emplaced at a depth of 7–14 km, formed in the same extensional tectonic setting as the Tayvallich lavas at 600 Ma. Geochemical and isotope parameters point to a largely crustal source. The intrusion belongs to a swarm of rift-related, A-type granitoids that originally stretched from the Appalachians to Scotland, and includes foliated granitoids in the Moine. The granitoids formed in response to the early break-up of Rodinia, c. 50 Ma before the development of the Iapetus Ocean.

Synopsis New British Geological Survey mapping has examined the stratigraphy and structure of Dalradian strata in the Gaick region of the Central Grampian Highlands of Scotland. In the north of that area, turbiditic strata in the Creag Dhubh Psammite Formation (Corrieyairack Subgroup) pass, via a well-defined sedimentary transition, into the stratigraphically younger Gaick Psammite Formation (Glen Spean Subgroup). This latter formation dominates the lithostratigraphy of the Gaick region, records shallow-water marine shelf conditions throughout, and was probably 1 to 2 km thick prior to deformation. Ordovician (Grampian) orogenesis affected the sedimentary rocks now arranged in a stack of Caledonian recumbent kilometre-scale F2 folds with gently ENE-plunging axes. Regional facing on these recumbent folds is typically sideways to the south. Southeast of the Gaick region, the folds, and thus facing, become progressively inclined and dip to the SE beneath the outcrop of the Appin Group Dalradian. No significant Fl folds, and hence no related facing changes, have been detected within this D2 fold stack. The structure of the Gaick–Drumochter area is therefore essentially a flat belt formed in the D2 deformation event and here named the Gaick Fold Complex. The D2 recumbent structures of this flat belt were rotated and steepened by D3 deformation into the Tummel Steep Belt to the SE, and adjacent to the Glen Banchor High to the NW. Tectonic transport in D2 in the Gaick Fold Complex is interpreted to be oriented on a north–south azimuth, similar to that in the Tay Nappe farther south. Such an interpretation implies that the NE–SW ‘Caledonian trend’ is a consequence of D3 deformation and reorientation, rather than a primary feature of the Grampian Orogeny.

Mineral waters in Britain show a wide range of 87 Sr/ 86 Sr isotope compositions ranging between 8 7 Sr/ 86 Sr = 0.7059 from Carboniferous volcanic rock sources in Dunbartonshire, Scotland to 87 Sr/ 86 Sr = 0.7207 in the Dalradian aquifer of Aberdeenshire, Scotland. The 87 Sr/ 86 Sr composition of the waters shows a general correlation with the aquifer rocks, resulting in the waters from older rocks having a more radiogenic signature than those from younger rocks. This wide range of values means that the Sr isotope composition of mineral water has applications in a number of types of studies. In the modern commercial context, it provides a way of fingerprinting the various mineral waters and hence provides a method for recognising and reducing fraud. From an environmental perspective, it provides the first spatial distribution of bio-available 87 Sr/ 86 Sr in Britain that can be used in modern, historical and archaeological studies.

Scotland has a magmatic record covering much of the period 3100–50 Ma. In this review, we pull together information on Scotland's igneous rocks into a continuous story, showing how magmatic activity has contributed to the country's structural development and assessing whether the effects of older magmatic events can be recognised in later episodes.The oldest igneous rocks are part of supracrustal sequences within the Lewisian Gneiss Complex, formed when Scotland was part of the supercontinent Kenorland. The supracrustal rocks were intruded between 3100 and 2800 Ma by granodiorites and tonalites, which were metamorphosed and deformed in a major tectonothermal event between 2700 and 2500 Ma. The break-up of Kenorland (2400–2200 Ma) was marked by the intrusion of mafic dyke swarms of tholeiitic affinity. The convergence of continental masses to form the supercontinent Columbia resulted, at ∼1900 Ma, in a series of subduction-related volcanic rocks and gabbro–anorthosite masses. Subsequent continent–continent collision formed a series of granite–pegmatite sheets at ∼1855 Ma and ∼1675 Ma and reworked much of the earlier rocks in the amphibolite facies. Columbia was breaking up by 1200 Ma, an event marked by remnants of basaltic magmatism in the NW of the country. Re-assembly of the continental fragments to form the supercontinent Rodinia resulted in the Grenville Orogeny, which in Scotland was marked by basement reworking but no confirmed magmatic activity. Early attempts to split Rodinia produced a rift-related, bimodal, mafic–felsic sequence in the Moine Supergroup of the Northern Highlands, at least some of the mafic rocks having mid-ocean ridge basalt affinities. Crustal thickening during a disputed orogenic event, the Knoydartian, may have caused regional migmatisation. The final break-up of Rodinia occurred in Scotland at ∼600 Ma, when very extensive tholeiitic magmatism characterised the later parts of the Dalradian Supergroup, while a series of granites intruded the Moine and Dalradian successions.Ordovician and Silurian times saw the closure of the Iapetus Ocean and the convergence of Laurentia, Avalonia and Baltica. The collision of a major arc system with Laurentia caused the Grampian event (480–465 Ma) of the Caledonian Orogeny, marked by ophiolite obduction, the generation of (largely) anatectic granites, volcanism in the Midland Valley and Southern Uplands, and intrusion of a major gabbro–granite suite in the NE. The late-Caledonian events (435–420 Ma) were largely post-collisional and were marked by the emplacement of alkaline igneous intrusions in the NW, calc-alkaline granitic intrusions over much of the country, widespread volcanic activity and regional dyke swarms. Laurentia, Avalonia and Baltica amalgamated to form the supercontinent Laurussia. Magmatic activity recommenced at 350 Ma, when intra-plate alkaline magmatism affected much of southern Scotland, in particular, through into Permian times. The alkaline magmatism was interrupted at ∼295 Ma by a short-lived event in which tholeiitic magmas were intruded as sills and dykes in a swarm ∼200 km wide. In the early Palaeogene, lithospheric attenuation related to proto-North Atlantic formation and the splitting of Pangaea was complemented by the arrival of the Iceland mantle plume. Huge volumes of mafic magma were emplaced as lava fields, central complexes and regional swarms, locally increasing crustal thickness by 30%

Synopsis The Green Beds are distinctive chlorite- and epidote-rich metasedimentary rocks, which collectively represent a series of important lithostratigraphical marker units in the Southern Highland Group of the dominantly late Precambrian Dalradian Supergroup of Scotland. In a recent re-evaluation of the nature and origin of the Green Beds, integrated mapping, sedimentological, petrological and geochemical studies have been carried out. The Green Beds form relatively thin but laterally extensive units within thick sequences of siliciclastic metasedimentary rocks, and were deposited from high-density turbidity currents and debris flows in a deep-sea slope apron or ramp setting. They comprise a physical mixture of basic and siliciclastic material, representing periodic influxes of basic igneous detritus into the ‘background’ siliciclastic sedimentary system. The mineralogy of the Green Beds is largely the result of metamorphism, although rare relict detrital minerals from both granitic and basic sources are preserved. Geochemical studies show that the Green Beds and other Southern Highland Group metasedimentary rocks have a similar trace element signature, indicating a common siliciclastic source area. Two suites of Dalradian lavas, the Tayvallich and Loch Avich pillowlavas, were also analysed. Although it is unlikely that these particular volcanic units were the source of the Green Bed basic material, it is believed that the mafic component of the Green Beds was derived from episodes of near-contemporaneous volcanic activity.

Synopsis A U–Pb monazite age of 474 ± 2 Ma has been determined for a weakly foliated granite boulder > 1 m in diameter from the Corsewall conglomerate, SW Scotland. The clast contains xenoliths and partly assimilated xenoliths of regionally metamorphosed rock from which the monazites have been derived. The age constrains the original emplacement of the pluton to between 474 ± 2 and c. 458 Ma, the age of the deposit in which the boulder is found. This age confirms an earlier Rb–Sr muscovite Ordovician age for boulders of this type and contradicts a pre-existing age of 1265 Ma used to suggest a source in Newfoundland and a post-Caradoc major lateral displacement of the Southern Uplands. The monazite age reaffirms that a basement-arc complex once existed north of the Southern Uplands and was the principal contributor of sediment to Ordovician–Silurian basins of southern Scotland. A source for the metamorphic detritus in the Dalradian block is unlikely, as the boulders would have been transported for > 150 km through zones of actively accreting terranes and marginal basins. The possibility of Archaean crust within this basement complex is further suggested by the Sm–Nd systematics of the granite boulders at Corsewall Point.

Provenance studies involving U / Pb analysis of detrital zircons have become increasingly popular through the application of microanalytical techniques. Adopted one-dimensional methods for the presentation of detrital zircon data, such as probability density distribution plots, generally require various degrees of filtering of discordant analyses. However, a uniform approach for filtering does not exist, making comparison of data from different samples and sedimentary units, as well as different studies of detrital populations, extremely difficult. The problem is further complicated by the need to switch from 207Pb / 206Pb based ages to 206Pb / 238U ages when zircon populations within a sample show a range of ages from Archean to Proterozoic or even Phanerozoic. Ion probe analysis of multiple spots within a single zircon crystal and calculation of an internal isochron can potentially eliminate problems associated with discordance, but this is a time consuming approach. Comparison of internal isochron ages for zircons generated by multiple ion probe intra-grain zircon analyses from a sample of psammite within the Dalradian Supergroup of Scotland with a data set from the same sample based on single grain ion probe analysis using various filtering methods suggests good agreement between the two approaches particularly when filtering is based on the probability of concordance. However, when precise information is required, such as the age of the youngest zircon grains so as to accurately constrain the maximum depositional age of the strata, then multiple analyses of individual zircon grains may be required.

Synopsis The Dawros–Currywongaun–Doughruagh Complex of NW Connemara comprises a deformed and metamorphosed suite of syntectonic mafic to ultramafic intrusions within folded, lower amphibolite grade metasediments of the Dalradian Supergroup. Field observations and thin-section petrography have identified examples of exceptionally well-preserved modal layering and primary igneous (cumulus) textures in central areas of two of the intrusions (Dawros and Currywongaun), together with macroscopic graded bedding, slumping, and scour structures that allow layer formation to be attributed to magmatic sedimentation. Layering is occasionally chaotically folded and deformed, and as evidence of associated solid state or plastic deformation is absent, this is thought to be due to synmagmatic deformation. A magmatic fabric that is observed almost everywhere throughout the intrusions generally increases in intensity towards their margins, giving a poorly defined internal strain zonation in both. The presence of layering in these relatively small syntectonic intrusives offers insights into the way that magma chamber processes operate in tectonically active environments by highlighting the structural and petrological constraints these environments impose on layer development. This has significant implications for perceived restrictions on layer development in terms of the types of intrusions they develop in.

Synopsis Growth histories have been determined from microstructural relationships and mineral chemistries of porphyroblasts of muscovite, Mn-rich garnet, almandine, biotite and chlorite. With data for albite, these chart the progressive development of biotite- and almandine-zone mineral assemblages during Barrovian metamorphism of Dalradian pelitic rocks near Loch Lomond in the SW Highlands of Scotland. Dimensionally aligned, fine-grained products of low greenschist facies dynamothermal metamorphism formed during D1 are included in early D2 Mn-rich garnets and in the cores of porphyroblasts of muscovite. Abundant albite porphyroblasts (An 0–3 ) also grew early in D2 enclosing trails of inclusions that define open folds, with some showing two mineral growth fabrics. Almandine grew later during D2 when temperature was at its peak. Matrix grain coarsening, the development of the regionally expressed schistosity and intense dissection of F2 folds took place during late D2. Inclusion-free overgrowths of almandine and plagioclase (An 4–17 ) grew during the end part of D2 to post-D2 along with porphyroblasts of muscovite, biotite and chlorite that are randomly disposed to the schistosity. Growth of the micaceous porphyroblasts is related to a change from the regionally expressed dynamothermal conditions of D2 to regional thermal growth conditions. Their development prior to D3 is shown by S3 minerals cutting across them and across the S2 schistosity. Both S3 and S4 developed during retrogressive low greenschist facies dynamothermal metamorphism. The timing of peak temperatures in the Loch Lomond district differs from that in the type area for Barrovian zones in the SE Scottish Highlands.

Synopsis The Dalradian Supergroup of the Scottish Highlands is a largely metasedimentary succession of Neoproterozoic to Early Cambrian age, metamorphosed during the Caledonian Orogeny. The rocks were deposited on the Laurentian margin during and following the break-up of Rodinia. This rift setting is evidenced, in the upper half of the succession, by the presence of several volcanic sequences. A significant development of these volcanic rocks occurs in the NE Grampian Highlands. There, the Blackwater Formation, within the Argyll Group of the Huntly area, is dominated by basic and ultrabasic metavolcanic rocks, with intercalated metasedimentary rocks. The rocks were metamorphosed in the amphibolite facies, with the Mg contents of their dominant amphiboles apparently reflecting those of the whole-rock protoliths. The protoliths ranged from ultrabasic picritic types (MgO up to 35 wt%) through high-magnesia basalts to basaltic andesites and andesites. The magmas feeding the volcanism were of tholeiitic affinity, broadly similar to metavolcanic rocks elsewhere in the Dalradian. Higher-Nb and lower-Nb groups can be distinguished. The inferred parental magmas were basalts with MgO c. 10 wt%. The ultrabasic rocks formed by accumulation of olivine and minor Cr-spinel within the parental basalts. Some more evolved rocks show evidence of minor (5–10 wt%) accumulation of Fe–Ti oxides. Primary magmas of the Blackwater metavolcanic rocks were generated from a mantle source, or sources, relatively enriched compared to the MORB source. It is speculated that all the Dalradian metavolcanic rocks represent varying degrees of mixing of magmas from this source and a depleted, MORB-like, mantle source.

The geochemistry of metamorphic quartz vein formation in Barrow’s index mineral zones north of Stonehaven, Scotland, was investigated in order to assess regional fluid flow and mass transfer. Metamorphic grade in the Dalradian metasediments increases to the north–northwest away from the Highland Boundary Fault (HBF) and associated ophiolitic rocks of the Highland Border Complex (HBC), passing through the Chlorite (Chl), Biotite (Bt), Garnet (Grt), Chloritoid (Cld), and Staurolite (St) zones. Syn-metamorphic fluid infiltration at 462±8.8 Ma (Breeding et al. in Am Mineral 89:1067, 2004) produced considerable quartz veining. Vein abundance varies from about 5 to 15 volume percent of the outcrops; veins tend to be more abundant in metapelitic layers than in metapsammitic ones. Metamorphic veins are surrounded by centimeter- to decimeter-wide zones of chemical and mineralogical alteration (selvages). Porphyroblasts, particularly Bt, Grt, Cld, and St, are typically larger in selvages than in wallrocks distal to veins. The altered selvages underwent fluid-driven addition of Na, Ca, and Sr, and loss of K, Rb, and Ba. Alteration is most intense within ∼750 m of the HBF, but is still very significant at the northern end of the field area some 2 km away. Mg/FeT (FeT=total iron) was either unchanged or increased due to alteration. Silica was added at some Chl and Bt zone localities near the HBF. Pb mass transfer was variable although Pb was added at a number of locations. Rare Earth elements (REE) were generally immobile, but light REE and possibly heavy REE were lost at one field site. The gain of Na and Ca and loss of K promoted the growth of plagioclase at the expense of micas (particularly muscovite) in selvages and wallrock inclusions throughout the field area and, probably, some calcite and/or dolomite growth directly adjacent to the HBF. The Ca gains were also critical for epidote production in the Bt zone. Gains of Ca and increases in Mg/FeT helped to stabilize Grt at the expense of Cld and St in some selvages. Hornblende and cummingtonite were discovered in strongly altered metapelitic rocks at one Cld zone locality. The metasomatism puts important constraints on the processes of mass transfer and suggests two models for regional fluid flow. In the first model, fluid flow in a direction of increasing temperature downward along the HBF added Na and Ca, and removed K from the Dalradian. In the second model, fluid flow upward from the HBC transported Na and Ca into the overlying Dalradian and, at the same time, stripped out K. The latter model is favored because it can most readily account for silica addition near the HBF, but neither model can be ruled out at present. In either case, the veins represent fractures that transmitted very large time-integrated fluid fluxes of at least ∼104 m3 (fluid)/m2 (rock). Consequently, the veins were conduits for regional fluid flow that caused considerable open-system chemical and mineralogical alteration during metamorphism.

Ion-microprobe U–Pb zircon geochronology demonstrates that magmatic rocks that cut the Slishwood Division are early Ordovician in age and relate to the earliest stages of the Grampian Orogeny. These tonalite and granite intrusions yield ages of 474 ± 5 Ma, 472 ± 6 Ma, 471 ± 5 Ma and 467 ± 6 Ma. A Sm–Nd mineral isochron of 457 ± 36 Ma confirms an Ordovician age for one of the tonalite bodies and demonstrates that they were affected by Ordovician metamorphism whereas biotite Rb–Sr mineral ages from two of the tonalite bodies show that the Slishwood Division had cooled below c. 350 °C at 449 ± 7 Ma. The early orogenic tonalite and granite intrusions are mylonitized in structures that overthrust the Dalradian Supergroup onto the Slishwood Division during major nappe formation and, thus, date the onset of such deformation at or before 471 ± 5 Ma, the age of the youngest intrusion where such field relations can be demonstrated.

Tephra-fall deposits in the Late Mesoproterozoic Sleat Group (Torridonian) from Skye, Scotland, are described for the first time. Two individual beds occur within the Loch na Dal Formation which represents sedimentation in a shallow marine environment. Each bed has a distinctive brown, crumbly, amorphous appearance in the field and has sharp contacts with its host metasediment. This unique lithology is identical to that of albitic schists described recently from the Southern Highland Group, Dalradian Supergroup, which were identified as air-fall tuffs.

Please click here to download the map associated with this article. In Ireland, a major NE-SW trending fault zone (the Fair Head—Clew Bay line) separates high-grade metamorphic rocks (the Dalradian Supergroup) that were originally deposited on an ancient continental margin from lower-grade rocks of oceanic affinity (the Clew Bay Complex). This fault zone continues towards the NE into Scotland, where it is known as the Highland Boundary Fault, and to the west into Newfoundland and the Appalachians, where it is termed the Baie Verte—Brompton line. As such, it is one of the longest fault zones in the Caledonian/Appalachian mountain belt. It has been active several times during this mountain-building episode, and has thus played an important role during the evolution of this orogenic belt. In Scotland and most of Ireland, this fault zone is generally very poorly exposed, and the relationships between the rock units on either side of this discontinuity have therefore proved controversial in the past. ...

The heavy mineral components of Ordovician greywacke turbidites of the Southern Uplands (subduction-accretion prism) Terrane were studied using high-resolution heavy mineral analysis. Diagnostic heavy minerals and the discovery of hitherto unreported species provide a new insight into the interplay of sediment fluxes, tectonic controls and provenance. Each formation is characterized by distinctive heavy mineral suites, indicating that the sediments of successive or laterally-equivalent formations were sourced from different and rapidly-changing lithologies. Appreciable variations were detected even in greywacke packages within an individual formation, indicating that these changes were taking place at a very fast rate. Detritus came from either an eroding Dalradian or a similar Barrovian metamorphic complex and its superjacent obducted ophiolite complex to the northeast, and from oceanic island arcs and collapsing seamounts to the south. The rapid appearance of metamorphic detritus, following a short period of ophiolite stripping, indicates that post-subduction-flip exposure of and erosion into the metamorphic complex was fast, suggesting extensional unroofing. The chemical similarity of detrital garnet with Scottish Grampian garnet may suggest derivation from that complex and, therefore, that the Southern Uplands Terrane has not travelled, sinistrally, more than about 200 km with respect to the Scottish Highlands. The absence of detrital staurolite in the greywackes raises the possibility that stripping either did not reach abundant staurolite levels or that staurolite was a relatively uncommon mineral in the source area. The former explanation is unlikely because, in western Ireland, a similar Dalradian stripping sequence involves the appearance of staurolite in the first arrival of metamorphic detritus. However, possibly, in western Ireland, the Dalradian source was extensionally unroofed to a much deeper structural level than was Scotland in the same time interval. Only the Shinnel Formation shows spatial detrital consistency, indicating that, by Shinnel times, the Dalradian hinterland had achieved a relatively uniform level of erosion. The discovery of lawsonite, as part of a ferroglaucophane-glaucophane blueschist assemblage in the Portpatrick Formation, is important yet enigmatic. Ordovician crosstie-winchite-ferroglaucophane-phengite assemblages are known throughout the northwestern Appalachian/Caledonian belt and may have provided the source for the Portpatrick blueschist-facies detritus. However, a possible source was from the Cadomian blueschist belt of Anglesey, then on the southern margin of the Iapetus Ocean. This is supported by garnets with sodic/calcic amphibole inclusions, unknown in the Dalradian. Perhaps Cadomian ‘terrane fragments’ were rifted from the southern margin of Iapetus to drift towards and collide with Laurentia to shed detritus into the Laurentian margin trench.

Geoff Tanner writes: Firstly, I would like to congratulate the authors for presenting a very carefully documented, and splendidly illustrated, account of the origin of the Stralinchy conglomerates. The aim of this contribution is to focus on one important question: is their controversial model for the Donegal Dalradian supported by studies of the stratigraphically equivalent Dalradian rocks of the SW Highlands of Scotland? If an orogenic unconformity is present in the Dalradian Supergroup, evidence for this should be found at the base of, or within, the Easdale Subgroup in this region. The field relationships seen at this stratigraphical level in Scotland are reviewed briefly, and an attempt made to reconcile the results with the Donegal model. Jura Quartzite–Easdale Subgroup relationship in SW Highlands of Scotland. The contact between the Jura Quartzite and the Scarba Conglomerate Formation (which represents the base of the Easdale Subgroup) is best seen in excellent coastal exposures between Jura and Loch Creran. It is exposed on the SE side of the island of Jura; is excised by faulting on the islands of Scarba and Lunga; and re-appears on the SE side of the Rubha Aird Peninsula, 7 km north of Oban. At Lussagiven on Jura [NR 6387 8666], the top of the Jura Quartzite is marked by cross-bedded quartzite followed by 23 m of thinly bedded, commonly gritty, sandstones with conglomerate lenses. These beds, which display cross-lamination, graded bedding and channel structures, pass upwards with perfect conformity into a 36 m thick unit of laminated slate with sandstone ribs, the Jura Slate member of the Scarba Conglomerate Formation. Gritty sandstone and conglomerate beds up to 1 m thick, that characterize the Scarba Conglomerate, appear 280m farther up sequence They contain pebbles to 3 cm; rip-up clasts of mudrock; and less common rafts of sandy sediment up …

The Palaeoproterozoic to early Neoproterozoic Annagh Gneiss Complex structurally underlies the Dalradian sequence in north Mayo, Ireland, and has been proposed as the depositional basement to Dalradian metasediments. The Annagh Gneiss Complex was deformed, metamorphosed and migmatized during the Grenville Orogeny and later reworked under amphibolite-facies conditions. This paper focuses on the timing of the post-Grenville events and particularly on the possible presence of post-Grenville, pre-Grampian deformation that could be attributed to the Knoydartian Orogeny. Seven U–Pb titanite analyses from Annagh Gneiss Complex gneisses have a weighted mean 207 Pb/ 206 Pb age of 963 ± 8 Ma, which dates cooling after the main Grenville metamorphism. Locally, a later phase of titanite growth at 943 ± 8 Ma post-dates the last phase of Grenville deformation. The weak discordance of the titanite data suggests that post-Grenville events had little effect on the U–Pb system in titanite. If the discordance was caused by a tectonic event, this is likely to have occurred during the early Ordovician Grampian Orogeny rather than in the Neoproterozoic. Within the Annagh Gneiss Complex, cross-cutting metadolerites provide a structural marker allowing post-Grenville deformation to be distinguished. In contrast, correlative metadolerites cutting the adjacent Dalradian metasediments share all Grampian deformation events affecting their host. Ar–Ar hornblende ages from the post-Grenville metadolerites indicate that reworking of the Annagh Gneiss Complex and the first episodes of Dalradian deformation occurred during the Grampian Orogeny in this part of Ireland. One sample yields a 475 ± 4 Ma Ar–Ar plateau age, which is interpreted to date Grampian deformation. Younger Ar–Ar hornblende and Rb–Sr mica ages record post-Grampian cooling. Neither field nor isotopic evidence for the Knoydartian Orogeny has been found in this part of Ireland.