St. Kilda: an illustrated account of the geology

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From: Harding, R.R. and Nancarrow, P.H.A. 1984. St. Kilda: an illustrated account of the geology. BGS Report Vol. 16, No. 7. Keyworth: British Geological Survey.].
Front cover.
Rear cover. St. Kilda: an illustrated account of the geology

St. Kilda: an illustrated account of the geology

Summary

Introduction

The Western Gabbro - field description

The Western Gabbro - petrology

The Cambir Dolerite

Breccia of gabbros and dolerites - field description

Breccia of gabbros and dolerites - petrology

Glen Bay Gabbro

Pegmatites

Metamorphism

Glen Bay Granite

Mullach Sgar Complex - field description

Mullach Sgar Complex - petrology

Conachair Granite

Minor intrusions

Quaternary deposits

The mineralogy of stream sediments of Hirta

The seabed and coastal features

Fracturing and faulting

Palaeomagnetism

The age of the Conachair Granite

Geochemistry of the major intrusions

Summary of the geological history of St Kilda

Acknowledgements

Specimen notation

References

Summary

The islands of St Kilda consist of a range of intrusive igneous rocks which were formed in a Tertiary volcano about 60 million years (Ma) ago. The oldest rock is the Western Gabbro (Ew), a banded and layered intrusion with an overall dip of 45°E in the north and 45°N in the south. At its eastern margin parts of the Gabbro have been intensely sheared and recrystallised prior to intrusion of the younger rocks of the Mullach Sgar Complex. The Cambir Dolerite intrudes the Western Gabbro on the western cliffs of the Cambir as thin veins which have a fine-grained metamorphic granoblastic texture. Blocks of gabbro, identical to the Western Gabbro, have been mixed with other gabbros and dolerites and form a widespread breccia (EK) which comprises most of Boreray, Soay and Glacan Mor. The gabbros may once have formed a large layered intrusion which underwent metamorphism, disintegration and finally intrusion by tholeiites and microbreccias. South of the tunnel in Glen Bay the igneous breccias (EK) is cut by the Glen Bay Gabbro (EG). The gabbro has been chilled to a glassy basalt against the breccias, and it itself divided into two parts by intrusion of the Glen Bay Granite (G). This represents the first evidence of granitic activity on St Kilda and immediately preceded formation of the Mullach Sgar Complex. The Complex consists of four major intrusive phases, each having a mafic and a felsic component, some resembling ring dykes and others extensively fragmented. The last major intrusion on St Kilda was the Conachair Granite (I), a leucocratic, very drusy rock intruded 55 Ma ago with an initial 87Sr/86Sr ratio of 0.7041. Late dykes and sheets of a range of dolerites and felsites cut the major intrusions and represent the last phase of igneous activity. Neither lavas nor Tertiary sediments have been found on St Kilda although minerals from contact and regional metamorphic environments occur in the stream sediments and probably represent the residues from rocks that originally enclosed the volcanic centre. Early faults trend NW–SE on Hirta and a later series of NE–SW tensional faults are partly responsible for such features as the Dun Passage and the Cambir neck. Palaeomagnetic data indicate that the major intrusions crystallised in a reversed-polarity magnetic field between 50 and 60 Ma ago, and that St Kilda has drifted north about 19° since that time.

The dominant topographical features of St Kilda are the result of Quaternary glaciation and a small glacier probably occupied Village Bay during the Devensian glacial maximum. Today St Kilda is part of a drowned landscape.

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

Acknowledgements

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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