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Data for U, Th, and K, determined by epithermal neutron activation methods, are reported for Lower and Middle Dalradian lithologies from Co. Donegal, Ireland. Abundances are identical to rocks with stratigraphical correlation in the Scottish Dalradian and appear not to have been affected by regional metamorphism to biotite and possibly even garnet grade during the Grampian orogenic phase, or later contact metamorphism in the aureole of the Main Donegal pluton. Uraniferous Dalradian lithologies have not been encountered in Donegal. Incorporation of Dalradian rocks, either by assimilation or as extensive raft trains within the Main Donegal granite, is therefore unlikely to have contributed to the development of known pegmatite-hosted U mineralization associated with this late Caledonian pluton.

Synopsis Moinian and Dalradian metasediments which surround the Findhorn Newer Granite Complex form a northwesterly extension of the Ballapell succession. The metasediments were first folded by regional-scale isoclinal F 1 folds following the Caledonian trend. These were then refolded by open, near-upright F 2 crossfolds, and later by tight, also near-upright F 3 folds trending N to NE which now often dominate the megascopic rock fabric. High grade metamorphism with widespread migmatization accompanied F 1 folding. Localized migmatization and recrystallization followed F 1 and F 2 folding, becoming more widespread after F 3 folding. A later metamorphism foliated and recrystallized a suite of early minor intrusions belonging to the Findhorn Newer Granite Complex. The Upper Findhorn rocks are situated centrally within the orthotectonic Caledonides. Their tectonic history is correlated with that of the Iltay and Ballapel Dalradians, but their thermal history is comparable with that of the Moines west of the Great Glen Fault.

SummaryMargarite occurs in quartz-bearing Dalradian graphitic schists both as a primary phase and as an alteration product of kyanite. The disposition of alteration products in these rocks demonstrates clearly the role of an externally imposed gas phase in determining margarite development.

The times of original fractionation of the Sm and Nd component of clastic sediments from a mantle source (≡ crustal residence age) have been estimated from Sm-Nd model ages calculated relative to a depleted mantle evolution. In this way the provenance and evolution of selected Precambrian and Phanerozoic sediments and metasediments from the British Isles have been examined. Whereas some Archaean and early Proterozoic sediments have Sm-Nd model ages that are close to their stratigraphic age, the Phanerozoic sediments analysed have model ages as much as 2.0 Ga in excess of their stratigraphic age. A more detailed study of Lower Palaeozoic sediments deposited on the northern margin of the Iapetus Ocean provides evidence for a marked change of provenance in the Ordovician after the deposition of the Dalradian Supergroup. A component with comparatively high143Nd/144Nd and Sm/Nd ratio (presumably basaltic) is present in the sediments throughout the accretionary prism. Crustal residence age estimates average about 1.5 Ga for both these Lower Palaeozoic sediments, and modern pelagic clays, and collectively fail to provide any evidence for significant continental growth during the Phanerozoic.

Synopsis Analysis of carbonate and pelitic mineral assemblages at the base of the Dalradian Series near Spean Bridge, Inverness-shire, leads to the conclusion that the temperature at the peak of regional metamorphism reached 535°C at a depth of 18.5 Km. In the light of the general parallelism of the garnet isograd (taken as nearly isothermal) and the base of the Dalradian, the thermal event is modelled in one dimension using an average heat production for the Dalradian of 4.0 × 10 −1 3 cal/cm 3 s and a temperature-dependent thermal conductivity with room-temperature average value of 7.8 × 10 −3 cal/cm s K. It is argued that the thermal inertia of the upper crustal rocks during metamorphism is offset by their concurrent erosion and that a steady state model is a reasonable approximation to the complex transient thermal history over the 20–40 m.y. period during which deformation, heating and erosion were taking place. The heat flow through the base of the Dalradian at the metamorphic climax was about 1.3–1.6 × 10 −6 cal/cm 2 s; this is attributed to heat production within an underlying 10–20 Km thick Moinian basement together with a normal, or slightly higher than normal, conductive flux from the mantle beneath. If a Lewisian-like lower continental crust underlies the Moinian it would have reached temperatures high enough to permit partial fusion and could be the source of calc-alkaline largely post-metamorphic-climax igneous rocks found in the area.

Synopsis Review of existing plate tectonic theories of the Lower Palaeozoic development of the north and northwest British Isles leads to the conclusion that ridge-trench impingement theory can be used to integrate all known geological observations within one unified plate tectonic theory for the area. Consideration of the consequences of the progressive impingement of a ridge-transform system with a northwesterly-dipping subduction zone in Arenig time gives explanations for: (a) termination of Dalradian sedimentation in the Arenig; (b) initiation of Barrovian metamorphism at the same time; (c) development and history of the Girvan-Ballantrae complex; (d) initiation of the Highland Boundary Fault as a dextral transform fault in the Arenig; (e) its absence in western Scotland; (f) the postulated presence of a boundary fault southeast of Connemara and perhaps at Pomeroy; (g) the peak of metamorphism (Barrovian and Buchan types) in the Dalradian and associated basic intrusions in the Arenig in both Scotland and Connemara; (h) the development of the Moine Thrust (and related foreland folds and thrusts) also in the Arenig and (i) the origin of the South Mayo trough on oceanic crust. Related events explained by this model include the location of the Donegal batholith, the generally low-grade metamorphism of Donegal and southwest Scotland, the location of the Great Glen Fault and all known faunal province and Ordovician sedimentological data from the Midland Valley and Southern Uplands. It is proposed that this Lower Ordovician sequence of related events be termed the Grampian Orogeny, distinct from the later Caledonian Orogeny, which is tectonically and geographically separate.

An isolated 50 × 7.5 m lens, apparently unique in Britain, lies within Glenfinnan Division pelites at Port Bheathain, Mull. It comprises 4 singular lithologies: (1) quartz-plagioclase-poor, carbonate-bearing, biotite-chlorite-epidote-garnet-amphibolites, with ⩽8 cm garnets; (2) thin, impure, iron-rich ‘limestone’ seams, carrying biotite, chlorite, epidote, garnet, hornblende and spinel; (3) laminated biotite-garnet-pelite and semipelite bands; (4) quartz-feldspar-poor, locally hornblendic, biotite-garnet-kyanite-rutile-staurolite-tourmaline-schists. Local concentrations of apatite, sphene and tourmaline occur. A common origin for all 4 lithologies is implied by (a) their apparently unique association within a single small lens; (b) very strong correlations between geochemically related (including ‘immobile’) elements (e.g. K-Rb, Ti-Y, Fe-Mg). The amphibolites have much lower Si, Cu and much higher Fe, Ti, V, Zr than typical ortho-amphibolites, and may be the first confirmed para -amphibolites from the Moines. The kyanite-schists similarly have lower Si, Cu, but also strikingly higher B, Al, Ti and P than analysed Lewisian, Moinian or Dalradian kyanite-schists. The lens might be a peculiar Lewisian pod, but seems as likely to represent a localised Moinian sedimentary environment, such as a hot spring, which deposited high ratios of clays: quartz + feldspars, whilst precipitating boron and carbonate minerals.

The Caledonian thrust belts of NW Scotland exhibit many features typical of foreland fold-and-thrust belts. The major structures propagated westwards towards the foreland, the thin carapace of Cambro-Ordovician sediments being accreted in imbricate and duplex zones beneath the major thrust sheets. On a broad scale, the rules of thrust geometry are obeyed but many local anomalies are found, particularly in the Assynt region. In Assynt, several large fault-bounded gravity-spreading structures, surge zones, have been identified. Extensional faults have been traced into strike-slip and then contractional faults. There are zones of extensional and contractional imbricate faults and extensional and contractional ductile flow. Similar structures have been found along the length of the Moine thrust belt, suggesting that gravity spreading was an important mechanism during the later stages of movement. Within the main Caledonian fold belt there are several major tectonic slides, which are recognized by a local increase in intensity of deformation. The majority are NW-verging and were originally gently dipping. In the Highlands SE of the Great Glen there are early major SE-verging recumbent folds associated with considerable crustal thickening reflected in high pressure (~12kbar) metamorphism. Many of the major slides may have formed by spreading away from this thickened crust. The Caledonian deformation and metamorphism occurred later in the Moines and Moine thrust zone than in the Dalradian and Grampian Moines to the SE of the Great Glen. This may reflect different levels of deformation; alternatively, it may argue for a large strike-slip displacement on the Great Glen fault, juxtaposing crustal blocks which had experienced different ages of Caledonian deformation and metamorphism. The late movement on the Moine thrust involved spreading from an area of thickened Caledonian crust formed either by the stair-step trajectory of the basal thrust to the Moines and, or, by thickening due to intrusion of large masses of late Caledonian granites

The pre-Carboniferous Midland Valley of Scotland comprises three tectonic elements: an arc, a proximal fore-arc basin and a marginal basin. These tectonic elements have been juxtaposed by strike-slip and thrust faulting, both of which have effected a 300% reduction in the width of the orogenic belt.Rocks which span Arenig to Late Devonian or Early Carboniferous times and which are found S of the Highland Boundary fault have no clasts of certain Dalradian provenance despite substantial uplift of the Dalradian block at this time. This, combined with other evidence, suggests the Midland Valley to have been remote from this rapidly uplifting terrane. The Dalradian block, eroded down by c. 410 Ma was thrust southeastwards in Late Devonian–Early Carboniferous times. However, this thrust movement was minor, yielding little sediment, but it caused Dalradian rocks to cover the northern margin of the Midland Valley where (1) the source for part of the Old Red Sandstone rocks existed and (2) the faults along which the Midland Valley block was transported to dock against the Dalradian block are thought to be present. The existing Highland Boundary fault is therefore seen as a late Old Red Sandstone reverse fault which covered more significant older structures.

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.

Synopsis A Rb–Sr isochron for biotites from five unfoliated acid pegmatites in the limestones of the Glen Urquhart complex and the surrounding Moine psammitic gneisses has yielded an age of 428 ± 5·Ma (λ 87 Rb = 1·42 × 10 −11 a −1 ), which lies within the range of late Caledonian pegmatite ages reported from elsewhere in the Northern Highlands. It leaves open the possibility that the limestones and kyanite-schists of the complex itself could be Lewisian, Moine, or Dalradian.

Systematic variations in both timing and conditions of metamorphism are observed in the Dalradian rocks of Central Perthshire, Scotland. Early (D2–D3 and syn-D3) relatively low-pressure metamorphism (550 °C, 7 kb) occurs in the Flat belt rocks of the Tay Nappe to the south, and later (post-D3) higher-pressure conditions (550 °C, 9 kb) dominate the Steep belt to the north. The latter appear to be superimposed on earlier low pressure assemblages (chloritoid–biotite) with the pressure increase probably relating to formation of the Tummel Steep Belt, possibly near 460 Ma. This polymetamorphism, in addition to the generally high pressures of Barrovian metamorphism throughout Perthshire, may account for the poor development of the metamorphic zonal pattern in this region compared to the eastern Dalradian.

A Rb-Sr whole-rock isochron age of 487 ± 6 Ma for the Slieve Gamph Igneous Complex is interpreted as a minimum age for the regional metamorphism of the ?pre-Dalradian country rock. It is also believed to date deformation in the local Dalradian, and thus raises the question of diachronous deformation and metamorphism in the Dalradian of Ireland and Scotland.

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 ( 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 ( 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.

The strain associated with the Horn Head Slide, a major tectonic break in the Dalradian rocks of NW Ireland, is recorded by pebbles in an adjacent quartzite horizon. Mean X/Y ratios of the deformed pebble shapes in excess of 8.0 are seen closest to the slide and the field of three-dimensional shapes lies along the K = 1 line. The usual methods of separating initial shape ratio ( R i ) and tectonic strain ratio ( R s ) from the deformed shape ratio ( R f ) of ellipsoidal markers are based on measurements of variation in fluctuation (e.g., the ( R f /φ technique). However, due to the high X/Y strains in this situation and since the pebbles initially lay parallel to bedding and to a principal plane of the finite strain ellipsoid, fluctuation is generally very low. Thus, except for the least deformed X/Y data, the R f /φ technique is inapplicable and other methods are used. For X/Y data with mean ( R f > 4.0 : R s is calculated as the harmonic mean of R f ; maximum R i values only are obtained from the range of R f data. For all Y/Z and X/Z data: R i is calculated from the logarithmic range ( ω log ) of R f ; R s is simply obtained from the geometric mean of R f modified by R i . It is concluded from this that a varying prolate tectonic strain ( K ∼- 1.5 ) reaching X/Y values in excess of 8.00 was coaxially superimposed on an initial oblate shape fabric to produce the present field of deformed pebbles in the quartzite near the slide.

Approximately 700 samples from five slate, three marble and five quartzite units have been analysed for 26 elements to determine the use of geochemistry in stratigraphic correlation. Statistical treatment of the data has established that for all the slates and marbles, and certain of the quartzites, geochemistry is a viable method of distinguishing units of similar lithology. The most useful elements for distinguishing the four main slate units are P, Cr. Zn, Cu and, to a lesser extent, Rb, Sr, Y, Nb, Ba, La and Ce. Sr may indicate climatic changes or variable organic activity. The three marble units were considered as three limestone and three dolostone types. Dolostones are distinguished by high insoluble residue contents and the elements that distinguish between the dolostones are heavily influenced by these. Limestones, however, have very large differences in Sr. SiO2, Al2O3, K2O, Cr, Mn, Cu, Rb, Sr, Y and Zr have been used in discrimint function analysis. These parameters are strongly controlled by the insoluble residue with Al2O3, K2O, Cr and Rb being correlated with shaley impurities and SiO2, TiO2 and Zr with sandy impurities. Sr, Pb, Y and Zn, and to a lesser extent S and Mn, appear to be associated with the carbonate fraction.The quartzites were found to be of three major types: (1) a very variable deltaic deposit (Appin Quartzite Formation), (2) marine bodies of immature quartz sands (Glen Coe and Eilde Quartzite formations) and (3) highly mature quartz sands (Binnein Quartzite Formation and northerly outcrops of the Glen Coe Quartzite Formation). It is possible to distinguish these three types on the basis of some trace elements (Zn, Rb, Sr, Y, Ba, La, Ce) but it is not feasible to distinguish between the Glen Coe and Eilde quartzites purely on geochemistry. The Eilde Flags, an immature estuarine sandstone, has a geochemistry intermediate between that of the quartzites and the pelites, although with higher CaO, Zr and Ba than either.

The Neogene evolution of the Central Swiss Alps is outlined and used as an analogue for the Grampian Orogeny of Scotland. A direct comparison is made between the three tectonothermal stages which characterize the Alps and similar events in the Scottish Highlands. Geometric similarities are emphasised using geological cross-sections. Kinematic and mechanistic similarities are inferred. A net convergence model which links the metamorphic and uplift–denudation history of the Alps to its tectonic development in a crustal-scale duplex is argued to describe appropriately the history of the Dalradian and northern Scottish Highlands.

Evidence is presented for a late Proterozoic, tectonothermal event which affected the rocks of the Moine assemblage in the Central Highland region of the Scottish Caledonides c. 750 Ma ago. This is about 200 Ma before the early Palaeozoic Grampian orogeny, whose effects are superimposed on the Moine rocks as well as dominating the tectonism in the Dalradian Supergroup.Field and isotopic studies are integrated in zones of ductile thrusting (sliding) which are typified by belts of tectonic schists with related swarms of quartz and muscovite-bearing pegmatite veins. Of particular significance is a ductile thrust (the Grampian slide) which separates deeper crustal rocks (the Central Highland division), interpreted as showing the imprint of the Grenville orogeny, from shallower rocks (the Grampian division) representing a supracrustal assemblage formed between the Grenville and the c. 750 Ma events.The Grampian slide is the structurally highest member of a system of related, previously unrecorded slides affecting the Central Highland division. New structural, petrographic and Rb-Sr isotopic data, obtained largely from a recently recognised inlier of the Central Highland division at Laggan, bear out that the quartz and pegmatite veins are segregations formed during ductile shearing under amphibolite facies conditions. Muscovites from these veins yield ages between 780 and 730 Ma, and a regression analysis of tectonic schists and the muscovites gives an age of 740 ± 40 Ma. These data substantiate our previous hypothesis that the deeper-seated Moine rocks were affected by a distinct orogenic event at 750 ± 30 Ma.In the Northern Highlands, similar vein swarms are related to the Sgurr Beag slide and to belts of previously unrecorded tectonic schists in the Glenfinnan division of the Moine assemblage. A new, 755 ± 8 Ma age obtained from such a tectonic schist at Kinloch Hourn, combined with previous, similar age data from lensoid pegmatites, imply that the c. 750 Ma event may have also affected the Morar and Glenfinnan divisions.

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.

The magnetic fabrics of 235 samples from 31 localities in Argyllshire, Scotland were determined to study the development of the Caledonian tectonic fabric in the southwest Highlands of Scotland. The regional fabric indicates a strong NE-SW compressional foliation due to the primary deformational phases, which in parts has been overprinted by secondary deformations. A detailed comparison of the anisotropy data and the available strain data shows that the two fabric ellipsoids are co-axial, and that their axial mean ratios seem to be related by an empirical power relationship of the type: Xi Xj = li lj a (for i = 1,2,3; j = 1,2,3 and i ≠ j) where χi and χj are orthogonal principal axes and li and lj are the corresponding orthogonal principal strain axes. The exponent a for the sites from Scotland is 0.088 ± 0.017 compared with 0.142 ± 0.001 and 0.145 ± 0.005 found in the Caledonian slates of the English Lake District and the Welsh slate belt.

Synopsis Psammitic and semi-pelitic metasediments in the Corrieyairack and Loch Killin areas form part of the Grampian Division and underlie the Dalradian Leven Schist of Glen Roy. Two stratigraphic successions, the Corrieyairack and Glenshirra Successions, have been recognized which share a common structural and metamorphic history, although they are separated by a well-defined zone of high strain, here termed the Gair-beinn Slide. An early foliation (S 1 ) associated with the development of isoclinal F 1 folds and movement on the Gairbeinn Slide, is refolded by tight near-upright, NE-SW trending major F 2 folds, in particular the Corrieyairack Syncline. These structures are refolded by open F 3 folds with variably oriented axial traces.

Synopsis Stratabound Cu + Zn mineralization at Creag Bhocan, near Tyndrum, represents the proximal equivalent of previously reported distal exhalites in the Ben Challum Quartzite of the Argyll Group Dalradian. The Creag Bhocan mineralization is characterised by pyrite, chalcopyrite and sphalerite hosted by chloritic schists and albitic quartzites. Mineral inclusions within syngenetic pyrite suggest ore formation at temperatures between 250°C and 350°C from reducing, acidic (pH3–5) hydrothermal fluids, in which H 2 S was the dominant aqueous sulphur species. Affinities with the volcanic-associated massive sulphide (VMS) group of deposits are suggested for the Creag Bhocan mineralization. High heat flow and minor intrusives associated with crustal extension are thought to have driven the circulation system, and the mineralization within the Ben Challum Quartzite as a whole has many characteristics similar to those of Besshi-type deposits.

In this note we draw attention to data which may require major revision of Dalradian stratigraphy and structure in northeast Scotland. The Central Highland sequence of the Dalradian and the Keith Division (Read 1955) of Banffshire are separated from the metasediments of the Banff Division by gneisses. These are referred to as the Cowhythe Gneiss (Read 1923), Queens Hill Gneiss (Harris & Pitcher 1975), Donside and Ellon Gneiss (Read 1955) and Inzie Head Gneiss (Read & Farquhar 1956) after the main areas of outcrop. The reader is referred to Read (1955, pl. 1) where the general distribution of the gneisses is shown. Their relationships to the Central Highland succession are often complicated by the large granitic intrusions of the post-tectonic Younger Granite suite. The generally accepted model for the region interprets the gneisses as migmatites developed from the Ben Lui and Ben Lawers schists (e.g. Read 1955, Harris & Pitcher 1975). Further they are considered to represent the migmatitic core of the Tay Nappe (e.g. Ashworth 1975). Read (1955) originally considered that the rocks of his Banff Division lay on a lag-surface (the Boyne Line) above the gneisses. The great recumbent fold of which the Banff Division formed the upper limb was called the Banff Nappe, and was considered to be a continuation of the Tay Nappe (Read 1955). Recent investigations by the authors reveal a sharp tectonic break between the Dalradian rocks of the Central Highlands/Keith Division and the gneisses. This break is marked by discordance and a major

Synopsis The Moinian and Dalradian stratigraphy of the Glen Roy-Lismore area is systematically described and its terminology and definition is revised. The highest formation of the succession, the Lismore Limestone Formation, is of Lower Dalradian age and stratigraphically underlies the Portaskaig Boulder Bed. At Loch Leven the Lochaber Group is 4 km thick and the entire Lower Dalradian succession 7 km thick. To the north and west the quartzite units wedge out and the succession thins to about 5 km. This regional disparity is attributed to major facies changes and not structural modification as previously maintained (Bailey, 1934). There is a sedimentary contact between the Moinian and Dalradian Assemblages across most of the area. A detailed geological map of Lismore Island is presented together with a stratigraphic table of the Lismore Limestone Formation, which comprises a thick, partly cyclical sequence of alternating limestones and slates. The Eilde Flags Formation, the Lochaber Group and the Ballachulish Group constitute three lithological associations, each diagnostic of particular depositional conditions within a shallow shelf sea environment. The chemical compositions of the limestones and slates are tabulated and briefly discussed.

RbSr and UPb isotope analyses are reported for two pre-metamorphic Caledonian granites which intrude Dalradian rocks in the Central Highlands of Scotland. These data indicate that the origin of the granitic magmas involved partial fusion of old crustal material. UPb systems of zircon size and magnetic fractions from the Ben Vuirich granite are strongly discordant. However, U/Pb isotopic ratios precisely define a chord which intersects concordia at 514−7+6 m.y. and 1316−25+26 m.y. Geological constraints suggest that the lower intersection records the post-F2, pre-M3 emplacement age of the granite. The upper intersection reflects the presence of old zircon xenocrysts incorporated into the granite magma without complete isotopic resetting. The ultimate source of these xenocrysts is probably a metamorphic basement complex which formed about 1320 m.y. ago, but the immediate source region of the granites could have been Dalradian sediments derived therefrom. RbSr whole-rock systems of the Ben Vuirich granite are also strongly discordant, although 8 out of 13 data points scatter about an “errorchron” of 564 ± 24 m.y. with an initial87Sr/86Sr ratio of about 0.716. This is interpreted as a spurious result due to incomplete homogenization of Sr isotopes in the source region during partial fusion. Initial87Sr/86Sr ratios at the time of emplacement indicated by the zircon data ranged from 0.7173 to 0.7191. Whole-rock samples from the Dunfallandy Hill granite have Rb/Sr ratios 2–3 times higher than those from Ben Vuirich and define a reasonably good isochron age of 491 ± 15 m.y. with an initial87Sr/86Sr of 0.7185 ± 0.0008. This may date granite emplacement or subsequent resetting of the high Rb/Sr rocks during Caledonian metamorphism. RbSr systematics indicate that the crustal source regions of these and other Caledonian granites separated from the upper mantle at least ca. 800 m.y. ago and probably ca. 1300 m.y. ago, thus confirming the interpretation of the upper intersection age of the zircon UPb data.

A stratigraphic framework is proposed for the Late Proterozoic Grampian Group of the Scottish Highlands. The Grampian Group is divided into three subgroups, each defined by distinctive lithofacies associations reflecting different environments of deposition. The thin Ord Ban Subgroup, which is locally developed at the base of the Grampian Group on Speyside and Strath Dearn, consists of shallow marine shelf sediments in association with concordant amphibolite sheets. Elsewhere the Corrieyairack Subgroup, which is thickest around Loch Laggan, forms the lower portion of the Grampian Group and comprises a thick turbiditic clastic sequence laid down in deeper water during rapid basin subsidence. The Glen Spean Subgroup comprises the upper part of the Grampian Group and is thickest in the Atholl District. It is composed of shallow water tidal and deltaic deposits, marking a shallowing of the Grampian Group basin. Lateral facies changes and diachronous contacts are indicative of differential subsidence and the Grampian Group basin apparently developed and filled before the onset of regional subsidence which heralded deposition of the overlying Dalradian sediments. The possibility that the Grampian Group was unconformable on the structurally underly-ing Central Highland Division and Glenshirra Succession is currently unresolved.

Synopsis The zircon U-Pb age of 510±10 m.y. of the Cashel-Lough Wheelaun basic intrusion establishes a point early in the time span of the Dalradian metamorphism in Connemara; the intrusion is post F 1 and late M 1 but pre-F 2 . This is also a maximum age for the base of the Lower Llanvirn and can be linked with Rb-Sr results if the 5.0 × 10 10 y Rb 87 half-life is used. The metasedimentary zircon results are compatible with an isotopic disturbance(s) between 400 and 510 m.y. and suggest that a large proportion of the source rocks were older than 1200 m.y. The U-Pb isotopic relationships of the metasedimentary zircons have similar features to those of cogenetic zircon suites.

Summary The author considers that, although Dr. D. B. McIntyre9s geometrical reconstruction of the structures observed in mid-Strathspey (1951, Q.J.G.S. 107 , 1–22) may be essentially correct, his kinematic interpretation of the structures is open to question. Dr. Mclntyre9s evidence is discussed briefly. Sander9s evidence for his theory of the relation of plane of symmetry to direction of movement is also reviewed. Structures in the Caledonides of Norway cannot be brought into agreement with Sander9s theory, and it is concluded that the theory needs modification. A plane of symmetry may be formed at right angles to a lineation which is parallel to the direction of movement. Linear structures reflect local conditions of deformation and may under certain conditions be formed at any angle with the principal direction of movement. Brief remarks are given on the formation of a lineation and on the general picture of linear structures in the Caledonides. Comments are made on the occurrence of minor linear structures in the Scottish Highlands, and their importance. Variation in trend of linear structures in the Highlands is briefly discussed. It is suggested that when the linear structures of the Highlands are more completely known it may be possible to explain all structures as having been formed during the Caledonian orogeny. The Moinian and the Dalradian may both be Caledonian geosynclinal sediments, corresponding to the Norwegian Sparagmite Formation and Soros Group respectively.

Minor folds with axial directions at a marked angle to the regional fold plunge have been observed on the Craignish peninsula, western Scotland, and on the Rosguill peninsula, northwest Donegal, western Ireland, and are described and discussed as discordant structures. The folds of the Craignish peninsula are thought to have resulted from rotation of a detached structure, originally concordant, into a new position. Some of those of the Rosguill peninsula are thought to have been produced by the same process; others are attributed to refolding which caused development of a new schistosity in the matrix. Most of the discordant structures are no longer attached to their original roots, and in such cases of detached structures, care should be used in analysis.

Synopsis The occurrences of fossils, mainly graptolites, in sediments deposited prior and subsequent to metamorphism of rocks of the Dalradian Series are reviewed and discussed. The available evidence suggests that this event had terminated by early Arenig time, at the latest, in western Ireland and south-west Scotland, but occurred, or persisted, later in northern Scotland.

The late Pre-Cambrian Portaskaig Boulder Bed Series shows a very uniform sequence for some 600 km along the Caledonian strike, from Connemara on the west coast of Ireland to Aberdeenshire in Scotland. The Series, which is divided into four groups of beds, comprises numerous individual boulder beds separated by stratified sediments. The lower boulder beds are carbonate-bearing and contain abundant intrabasinal fragments of dolomite and few extrabasinal crystalline rocks while the higher beds are psammitic and contain mainly extrabasinal crystalline rocks. Varve-like banding, involutions and sandstone polygons suggest periglacial conditions. It is concluded that the boulder beds represent subaqueous tills deposited on unconsolidated sediments. Contemporaneous deformation of some of these sediments is attributed to the movement of ice, although the possibility that sliding of the unconsolidated sediments occurred locally is considered.

Summary The region is homo-axial; the plunge of the fold-axes is to the south-east at nearly 30°. A profile nearly seven miles deep has been constructed by the use of the axial method of tectonic analysis developed by Lugeon, Argand, Wegmann, and others. This profile shows the tectonic relations of the Grantown Series, the Moine, and the Dalradian. Given the profile and the plunge, the general dips can be predicted; these coincide with the observed data. Metamorphism grades from phyllites to migmatites from the highest levels to the deepest. The majority, if not all, of the contacts are tectonic. Gigantic tectonic inclusions of Moine swim in the Dalradian; it is possible that similar inclusions of Dalradian occur in the Moine. The Moine and Dalradian suffered the same movement, the sense of which appears to have been towards the south-west. Lineation is perpendicular to this direction.

The area discussed is depicted on Sheet 56 of the 1-inch map of the Geological Survey of Scotland. This sheet was geologically surveyed by James Geikie, H. M. Skae, and G. Barrow, and is not accompanied by a Memoir. The district shown in Fig. 1 has received the scantiest notice in geological literature. Fundamentally, its rocks are metamorphosed sediments of the Dalradian Series, invaded by intrusions of various characters and ages. Their apparent sequence, as set forth on Sheet 56 and by Bailey (1925, p. 673, and map opposite p. 698), is as follows:— This table may show the true or the inverted succession (Bailey, 1925, p. 672, footnote). Bailey9s terminology is here adopted. “Basic Older Igneous Rocks” are intercalated with both Ben Lui and Ben Lawers sediments of Glen Shee. Further, the region includes a mass of “Older Granite” which, by invasion of the Ben Lui Schist, produced a complex termed the “Duchray Hill Gneiss”. Sheet 56 shows the gneiss as a band, which is continued on the adjoining Sheets Nos. 55 and 65 (cf. Read, 1928, map on p. 768). In the Memoirs on Sheets 55 and 65 the gneiss is briefly described and reference made to the type area of Duchray Hill (Barrow, 1905, pp. 101–2; 1912, pp. 69–70). Bailey (1925, p. 676) has verified the igneous nature of much of the material exposed on this hill. Read (1927, pp. 337–8) has compared the composite oligoclase-biotite-gneisses of Cromar, in their respect to their origin, with the Duchray

Among other parallels to the structures which he regarded as “crush-conglomerates” in the Manx slates, G. W. Lamplugh (1895, 586–7) mentions beds with isolated fragments in the Bray Series, at Howth, Co. Dublin. The age of this Series of slates and quartzites is rather conjectural. Fossil evidence, founded on such forms as Oldhamia, though supplemented by W. J. Kyan and T. Hallissy, who suggest Middle Cambrian (1912, 249), is inconclusive. More recently, E. Greenly proposes a pre-Cambrian age (1919, 896). The alternating (or greywacke) type of sedimentation seen in the beds recurs many times during Dalradian, Cambrian, Ordovician, and Silurian. Two schools in the interpretation of the Series have to be considered. The older school, with J. Kelly (1853, 240, 253–262) and G. H. Kinahan (1878, 12, 14) as its adherents, ascribed the fragmentation in the shaly beds to volcanic agencies, and, since they failed to distinguish cleavage from bedding, regarded many of the quartzites as intrusive. The younger school, including A. McHenry and W. W. Watts (1898, 17) and Lamplugh (1903, 6), described the breccias as produced by friction during regional earth-movements. The latter, as W. J. Sollas puts it (1893, 98), possibly extended from post-Ordovician to post-Carboniferous. While the effect of later crushing and faulting must not be overlooked, it seems necessary i n the light of recent work by E. B. Bailey (1930, 86–90), Bailey and J. Weir (1932, 429–458), and S. M. K. Henderson (1935, 487–509), t o invoke probable seismicity and contemporaneous slumping to explain the alternating beds and associated shaly breccia. A new suggestion will be advanced regarding the quartzites.

Previous literature. 1819. Macculloch, J: “A Description of the Western Islands of Scotland and the Isle of Man, etc.” 1852. Nicol, J: “On the Geology of the Southern Portion of Cantyre,” Q.J.G.S., viii, pp. 406-425. 1864. Hull, E.: “The Raised Beach of Cantyre” Geol. Mag., iii, No. 1, pp. 5-10. 1865. Thomson, J.: “On the Geology of the Campbeltown District,” Trans. Geol. Soc. Glasgow, ii, pt. 1, pp. 76-89. Young, J. Wallace: “Analysis of Minerals from Campbeltown,” Trans. Geol. Soc. Glasgow, ii, pt. 1, p. 89. 1898. Paterson, J., and Renwick, J.: “Report of a Visit to Sanda and Glunimore,” Trans. Nat. Hist. Soc. Glasgow, v. pt. 2, pp. 201-203. 1. District Described. Southern Kintyre comprises the district S. of the main road between Campbeltown and the northern end of Machrihanish Bay. The area is naturally divided into two parts: 1. S. of an E.–W. line through Campbeltown, Kintyre forms a rectangular area bounded to the W. by a straight and mountainous N.–S. coast, to the S. by a slightly concave shoreline running in a general E.–W. direction, and to the E. it ends off sharply in steep cliffs of O.R.S. 2. Between the E.–W. line through Campbeltown and the northern limit of the district is a triangular area of low ground covered with recent deposits. This paper includes a description of the islands of Davarr and Sanda. 2. Topography. (a). Coastline. Two types This 250-word extract was created in the absence of an abstract

This paper deals with the succession in the ancient (Dalradian) rocks of a part of the Western Highlands of Scotland, in the immediate neighbourhood of the Great Glen Fault. Along the line of the fault the length of the area is a little over 100 miles, and it extends to 30 miles south-east of it. The greater part of this region, however, is occupied by later igneous rocks or by the sea, which exposes a vast series of coast sections. It presents special advantages to a student of the ancient formations owing to the fact that the rocks are nearly all in a low stage of metamorphism, often resembling ordinary Lower Palaezoic rocks; while the members of the small portions which reach the Biotite Zone or, near Loch Leven and Dalmally, the Garnet Zone can easily be followed into the less altered stages. Further, most of the area is readily accessible and the student has the boon of a complete set of excellent 1-inch maps and memoirs (see index-map, fig. 1) published by the Geological Survey. Without this detailed Survey work, recording the various rock-types in all places, no general synthesis would be possible, as the complexity of structure is too great for any isolated worker. Some such synthesis is very necessary, as no section of the Highlands offers better chances of a clue to the intricate problems of the Scottish Archaean. Up to the present there has been little agreement as to the connexions between adjacent sheets. Islay was

Synopsis The Bonahaven Dolomite (?295 m thick) lies in the Dalradian sequence between the Port Askaig Tillite (beneath) and the Jura Quartzite (above) and is best developed on Islay (Inner Hebrides). The sequence in the formation at five coastal outcrops is figured and four members erected. Member 3 (?150 m thick) contains stromatolite structures at ten horizons. These are described in terms of their bed geometry (laterally continuous, discontinuous), lamination (0.5–2 mm thick) and growth structures (stratiform, laterally linked hemispheroids, columnar, irregular). Ptygmatic sandstone crack–structures are ubiquitous in thinly interbedded sandstone/siltstone beds; they are thought to be subaqueous contraction cracks. The apparent palaeoclimatological contradiction posed by a dolomite overlying a tillite is discussed, and it is suggested that late Precambrian dolomites may not have needed a warm climate to form.

The epidioritc and quartzite of the Malin Head district, (Ireland) are considered by Holmes and Reynolds [7] to be metasomatically transformed into skarn-rocks and mica-schist respectively. The trace element contents of these rocks were investigated using semi-quantitative methods in order to study the behaviour of the different trace elements during the metasomatic changes which have taken place. The elements which have been determined are Rb, Ba, Ag and Pb; Sr, Y and La; Li, Cr, Ni, Co, V, Cu, Sc, Sn and Mo; Ga; Zr, Be, Tl, Ge and In. The trace elements follow the major elements for which they can substitute in favourable crystal lattices, the substitution being in accord with Goldschmidts rules. Rb, Ba and probably Pb and Ag follow and substitute for K; Sr and Y for Ca and probably K; Li, Cr, Ni, Co, V, Cu, Sc, Sn and Mo for Mg, Fe 2 and Fe 3 ; and Ga for Al.

Synopsis Field and textural evidence from a coastal traverse immediately west of Macduff indicates that the main phase of regional metamorphism separates in time the movements which produced the slaty cleavage and folds at Macduff from those which produced the strain-slip cleavage and monoclines on the steep western limb of the Boyndie Syncline. The slaty cleavage of the Macduff Slates is, therefore, thought to be equivalent to the early movements in the rest of the Dalradian; an important conclusion in the light of the recent discovery of Lower Ordovician fossils in the Macduff Slates.

In the Dalradian rocks of the Loch Tummel area it is possible to establish a series of metamorphic events which are both syntectonic and post-tectonic in relation to the polyphasal sequence of structures established in the area. Mineral assemblages which have developed during successive phases of metamorphism have been distinguished by their textural relationships, and are assigned to the appropriate metamorphic facies. Comparisons have been made with the metamorphic sequence established by Rast in the Schichallion area.

The Main Donegal Granite is described in its regional setting and details are given of the envelope of Dalradian metasediments and the relation to the earlier members of the granite complex. A separation is made into the Main Granite Proper and the structureless Trawenagh Bay Granite, covering areas of 140 and 20 square miles respectively. The Main Granite Proper is a biotite-granite showing a medium-grained, grey, apparently marginal type in the south-east of its outcrop, and a coarser, lighter-coloured variety in the north and west. Pegmatite forms a roof facies in the south-west. Flow structures are shown by mineral orientation and a perfect banding ; the latter is produced by granites of varying grain and biotite content, the finer-grained biotite-richer type being of earlier consolidation and possibly constituting a roof phase. Banding is everywhere steep and along the NE.–SW. length of the Main Granite Proper. Within the Granite, perfectly aligned trains of free-swimming rafts of country rocks, seen to be derived from roofs or walls, extend for some dozen or more miles in the Granite and agree perfectly with its flow pattern. The contact-zones of the Main Granite Proper show marked sheeting by granite and, in these zones, high-grade contact schists are produced with staurolite and garnet—deformation, and heating have acted together. The marginal parts of the Granite and the adjacent contact-zones show an intense shoar structure, lineation and mullioning, the planar structures running vertically NE.–SW. and the linear elements plunging gently north-eastwards in the north-eastern part of the outcrop and south-westwards in the south-western. The production of these structures was contemporaneous with that of the contact schists and is related to the emplacement of the Main Granito Proper. The phenomena shown by the Main Granite Proper and by its roof and walls are consistent with the almost horizontal flow, from north-east to south-west, of viscous magma in a long narrow body beneath a pronged roof and between steep walls. A broad divergence of raft-trains as they are followed into the granite shows that it cannot be of replacement origin, nor can the room problem be solved by major stoping. The authors propose that lateral magmatic wedging is the mechanism of emplacement this great granite body.

SummaryThe Loch Awe Syncline (Bailey, 1913), affecting the Middle and Upper Iltay Dalradian Successions of Argyllshire, is recognised as an F1 structure. The present work shows that the west limb is arched over a pre-O.R.S., late Caledonian monoform responsible for the fan of first cleavage over the area investigated. Reflecting the style of the earlier Ben Lui-Tarbert Monoform (Roberts & Treagus, 1964), the newly described Kilmelford–Turnalt Monoform adds to the symmetry of the Iltay Nappe Complex. “Mushroom” Structure (Rast, 1963).

Thirty-eight whole-rock samples of Connemara granites give the following Rb–Sr whole-rock ages: Galway granite, 384 ± 1 m.y.; Omey granite, 3884 ± 17m.y.; Roundstone granite, 3954 ± 80m.y.; Inish granite, 404 ± 8m.y.; Oughterard granite, 510 ± 35 m.y.; potash-feldspar gneisses, 725 ± 175 m.y. As the field evidence shows that the Oughterard granite was intruded into the Dalradian metasediments post-F4 in the Connemara fold-sequence, and possibly post-F5, this dates the fold phases F1 to F4 and the metamorphisms M5 to M1 as pre-510 ± 35m.y. The older isotopic age of the potash-feldspar gneisses agrees with their field-deduced age of post-F2pre-F3. Muscovite, potash feldspar, and whole-rock Rb–Sr analyses show that the Oughterard granite was metamorphosed about 444 ± 7 m.y. ago, which is close to the K–Ar age obtained from three hornblendes separated from potash-feldspar gneiss. These results indicate that the post-500 m.y. dates obtained from the Connemara Dalradian rocks, and therefore presumably the Scottish Dalradian also, are the dates of late metamorphisms. While the possibility of Pre-Cambrian metamorphism and folding in the Dalradian is apparent, the results are equally in agreement with a Cambrian age for the main metamorphisms M1 to M5 and foldings F1 to F4, especially in view of the present uncertainty about the age of the Cambrian period. The base of the Arenig would seem to be younger than 510 ± 35m.y.

Synopsis The interpretation of Bailey and Macgregor (1912) and Bailey (1922) is revised. The presence of three major, isoclinal and recumbent folds is demonstrated. Their geometry and facing is described by reference to minor structures, sedimentary structures and the overall stratigraphy. The Glen Orchy Antiform, around which the primary structures are folded, possibly developed together with the primary folds. It is not associated with any minor secondary structures. The structure of the Glen Orchy area raises important questions in the interpretation of the overall structure of the Moine and Dalradian.

Among the most controversial problems of British geology are those concerning the metamorphic rocks which make up the greater part of the Scottish Highlands. The writer, in common with many geologists whose work has brought them into contact with the region, has long felt that the discussion and solution of these problems would be greatly facilitated by a nomenclature less involved and more consistent than that at present in use. A simplified nomenclature, moreover, would do much to stimulate and hold the attention of students and general readers.

Gray and white quartzites occur in association with metamorphosed limestones and garnetiferous mica schists near Blair Atholl, northeast Scotland. The quartzites are irregular in texture, with quartz frequently occurring in radial aggregates, and are quite different from quartzites normally found in the Dalradian series. It is suggested that they were derived from chert or jasper, rather than sandstone.

After the Connemara schists, of Dalradian age, had been folded (F1) and metamorphosed to sillimanite grade, they were strongly hornfelsed by the intrusion of basic magma. From this magma crystallized peridotites, norites, pyroxenites and gabbros, all with An80–87 plagioclase. Movements then caused the magma to break up these ultrabasic rocks and crystallization of basic rocks occurred. They are labradorite (An55–65)-hornblende-relic clinopyroxene rocks that grade into quartz-hornblende-labradorite (An55)-gneiss, quartz-hornblende-labradorite (An55)- and andesine (An40)-gneiss and quartz-andesine (An40)-hornblende-biotite-gneiss. These gneisses formed during strong F2 and F3 movements, and probably during amphibolite-facies metamorphism, and they broke up, injected, metasomatized and amphibolized the earlier basic and ultrabasic rocks, giving a heterogeneous migmatite that commonly contains two plagioclases, An40 and An55. Quartzo-feldspath-ization of metasediment occurred, possibly up to a mile from the migmatites. This, and recrystallization, formed quartz-andesine (An40)-biotite-gneiss that merges into the magmatically crystallized gneisses as do gneisses formed by silicification of basic and ultrabasic rocks. Last to crystallize was K-feldspar gneiss, possibly formed by segregation of residual magma into one zone. This migmatite complex contains evidence of both magmatic and metamorphic crystallization of hornblende and the relative importance of each process remains to be determined. Tight F3folding formed the Cashel syncline, and a series of complementary folds. Later retrogressive metamorphism caused widespread chloritization, sericitization and saussuritization. Last of all was the intrusion of the Galway and Roundstone granites followed by their associated dykes. Twenty-four chemical analyses and nearly five hundred plagioclase determinations are utilized.

Sedimentary dikes occur in the exposures of the Appin phyllite-limestone along the shore of Loch Leven south of Fort William, western Scotland. Both downward and upward injected dikes are present; the former are probably contemporaneous with deposition of the beds, the latter post-depositional. The dikes are invariably associated with slumped rocks that are typical of unstable marginal geosynclinal areas. The term "cavity filling dikes" suggested by Fairbridge in 1946 is not recommended, but his suggestion that sedimentary dikes are associated with slumped deposits of marginal geosynclinal areas is confirmed.