St. Kilda: an illustrated account of the geology

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St. Kilda: an illustrated account of the geology

Front cover.



The Western Gabbro

The Cambir Dolerite

Breccia of gabbros and dolerites

The Glen Bay Gabbro



The Glen Bay Granite

The Mullach Sgar Complex

The Conachair Granite

The minor intrusions

The Quaternary deposits

The mineralogy of stream sediments of Hirta

The seabed and coastal features

Fracturing and faulting


The age of the Conachair Granite

Geochemistry of the major intrusions

Summary of the geological history of St Kilda


Specimen notation


Geological history

1 Formation of early mafic complex
A Initiation of magmatism in the vicinity of St Kilda, located at the intersection of lines of weakness in the Precambrian crust. Crystal accumulation from mafic magma to produce the Western Gabbro EW
B Intrusion of the Cambir Dolerite into solid but still hot EW to form basic granulites and spinel-amphibole assemblages.
C Intrusion of dolerites and gabbros and disintegration of EW with general uplift of these rocks during formation of the igneous breccia EK. Intrusion of basalts and explosion microbreccias at a high level (probably less than 5 km deep) accompanying surface volcanism. Hydrothermal activity during cooling and solidification of the EK breccia.
2 Formation of complex comprising a range of mafic rocks and major granitic components
A Renewed surface volcanism followed by collapse of block of cold EK breccia into a basaltic magma chamber formed at a high level in the crust. Development of thick chilled zone round this block and formation of the Glen Bay Gabbro.
B Crushing of Glen Bay Gabbro followed by intrusion of Glen Bay Granite and then by porphyritic felsite dyke.
C Development of Mullach Sgar Complex involving at least four major intrusive phases each with felsic and mafic components. Intrusion of Glen Bay Dykes.
D Intrusion of Conachair Granite at a high level in the crust, with N or NNW fracturing of the Mullach Sgar Complex.
3 Uplift, loss of volatiles, minor intrusion and cooling
A Consolidation of Conachair Granite followed by NW faulting and hydrothermal activity at temperatures between 260° and 100°C.
B Intrusion of dolerites and felsites as cone sheets and dykes; NE faulting.
C Zeolite-grade hydrothermal alteration accompanying final cooling of the St Kilda complex in reversed polarity geomagnetic field.
4 Erosion
A Uplift and erosion during much of Tertiary time, with circumstantial evidence of some deposition.
B Erosion by glaciers and deposition of glacial and periglacial sediments; rise in sea level.

Specimens referred to in the text

Prefix Collection

S — Rock from the Scottish Sliced Rock Collection housed at BGS, Murchison House, Edinburgh

MR — Rock from the Museum Reserve Collection housed at BGS, Exhibition Road, London

HM — Rock from the Hunterian Museum, Glasgow

CC — Rock from the Cockburn Collection, Royal Scottish Museum, Edinburgh


We wish to acknowledge the generous help afforded by the many people in different organisations in carrying out the St Kilda project. In particular Dr J. Morton Boyd, R. N. Campbell, C. Brown and Dr M. E. Ball of the Nature Conservancy positively encouraged the project as did Mr D. MacLehose of the National Trust for Scotland. The local knowledge of Wally Wright, the Warden on St Kilda representing both organisations, made possible more extensive mapping and collecting than we had hoped for, and the rock-climbing skills of Stewart Murray enabled access to many of the islands. The assistance and cooperation of the St Kilda Detachment, Royal Artillery, Guided Weapon Range under (at various times) Captains M. S. Forsyth, A. Cameron, and D. J. A. Cooke are much appreciated, and the willingness and expertise of our boat captain Andy Miller Mundy and his crew in undertaking the hazardous survey work near the cliffs is now legendary. We are very grateful for submarine samples collected by G. Ridley and his team (1979) and by Dr P. Kokelaar (1983), and their assistance has added significantly to ideas both about extent of intrusions and about Quaternary geology.

Specimens and thin sections were prepared in the Petrology Unit by C. W. Wheatley and R. D. Fakes, and Mr R. K. Harrison Dr M. T. Styles and Mr B. R. Young have assisted materially with discussion and X-ray data. We are grateful to R. T. Smith (Metalliferous Minerals and Applied Geochemistry Unit of BGS) for training and advice in geochemical sampling, and aspects of the Quaternary geology owe much to ideas discussed with both Dr J. D. Peacock (Highlands Unit of BGS) and Dr D. G. Sutherland (Edinburgh University). All the photographs were processed in the Photographic Department and special thanks are due to J. M. Pulsford, H. J. Evans, C. J. Jeffery and R. E. Collins for the help they gave at all stages in the project. Miss L. Wahl drew the map and the diagrams and made many valuable suggestions for their improvement, and the final production of the map and report owes much to G. F. Inzani, J. B. A. Evans, Gill Cutress and Dr T. J. Dhonau.


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