Northwestern Scotland Research Articles

Helen's Reef: a microgabbroic intrusion in the Rockall intrusive centre, Rockall Bank

Rockall Island is composed of 52 ± 9 Ma (m.y.) aegirine granite and forms part of a nearly planated intrusive complex. The nearby Helen's Reef lies within the complex but is composed of Cretaceous (81 ± 3 Ma) microgabbros. The petrography and chemistry of the microgabbros differ from Mid-Ocean Ridge tholeiites but are similar to the Tertiary gabbroic rocks of northwestern Scotland. The relationship of the Cretaceous igneous activity to the evolution of the North Atlantic Ocean is discussed briefly.

Early precambrian trondhjemitic suites in Western Australia and Northwestern Scotland, and the geochemical evolution of shields

Sodium-rich trondhjemites and quartz keratophyres, retained as pebbles in Archaean metaconglomerates, represent some of the oldest components (up to 3 by) of the Kalgoorlie System, Western Australia. New determinations indicate high Si, Na, Na/K, Ba/Rb, Ba/K, Sr/Ca+K, Ni, Ni/Mg, and Cr/Mg, and low K, Rb, Rb/Sr, and Y, in comparison to Taylor's (1965) [1] and other' average granites and to Nockolds and Allen's (1953) [2] acid calc-alkaline rocks. Syngeosynclinal quartz keratophyres (2.6 by) studied by O'Beirne (1968) [3] show similar comparisons. Similarly, high-grade acid and intermediate gneisses of the Lewisian, Scotland, have high Na/K, K/Rb, Ba/Rb, Ba/K, Sr/Ca+K, Ni, Ni/Mg, Cr, Cr/Mg, and low K, Rb, Rb/Sr, Y and Y/Ca. Secular decrease in Na/K, K/Rb and Sr/Rb is shown by granite series in Western Australia, Transvaal, Rhodesia and the Canadian Shield. The earliest members of these series compare with sodic acid plutons of island arcs, in particular Fiji, an archipelago showing a pattern of evolution analogous to that of the proposed model (Glikson, 1971) [4]. Evidence for an origin of Archaean sodic granites from below a basic crust is furnished by the abundance of basic xenoliths, by relationships in agmatite-gneiss terrains in Greenland, Scotland, and India, by K levels lower than those of Archaean metasediments, and by low initial 87Sr/ 86Sr ratios. The abundance of euhedral quartz phenocrysts in the quartz keratophyres supports origin through partial melting of eclogite [5]. In high-grade belts, the originally low abundances of K and Rb were accentuated by synmetamorphic depletion, with which the formation of late-stage K-granites may have been associated. The correlation of chemical data enables an insight into spatial and temporal relationships between low- and high-grade early Precambrian terrains, and on the geochemical evolution of shields.

Zircon U–Pb ages of granulites from the Central Region of the Lewisian, northwestern Scotland

SummaryThe overprinting of the high grade rocks of the Scourian episode, during the Inverian and Laxfordian episodes, has so far prevented a reliable estimate being made of the age of the Scourian granulite–facies metamorphism in the Central Region of the Lewisian of northwestern Scotland. In an attempt to solve this problem the U–Pb isotopic systems of zircons from two samples of hypersthene granulite from the Kylesku group have been examined. These zircons have discordant apparent ages complicating the interpretation of the results. However, by measuring a number of size fractions from each of the two zircon suites it has been possible to demonstrate that a major isotopic event or events took place approximately 2700 m.y. ago in these granulite–facies rocks. It has also been shown that the two discordance trends found for the two zircon populations can be explained in terms of episodic disturbances of the U–Pb isotopic systems at times which correspond to the known ages of ‘Laxfordian’ and ‘Caledonian’ events.

The absolute time scale and the subdivision of Precambrian rocks in Scotland

Abstract The remnants of three major orogenic cycles have been recognised within the Lewisian complex of north-western Scotland, the Scourian, Inverian and Laxfordian orogenies having taken place, respectively, about 2600 + to 2460 m. y., 2200 to ?2000 m. y. and 1600 to 1300 m. y. ago. After the deposition of the Torridonian (and Moine) series, the Knoydartian orogeny took place about ?950 to 740 m. y. ago. Evidence from the Precambrian of Canada and the northern United States of America indicates the existence, of five orogenic episodes—2730 to 2450 m. y. (Kenoran), 2200 to 2100 m. y. (?Penokean), 1900 to 1700 m. y. (Hudsonian), 1520? to 1220? m. y. (Elsonian) and 1090 to 770 m. y. (Grenville). The marked correspondence of dates for the Precambrian orogenic episodes of Scotland and Canada and their similarity with those of peaks of world wide mineral ages (Gastil 1960) and with dates for orogenic episodes in Sweden (Welin 1966) and the Baltic Shield (Polkanov & Gerling 1961) suggests an overall correlati...

Bilateral Asymmetry: a Larval Polymorphism in some Argid Sawflies (Hymenoptera, Symphyta)

BILATERAL asymmetry is virtually unknown among insects, other than as a rare aberration. Pasteels1 has briefly noted the occurrence of colour asymmetry in larvae of Arge ustulata (L.) in Belgium and the Tyrol. Intensive collecting in south-eastern England and northwestern Scotland has shown British populations of A. fuscipes (Fallen) and A. ustulata feeding on birches (Betula pendula Roth, and B. pubescens Ehrh.) to consist of symmetrical and asymmetrical individuals. Both forms appear to be present in proportions too large to be attributable to recurrent mutation alone. Unfortunately, however, samples from particular localities have so far been too small to allow accurate frequency determinations. Nevertheless, assuming genetic control, this phenomenon, would appear to constitute a true polymorphism (Ford2).

STRUCTURES IN SHALLOW‐WATER PRECAMBRIAN SEDIMENTS FROM NORTHWEST SCOTLAND

The paper deals with sedimentary structures in shallow‐water Precambrian deposits of Lower Torridonian age in the island of Skye in northwestern Scotland. The structures described include irregular lamination, ripple bedding, festoon bedding and structures produced by local erosion and reworking. Evidence of penecontemporaneous disturbance thought to be due to the movements of quicksands is briefly referred to. Variations in lithology and structure within the succession are attributed to local differences in conditions within a possible environment of sedimentation in an intertidal zone with off‐shore sandbanks.

V.—Further Observations on the Margin of the Laxfordian Complex of the Lewisian near Loch Laxford, Sutherland

SynopsisThe paper is concerned with one of the marginal parts of the Laxfordian complex of the Lewisian of north-western Scotland where the older Scourian complex (c. 2500 million years) was reactivated during the Laxfordian orogeny (c. 1600 million years). The effects of Laxfordian deformation are discussed with reference to the distortion of Scourian structures and post-Scourian dolerite dykes and to the orientation of newly-developed structures. The arrangement of structures N.E. of the Laxfordian metamorphic front is thought to result from rotation of pre-existing planar elements during compression on north-east-south-west lines, without the development of very large folds. The connection between the style and arrangement of Laxfordian structures and the metamorphic state of the rocks at the time of deformation is illustrated with reference to both local and regional variations, and some implications of the regional changes in the structural pattern are discussed.

Concentration of Chloride in Fresh Waters and Rain Water

A STUDY of analyses of the waters of a number of freshwater lochs in the north-west of Scotland, sampled in 1955, showed that the chloride concentrations (Cl) varied with the distance (D) of the loch from the coast, the most suitable equation relating these parameters being of the form Cl = K.D−n, where Kand n are constants. This form of equation was used by Gorham1 to describe similar data obtained from tarns in the English Lake District. A second series of lochs in north-western Scotland, sampled in 1960, and an earlier series of samples from lochs on the island of Rhum, western Scotland, taken in 1958, have also provided data fitting the general equation adequately.

Current-Bedding in the Moine Series of North-Western Scotland

AbstractThis note records a number of observations of orientation of current-bedding in the Moine Series of Inverness-shire, Wester Ross and Sutherland, in the North-Western Highlands of Scotland. The foreset beds have been found to have a dominant dip towards the north, thus suggesting that the Currents responsible for the transport of the material forming these beds flowed from the south.

On a Marine Holocene Fauna in North-Western Scotland

THE country mapped by the officers of the Geological Survey of Scotland on Sheet 71 includes a great many examples of raised beaches, and in the Memoir describing that country (Geology of Glenelg, Lochalsh and South-east Skye, 1910), the heights of the raised beaches are given as occurring up to about 100 ft. above the sea. The suggestion is also made (p. 168) that in the neighbourhood of Loch Alsh, a raised beach varying in height from 30 to 40 ft. along the outer coast seemed to rise in altitude when traced up Loch Alsh away from the open sea; the description continues- At a point a few yards above the junction of the Udalain stream with the sea, Mr Clough and Dr Crampton found a deposit of shelly clay which probably represents the sublittoral zone of this This locality is important, because here one might expect to obtain evidence of the relation between the raised beaches and the later stages of glaciation, and the reported occurrence of a marine fauna at this point made further investigation the more desirable. As I was passing along the shores of Loch Alsh on my way to the Outer Hebrides in 1935, I decided to examine this shelly clay, and the results, which are set out in the present paper, are not in the least what I expected. It was not possible in the time available to prove the stratigraphical relation between the shelly clay and any particular raised beach, but the fauna of the clay is entirely recent in character, and as the depth of deposition represented is not more than 2 or 3 fathoms, it is almost certain that this clay is not contemporaneous with a high-level raised beach. This negative result is disappointing, but the fauna of this clay is worthy of description, especially for comparison with that described in this same Survey Memoir from Loch na Dal, in Skye. I am extremely grateful to Dr W. A. Macfadyen for extracting and identifying the Foraminifera, and for his suggestions about the Foraminifera in the clay from Loch na Dal; I also wish to thank Dr F. W. Anderson for advice about the Loch na Dal fauna, and Mr J. R. le B. Tomlin for help in the identification of the Mollusca.

IV.—Moine Pebbles in Torridonian Conglomerates

The relation of the Torridon Sandstone to the Moine Gneiss or ‘Eastern Schists’ is one of the primary questions in thegeology of the Scottish Highlands. These two widespread series ofrocks occur on opposite sides of the great overthrusts in North-Western Scotland; and another remarkable feature of their distributionis that though the Torridon Sandstone often rests directly upon theLewisian Gneiss, it never occurs on the Moine Gneiss. The view hastherefore been suggested that the Moine rocks are the easternmetamorphosed continuation of the Torridonian. Some altered Torridon Sandstones certainly resemble the rocks of the Moine Series.Dr. Home, in his address to the British Association in 1901, quotedthe authority of Dr. Teall and Dr. Peach for the resemblance ofaltered Torridon Sandstone to the Moine; and he again remarked thisresemblance in the memoir on the North-West Highlands. The lateW. Gunn went further, and in the same work claimed (p. 612) that “east of Dundonnell good evidence can be adduced that alteredTorridon Sandstone has entered largely into the composition of the Eastern schists”. The recent memoir on the Fannich Mountains represents some of the flaggy granulites of that district as due “to the crushing of Torridonn grit”.