Sheet Swarm Research Articles

Proceedings

16–19 September 1974 Engineering Group Regional Meeting (Liverpool) 1 October 1974 Geological Information Group Meeting Information flow in the natural resources industries 2 October 1974 Ordinary General Meeting Intrusive sheet swarms and the identity of crustal layer 3 in Iceland. G. P. L. Walker. A new concept of the evolution of the British Tertiary intrusive centres. G. P. L. Walker. Sedimentation and Tectonlsm (Permo–Trias) on W margin N Minch basin of Stornoway. R. Steel & A. Wilson. These papers are scheduled for the March 1975 issue of the Journal. 24 October 1974 Hydrogeological Group Meeting The Scope of Hydrogeology 30 October 1974 Ordinary General Meeting Initiation and Growth of East Antarctic Ice Sheet. D. J. Drewry. Permian Beds of offshore and adjacent coastal areas of Durham and Northumberland. D. Magraw. The geological results of seismic refraction surveys in the Bristol Channel, 1970–1973 M. Brooks & D. G. James. These papers are scheduled for future issues of the Journal. 5 November 1974 Geological Information Group Meeting Geological Information Structure 6 November 1974 Volcanic Studies Group Meeting Volcanology research in progress: a discussion meeting Larvikites of the Oslo Province, Norway I.G. Meighan. The larvikitic syenites of the Permian Oslo Province are cumulates exhibiting steep igneous lamination (45°–90°) with occasional parallel, small-scale rhythmic layering. In the large, southernmost larvikite complex, these structures dip inwards, and were formed by convective circulation of magma in a subvolcanic reservoir. Major cryptic layering is absent, and internal contacts indicate several injections of magma,

Intrusive sheet swarms and the identity of Crustal Layer 3 in Iceland

The remarkable swarm of inclined basic intrusive sheets found in the deeply dissected part of SE Iceland is believed to constitute part of a widespread layer which, in other parts of the country, occurs below sea level as the 6.35 km s −1 crustal layer revealed by seismic refraction studies. A simple mechanism for the generation of the sheet swarm is proposed, based on the contrasted density gradients of crust and uprising magma: basic magma rises to the surface when its density is everywhere less than the bulk density of the rocks it cuts, otherwise it is often diverted laterally to form an intrusive sheet where its density equals that of the country rock. Once initiated, density relations in the crust are such that the swarm is strongly self-perpetuating in nature. It is believed that more than half of the uprising magma in Iceland has been so diverted to form the sheet swarm. One corollary is that the crust in Iceland “filters” the magmas entering it so that only the lighter or those which rise at a high volumetric rate succeed in passing through to the surface. Confluent sheet gabbro intrusions may develop when the frequency of uprise of magma batches is high. Sheet swarm cupolas or perched swarms also occur in central volcanoes, in which the low-density acid volcanic rocks “capture” uprising magmas.

Cone sheet swarm, resurgence of tejeda caldera, and the early geologic history of gran Canaria

The southwestern half of Gran Canaria probably represents the lower part of a large Tertiary volcano. The following stages in its development are recognized: alkali olivine basalt shield volcano; caldera collapse and emission of large quantities of alkali trachytic to soda rhyolitic ash flows; building of a central volcano inside the caldera, and high level intrusion and resurgent doming of the central volcano’s substructure by thick trachytic sills, a central syenite stock, and numerous trachytic cone sheets. The syenite stock and the younger cone sheet system inside the caldera suggest that two stages of doming followed caldera collapse.

XIII.—Granites and Associated Rocks of the Eastern Part of the Western Redhills Complex, Isle of Skye

SynopsisThe country lying between the head of Loch Ainort and the northern end of the Blaven Range in central Skye forms part of a Tertiary igneous complex. Country rocks in the area are Jurassic sediments. The earliest igneous rocks are basalt lavas which show the effects of thermal metamorphism by a later intrusion of eucrite and gabbro. This latter, which is part of the Blaven mass, has features of a layered intrusion such as igneous lamination, repetition of layers rich in certain minerals and comparable textures. It may be correlated with zones of the Cuillin layered intrusion. The eucrites are cut by a cone sheet swarm. Adjacent to the eucrite is a vent breccia which contains eucrite blocks, is not cut by cone sheets and is, therefore, of later age than the eucrite. The major part of the area under discussion is formed from four granitic ring dykes and a hybrid suite with ring dyke form, all of which belong to the Western Redhills intrusive complex; in addition, part of a fifth granitic ring dyke, which probably belongs to the Eastern Redhills intrusive complex is included in the area. The granites are mainly fayalite-hedenbergite types with a variety of phenocrysts of felspar and features characteristic of high level intrusions such as granophyric texture and varying optical and chemical properties of the felspar indicating higher temperature modifications. Three new chemical analyses of the granites and one of ferrohedenbergitic pyroxene are presented. The granites may be regarded as partial remelts of the underlying basement gneiss and a simple investigation of thermal requirements indicates that the heat necessary for the partial melting could be provided by a mass of basic rock presumed from geophysical evidence to underlie the complex. Various acid and basic minor intrusions in the area are also described.