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

The Paleoproterozoic schists of the Leverburgh Belt, South Harris and the Neoproterozoic\ud carbonaceous metasediments of the Dalradian Supergroup were deposited during the two most\ud significant periods of black shale deposition globally. Hosted within these metasedimentary rocks\ud are graphite-bearing mineralised plutons, formed during orogenic events. The assimilation of\ud carbonaceous lithologies during magmatic pluton emplacement is a commonly recognised mechanism\ud in the formation of many metal and semi-metal-enriched deposits. Graphite mineralisation as a result\ud of carbon assimilation is a feature often associated with these mineral deposits, though the source of\ud the carbon and any associated metal deposits is not always understood. In this study, carbon and\ud sulphur isotope analyses demonstrate that the crustal assimilation of the Paleoproterozoic host rocks\ud took place during magmatic emplacement and provided the source of carbon and sulphur during\ud mineralisation of the plutons. Minor enrichments of trace elements are present in the South Harris\ud plutonic lithologies, indicating that mobilisation and enrichment occurred during assimilation of\ud the schists. Petrographic and elemental analysis of a Dalradian-hosted Ordovician pluton indicates\ud a similar but more substantial enrichment of these trace elements during crustal assimilation. The\ud timing and depth of assimilation appear to play key roles in the extent of graphite and associated\ud trace element enrichments.

Sediment provenance analysis is important in reconstructing chronological and spatial relationships between source area erosion/exhumation and sediment deposition in adjacent basins. Here we provide new provenance data from conglomerate clast populations, sandstone petrography, heavy mineral assemblages and U–Pb detrital zircon geochronology from the 9 km-thick, Siluro-Devonian Lower Old Red Sandstone (LORS), northern Midland Valley Basin (MVB), Scotland. The MVB developed in the foreland to the Caledonian Orogeny and comprises a mixed succession of conglomerate and pebbly sandstone interbedded with volcanic lithologies. Analysis of 554 samples (137 petrographical thin sections across 22 formations/members; 390 conglomerate clast recordings across 7 locations; 17 heavy mineral and 10 detrital zircon samples) from the entire succession shows a proximal, consistent source or sources located to the east or NE. These results indicate that sediment derived directly from areas of Scandian uplift of Norwegian lithologies is unlikely to have made a significant contribution to the LORS of the northern MVB and the provenance instead lies more proximally in both the neighbouring Dalradian Supergroup metasedimentary rocks and other contemporaneous lithologies. The coarse-grained nature of the sedimentary rocks suggests consistent and continued uplift of these lithologies throughout deposition of the LORS. Integration of these provenance techniques allows robust interpretation of basin fill from a tectonically rejuvenating source in the Caledonian foreland. Supplementary material: MVB LORS palaeocurrent data; List of sample locations is available at https://doi.org/10.6084/m9.figshare.c.4661966

The circum-North Atlantic region archives three major late-Mesoproterozoic to Neoproterozoic tectonic episodes, the Grenville-Sveconorwegian and Renlandian orogenies followed by rifting and formation of the Iapetus Ocean, and each is bracketed by sedimentary successions that define three megasequences. In this context, we summarise sedimentological and geochronological data and propose a new stratigraphic framework for the iconic Torridonian-Moine-Dalradian successions and related units in Scotland. The Iona, Sleat, Torridon and Morar groups of the Scottish mainland and Inner Hebrides, and the Westing, Sand Voe and Yell Sound groups in Shetland, form the newly named Wester Ross Supergroup. They were deposited c. 1000–950 Ma within a foreland basin to the Grenville Orogen and, collectively, are in Megasequence 1. Some of these units record Renlandian orogenesis at c. 960-920 Ma. The newly named Loch Ness Supergroup consists of the Glenfinnan, Loch Eil and Badenoch groups of the Scottish mainland, deposited c. 900–870 Ma and are assigned to Megasequence 2. These units record Knoydartian orogenesis c. 820-725 Ma. The regionally extensive Dalradian Supergroup belongs to Megasequence 3; it was deposited c.

Temporary excavations during the construction of the Glendoe Hydro Scheme above Loch Ness in the Highlands of Scotland exposed a clay-rich fault gouge in Dalradian Supergroup psammite. The gouge coincides with the mapped trace of the subvertical Sronlairig Fault, a feature related in part to the Great Glen and Ericht–Laidon faults, which had been interpreted to result from brittle deformation during the Caledonian orogeny (c. 420–390 Ma). Exposure of this mica-rich gouge represented an exceptional opportunity to constrain the timing of the gouge-producing movement on the Sronlairig Fault using isotopic analysis to date the growth of authigenic (essentially synkinematic) clay mineralization. A series of fine-size separates was isolated prior to K–Ar analysis. Novel, capillary-encapsulated X-ray diffraction analysis was employed to ensure nearly perfect, random orientation and to facilitate the identification and quantification of mica polytypes. Coarser size fractions are composed of greater proportions of the 2M1 illite polytype. Finer size fractions show increasing proportions of the 1M illite polytype, with no evidence of 2M1 illite in the finest fractions. A series of Illite Age Analysis plots produced excellent R2 values with calculated mean ages of 296 ± 7 Ma (Late Carboniferous–Early Permian) for the oldest (2M1) illite and 145 ± 7 Ma (Late Jurassic–Early Cretaceous) for the youngest (1M) illite. The Late Carboniferous–Early Permian (Faulting event 1) age may represent resetting of earlier-formed micas or authigenesis during dextral displacement of the Great Glen Fault Zone (GGFZ). Contemporaneous WNW(NW)–ESE(SE) extension was important for basin development and hydrocarbon migration in the Pentland Firth and Moray Firth regions. The Late Jurassic–Early Cretaceous (Faulting event 2) age corresponds with Moray Firth Basin development and indicates that the GGFZ and related structures may have acted to partition the active extension in the Moray Firth region from relative inactivity in the Pentland Firth area at this time. These new age dates demonstrate the long-lived geological activity on the GGFZ, particularly so in post-Caledonian times where other isotopic evidence for younger tectonic overprints is lacking.

Reconstructing regional geological histories is challenging where basins have limited and/or fragmentary preservation. Several isolated outlier basins of Lower Old Red Sandstone (LORS) occur on the Scottish Grampian terrane; however, their sedimentology and relationship to other similar-aged LORS deposits has been poorly constrained, as has their significance in the wider Caledonian orogenic framework. Here we present a combination of new sedimentological and multidisciplinary provenance analyses for the LORS outliers at Aberdeen, Cabrach, New Aberdour, Rhynie and Tomintoul. Three facies associations are identified, the deposits of locally derived conglomeratic alluvial fans, which pass upward into fluvial channel and floodplain facies associations. Provenance data from conglomerate clast populations, sandstone petrography, heavy mineral assemblages, and U–Pb detrital zircon geochronology indicate derivation from the surrounding Dalradian Supergroup, with local influence from contemporaneous volcanic or plutonic lithologies. These data suggest a similar provenance to that of the northern Midland Valley Basin LORS, and a direct relationship between the LORS north and south of the Highland Boundary Fault. This indicates that the preserved outliers represent fragments of wider, unpreserved LORS cover that accumulated between the late phases of the Caledonian Orogeny and onset of post-orogenic collapse in the Mid-Devonian.

Early Ordovician collision of the Lough Nafooey Arc (part of the Baie Verte Oceanic Tract) with the passive continental margin of Laurentia peaked at c. 475 Ma in Scotland and Ireland and was followed by subduction polarity reversal. We examined Upper Ordovician–Silurian sedimentary rocks from western Ireland to see whether collision was followed by renewed arc magmatism. Despite the scarcity of dated igneous intrusions between the Grampian (c. 470 Ma) and Acadian (c. 420 Ma) orogenies in Ireland, detrital zircons show a continuity of activity peaking at 480–440 Ma, implying no hiatus in regional magmatism. Differences in zircon U–Pb age spectra highlight the isolation of basins in the southern Killary Harbour area from those north of the South Mayo Trough. These latter rocks were largely derived by erosion from Moine and Upper Dalradian sources. By contrast, the Killary Harbour Basin shows a decreasing influence from the Dalradian after c. 436 Ma and an increasing influence of contemporaneous magmatic zircons. These were transported from sources along-strike from the present NE, probably at the southern end of the Scandian Mountains in SE Greenland. The western Irish basins formed as pull-apart basins in a forearc setting and are analogous to Cenozoic pull-apart basins in Sumatra. Supplementary material: U-Pb zircon analytical data is available at a decreasing influence from the Dalradian after c. 436 Ma and an increasing influence of contemporaneous https://doi.org/10.6084/m9.figshare.c.5209849

The Loch Bà ring-dyke and the associated Centre 3 granites represent the main events of the final phase of activity at the Palaeogene Mull igneous complex. The Loch Bà ring-dyke is one of the best exposed ring-intrusions in the world and records intense interaction between rhyolitic and basaltic magma. To reconstruct the evolutionary history of the Centre 3 magmas, we present new major- and trace-element, and new Sr isotope data as well as the first Nd and Pb isotope data for the felsic and mafic components of the Loch Bà intrusion and associated Centre 3 granites. We also report new Sr, Nd and Pb isotope data for the various crustal compositions from the region, including Moine and Dalradian metasedimentary rocks, Lewisian gneiss, and Iona Group metasediments. Isotope data for the Loch Bà rhyolite (87Sr/86Sri = 0.716) imply a considerable contribution of local Moine-type metasedimentary crust (87Sr/86Sr = 0.717–0.736), whereas Loch Bà mafic inclusions (87Sr/86Sri = 0.704–0.707) are closer to established mantle values, implying that felsic melts of dominantly crustal origin mixed with newly arriving basalt. The Centre 3 microgranites (87Sr/86Sri = 0.709–0.716), are less intensely affected by crustal assimilation relative to the Loch Bá rhyolite. Pb-isotope data confirm incorporation of Moine metasediments within the Centre 3 granites. Remarkably, the combined Sr–Nd–Pb data indicate that Centre 3 magmas record no detectable interaction with underlying deep Lewisian gneiss basement, in contrast to Centre 1 and 2 lithologies. This implies that Centre 3 magmas ascended through previously depleted or insulated feeding channels into upper-crustal reservoirs where they resided within and interacted with fertile Moine-type upper crust prior to eruption or final emplacement.

The Hebridean islands of Islay, Jura and Colonsay are mainly composed of Neoproterozoic metasedimentary rocks of the Dalradian supergroup, whereas Tiree and Coll are underlain by Archaean rocks of the Lewisian Gneiss Complex. Most of the terrain is low-lying; only the three peaks of the Paps of Jura exceed 600 m. Successive episodes of ice-sheet glaciation have produced areas of ice-moulded topography, and retreat of the last (Late Devensian) ice sheet was accompanied by the deposition of outwash deposits and moraines, including the longest medial moraine in Scotland. The most impressive and internationally renowned landforms on the islands are those related to coastal erosion and deposition: high-level glacimarine deposits; ancient (possibly Pliocene) glaciated strandflat; extensive areas of high rock platforms and intertidal platforms that demonstrably pre-date the last ice sheet; an isostatically tilted rock platform that formed under Lateglacial periglacial conditions; magnificent flights of Lateglacial shingle ridges extending up to 37 m above present sea level; and lower Holocene raised beaches locally backed by sand dunes and areas of machair (calcareous aeolian shell-sand) deposits. Inland, the quartzite flanks of the Paps of Jura are scarred by paraglacial landslides possibly triggered by seismic activity that accompanied rapid glacio-isostatic uplift.

The Connemara Marble of Ireland is a distinctive green decorative stone used in world architecture. These stones were imported and widely used in the Carnegie Institute Extension built in 1907 which is acknowledged as one of the America's finest Gilded Age Beaux-Arts structures. This report documents the use, quantities, and locations of Connemara Marble in the Carnegie Institute and through a review of historic primary source documents, secondary sources articles, on-site inspections, sampling, testing, and microscopic analysis has determined the stone's mineralogy and petrology, color types, and the specific location and identify of the actual quarry site and the lithostratigraphy from which these materials originated. In the 1970's the Carnegie Institute was listed in the National Register of Historic Places by the United States Department of the Interior and named a Historic Landmark by the Pittsburgh History Landmarks Foundation. In 2019, the Connemara Marble was proposed as a Global Heritage Stone Resource to the International Union of Geological Sciences. The Carnegie Institute incorporates 32 varieties of igneous, metamorphic, and sedimentary rocks types from Algeria, Croatia, France, Greece, Italy, Ireland (i.e., the Connemara Marble), and the United States of America. The building's architects utilized four-color types of Connemara Marble in 13 prominent interior spaces of the building. The Connemara Marble formed from Precambrian Dalradian carbonates during the lower Ordovician Grampian orogeny, some 470 million years ago in western Ireland. Approximately, 366 square meters of floor tiles and wall inlays were fabricated out of some 60 metric tons of Connemara Marble blocks extracted from the Streamstown quarry near Clifden, County Galway.

During metamorphism, large quantities of fluids are mobilized and transported through the crust. Such fluids may interact with surrounding rock and serve as a medium for chemical transport. In this study, we use coupled carbonation and oxygen isotope fronts to estimate fluid and carbon fluxes for metamorphic fluid flow in vein pathways in the Dalradian metasediments that crop out in Glen Esk, SE Scottish Highlands. Comparison of wall rock and vein oxygen isotope ratios indicate that the Barrovian sequence in Glen Esk was infiltrated by at least two separate fluid flow events, where fluid flow from the North Esk Fault overprints an earlier fluid flow event, for which the syn-metamorphic magmatic intrusions north of Glen Esk are a probable fluid source. Advection is the dominating mode of fluid transportation in veins. By advection-dispersion and advection-diffusion modeling, we estimate a time-averaged fluid flux of 0.0005 to 0.0135 m3 · m−2 · yr−1 and a carbon flux of 0.04 to 0.71 mol C · m−2 · yr−1for fluid flow from the North Esk fault into the Dalradian metasediments. The duration of this fluid flow event is estimated to between 11 and 230 kyr. Our results also indicate that hydrodynamic dispersion was the main reason for broadening of the oxygen isotope front.

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.

Detrital zircons from four samples of upper Dalradian metasedimentary rocks from the Buchan Block in the NE Grampian Highlands of Scotland were analysed by laser ablation inductively coupled plasma mass spectrometry to establish their U–Pb age and trace element composition. The analysed grains (magmatic cores) mainly yield concordant ages ranging from Neoproterozoic to Eoarchaean. Kernel density plots of the data show pronounced peaks in the late Mesoproterozoic, Palaeoproterozoic and Neoarchaean eras. The data are indistinguishable from detrital zircon age spectra from Dalradian rocks elsewhere, an interpretation supported by application of a non-parametric multidimensional scaling algorithm, and are consistent with a Laurentian source. Similar to existing studies from other Dalradian rocks, the age spectra from the Buchan Block reveal an increase in the relative proportion of older detritus with time, suggesting derivation from late Mesoproterozoic (Grenville) then Palaeoproterozoic orogens before widespread exposure and denudation of their Archaean basement rocks. Application of a novel approach to estimate the most likely time of radiogenic-Pb loss indicates that some detrital zircon grains were affected by element mobility around 470–450 Ma as a result of Grampian orogenesis. Supplementary material: Laser ablation inductively coupled plasma mass spectrometry U–Pb and trace element data, a matrix showing results of Kolmogorov–Smirnov (K–S) test, cathodoluminescence imaging of zircons and selected trace element plots are available at http://doi.org/10.6084/m9.figshare.c.3153931

The main conclusion of this study is that non-coaxial strain acting parallel to a flat-lying D 1 spaced cleavage was responsible for the formation of the D 2 spaced crenulation (shear band) cleavage in Dalradian rocks of Neoproterozoic-Lower Ordovician age in the SW Highlands, Scotland. The cm-dm-scale D 2 microlithons are asymmetric; have a geometrically distinctive nose and tail; and show a thickened central portion resulting from back-rotation of the constituent D 1 microlithons. The current terminology used to describe crenulation cleavages is reviewed and updated. Aided by exceptional 3D exposures, it is shown how embryonic D 2 flexural-slip folds developed into a spaced cleavage comprising fold-pair domains wrapped by anastomosing cleavage seams. The bulk strain was partitioned into low-strain domains separated by zones of high non-coaxial strain. This new model provides a template for determining the sense of shear in both low-strain situations and in ductile, higher strain zones where other indicators, such as shear folds, give ambiguous results. Analogous structures include tectonic lozenges in shear zones, and flexural-slip duplexes. Disputes over the sense and direction of shear during emplacement of the Tay Nappe, and the apparently intractable conflict between minor fold asymmetry and shear sense, appear to be resolved.

The petrochemistry of the major elements (33 new rock analyses) of the Dalradian Glencolumbkille metadolerite sills, now amphibolite, shows them to have been quartz tholeiites like those of most of Donegal. Their compositions overlap with those of Connemara (Co. Galway) and Knapdale (Scotland), except the latter two also extend into the olivine tholeiite field. The syn-D2 (~470Ma) sheared schistose margins of the Glencolumbkille bodies are garnet amphibolites, unlike the generally garnet-free centres, and have suffered fluid-derived metasomatism and garnet growth. Garnets are postulated to have been nucleated by the activation energy from the shearing, but the main garnet growth and metamorphism was under static post-D2 pre-D3 conditions in certain favourable rock compositions. Garnet growth in the Glencolumbkille and Connemara amphibolites was generally restricted to rocks with low Mg/Fe, low Fe2O3/FeO and high MnO compositions, whether original or metasomatic. The long-standing puzzle as to why amphibolites that were originally dolerite sills in the Dalradian succession are clustered at certain horizons (at Glencolumbkille in and near the 654–635Ma Marinoan glaciation Portaskaig Tillite Formation) is examined. The clustering is suggested to be partly explained by recent structural research showing that the intrusion of widespread lateral sheets of basaltic magma is restricted to closely interbedded competent beds with thin incompetent pelites. The former act as lids, while the latter allow lateral expansion of the original sills. The whole magmatic suite studied intruded sediments deposited before 600Ma. It is thought to be part of the major ~600Ma magmatism and crustal extension that affected the Scottish Highlands and eastern North America as the supercontinent Rodinia split apart and the Iapetus Ocean opened. Of the metabasites, only the Knapdale ones have been U-Pb dated, yielding a magmatic 600Ma age and thus definitely shown to be pre-D1.

As discussed in Chapter 11 of this Report, some authors have considered that the uppermost part of the Southern Highland Group of the Dalradian in Scotland might be Lower Palaeozoic, possibly Early Cambrian to Early Ordovician. The age of the Irish Dalradian is also not fully clear (Daly 2001). Thus, in Ireland, strata that are clearly Cambrian in age are restricted in outcrop, being confined to some areas of the southeast (Fig. 16) in the Leinster Terrane (Murphy et al. 1991; Woodcock 2000; Holland 2001, 2009). There they comprise the Bray and Cahore Groups and part of the Lower Palaeozoic Ribband Group (Figs 18, 19). In addition, in southernmost Leinster (Fig. 20), palynological studies have now shown that the Cullenstown Formation is Cambrian. In the same area, the Ballycogly Group mylonites, which occur along the boundary between the Leinster Terrane and the Precambrian basement of the Rosslare Terrane, are considered to be Cambrian (Tietzsch-Tyler & Sleeman 1994a) although there is no direct biostratigraphical evidence.

The Port Askaig Formation (PAF) is a diamictite-bearing succession in the Dalradian Supergroup of Scotland that provides an excellent archive of a Cryogenian glaciation in the Garvellach Islands and Islay, Argyll. The formation is ∼1100 m thick, comprises 5 members and includes 47 diamictite beds, interbedded with siltstones, dolostones and sandstones. Here we document seven features of the PAF that indicate its relative stratigraphic completeness. There are gradual, progressive changes up-section in the lithologies of the diamictites, their interbeds, and clast lithologies. The sharp basal surfaces of the diamictites each show the same, repeated pattern of environmental change, from non-glacial to glacial. Many of the top surfaces of the diamictites show evidence of periglacial conditions. The succession in the PAF records a total of 76 climatically-related stratigraphic episodes: 28 glacial episodes, 25 periglacial episodes and 23 non-glacial episodes. Parts of Member 1 (Diamictites 1–12 and Diamictites 16–18) and Member 2 (Diamictite 31 to the base of Member 3) are most compete on the east coast of Garbh Eileach. The PAF in the Garvellach Islands occurs within a succession that is several kilometres thick, as newly revealed by sea-floor mapping. Compared with other Cryogenian and Phanerozoic glacial successions, the PAF is exceptional in its combination of formation thickness, the number of climatically-related stratigraphic episodes, and the considerable thickness of its host supergroup. Furthermore, these indicators of relative stratigraphic completeness provide evidence that the base of the PAF on the east coast of Garbh Eileach is a succession without a major break in deposition, supporting the account of the strata at and below the base of the PAF in the companion article by Fairchild et al. (2018) .

The final stages of the emplacement of the Dalradian block into Scotland are recorded in the Old Red Sandstone rocks that lie faulted against the main Dalradian belt. In the case of the Balmaha–Queen Elizabeth Forest sequence this involves the Gualann Fault, a splay of the Highland Boundary Fault, which transects c . 6.5 km of Lower Old Red Sandstone conglomerates and sandstones between Balmaha and Aberfoyle. A sequence of coarse conglomerates at Balmaha, near the top of the succession, derived from the SE, was deposited unconformably on the Highland Border Complex. This conglomerate has been studied in detail and the sequence is interpreted as an oblique section through an alluvial fan. Fitting the average dimensions of other alluvial fans to the unfolded cross-section suggests that it once extended some 5 km to the north, over the position of the Dalradian block. The conglomerate–sandstone sequence beneath was also deposited in alluvial fans that accumulated in a strike-slip, extensional basin in which units of conglomerate become progressively younger to the SW. Immediately NW of the Gualann Fault, in the Queen Elizabeth Forest, sub-horizontal red sandstones and breccias rest unconformably on the Highland Border Complex and were probably deposited beyond the reach of the coarse conglomerates. It is suggested that the Gualann Fault was initially a very low-angle or flat thrust, which was rotated in mid-Devonian times during the development of the steep NW limb of the Strathmore Syncline. It brought sub-horizontal breccias and sandstones, originally deposited 10–15 km to the NW, well out of the influence of the conglomerates on the SE side of the fault, into juxtaposition with folded coarse conglomerates south of the Fault. As the Gualann Fault is a splay of the Highland Boundary Fault this implies that the latter may also be a thrust that brought the Dalradian block over the Highland Border rocks. The whole sequence, from Balmaha to the Queen Elizabeth Forest, reflects the unroofing of an existing source of conglomerates to eventually erode their quartzite basement. The Highland Boundary Fault, although a fairly straight and prominent feature to the NE, becomes displaced and difficult to follow in the area to the SW where it is accompanied by many splays trending roughly N–S. The area, in and around the Firth of Clyde, has been regarded as the product of a locked phase of the fault movement that produced an extensional basin in Upper Old Red Sandstone times, but is also likely to have generated the more northern basins seen here. There are a few records of conglomerates and associated rocks cut by the Highland Boundary Fault to the NE, suggesting that Old Red Sandstone basins extended well beyond the present Highland Boundary Fault. These basins may also have relevance to the timing of the incoming of the Dalradian block, absent from Scotland during earlier periods.

Calcite pseudomorphs have replaced euhedral ikaite (CaCO 3 .6H 2 O) porphyroblasts in Dalradian calcareous slates and metadolostones of western Scotland, with a volume decrease of at least 47%. Porphyroblast–fabric relationships indicate that the initial growth of ikaite post-dates a penetrative tectonic fabric developed during upright folding. This is the first reported occurrence of metamorphic ikaite porphyroblasts and points towards growth within the slates during an ultra-low-temperature metamorphism with an exceptionally low geothermal gradient. This event is associated with the penetration of long-lived and extreme permafrost deep into subaerially exposed bedrock during Neoproterozoic glaciation. The presence of the well-preserved pseudomorphs within the Easdale slates of the Argyll Group implies that a Neoproterozoic orogenic unconformity exists above the stratigraphic position of these rocks.

Stable isotope ratios can provide important evidence for estimating groundwater temperatures during the formation of clay minerals in response to chemical weathering of rocks at the landsurface. In this paper, we investigate weathering kaolins found in Buchan, NE Scotland. Stable oxygen and hydrogen isotopes for kaolins from weathered clasts in post-Cretaceous fluvial gravels and from clay-rich saprolites developed in Dalradian metamorphic and Caledonian igneous rocks indicate weathering under warm groundwater temperatures of 23 ± 5 °C. Comparisons with Cenozoic palaeotemperatures derived from North Sea sediments indicate that weathering took place under humid tropical climates of the Palaeocene–Eocene. The > 25 m deep weathering profiles in the Buchan Gravels indicate that these coarse, quartzite- and flint-bearing gravels are older than previously thought and were likely deposited by rivers which also fed thick Palaeocene–Early Eocene deep-water sand fans in the western North Sea basin. A later phase of kaolin weathering at lower temperatures of 15 ± 5 °C took place in the Middle Miocene. Kaolinitic weathering remnants are largely confined to elevations of > 100 m in eastern Buchan and define a well-preserved, deeply weathered Cenozoic landscape in north-east Scotland.

This paper presents recently collected swath bathymetry from the Firth of Lorn. 553 km2 of data were collected during 2012–2013 as part of the INIS Hydro (Ireland, Northern Ireland and Scotland Hydrographic Survey) programme.The area proves to consist of bedrock-dominated seabed, divided into narrow, stratigraphically-constrained and glacially-over-deepened basins. The bedrock is composed of late Proterozoic Dalradian metasediments overlain unconformably by Old Red Sandstone (ORS) sediments and lavas of ?Silurian-age. The central region of the Firth of Lorn is dominated by a vertical cliff, up to 150 m high and extending for approximately 24 km. This feature, here termed the Insh Fault, may have originated as a Dalradian extensional fault, been reactivated as an ORS feature and now forms a fault-line scarp with resistant ORS rocks on the downthrown side, flanking the more deeply eroded metasediments exposed in the basin. Tertiary intrusives are common, in particular, swarms of Paleocene dolerite dykes exposed on the sediment-free bedrock surfaces, and can be traced for many kilometres.Evidence for past glaciation is widespread, manifest in the extensive erosion of the bedrock platforms and the abundance of well-preserved moraines and over-deepened basins. The survey region includes the Corryvreckan Whirlpool and Great Race, beneath the tidal flows of which occur submarine dunes.

Synopsis Most clast lithologies within the basal conglomerate of the Iona Group, exposed on Iona, Inner Hebrides, SW Scotland, resemble the adjacent Archaean–Palaeoproterozoic Lewisian Gneiss Complex, supporting the hypothesis that the Iona Group unconformably overlies Lewisian gneisses on Iona. Moreover, a whole-rock Sm–Nd t DM age of 2512 Ma from one of these clasts supports its lithological correlation with granitic pegmatites in the underlying gneisses, while a second quartz-rich paragneiss clast with a Sm–Nd t DM age of 2745 Ma, is similar to Lewisian paragneisses on Coll. However, whole-rock t DM ages of 1755–2060 Ma and pronounced c . 1780–1800 Ma peaks in U–Pb detrital zircon and titanite data for Iona Group metasediments suggest additional distal sources of Laurentian affinity including the Rhinns Complex as well as Labradorian, Pinwarian and Archaean detritus. The Iona Group was deposited and deformed after c . 1490 Ma and before c . 422 Ma, constrained by the youngest detrital zircon and titanite and the intrusion of the Ross of Mull Granite, respectively. The detrital data permit correlation of the Iona Group with the Colonsay Group and the Glenshirra Subgroup of the Dalradian Grampian Group, although correlation with parts of the Torridonian cannot be excluded. Supplementary material: Full analytical results areavailable at http://www.geolsoc.org.uk/SUP18768.

We appreciate funding and support from the Research School of Earth Sciences, Australian National University for the SHRIMP analyses performed there. The type locality for high-temperature, low-pressure regional metamorphism, the Buchan Block in NE Scotland, exhibits profound differences to the rest of the Grampian Terrane. These differences have led some to regard the Buchan Block as an exotic crustal fragment comprising Precambrian basement gneisses and cover rocks thrust into their current position during Grampian orogenesis. Although rocks of the Buchan Block are now generally correlated with Dalradian strata elsewhere, the origin of the gneisses and the cause of the high heat flow and associated magmatism is debated. We report SIMS U–Pb and Hf isotopic data in zircon from high-grade rocks from the northeast (Inzie Head Gneiss) and northwest (Portsoy) corners of the Buchan Block. Around Inzie Head, upper amphibolite to granulite facies metasedimentary gneisses coexist with diorite sheets that were emplaced contemporaneously with partial melting of their host rocks, at least locally. U–Pb geochronology indicates a crystallization age for the diorite of 486 ± 9 Ma. Highly-deformed diorites within the Portsoy Gabbro have a crystallization age of 493 ± 8 Ma. Ages of ca. 490 Ma for magmatism and high-grade metamorphism, which are broadly contemporaneous with ophiolite obduction and the onset of orogenesis, are significantly older than the established peak of Grampian metamorphism (ca. 470 Ma).We propose a new model for the Grampian Orogeny involving punctuated tectonothermal activity due to tectonic switching during accretionary orogenesis. Rollback of a NW-dipping subduction zone at ca. 490 Ma produced a back-arc environment (the Buchan Block) with associated arc magmatism and high dT/dP metamorphism. Arrival of an outboard arc resulted in shortening (the initial phase of the Grampian Orogeny) at ca. 488 Ma. Initiation of a new, NW-dipping subduction zone to the SE of the ca. 488 Ma suture, which then began to rollback at ca. 473 Ma, led to lithospheric-scale extension, decompression melting and advective heating of the middle crust, producing the widespread ca. 470 Ma Grampian (classic Barrovian and Buchan) regional metamorphism. Resumed hinge advance and the final phase of shortening cut off the heat supply at ca. 465 Ma, marking the end of the Grampian Orogeny. Publisher PDF