Dalradian Basin Research Articles

Core surprise: What's inside a plate boundary?

Despite the fact that 90% of global seismicity occurs at plate boundary faults, our understanding of their internal structure is lacking. It’s not easy to see inside a plate boundary fault – typically composed of a high-strain fault core surrounded by a fractured damage zone – and when we can, it often requires expensive drilling projects that yield limited information on the internal structure of the whole fault. Understanding the internal structure of large faults is crucial, because their chemical and mechanical properties control how and where earthquakes rupture, nucleate and propagate. This in turn limits the size of the earthquake or the amount of radiated seismic energy, and consequently the severity of surface damage. The 1999 magnitude 7.7 earthquake along the Chelungpu plate boundary fault, for example – the second deadliest earthquake in Taiwan’s recorded history – saw significant variations in slip and ground motion at different locations along the fault which resulted in large local variations in casualties and damage. Subsequent field investigations related these variations to changes in the fault’s structure (i.e., clay width, geometry), which in turn controlled how the fault moved.

Basement palaeogeography of late Neoproterozoic Scotland: constraints from exotic clasts within the lower Dalradian Supergroup

Synopsis U–Pb geochronology of exotic igneous cobbles, together with detrital zircons from a nearby sedimentary heavy mineral band from the Glenshirra Group (basal Dalradian Supergroup), provide information on the age and nature of the adjacent crystalline basement that supplied detritus to the Dalradian Basin in the late Neoproterozoic. The igneous cobbles were sampled from a metaconglomerate interpreted as debris-flow deposits from a local igneous basement hinterland. A granite cobble, and detrital zircons from a nearby psammite, gave c . 1800 Ma ages, indicating derivation from a presumed local Ketilidian–Makkovikian source akin to that exposed on Inishtrahull, Colonsay and the Rhinns of Islay. A syenite clast yielded crystallization ages of c . 1500 Ma (zircon cores), followed by a subsequent metamorphism at 1470 Ma (zircon rims). This corresponds to coeval Mesoproterozoic events that affected the Pinwarian terrane of SE Canada and Newfoundland. The Moine Supergroup and Glen Banchor Succession are unlikely sources of detritus for the Glenshirra Group, as palaeodrainage across the line of the future Great Glen Fault (in its pre-displacement disposition) would largely derive from local basement to the west of the Moine Supergroup. Whilst the existence of nearby Ketilidian–Makkovikian basement has been noted previously, this study provides evidence that Pinwarian basement also lay adjacent to Scotland during the deposition of the early Dalradian basin fills. These data should be incorporated into palaeogeographical reconstructions at the time of the break-up of the Rodinian supercontinent in the late Neoproterozoic. Supplementary material: Full U-Pb LA-MC-ICP-MS analytical results are provided at is http://www.geolsoc.org.uk/SUP18692 .

Chapter 62 The Port Askaig Formation, Dalradian Supergroup, Scotland

Abstract The Port Askaig Formation (Fm.) is a thick glaciogenic succession within the Dalradian Supergroup that consists of over 700 m of variably dolomitic diamictite, conglomerate, sandstone mudstone and minor dolomite, and is bounded by mixed siliciclastic–carbonate successions of the Islay (Lossit) and Bonahaven formations. These strata are exposed in the metamorphic Caledonides of Scotland, although excellent preservation of sedimentary structures can be found at several sites. An extensional setting for this succession has been proposed based on stratigraphic and structural arguments. The available chemostratigraphic data include the Chemical Index of Alteration, δ 13 C and Sr-isotope values. Palaeomagnetic analyses have been shown to be subject to post-depositional Caledonian overprinting. There is also continued debate over the regional palaeogeographical reconstructions of the Scottish promontory for this time period. The succession is chronologically poorly constrained with U–Pb analyses of stratigraphically much higher or lower deposits. The thick succession is thought to record glacially influenced marine sedimentation and reworking of unstable sediments in a tectonically active setting with evidence of ice-margin fluctuations. Alternative palaeoenvironmental interpretations that focus on glacial terrestrial processes and emphasize climatic influence instead of tectonic activity have also been proposed. The overlying carbonate is a lithologically diverse coastal complex and so does not fit the Neoproterozoic norm. Research has to date focused on the stratigraphic and sedimentological aspects of this succession, as well as some of the broader palaeogeographical and structural features of the Dalradian basin. Future efforts should focus on the chronological, structural and palaeogeographical constraints of this succession.

Detrital zircon, detrital titanite and igneous clast U–Pb geochronology and basement–cover relationships of the Colonsay Group, SW Scotland: Laurentian provenance and correlation with the Neoproterozoic Dalradian Supergroup

A multidisciplinary provenance study, including in situ U–Pb dating of detrital titanite, was undertaken on the enigmatic low-grade metasediments of the Colonsay Group, SW Scotland to determine their source and assess previous correlations with major Neoproterozoic sedimentary sequences within the Scottish Caledonides. Sm–Nd model ages ( t DM) of the Colonsay Group range from 1664 to 2140 Ma, suggesting a Palaeoproterozoic source. Laser Ablation ICPMS U–Pb detrital zircon ages from six Colonsay Group formations are consistent with sources within Laurentia with predominant input from a ca. 1780 Ma source (Rhinns Complex) and some Grenvillian ( ca. 1.3–0.95 Ga), Pinwarian ( ca. 1.51–1.45 Ga), Labradorian ( ca. 1.71–1.62 Ga) and Ketilidian ( ca. 1.9–1.75 Ga) detritus. Archaean (> ca. 2.5 Ga) input is minimal. Clasts from the basal Octofad Sandstone Formation indicate input from ca. 1795 Ma and ca. 1400 Ma sources (U–Pb SIMS zircon ages). U–Pb (SIMS) analyses of detrital titanite record Grenville metamorphic events in the source terranes and suggest a slightly younger maximum depositional age of ca. 942 Ma compared with the youngest detrital zircon age of ca. 1025 Ma. The data support the interpretation that the Octofad Sandstone Formation is part of the Colonsay Group and that the Colonsay Group rests unconformably on the Rhinns Complex on Islay. Deposition of the Colonsay Group is, therefore, tied to the margins of Laurentia, possibly in the foreland to the Grenville Belt, as the Rhinns Complex does not record any Grenville deformation. The data are consistent with deposition of the Colonsay Group in a foreland basin related to the Rigolet phase ( ca. 1000–980 Ma) of the Grenville Orogeny. On the basis of detrital zircon U–Pb ages the Colonsay Group best correlates with the lowermost Dalradian Grampian Group of Scotland. This correlation has significant implications for Dalradian basin dynamics, timing of Dalradian deposition and timing of Dalradian deformation. In addition, the unconformable contact between the Colonsay Group and the Rhinns Complex on Islay would make it one of the few places where the basement to the Dalradian can be observed.

Sedimentology and stratigraphic affinities of Neoproterozoic coarse clastic successions, Glenshirra Group, Inverness-shire, Scotland

Synopsis Records of ancient environments and past basin histories can be preserved in metasedimentary successions, despite their subsequent deformation and metamorphism. In the Central Scottish Highlands SE of Loch Ness, the Garva Bridge Psammite and the Glen Buck Pebbly Psammite Formations (hitherto included within the Glenshirra Subgroup at the base of the Neoproterozoic Grampian Group) represent a continuum of alluvial fan to shallow water sediments, deposited in a SE thinning fan-delta clastic wedge. These sediments, derived from an uplifted granitoid hinterland to the west, contrast with the overlying marine sedimentary rocks of the Corrieyairack Subgroup, which were deposited by sediment gravity flows within a submarine slope setting. The Glen Buck Pebbly Psammite/Garva Bridge Psammite Formations and the Corrieyairack Subgroup represent two genetic stratigraphic sequences divided by a sharp sequence boundary that records a major reorganization in basin architecture. Hence, we propose that the Garva Bridge Psammite and Glen Buck Pebbly Psammite Formations be included within a separate Glenshirra Group, genetically unrelated to either the marine deposits of the immediately overlying Grampian Group or the earlier, locally migmatized (Moinian?) basement to the Central Highlands. The Glenshirra Group thus represents the earliest phase of post-Knoydartian extension, predating the main Dalradian basin development.

Giant cross-beds in the Neoproterozoic Port Askaig Formation, Scotland: implications for snowball Earth

The Neoproterozoic Port Askaig Formation is a thick succession of interbedded diamictite, conglomerate, sandstone and mudstone that records glacially influenced marine sedimentation in the extensional Dalradian basin. Giant cross-beds (individual units up to 11 m thick) are observed in sandstone at the base of Member III of the Port Askaig Formation. The cross-bedded sandstones are closely associated with other sandstone, diamictite and conglomerate facies and are characterized by both planar and trough-shaped foreset laminae with an average maximum dip angle of 14°. The cross-bedding has compound internal structure and indicates a predominant paleocurrent direction towards the south. The sandstone is medium-grained, well sorted and quartzitic. The giant cross-beds are interpreted to have formed as a result of the migration of large subaqueous simple and compound dunes in a marine setting. Based on sedimentological analysis of the sandstone and associated facies as well as the tectonic setting of the Port Askaig Formation, the giant dunes are thought to have formed under the influence of tidal currents and indicate open marine, ice-free depositional conditions. The sedimentology of the cross-bedded sandstone and its close association with diamictite and conglomerate indicate repeated ice margin advance and withdrawal, with periods of both ice-free and ice-influenced conditions. The environmental conditions recorded in these Port Askaig deposits appear inconsistent with ‘deep freeze’ or rapid meltback associated with a snowball Earth.

U–Pb geochronology of late Neoproterozoic augen granites in the Moine Supergroup, NW Scotland: dating of rift-related, felsic magmatism during supercontinent break-up?

Within the Caledonides of Caithness, Scotland, the Neoproterozoic metasedimentary rocks of the Moine Supergroup are intruded by minor sheets of strongly deformed granite. U–Pb sensitive high-resolution ion microprobe zircon ages of 599 ± 9 Ma (Berriedale augen granite) and 588 ± 8 Ma (Braeval augen granite) are interpreted to date emplacement during the late Neoproterozoic. These augen granites are therefore unrelated to either Knoydartian ( c . 870–790 Ma) or Caledonian ( c . 470–420 Ma) orogenesis in the Scottish Highlands. Intrusion was, however, broadly contemporaneous with late Neoproterozoic extension of the Laurentian margin during continental break-up and the opening of the Iapetus Ocean. In the context of evidence for rift-related mafic magmatism within the Dalradian basin in Scotland at c . 600 Ma, as well as contemporaneous anorogenic magmatism along the margins of the developing Iapetus Ocean in the Appalachians, we propose that the protoliths of the augen granites were probably emplaced during continental break-up. The new data broaden the extent of this late Neoproterozoic magmatic event in the Scottish Highlands: other deformed granites that are at present undated but have been assumed to be of Caledonian age therefore require reinvestigation.

Catastrophic mass failure of a Neoproterozoic glacially influenced continental margin, the Great Breccia, Port Askaig Formation, Scotland

Abstract The Great Breccia is a thick (over 50 m) diamictite that occurs close to the base of the Neoproterozoic Port Askaig Formation in Scotland, and contains matrix-supported clasts up to 100 m across in a poorly sorted dolomitic matrix. Detailed logging and mapping of cliff and shoreline exposures through the Great Breccia on the Garvellach Islands show that it is not a simple ‘megabreccia’ but consists of three sedimentary units including a megabreccia (Unit 1), diamictites interbedded with dolomitic conglomerate, sandstone and mudstone (Unit 2) and a diamictite with metre-sized dolomite clasts (Unit 3). The sedimentological characteristics of the Great Breccia indicate that it formed by a variety of subaqueous sediment gravity flow processes, including matrix-rich debris flows and turbidites. Clasts up to 100 m across contained within the megabreccia unit record catastrophic collapse and failure of local calcareous siliciclastic deposits; other diamictite units were deposited by debris flows. Conglomerates were deposited from high-concentration turbulent flows, and sandstones and fine-grained facies also exhibit characteristics typical of turbidites. Continued downslope remobilization of sediment in the basin following deposition of the Great Breccia is recorded by overlying deformed diamictites, detrital dolomites and siltstones of the Disrupted Beds. The Great Breccia and associated facies within the lowermost part of the Port Askaig Formation are similar to those found in carbonate megabreccia successions that record catastrophic mass failure of carbonate platforms. Failure events in the Port Askaig Formation most likely relate to localized faulting and subsidence of the Dalradian Basin. Although the Port Askaig Formation is considered a Neoproterozoic ‘glacial’ succession, there is little direct evidence for glacial influence on sedimentation in the Great Breccia, making it difficult to fully assess paleoclimatic conditions.

The Neoproterozoic Dalradian Supergroup of Scotland: an alternative hypothesis

The Dalradian Supergroup is interpreted traditionally as recording c. 300 m.y. of ‘episodic’ Neoproterozoic rifting. However, lower Dalradian (pre-Easdale Subgroup) facies architecture is incompatible with rift-basin fill, and no unambiguous Neoproterozoic extensional structures are present in those rocks. Consequently, no objective evidence exists to infer that Dalradian sedimentation was initiated during extensional tectonism. That, combined with the accumulating data for contractile deformation in Scotland at c. 870–800 Ma, the Knoydartian orogeny, permits the proposal of an alternative tectonostratigraphic evolution for the Dalradian. I propose that Dalradian basin genesis was initiated as a foredeep in response to Knoydartian orogenesis. The coarsening- and shallowing-upward, 6–8 km-thick Grampian Group–lower Lochaber Subgroup succession arguably represents a flysch to molasse Knoydartian foredeep overlain by a moderately stable post-orogenic shelfal sequence recorded by the relatively uniform thinner (c. 4 km) and compositionally more mature rocks of the upper Lochaber through Islay subgroups. Lithospheric-scale extensional tectonism and rifting did not occur until the late Neoproterozoic, as marked by the laterally variable, volcanic- and igneous-bearing Easdale Subgroup, and was followed by the late Neoproterozoic–early Palaeozoic Iapetan rift-to-drift transition through the Southern Highland Group.

Pseudomorphs after ikaite in a glaciomarine sequence in the Dalradian of Donegal, Ireland

Synopsis Ikaite, calcium carbonate hexahydrate (CaCO 3 .6H 2 O) has been reported from several localities world-wide in periglacial and glaciomarine settings. The mineral has been documented forming in seawater and lakewater at temperatures close to 0°C, but inverts to calcite at higher temperatures. Widespread occurrences of pseudomorphs after ikaite have been described from the geological record in glacial sequences. An isolated occurrence of similar pseudomorphs in Precambrian Dalradian rocks on the Garvellach Islands has been noted. A newly discovered occurrence in the Skelpoonagh Bay Limestone of the Appin Group Dalradian in south Donegal is reported here. This is in exactly the same part of the stratigraphy as the Garvellach example, suggesting that there may have been widespread development of pseudomorphs after ikaite within the Dalradian basin. The Donegal examples are preserved as slightly ferroan dolomite pseudomorphs up to 40 mm in length in a matrix of calcite. The occurrence of these structures immediately beneath the Dalradian Boulder Bed is consistent with its widely accepted glacial origin.

Origin of sulphur in metamorphosed stratabound mineralisation from the Argyll Group Dalradian of Scotland

ABSTRACTSulphur isotopic data are presented for the four horizons of stratabound mineralisation in the Argyll Group Dalradian (Vendian) of the Central Highlands of Scotland. The styles of mineralisation reflect the stratigraphic and tectonic evolution of the Dalradian basin. The SEDEX-type Ba + (Zn + Pb) Loch Lyon Horizon has δ34S values for pyrite of +17% and for baryte of +27%. The baryte sulphur source was Dalradian sea-water although the values were modified by isotopic exchange with an H2S-rich metamorphic fluid derived from the surrounding graphitic schists. The sulphur source for sulphide in the horizon was probably the underlying strata; sulphide values were only slightly affected by metamorphism. Sulphides in the Pyrite Horizon are isotopically indistinguishable from those in the remainder of the volcanogenic Ben Lawers Schist Formation (0 ± 4%); these values suggest that, at least in the Tyndrum area, sulphides in the Pyrite Horizon are of igneous origin. VMS-type Cu + Zn + Pb mineralisation in the Ben Challum Quartzite Formation has a narrow range of values around + ll%. The likely source of this sulphide sulphur is reduced sea-water sulphate, the isotopic values remaining consistent owing to the buffering effect of anhydrite in the underlying calcium-rich rocks.

Glacially- and tidally-influenced shallow marine sedimentation of the late Precambrian Port Askaig Formation, Scotland

The late Precambrian Port Askaig Formation forms part of the thick succession of metasediments and volcanics of the Dalradian Supergroup of northwest Britain. The glacianegnic Port Askaig Formation is well exposed on the Garvellach Islands in western Scotland and consists of a series of thick and regionally extensive diamictites interbedded with sandstones, siltstones, dolomites and minor conglomerates. Diamictite facies are similar to glaciomarine diamicts described from the Late Cenozoic Yakataga Formation in the Gulf of Alaska, formed by settling of fine-grained suspended sediment and ice-rafting in relatively shallow water (< 250 m) . Two particularly distinctive diamictite units, the Great Breccia and the Disrupted Beds are interpreted as the product of downslope resedimentation events; it is significant that the Port Askaig Formation was deposited during a period of increasing tectonic activity in the Dalradian Basin. Thick and extensive sand sequences interbedded with the diamictite facies are interpreted as tidally influenced shallow marine shelf deposits; sands may have been derived from deltaic sources on the basin margin and transported into the basin during storms. The entire Port Askaig Formation appears to represent an overall shallowing upwards sequence punctuated by smaller scale transgressive (diamictite) and regressive (sandstone) events. Interpretation of the Port Askaig Formation as a succession of glacially and tidally-influenced shelf sediments has significant implications for the reconstruction of palaeoclimatic histories and regional palaeogeographies during late Precambrian time.

The role of the Cruachan Lineament during Dalradian evolution

Synopsis Geological and geophysical evidence is reviewed for the existence of a trans-Caledonoid (NW–SE) lineament, the Cruachan Lineament, in the Scottish Caledonides, separating a high-density SW Highland block dominated by abundant late Precambrian Dalradian mafic rocks in which the Moine is absent from a low density Moine plus Dalradian Central Highland block poor in mafic rocks. Contrasted metamorphic P-T-time histories are predicted on opposite sides of the lineament. An alternative model of early Dalradian evolution is proposed in which basaltic volcanism and sedimentation in the SW Scottish Highlands occurred in a quasi-oceanic trans-Caledonoid pull-apart basin bounded along its NE side by a major fault now identified as the Cruachan Lineament. The Dalradian basin(s) may have experienced right-lateral translation relative to the crustal block to the SE along a dextral transcurrent fault zone possibly in the vicinity of the Highland Boundary Fault during late Precambrian time. In this model, the Dalradian terrain may provide no evidence regarding the early history of the NW side of the lapetus Ocean.

Chemistry of the Dalradian (Vendian-Cambrian) metalimestones, British Isles

Based on > 600 (∼ 300 new) analyses for up to 39 elements, chemical variations in all major Dalradian carbonate formations have been investigated. Based on actual SiO 2 and MgO distributions, samples are divided into pure carbonate rocks (< 25% SiO 2; 70% of analyses), impure carbonates (25–40% SiO 2; 22% of analyses) and skarns (Ca/Mg/Fe-silicate rocks; > 40% SiO 2; 8% of analyses). Pure carbonates are divided into limestones (< 7.5% MgO; 71% of analyses), dolostones (> 7.5% MgO; 27% of analyses) and ophicalcites (> 25% MgO; 2% of analyses). Impure carbonates and skarns have often been metasomatically altered from sediment compositions; only pure carbonates were assessed statistically. Element distributions are non-Gaussian (e.g., bimodal for Si, Ca, Sr and Mg; positively skewed near-Poisson for elements concentrated in accessory minerals). Outlying values are found for many (especially metallic) elements, probably reflecting local mineralisation. Graphical and non-parametric statistical techniques were therefore used to delineate the following stratigraphical and lateral variations: 1. (1) Except for thin dolostone beds in the Crinan and Islay Subgroups, dolostone/limestone ratios decrease up the succession, from ∼ 2:1 to near zero (Lower to Upper Dalradian). 2. (2) Dolostones have stochastically higher Si, Fe, V, Cr, Zn, Y, Zr, Ba and Pb than limestones, suggesting that dolomitisation has preferentially affected impure carbonate sediments. Dolostones have lower Sr because dolomite tolerates less Sr than calcite. 3. (3) Many supposed laterally equivalent carbonate formations are chemically heterogeneous — especially in the Blair Atholl Subgroup, where facies changes readily occurred in a shallow-water environment. 4. (4) In the Loch Tay Limestone, confidently traceable for 225 km along strike, Ca, Zn, Rb, Pb and Th tend to decrease, and Si and Mg to increase from SW to NE. Metamorphic grade increases slightly in the same direction, but since non-volatile element contents all increase with prograde decarbonation, these changes are likely to be pre-metamorphic. 5. (5) If ranks are assigned to major carbonate horizons, from 1 = oldest (e.g., Ballachulish Limestone), to 8 = youngest (e.g., Largie Limestone), V, Cr, ?Co, Ni, Zn and Pb contents of 90 dolostones are found to increase up the Dalradian sequence. 6. (6) By contrast, maximum contents of 16 elements (e.g., V, Rb, Al, Ti) in 350 limestones increase inwards from Lower and Upper towards less pure Middle Dalradian limestones. Observations (4) and (6) are consistent with the greater instability and incidence of volcanism in the Middle Dalradian basin. Observation (5) appears to reflect an association of dolomitisation with increasing, syngenetic Pb/Zn mineralisation. Many stratigraphical correlations suggested by field workers (e.g., Lismore/Blair Atholl, Culdaff/Tayvallich Limestones) are upheld by chemical comparison, whereas others may require revision (e.g., Sandend “group” may correlate with the Ballachulish, not Lismore Limestone). The Kinlochlaggan Limestone is most probably Lower Dalradian.

Agalmatolite and the maturity of sandstones of the Appin and Argyll groups and Eriboll Sandstone

Synopsis Agalmatolite is a term used to describe a soft waxy rock amenable to carving composed of massive, felted muscovite with very minor quartz and feldspar. The term is derived from αγαλμα meaning sculpture or statue. The occurrence of agalmatolite was recognised, during the initial geological survey of the NW Highlands, as resulting from a “peculiar Precambrian weathering of the Lewisian” which occurs beneath the Cambrian unconformity. We have examined the effects of this weathering on the Lewisian gneisses in a zone of alteration extending two to three metres below the Cambrian unconformity on the west side of Loch Eriboll on the north coast of Scotland. The mafic minerals, a large proportion of feldspar and even some quartz have been altered to a green muscovite in what we take to be a palaeosaprolite resulting from alkaline weathering. We imagine a loose Precambrian quartz-rich soil being river-borne a short distance to the Dalradian basin or reworked to form the Eriboll Sandstones, a supposition strongly supported by the intermittent development of discontinuous layers of green micaceous shale in the Eriboll Sandstone reminiscent in colour and composition of the agalmatolite. The alumina dissolved from the Lewisian is transported as a colloid or in solution in groundwater to the sea where it reacts with dissolved silica and K + , Na + or Ba ++ to form huge bodies of authigenic or hydrothermal-sedimentary feldspar now preserved in the Fucoid Beds and in the Middle Dalradian.

Moine─Dalradian relationships and their palaeotectonic significance

Regional geochemical and lithogeochemical data, when reviewed in relation to the development of the Caledonian orogen in Scotland, indicate that the Moine-Dalradian boundary coincides with a long-lived crustal discontinuity. It is suggested that this boundary follows the original southeastern margin of an old continental slab in which the Lewisian basement was overlain by a thick layer of ‘Old Moine’ metasediments affected by pre-Caledonian (750 Ma or over) deformation and metamorphism. Early Caledonian ‘Young Moine’ sediments on this slab resemble the underlying metasediments in lithofacies but are somewhat poorer in Zr and Y. To the southeast of the boundary, a thick Dalradian succession accumulated in a marine ensialic basin on a thinned basement of granulites and gneisses. In the Dalradian, elements of basic-ultrabasic association are high, especially in the upper Argyll and Southern Highland Groups where they are associated with products of basic volcanicity. Stratabound Ba, Pb and Zn mineral deposits occur widely not far below the volcanic horizon, and magmatism and mineralization are attributed to the opening of a palaeo-oceanic rift within the Dalradian basin. The geochemistry of the Torridonian, Old Moine and Young Moine detrital sediments suggests derivation from a common source dominated by intermediate-acid calc-alkaline rocks. Low to moderate large-ion litho-phile (l. i. l.) element levels suggest variable degrees of depletion caused by deep metamorphism of the source rocks. Isotopic data show that the sourceland was Archaean and early Proterozoic, and suggest that it may have resembled the Ketilidian and pre-Ketilidian of southern Greenland. The geochemical influence of this ancient western sourceland can be recognized throughout the Dalradian succession, constraining models that involve the availability of young island arc or exotic tectonic materials during the filling of the Dalradian basin. The mechanical strength, relatively low density and high heat production of the Old Moine rocks enhanced the contrast between the crustal slab incorporating a thick Moine layer and the main Dalradian basin beneath which this layer is absent. Tectonic and metamorphic develop­ments differed in the adjacent regions during orogeny, and deep discon­tinuities at the Moine-Dalradian boundary acted as conduits for Late Caledonian appinitic and metalliferous granites rising from sub-crustal sources.

The ending of the Caledonian orogeny in Scotland

The geological evolution of the metamorphic Caledonides during the 100-million-year period from the phase of crustal thickening initiated by the Grampian orogeny to the emplacement of late granites at c. 400 Ma is reviewed. Shortening and westward transport associated with the Grampian orogeny were diachronous; deformation began in the weak Dalradian basin and extended north-westward into the area of crystalline Moine rocks. An important change in tectonic regime, coinciding roughly with the closing of the Iapetus Ocean, was recorded at c. 425 Ma by the onset of sinistral transcurrent motions on NE–SW lines parallel to the Caledonian belt. Regional uplift of the thickened crustal welt, accompanied by deep erosion, led to the removal of 20–30 km of overburden from the Highlands, mainly in Ordovician and Silurian times. Known Lower Palaeozoic deposits of Scotland cannot account for the volume of sediment so produced and it is suggested that other basins were located in a major transform zone beneath the northern North Sea. The onset of late Caledonian magmatism coincided with the start of the post-collisional regime of strike-slip motion. The distribution of granites and appinites was influenced by transverse (NW–SE) dislocations in the lower crust and the emplacement of individual intrusions was often fault-controlled. The deep transverse lineaments acted as conduits for both sub-crustal and crustal melts over periods of up to 25 million years. It is suggested that mantle rocks metasomatized by fluids generated at an underlying subduction zone were the main source of the sub-crustal melts, and that magmatism was triggered from above by vertical and horizontal block movements. This paper is a contribution to IGCP Project 27.

Sulfur isotope study of the Aberfeldy barite, zinc, lead deposit and minor sulfide mineralization in the Dalradian metamorphic terrain, Scotland

At Aberfeldy the bulk of barite sulfur (delta 34 S = 27-36 per mil) was derived during periods of rapid surface and subsurface (?) mixing of hot ( approximately 200 degrees C?) brine with Vendian sea water, enriched in 34 S as a result of bacterial sulfate reduction within the enclosed Dalradian basin. During periods of slower brine-seawater mixing, greater proportion of isotopically light sulfate was derived from the slow oxidation of exhalative reduced sulfur (delta 34 S <28 per mil). The delta 34 S distribution is bimodal (31 and 35 per mil) and individual barite layers become isotopically light upward. Pyrite in the footwall and hanging-wall schists at Aberfeldy incorporates sulfur of bacteriogenic origin. Indications that no equilibrium exchange occurred between barite-pyrite and sulfide-sulfide pairs during greenschist-amphibolite facies metamorphism.--Modified journal abstract.

Stratigraphy, subduction and strike-slip faulting in the Mona Complex of North Wales—a review

The Mona Complex of Anglesey and Llŷn can be divided into: (1) gneisses and granites; (2) Penmynydd schists; (3) Monian Supergroup (= Bedded Succession of Greenly, 1919). Monian stratigraphy has been repeatedly revised yet important problems remain in determining relative and absolute ages of rocks within the Complex. Some Monian rocks long believed to be Precambrian have recently yielded Cambrian palynomorphs (Gwna Group) and radiometric dates as young as 518 ± 17 Ma (Sarn granite). Given the uncertainty which exists over the age of the earliest Cambrian sedimentation in the Welsh Basin (somewhere between 590–510 Ma), there need be no conflict between Monian age data and the field evidence which indicates the Mona Complex to be older than the Welsh Basin. The occurrence in Anglesey of blueschist, melange and ‘ophiolites’ (?), with nearby ignimbrites, has resulted in Monian geology being explained in terms of palaeo-subduction. The evidence for this is weak. No-one has yet proved the existence of a Monian accretionary prism, or forearc basin, or volcanic are. All previous models invoking Monian subduction are rejected as oversimplistic. Like the Ganderian of Newfoundland, Monian rocks once lay on the NW margin of the Avalonian plate. Most previous subduction models assume this location to be synonymous with the SE margin of the Iapetus Ocean. Yet the Monian Supergroup, with its enigmatic provenance, may have been deposited before Iapetus opened. The source area could even have been the ‘vanished continent’ possibly removed by transcurrent faulting from a position S of the Dalradian Basin. The potential importance of early transcurrent movement to Monian evolution has been overlooked. Internally, much of the complex appears as a tectonic collage with steeply dipping mylonitic ‘Penmynydd’ schist zones of probably early Cambrian age separating strips of deformed low grade melange from continental crustal basement. The Complex has little in common with adjacent areas and may represent part of an exotic terrain moved to approximately its present position during the Cambrian.

Cambrian and late Precambrian basaltic igneous activity in the Scottish Dalradian: a review

SummaryIn the evolution of the Scottish Dalradian basin, the climax of widespread tholeiitic magmatism pre-dated or coincided with major basement faulting and the onset of deep marine sedimentation in Lower Cambrian times. Earlier basin-deepening and precursory igneous activity had already occurred. Major and trace element chemistry of low-grade metabasites indicates their affinity with basalts from ocean-spreading centres, or transition to ‘within-plate’ types. Geological evidence favours a tectonic setting for this igneous suite and the Highland Border Series on the NW margin of an actively evolving Lower Cambrian ocean basin, possibly a marginal basin to the Iapetus Ocean System.