Igneous rocks of south-west England

P. A. Floyd

Department of Geology, University of Keele.

C. S. Exley Department of Geology, University of Keele.

M. T. Styles British Geological Survey, Keyworth, Nottingham.

GCR Editors: W. A. Wimbledon and P. H. Banham

Chapman & Hall

London, Glasgow, New York, Tokyo, Melbourne, Madras

Published by Chapman & Hall, 2–6 Boundary Row, London SE1 8HN

Chapman & Hall, 2–6 Boundary Row, London SE1 8HN, UK

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First edition 1993

© 1993 Joint Nature Conservation Committee

Typeset in 10/12pt Garamond by Columns Design & Production Services Ltd, Reading Printed in Great Britain at the University Press, Cambridge

ISBN 0 412 48850 7

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Contents

Acknowledgements

Access to the countryside Preface

1 The igneous rocks of south-west England

Introduction and site synthesis

2 Geological framework

The regional setting – Variscan Orogen

The local setting – Rhenohercynian zone of south-west England

Devonian volcanic activity

Carboniferous volcanic activity

Carboniferous–Permian plutonic activity

Post-orogenic volcanic activity

3 Lizard and Start Complexes (Group A sites)

Introduction

List of sites

Lithological and chemical variation

A1 Lizard Point [SW 695 116]–[SW 706 115]

A2 Kennack Sands [SW 734 165]

A3 Polbarrow–The Balk [SW 717 135]–[SW 715 128]

A4 Kynance Cove [SW 684 133]

A5 Coverack Cove–Dolor Point [SW 784 187]–[SW 785 181]

A6 Porthoustock Point [SW 810 217]

A7 Porthallow Cove–Porthkerris Cove [SW 798 232]–[SW 806 226]

A8 Lankidden [SW 756 164]

A9 Mullion Island [SW 660 175]

A10 Elender Cove–Black Cove, Prawle Point [SX 769 353]–[SX 769 356]

4 Pre-orogenic volcanics (Group B sites)

Introduction

List of sites

Lithological and chemical variation

B1 Porthleven [SW 628 254]–[SW 634 250]

B2 Cudden Point–Prussia Cove [SW 548 275]–[SW 555 278]

B3 Penlee Point [SW 474 269]

B4 Carrick Du–Clodgy Point [SW 507 414]–[SW 512 410]

B5 Gurnard's Head [SW 432 387]

B6 Botallack Head–Porth Ledden [SW 362 339]–[SW 355 322]

B7 Tater-du [SW 440 230]

B8 Pentire Point–Rumps Point [SW 923 805]–[SW 935 812]

B9 Chipley Quarries [SX 807 712]

B10 Dinas Head–Trevose Head [SW 847 761]–[SW 850 766]

B11 Trevone Bay [SW 890 762]

B12 Clicker Tor Quarry [SX 285 614]

B13 Polyphant [SX 262 822]

B14 Tintagel Head–Bossiney Haven [SX 047 892]–[SX 066 895]

B15 Brent Tor [SX 471 804]

B16 Greystone Quarry [SX 364 807]

B17 Pitts Cleave Quarry [SX 501 761]

B18 Trusham Quarry [SX 846 807]

B19 Ryecroft Quarry [SX 843 847]

5 Cornubian granite batholith (Group C sites)

Introduction

List of sites

Lithological and chemical variation

Petrogenesis

Cl Haytor Rocks area [SX 758 773]

C2 Birch Tor [SX 686 814]

C3 De Lank Quarries [SX 101 755]

C4 Luxulyan Quarry (Golden Point, Tregarden) [SX 054 591]

C5 Leusdon Common [SX 704 729]

C6 Burrator Quarries [SX 549 677]

C7 Rinsey Cove (Porthcew) [SW 593 269]

C8 Cape Cornwall area [SW 352 318]

C9 Porthmeor Cove [SW 425 376]

C10 Wheal Martyn [SX 003 556]

C11 Carn Grey Rock and Quarry [SX 033 551]

C12 Tregargus Quarries [SW 949 541]

C13 St Mewan Beacon [SW 985 534]

C14 Roche Rock [SW 991 596]

C15 Megiliggar Rocks [SW 609 266]

C16 Meldon Aplite Quarries [SX 567 921]

C17 Praa Sands (Folly Rocks) [SW 573 280]

C18 Cameron (Beacon) Quarry [SW 704 506]

C19 Cligga Head area [SW 738 536]

6 Post-orogenic volcanics (Group D sites)

Introduction

List of sites

Lithological and chemical variation

D1 Kingsand Beach [SX 435 506]

D2 Webberton Cross Quarry [SX 875 871]

D3 Posbury Clump Quarry [SX 815 978]

D4 Hannaborough Quarry [SS 529 029]

D5 Killerton Park and quarries [SS 971 005]

References

Glossary

Index

Acknowledgements

Work on this volume was initiated by the Nature Conservancy Council and has been seen to completion by the Joint Nature Conservation Committee on behalf of the three country agencies, English Nature, Scottish Natural Heritage and the Countryside Council for Wales. Since the Geological Conservation Review was initiated in 1977 by Dr G.P. Black, then Head of the Geology and Physiography Section of the Nature Conservancy Council, many specialists in addition to the authors have been involved in the assessment and selection of sites; this vital work is gratefully acknowledged.

Over many years the authors have benefited from the guidance of the following: past and present members of the Publications Management and Advisory Committees for their support and advice; Dr R.P. Barnes, Dr AJJ. Goode and Dr R.T. Taylor of the British Geological Survey; Dr B. Rice-Birchall of Keele University and British Ceramic Research Ltd.; Professor C.M. Bristow of the Camborne School of Mines; Mr P. Hawken of the China Clay Museum, St Austell; Mr D.C. Methven, Manager of the De Lank Quarries; Mr A.D. Francis and Mr J. Howe of English China Clays International Ltd.; Dr M. Stone of the University of Exeter; Mr C.V. Smale of the Goonvean and Rostowrack China Clay Co. Ltd.; Mr G.J. Lees and Dr R.A. Roach of Keele University; Mr J. Symonds, Manager of Luxulyan Quarry.

Dr A.J. Barber, Dr A. Hall and Dr M.F. Thirlwall of Royal Holloway and Bedford New College, University of London, kindly made many helpful comments when they refereed the text.

Thanks are also due to the GCR publication production team: Dr D. O'Halloran (Project Manager); Valerie Wyld (Sub-editor); Nicholas D.W. Davey (Scientific Officer and Cartographic editor); and Chapman and Hall for their help and advice at the final stages of publication. Cartographic drafting was done by Silhouette (Peterborough).

Access to the countryside

This volume is not intended for use as a field guide. The description or mention of any site should not be taken as an indication that access to a site is open or that a right of way exists. Most sites described are in private ownership, and their inclusion herein is solely for the purpose of justifying their conservation. Their description or appearance on a map in this work should in no way be construed as an invitation to visit. Prior consent for visits should always be obtained from the landowner and/or occupier.

Information on conservation matters, including site ownership, relating to Sites of Special Scientific Interest (SSSIs) or National Nature Reserves (NNRs) in particular counties or districts may be obtained from the relevant country conservation agency headquarters listed below:

English Nature, Northminster House, Peterborough PE1 1UA.

Scottish Natural Heritage, 12 Hope Terrace, Edinburgh EH9 2AS.

Countryside Council for Wales,

Plas Penrhos,

Ffordd Penrhos,

Bangor,

Gwynedd LL57 2LQ.

Preface

This volume illustrates some of the significant aspects of magmatic activity from Devonian (408 million years ago) to early Permian (270 million years ago) times in SW England. This period covers the progressive development of the Variscan mountain-building episode, from initial basin formation to final deformation and the subsequent development of a fold mountain belt – the Variscan Orogen. Both extrusive (volcanic) and intrusive (plutonic) rocks are found in the orogen, and chart the various stages of its magmatic development.

The sites described in this volume are key localities selected for conservation because they are representative of the magmatic history of the orogen from initiation to stabilization. Some of the earliest volcanic activity in the Devonian is represented by submarine basaltic and rhyolitic lavas developed in subsiding basins, caused by the attenuation of the existing continental crust. In some cases, extensive rifting and attendant magmatism produced narrow zones of true oceanic crust, whereas elsewhere basaltic volcanism is related to fractures in the continental crust at the margins of the basins. After the filling of the sedimentary basins, and their deformation caused by crustal shortening (late Carboniferous Period), further activity is manifested by the emplacement of the Cornubian granites and later minor basaltic volcanism in the early Permian.

Accounts of the constituent parts of this history have enriched geological literature from the nineteenth century onwards, and have contributed to the advancement and understanding of magmatic and tectonic processes. South-west England contains examples of the composition and emplacement of ancient ocean crust (ophiolites), the diversity and formation of submarine lavas, the emplace­ment of multiple granite intrusions and their effects on the surrounding rocks, and of the nature of economically important post-magmatic alteration processes and mineralization.

P.A. Floyd, C.S. Exley and M.T. Styles

References