Janet Watson

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

This 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.

Regional geochemistry of uranium as a guide to deposit formation

Geochemical maps of northern Scotland prepared by the Institute of Geological Sciences show the distribution of uranium based on the analysis of stream sediment and water samples by the delayed neutron method. These maps not only indicate the relative concentration of the element in the principal tectonic units but also identify areas of uranium mineralization in the bedrock. Evaluation of the geochemical maps in the light of known geophysical and geological data suggests that both the Galedonides of northern Scotland and the Hebridian craton to the west of the Caledonian orogenic front are underlain by a thick layer of basement depleted in uranium and other incompatible elements. Enrichment of uranium has occurred only where the basement has been disrupted either as a result of magmatism or by deep faulting. Thus, most granites in the area surveyed contain only average abundances of U, but large values characterize certain early Proterozoic and Caledonian granites which appear to have derived at least part of their substance from the mantle. Uranium enrichment in the non-marine Old Red Sandstone overlying the metamorphic Galedonides is most marked in the vicinity of deep faults which are thought to have separated areas of emergence from subsiding sedimentary basins in Old Red Sandstone times. The style of mineralization thus reflects both regional and local factors.

The Geological Record

The rocks which make up the earth’ s crust are the principal documents of geological history. Every aspect of these rocks, their composition and physical properties as well as their distribution and mutual relationships, is indicative of their origin and it is the function of the geologist to establish from observation a basis for the interpretation of earth history. The crust as a whole has been changing since the earliest geological times and its present-day make-up is only intelligible when examined in relation to its past evolution. The historical approach, the distinctive trait in geological thinking, is therefore fundamental to the understanding of the earth sciences.

Review Lecture: Ore-Deposition Through Geological Time

Economic mineral deposits represent abnormal concentrations of metals which must be regarded as records of unusual geological events. The recognition of any long-term changes in styles of mineralization must depend on the identification of anomalies within the geological régimes characteristic of successive stages of the Earth’s history. This question will be discussed in relation to recent developments in the Earth sciences. The recognition of structural and chemical inhomogeneities in the lithospheric mantle suggests that mapping of mantle age-provinces may become possible; such studies bear on the significance of certain metallogenic provinces. Coordinated geochemical, structural and palaeomagnetic studies which are throwing light on the evolution of early tectonic systems should help to illuminate the significance of changes in style of mineralization at the Archaean/Proterozoic boundary, as well as the distribution of some types of Proterozoic deposits. Geochemical evidence concerning the changing rôle of organic processes in sedimentation and diagenesis has a bearing on the origin of sedimentary ores, especially over the controversial period characterized by accumulation of banded iron-formations.

Caledonian granites in relation to regional geochemistry in northern Scotland

Summary Regional geochemical maps based on stream sediment sampling provide means of comparing the relative abundances of trace elements in granitic bodies with those characteristic of the enclosing Moine and Dalradian country rocks. Migmatites, vein complexes and pre-Devonian forceful intrusions in northern Scotland differ little from their country rocks on these maps (apart from higher levels of Sr): this observation supports the widely held view that they are derived mainly from remelting of Proterozoic crustal materials. The latest mainly Devonian granites and alkaline complexes tend to show levels of U, Li, Be and Rb well above the background values of the province and from this and other evidence it is suggested that their parent magmas were derived at least in part from material underplated on the crust, and perhaps metasomatically enriched in incompatible elements through mantle processes connected with subduction. The magmas were released during phases of deep fracturing. There are indications of compositional variations related to distance from the Caledonian suture.

Regional geochemistry of the Northern Highlands of Scotland

Summary The distribution of trace-elements between the Lewisian basement and the Caledonian cover-formations can be assessed from regional geochemical maps based on stream-sediment sampling. The Lewisian basement is characterised by depletion in heat-producing elements, U, K, Rb, and in Pb, Be and by high values of Sr, Mg and first row transition elements. The range of Zr, Ba, B and Y levels are comparable with those over the Moine. Regional variations relate to crustal level and to the presence of zones of high deformation. Anomalies for Mo, Zr and U are associated with granite and pegmatites in some of these zones. Within the Moine nappe the zone of tectonically emplaced Lewisian slices divides an area where the bulk geochemistry is consistent with that of continental arkoses—the Morar Division—from the Glenfinnan and Loch Eil divisions which have geochemical characteristics closer to those of greywacke facies. No other geochemical discontinuity has been recognised in the Moine but the boundary with the Dalradian is marked by an increase in boron. Clear misfits of element distribution across the Great Glen fault are apparent but the data so far available could be interpreted in terms of either sinistral or dextral displacement.

Metallogenesis in Relation to Mantle Heterogeneity [and Discussion]

Regional variations in the type and abundance of metalliferous mineral deposits formed at a given time depend on environmental factors operating not only at the sites of deposition but also in the source regions supplying the metals. The extent to which chemical heterogeneities in the mantle may have helped to determine such patterns of variation can be most effectively explored by concentrating on types of ore deposit that are genetically related to magmatism originating in the mantle. Attention is focused particularly on regions where phases of mineralization involving the same metal or group of metals have been repeated over a long time-span and/or in widely different tectonic settings. Chromium, platinum, nickel, tin and uranium are of interest in this context. Although the two latter elements are often considered to have crustal sources, it is suggested that mantle-derived volatiles have played a part in the processes leading to their enrichment in certain acid magmas.

European Shield-Areas

We will preface the account of the shield-areas of each continent with a short summary of the make-up of the continent as a whole, illustrated by a diagrammatic map for reference in later parts of the book. Accordingly, Fig. 2.1 shows the principal structural units of Europe in which the main events in the history of the continent are recorded.

The Cratons of South America and Antarctica

To deal with the early histories of the continents of South America and Antarctica in the one chapter is not a matter of pure expediency. It has long been recognised that the Andean fold-belt comes ashore in Grahamland (the Antarctic Peninsula) and, with the increase of exploration, many fundamental similarities in the constitution of the two regions have become manifest. Before embarking on our account of the Precambrian rocks of the two continents, we shall as usual give summaries of their geological histories, from which these similarities will become apparent. Geological knowledge of these difficult terrains is necessarily uneven so that our generalised descriptions must be provisional. For the same reason, we shall carry our descriptions on to include the latest Precambrian and earliest Palaeozoic rocks, rather than leaving these to be treated separately in Part II.

North Atlantic Shield-Areas

The scattered land-areas which lie within and on the borders of the northern Atlantic fall into place as portions of a simpler structure that have been dispersed by continental drift in comparatively recent geological times. The continental fragments which form the bulk of these lands, when reassembled to make allowance for the effects of drift, form three great structural entities (Fig. 3.1): n n(a) n nThe Caledonian— Appalachian system of Palaeozoic mobile belts. n n n n n(b) n nThe more ancient shield and platform areas which constituted the stable foreland on the western side of the Caledonian belts. n n n n n(c) n nA fragmentary cover of younger rocks superimposed on these structures and largely concentrated along the Atlantic margins of the continents; these margins are fringed in some places by Mesozoic or younger sedimentary successions and in others by late Mesozoic—Tertiary lava-plateaux and associated igneous intrusions — the components of the classic North Atlantic or Thulean igneous province.