Dalradian Schists Research Articles

The influence of microscale lithological layering and fluid availability on the metamorphic development of garnet and zircon: insights into dissolution–reprecipitation processes

AbstractThe response of garnet and zircon to prograde amphibolite-facies metamorphism in late Proterozoic mica schists from the Scottish Highlands has been investigated. Spatial analysis of zircon populations using scanning electron microscopy was undertaken in Dalradian Schists that have undergone a sequence of prograde garnet growth and localised breakdown reactions involving coupled dissolution–reprecipitation. Fluid availability and matrix permeability strongly control this metamorphic response and different generations of garnet contain radically different populations of metamorphic micro-zircon and associated changes in the detrital zircon population. Micro-zircon abundance increases during garnet growth, whereas that of detrital zircon decreases. The mineralogy of the matrix influences zircon abundance in porphyroblast phases, where garnet overgrows a micaceous matrix zircon-rich garnet forms and where it overgrows a quartzofeldspathic matrix the result is zircon-poor garnet. Following garnet growth, micro-zircon abundance decreases at each stage of the prograde reaction history, with sillimanite-zone schists containing the lowest abundance, suggesting micro-zircons are texturally less stable at staurolite- and sillimanite-grade metamorphism. Micro-zircons are distributed evenly across host minerals in the matrix, with the exception of retrograde chlorite where micro-zircons are absent due to fluids removing Zr before new zircon can precipitate. There is an overall decrease in the mode of zircon at each stage of the reaction history, indicating that zircon is a highly reactive phase during amphibolite-facies metamorphism and is very sensitive to individual prograde and retrograde reactions.

A new map and interpretation of the geology of part of Joyces Country, Counties Galway and Mayo

A new detailed (1:25,000) geological map, cross-sections and descriptions are presented of a large part of Joyces Country, consisting of Connemara Dalradian Schists unconformably overlain by both Silurian (Upper Llandovery to Wenlock) and Carboniferous (Tournaisian to Lower Visean) rocks; only the edge of the last has been mapped. The Dalradian rocks contain D2, D3 and D4 folds and are of staurolite to sillimanite grade with late andalusite and cordierite, constrained to between 470 and 463 Ma. Major D3 folds are folded by the D4 Connemara Synform and the complementary D4 Joyces Antiform, which predate the intrusion of the 462 Ma Oughterard Granite. The folding of the Silurian rocks, termed Scandian as it was intra-Silurian, is identified precisely as a short late Wenlock to early Ludlow episode, (430 to 424 Ma; Gradstein et al. 2012), probably Gorstian Stage (427 ± 3 Ma), and hence not end-Silurian (419 Ma). The mechanism whereby the Scandian open folding of the Silurian rocks took place above the relatively high-level rigid Dalradian basement in the northern part of the area is shown to be by closely-spaced nearly vertical E–W faulting and shearing along the strike of the sub-vertical foliation in the Dalradian Schists. This controlled the trend of the folds in the Silurian rocks. At deeper levels in the basement in the southern part of the area, far fewer, and more widely spaced faults involving larger movements are probably related to syn-Scandian folding. At least five major folds in the exposed Silurian are identified. There would have been more folds (now eroded away) in the south of the area above the Dalradian rocks. The area is crucial in exposing part of the brittle-ductile transition in the Dalradian during the Scandian folding and the solution of the reciprocal problems of how the basement controlled the folding of the Silurian rocks and how their folding affected the Connemara basement.

A history of the Shetland Ophiolite Complex

Synopsis The Shetland ophiolite was derived from an intra-continental, ocean-floored basin that opened shortly after 600 Ma on the eastern edge of Laurentia, during the tensional opening of the Iapetus Ocean. The precursor sedimentary and volcanic extensional basin is exposed in the south of Shetland and can be followed northeastwards by offshore geophysical anomalies. Eastward subduction of the floor of the basin led to hydration of the descending slab and the overlying mantle wedge. At about 500 Ma pressure from the east caused westward obduction of part of the subducting slab. The obduction thrust originated in the overlying mantle wedge and cut through the descending slab from above into underlying mantle. The result is a nappe cutting steeply through the sub-ocean-floor layers. The obducted nappe over-rode highly metamorphosed Shetland–Dalradian schists and gneisses and now forms the Lower Nappe of the Shetland Ophiolite Complex. The exposed pseudo-stratigraphic succession extends from several kilometres deep in the mantle, as exposed in the west, through the lower crust to the base of the sheeted dykes in the east, before passing beneath the sea; a surface distance of some 6 km. The ophiolitic pseudo-stratigraphy occurs as steeply dipping layers within the nappe, which gravity modelling shows to be about 3 km thick. The section through the ophiolite is lying on its side. After its obduction, erosion of the nappe and nearby metamorphic basement led to deposition of fine-grained, laminated siliceous sediments, locally graphitic and conglomeratic. Resumed pressure from the east during the Scandian orogeny telescoped the Lower Nappe, thereby emplacing an Upper Nappe over the Lower Nappe. The two nappes are separated by an imbricate zone incorporating siliceous sediments, ophiolitic hornblende schist and slices of the ophiolite nappes. The Skaw granite was intruded at about 425.6 Ma during this phase.

Fluid disturbed hornblende K-Ar ages from the Dalradian rocks of Connemara, Western Ireland

The results of K-Ar age determinations and hydrogen and oxygen stable isotope measurements on hornblendes from the poly-metamorphic Dalradian schists of Connemara, western Ireland indicate that the hornblende K-Ar systems have been significantly disturbed by low temperature fluid reaction. Correlations between K-Ar ages, δD values, structural water contents and the occupancy of the hornblende structural ‘A’ sites suggest that excess water as H 3 O + occupies vacant ‘A’ site positions during stable isotope exchange and displaces Ar * causing lowering of the K-Ar ages. This process occurs below the normally accepted closure temperature for Ar in hornblende and is not detectable by optical or X-ray investigation of the mineral. This process may not be unique to Connemara hornblendes and so is of potential importance to geochronology in other metamorphic terranes.

The geology of SW Connemara, Ireland: a fold and thrust Dalradian and metagabbroic-gneiss complex

Sillimanite grade Dalradian schists composed of pelites, impure psammites, quartzites and calcite marbles, all containing amphibolites, were intruded and hornfelsed (giving hornfels sillimanite, garnet and cordierite) by a plutonic basic to acid igneous rock series now metamorphosed and folded. Important folding (denoted D 2 , D 3 , D 4 , and D 5 ) as been recognized with a major southward facing D 2 , anticline, the Cashel antiform, being repeatedly folded by D 3 , which was a very strong phase, and accompanied and followed by D 3 -D 4 , major ductile slides and thrusting. Although the meta-igneous suite includes rare peridotites (olivine Fo 75-87 ,) and anorthosites, it is mostly hornblende-augite-labradorite or bytownite-metagabbro and later quartz diorite or trondhjemite gneiss, which has injected the earlier more basic rocks. Variably preserved igneous layering in the basic rocks indicates that the whole intrusive basic sheet is flat-lying and inverted, lying on the inverted limb of the Cashel antiform. About 460 Ma ago the D 3 -D 4 , Mannin thrust carried the igneous complex, including the largely crystallized gneisses and the cover of Dalradian metasediment, over a now largely mylonitized set of acid volcanic rocks (rhyolites or ignimbrites) which enclose a separate suite of massive amphibolites intruded or extruded into the acid volcanic rocks before the thrusting. The base of the overthrust metagabbro sheet, which had cooled to albite-epidote-amphibolite facies conditions, was ground down into a fine-grained NNW lineated amphibolite. The volcanic rocks, the Mannin thrust, and the overlying basic complex, were folded by D 4 , and D 5 , into a domed structure. About 400 Ma the aplopegmatitic facies of the Inish Granite intruded and caused hornfelsing and folding while the Galway Granite was intruded in the south. Lamprophyric sills and dykes and porphyry dykes were followed by the final phase of the Inish Granite. The southward facing folding and thrusting of the Connemara Dalradian block and the gabbro-gneiss emplacement are major elements in the Caledonide history of the British Isles. It is postulated that there was a tectonic and genetic connection between the gabbro-gneiss intrusion into these Dalradian rocks, the unusually rapid decrease in metamorphic pressure already documented for Connemara and the exceptionally southern position of the Connemara block.

Pressures of metamorphism in the thermal aureole of the Etive Granite Complex

Synopsis The Etive Complex was intruded into low grade Moine and Dalradian schists, including psammites, metabasites, calc-silicates and metapelites. Pressures of the contact metamorphism in the southern section of the thermal aureole have been calculated using an internally consistent dataset derived from published phase equilibrium data. Calculated pressures lie within the range 1.0 to 2.0 kb, corresponding to 3 to 6 km of overburden during intrusion. Pressures derived from a quartz-bearing metapelite, a SiO 2- undersaturated metapelite and a calc-silicate are in good agreement both with each other and with regional constraints. The average rate of uplift between the Caledonian regional metamorphism between 475 and 500 Ma and the intrusion of the Etive Complex at about 400 Ma was 0.1 to 0.2 mm per year.

Local Modification of Gas Composition by Graphite-Iron Oxide Buffering

French1 and Eugster and Skippen2 have demonstrated the considerable variation in proportions of gas species which may be produced in the system C-O-H when oxygen fugacity is buffered by both graphite and by differing iron redox assemblages. Some Dalradian schists of the sillimanite zone in NE Scotland show mineral associations which may be interpreted in terms of such variation. The rocks which outcrop over an area about 1 km square at the Buck of the Cabrach, Banffshire, contain the essential assemblage musco-vite-biotite-quartz-plagioclase-cordierite-andalusite (sillimanite). The presence of two aluminium silicate polymorphs indicates disequilibrium and has been interpreted as a record of temperature migration across the andalusite/sillimanite univariant boundary during recrystallisation3,4.

Structures in the southern part of the Cabrach Igneous Area, Banffshire

Abstract: Bordering the gabbro area of the Cabrach on the south, a belt of serpentine extends for nearly five miles (8 km.) from west to east, and is faulted against Dalradian schists at its southern margin. The ultrabasic rocks are extensively sheared, on predominantly vertical surfaces which lie between 075 ° and 095 ° , and are crossed by north-east faults. Structures within the serpentine belt indicate that it was subjected to sinistral wrench movements, after the emplacement of the serpentines. Many intrusive acid vein-rocks occur in the serpentines. The ultrabasic masses are faulted against hypersthene-gabbos which adjoin on the north; transitional types between gabbro and serpentine have not been recorded. The serpentines occupy a fault-zone, which is considered to be a continuation of the dislocation boundary of the Insch Igneous Complex, as seen to the east of the RhynieBogie Old Red Sandstone outlier.

III.— Note on the chemical composition of conglomerates of Lower and Upper Old Red Sandstone age from Ardmore Peninsula, Dunbartonshire

Brick-red sandstones and conglomerates of Upper Old Red Sandstone age unconformably overlie chocolate-brown to purple-brown sandstones and conglomerates of Lower Old Red Sandstone age on Ardmore Peninsula, Dunbartonshire. The unconformity is well shown on the western side of the peninsula in the tidal area of the River Clyde (Nat. Grid: NS 788313) and a conglomerate from each formation has been analysed (Table 1). The specimens crushed for analysis were collected as close as possible to the plane of unconformity by Mr. Edward Scorgie during a study of the peninsula ( Trans. geol. Soc. Glasg., 22, 295). Fragments greater than one inch across were separated and not included in the crushed samples. The Lower Old Red Sandstone conglomerate is made up essentially of fragments of quartzite set in a siliceous matrix with a red iron oxide cement. The fragments in the Upper Old Red Sandstone conglomerate are predominantly low-grade Dalradian schists containing variable amounts of quartz, chlorite and muscovite. The matrix is dominantly siliceous and is cemented by calcite and red iron oxide. These petrographic characters are reflected in the chemical analyses: the relatively greater amount of SiO2 in the Lower Old Red Sandstone conglomerate (Table 1—A) is the result of the abundance of quartzite fragments while the relatively greater proportions of TiO2, Al2O3, FeO, MgO and K2O in the Upper Old Red Sandstone conglomerate (Table 1—B) result from the presence of schist fragments. The CaO and CO2 proportions reflect the carbonate fraction of the matrix, which is more abundant in the This 250-word extract was created in the absence of an abstract

A contribution to the mineralogy of chloritoid

SummaryChloritoids from the Saxa Vord schists in the Shetland Isles and the Dalradian schists north of Stonehaven have been studied optically and chemically. In both eases monoclinic and triclinic crystals occur with α = b in the monoclinic varieties. Twinning is common with twin axes [110] and [230]. The formula of chloritoid is considered in relation to the structure recently suggested by Brindley and Harrison (1952).

“Hutton's Unconformity,” Isle of Arran

AbstractIt is shown that “Hutton's Unconformity” on the Isle of Arran has been misinterpreted. The assumed plane of unconformity is in reality the plane separating the Dalradian schists from the upper-most zone of schists impregnated with and cemented by calcareous material. The true plane of unconformity is situated some 4 feet above this plane.

XVII.— The Stratigraphical Order of the Dalradian Schists near the Highland Border in Angus and Kincardine

1. Introduction. The area (Fig. 1) which is the subject of the present communication includes a north-easterly oriented belt of country forming the south-eastern flank of the Grampians between the Kincardineshire coast north of Stonehaven and the River South Esk (Glen Clova) in Angus. It is drained by a number of streams flowing south-eastwards to the plains of Strathmore and the Howe of the Mearns, of which the most important are the North Esk, its tributary the West Water, the Noran Water (Glen Ogil) and the South Esk. Between the North and South Esk a few of the hills exceed 2000 feet, but farther north-east heights of from 1000 to 1500 feet are more common. In spite of this considerable degree of relief, exposures are poor, apart from those in the rivers mentioned and along the coast. This is due to the comparatively gentle slope of the hills, which are largely clothed in grass and heather, and to the thick glacial deposits which cover much of the area. This belt of country is formed mainly of Dalradian schists. They are bounded to the south-east partly by the Highland Boundary Fault, which brings down rocks of Downtonian and Lower Old Red Sandstone age, and partly by faults, probably of lesser magnitude, separating them from the Highland Border Series of Cambrian and Ordovician age. The first detailed account of the Highland Border rocks was by Barrow (1901) who divided them into an older Jasper and Green-Rock Series and a younger Margie Series. This 250-word extract was created in the absence of an abstract The author wishes to record his appreciation of the assistance and helpful criticism of colleagues at the Scottish Office of H.M. Geological Survey. Thanks are particulary due to Mr. G. A. Goodlet for the drawings for Figs. 4 and 5.

The Garabal Hill-Glen Fyne Igneous Complex

I. Introduction The Garabal Hill-Glen Fyne igneous complex lies just west of the head of Loch Lomond and stretches westward into Glen Fyne. The complex occupies an area of some 12½ square miles embraced by sheets 37, 38, 45 and 46 of the one-inch Geological Survey map of Scotland. It breaks through various Dalradian schists already folded and regionally metamorphosed and these suffered thermal metamorphism when the rocks of the complex were intruded. The age of the complex is generally accepted as Lower Old Red Sandstone, owing to its close resemblance to other igneous masses of that age in Scotland. The earliest account of the area is given by Dakyns and Teall (1892) in a paper which has since become classical. The boundaries of the massif were accurately delineated, some of the rock types were mapped, briefly described and chemically analysed, and the hypothesis put forward that they had been derived from some parent magma by a process of differentiation operating at depth. A description of part of the western portion of the mass and of the thermal metamorphism induced there in the surrounding schists was published in the Cowal Geological Survey memoir (1897, pp. 96–101) and in the mid-Argyll memoir (1905, pp. 92–3). Finally, a paper by Wyllie and Scott appeared in 1913, dealing mainly with the ultrabasic rocks of the complex but including some data on the basic and intermediate rocks. The variation in the ultrabasic and dioritic rocks was ascribed to local variation

Further evidence of contemporaneous volcanic action during the period of the Dalradian Schists

In September 1910, while on holiday in Strathdon, I discovered an outcrop of a peculiar rock which I think worthy of a short description. It was found in contact with the beautiful foliated olivine gabbro at Pooldhulie Bridge on the Don, which has been described and figured by Dr Teall in the Geological Survey's Memoir on Sheet 75, p. 22. The outcrop extends from immediately under the bridge for about 15 yards westwards, and being actually in the bed of the river, would probably be inaccessible in ordinary states of the water. At the date of my visit the river was very low, and one could easily step from the foliated gabbro on to the exposed ledge. It really rests against the gabbro, and appears to dip away from it southwards, at an angle of about 45°. As regards its macroscopic characters, the rock on a fresh fracture is of a somewhat mottled, greenish-grey colour, of flinty character, breaking with a hackly fracture, and shows evident colour banding in darker and lighter colours. Along and parallel to the banding are to be seen numerous porphyritic crystals, many of them more or less rounded. These are easily diagnosed as felspars. On the whole, the rock presents to the naked eye the characters of a rhyolite, and this was the first opinion entertained as to its nature. Its specific gravity was found to be 2·84. As regards its microscopic characters, two sections were cut, one as far as possible parallel to the

XXIV. The Classification of the Post-Carboniferous Intrusive Igneous Rocks of the West of Scotland

I. Introduction. No one studying the geological maps of Scotland, on whatever scale, can fail to be impressed by the enormous amount of igneous activity they disclose in the geological history of that country. The broad areas of pink or brown, indicating great lava plateaux, and the splashes and stripes of crimson, indicating intrusive rocks, are literally everywhere. Not only are the igneous rocks widely distributed in space, but also in time. Practically every formation up to the Permian supplies its quota. In the Archaean we have the great foundation of gneiss, with its inextricable tangle of acid and basic igneous rocks. The Dalradian schists and gneisses of the Grampians are everywhere penetrated by igneous masses now largely metamorphosed to massive epidiorites and hornblende schists. The Cambrian of the north-west Highlands contains an interesting complex of highly alkaline rocks, including borolanite and several other rare types. The Ordovician, Devonian, Carboniferous, and Permian in Scotland each exhibit a great series of extrusive and intrusive igneous rocks. Finally, after a long period of quiescence in the Mesozoic, an enormous outburst of igneous activity in the Tertiary gave rise to the great lava plateaux of the Western Isles. This richness in material makes Scotland a petrographer's paradise. The amount of work that has been done, however, in spite of the labours of Zirkel, Allport, Judd, Geikie, Teall, Harker and others, is very little in comparison with the great amount that still remains for petrographers to do. “No adequate investigation,” This 250-word extract was created in the absence of an abstract

On the Crush-Conglomerates of Argyllshire

I. Introductory In a former communication to this Society dealing with the Dalradian schists of Argyllshire, I described a deposit known as the Boulder-bed, which occurs in the Highland Series along a horizon extending from Aberdeenshire to Islay. I pointed out that this abnormal conglomerate owed its origin to aqueous deposition, and supported that contention by showing that, in various localities along its outcrop, it contains boulders of foreign material. At that time, with the exception of the Isle of Man, where the phenomena of crush-conglomerates had been brought to light by the researches of Mr. G. W. Lamplugh, the occurrence of pseudo-conglomerates had not been recorded in Britain. My subsequent work, however, on the Geological Survey in Cornwall, revealed their existence in that county on a large scale; this was described in the Summary of Progress of the Geological Survey for 1899 (pp. 89 et seqq. ), and in a paper read before the Geological Society of Cornwall last November. On continuing my work in the Highlands last summer I was able to show that these structures occur in Scotland among the Dalradian Series, where they bear so marked a resemblance to many portions of the Highland Boulder-bed that they may readily be confounded together, especially as they are both met with on the same horizon. In my above-mentioned communication to this Society, I sought to demonstrate that the typical crystalline schists of the Central Highlands, when followed into the Loch Awe Basin, pass laterally into sediments which are comparatively