Contents

Acknowledgements

1 Introduction

1.1 Aim and scope

1.2 Approach to metamorphic rocks

1.3 A few helpful concepts

1.4 Finding your way through the book

1.5 Further reading

2 Background

2.1 The stages of work

2.2 Origins of metamorphic rock structure

2.3 Names and categories of rocks

3 Mapping metamorphic rocks

3.1 Use of field maps and field notebooks

3.2 Defining and mapping formations and markers

3.3 Contacts and boundaries of metamorphic rocks

4 Banding

4.1 Gross banding

4.2 Fine banding and striping

4.3 Three dimensions

5 Minerals, rock-types, compositions and grades

5.1 Minerals

5.2 Fine-grained material

5.3 Rock-type names

5.4 Reporting rock-types

5.5 Compositional category and metamorphic grade

6 Textures, fabrics, cleavage and schistosity

6.1 General

6.2 Textures

6.3 Fabric, cleavage and schistosity types

6.4 Deformation fabrics traversing a band

6.5 Deformation fabrics and folds

7 Scattered entities: pods, boudins, augen, pseudomorphs, veins and pegmatites

7.1 Boudins and shear-pods

7.2 Augen, flaser and large mineral grains

7.3 Pseudomorphs

7.4 Veins and pegmatites

8 Contacts and reaction zones

8.1 Igneous contacts — aureoles and metasomatism

8.2 Reaction zones and chemical changes at contacts

9 Faults, mylonites and cataclasites

9.1 Faults

9.2 Fault and shear-zone rock-types

10 Reference tables and checklists

10.1 Compositional categories and their grade indicators

10.2 Minerals

10.4 Checklist for recording textures and fabrics (facing inside back cover)

10.5 Checklist of rock features (inside back cover)

Rocks illustrated in this book are from the Western Alps or from Ireland, unless specified otherwise.

The scale of photographs is generally shown by inclusion of a millimetre rule or a white 10 cm bar. For other cases, scale is stated in the figure caption.

Hardness is measured according to Mohs’ scale, set out below. Other objects (such as knives or coins) can be used to test the hardness of minerals once their own hardnesses have been determined.

Copyright © 1984 Norman Fry

Reprinted 1992, 1993, 1995, 1996, 1997 by John Wiley & Sons Ltd, Baffins Lane, Chichester, West Sussex PO19 1UD, England

First published 1984 and reprinted 1985, 1988, 1989 by Open University Press

All rights reserved

No part of this book may be reproduced by any means, or transmitted, or translated into a machine language without the written permission of the publisher.

Other Wiley Editorial Offices

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John Wiley & Sons (SEA) Pte Ltd, 37 Jalan Pemimpin #05-04, Block B, Union Industrial Building, Singapore 129809

British Library Cataloguing in Publication Data

Fry, Norman

The field description of metamorphic rocks.—

(The Geological Society of London

Handbook Series)

I. Title II. Series

552’.5 QE475.A2

ISBN 0 471 93221 3

Acknowledgements

The preparation of this book has been assisted by many people, often unknowingly. They include my colleagues in the Geology Department of the University College of Swansea, the Editor and Authors of the Geological Society of London Handbooks, my family and friends, and those geologists working in the Western Alps during the summer of 1981 who got together and discussed geology and fieldwork on many occasions.

1

Introduction

1.1 Aim and scope

This book is about describing metamorphic rocks and rock-masses. It is primarily for use in the field, when describing those aspects of metamorphic rocks which are discernible with only basic equipment (handlens, hammer, clinometer, etc.). It has been written with final-year undergraduate students in mind, but should be helpful to any undergraduate, graduate student, practising geologist or amateur faced with making a general description of an area of metamorphic rocks. This book provides a systematic framework, enabling readers to produce useful and broadly similar descriptions, despite possible differences of background or specialist interest. It does not provide metamorphic specialists with assistance in the detailed interpretation of metamorphism.

This volume is a companion to handbooks on the field description of sedimentary and igneous rocks. It therefore does not cover pre-metamorphic features of obvious sedimentary or igneous origin which may sometimes be preserved in metamorphic rocks. The reader will have to decide whether to refer to this book alone or to the handbook set, in areas where pre-metamorphic features are preserved.

Describable features of metamorphic rock-masses may be:

As the first three all require microscopic and chemical techniques for specialist study, there is a practical limit to their non-specialist description in the field. The limit to what may be expected in the way of structural description is less obvious. It is assumed here that production of a map is essential and one chapter has been given to considering the problems which can arise when mapping in metamorphic terrains. The companion handbook, Basic Geological Mapping, should be referred to for mapping techniques. Qualitative relationships between structural and metamorphic aspects of a rock-mass are important, and this book gives guidance on their treatment. Quantitative geometry and mechanisms of deformation are not dealt with, being considered beyond the scope of a non-specialist description.

1.2 Approach to metamorphic rocks

There is a widespread belief that metamorphic rocks are the most difficult rocks to understand. The techniques used in laboratory studies of metamorphic petrology can certainly seem mysterious, and they clearly do not provide any basis for describing rocks in the field. If we were to approach a rock exposure with questions such as “What is its paragenesis?”, “What metamorphic facies does it belong to?”, “Is it granoblastic?”, we would probably come away with an empty notebook, feeling bewilderment and inadequacy. So what should we do?

First of all, we can abandon mystifying language. Questions about ‘facies’ urge us to jump to hypothetical interpretation when we should still be making a comprehensive description of mineral evidence. Textural terms, like ‘granoblastic’, urge us to put into words things which, being visual, are far better recorded as drawings. So do not be mystified by people who would have you believe that it is clever to interpret metamorphism on inadequate evidence, or to use their big words when these actually convey less information than your non-verbal records. This book will not mention ‘paragenesis’, ‘metamorphic facies’ or ‘granoblastic’ again. Instead, it will provide schemes for the recording of visible minerals and textures without them, and for making simple deductions about metamorphic conditions.

There are also large quantities of metamorphic rock in which minerals and textures are not visible. We have to describe what can be seen, not to worry about what cannot. If the visible features are of a different nature (veins, for example), then they are what we should describe (with the aid of the section of this book dealing with veins).

The genuine difficulty underlying work on metamorphic rocks is quite simply their variety. They cannot all be described in the same way. Thus, Chapter 6 will give alternatives for the way that composition should be described, according to whether the rock is ‘fine-grained’ (grains too small to be visible with a × 10 handlens) or ‘coarse-grained’ (grains visible). Then there are features (e.g. pseudomorphs) which may turn up occasionally in all kinds of metamorphic rock. The only advice which is always applicable is simply to ask yourself “Are pseudomorphs present?” If so, describe them (with the aid of the section on pseudomorphs). If not, move on to other things.

Clearly, this book has a duty to deal with features having as wide a variety as do metamorphic rocks. There are checklists to help a reader find the sections on particular rock features, but inevitably the reader is going to have to decide which of these sections are relevant to his rocks.

1.3 A few helpful concepts

Before explaining the structure of this handbook, there are a few matters of terminology and approach to be made clear. This book will distinguish between ‘record’, ‘inference’, ‘synthesis’ and ‘interpretation’. A good rock-mass description does not just represent field records. It organizes them and edits them, making use both of direct inferences (e.g. that a micaschist is a metamorphosed sedimentary rock) and of synthetic statements. These statements of patterns discerned in various aspects of the rocks (composition, structure, metamorphism, etc.) together with any resulting correlations, make the description one of rock-mass geology, not merely of rocks. Avoid interpretations, that is to say, statements of processes which may ‘explain’ the syntheses. Remember that an excellent description is distinguished by the extent to which an over-all geometry and geological history of the rock-mass are established through correlation, without recourse to hypothetical genetic interpretations. Section 2.1 runs through the different tasks involved in making a description of a metamorphic rock-mass.

There are also distinctions to be made between different ideas of what a rock is. Ask somebody the question “What is this rock?”, and you may get the perfectly valid reply “It is a milestone.” As a geologist you would probably expect an answer more like “Old Red Sandstone” or “Arkosic sandstone.” These examples illustrate the point that in speaking of ‘a rock’ we may be referring to an object, to a distinctive body of rock or to a material. Where necessary we can distinguish these concepts by use of the terms rock, rock-unit and rock-type respectively. These distinctions become particularly important in the different ways that such things are given names — a point which will be developed further in Section 2.3.

A final distinction to be mentioned at this stage is between ‘outcrop’ and ‘exposure’. Although these words are often used as synonyms we sometimes need two definitions. The area where the solid rock’s top surface cuts through a particular rock-unit can be strictly referred to as that unit’s outcrop even if totally hidden by soil, sediment, vegetation or standing water. The parts of the outcrop which are not hidden by any overlying material are exposed, i.e. they are visible exposures.

1.4 Finding your way through the book

From the list of contents you will see that this text proceeds, like a description of a rock-mass, from the general to the particular. It starts with instruction on how a rock-mass may be studied (Chapters 1–3), continues by considering in turn the different field features of metamorphic rocks (Chapters 4–9), and ends with some tables and lists for immediate reference at a rock exposure (Chapter 10).

The chapter you are now reading (Chapter 1) is introductory, and with the possible exception of the Tables, it will probably only need to be read again if this book has been left unused for a time. Table 1.1 may be useful at the start of fieldwork for directing attention to chapters and sections relevant to particular types of metamorphic terrain. The introductions to particular features at the beginnings of these chapters and sections should be read at that stage. As fieldwork progresses the checklists of Chapter 10 are likely to become the more useful means of reference to particular chapters and sections, within which the phrase “in the field” introduces direct instructions on what to record.

Chapter 2 (entitled ‘Background’) deals with three different subjects, in sections which could have been presented as separate chapters. The common feature of these sections is that they are rather intellectual, and ought to be read and considered carefully. They provide, and perhaps straighten out, some ideas and concepts necessary for thorough fieldwork on metamorphic rocks. As Chapter 2 may be heavy going, avoid reading it in a rush, and use opportunities such as the journey to the field area or the time when fieldwork is halted by bad weather to read through it again.

Although Chapter 3 (‘Mapping metamorphic rocks’) may be treated similarly, it also contains specific advice (for example, about notebooks and photographs) of more direct use in the field, particularly during the first few days of fieldwork.

Table 1.1 Reference to Chapters and Sections for particular rock-mass types

Of the Chapters on rock features, 5 and 6 deal with the core of the matter of rock description — minerals, rock-types, textures and related properties — and are relevant to all metamorphic rocks. Chapter 7 considers objects such as augen and veins which occur within some metamorphic rocks, and Chapters 8 and 9 deal with the various forms of alteration which may be associated with igneous activity, contacts or movement zones. Taken in this order, the context of each feature to be considered can be described in terms of those previously dealt with. It is therefore a suitable order in which to study and describe the aspects of a metamorphic rock-mass.

1.5 Further reading

Readers may wish to undertake further reading, or to extend their study of various aspects of the geology of their field areas. To facilitate this, a number of topics are dealt with in this book in a manner which accords with particular publications listed in Section A of Table 1.2.

Further publications chosen as being of potential interest to users of this book are shown in Section B of Table 1.2. Hobbs et al. places a somewhat similar treatment of deformation fabrics in a detailed structural context. Readers intending to undertake full structural studies are recommended to refer to Ramsay and Huber. For the field description of igneous rocks, and structural mapping, there are the companion books of this series.

Table 1.2 Selected further reading

2

Background

2.1 The stages of work

2.1.1 Preparation for fieldwork Check:

Read the handbooks of this series on mapping and on the description of different rocks, according to the types to be dealt with. (For many metamorphic areas, that means all of them.)

Note any conventions to be followed (arising from the definition of your task, from ‘house style’ or from these handbooks) regarding rock names, records of orientations, locations (e.g. grid references), units (e.g. SI only), scales for sketches or photographs (e.g. 1 metre, not a hammer or a lens-cap or a local coin), map colours and symbols, forms of local place-names (in areas of more than one language or script) and labelling of samples.

If you are new to this kind of rock-mass description, and if there is time, try making a description of any small rock exposure showing some of the features which are specifically dealt with in this book. This may help your ability to organize time in the field by showing which tasks are easy and which are not.

2.1.2 Reconnaissance

If the size of the area and the nature of the terrain allow, walk (or drive, or ride) over the area, and try to get views of as much of it as possible from higher ground. Note where it is difficult to move around on account of the topography or vegetation, the main physical barriers (e.g. rivers, cliffs), and the main routes of access to the different parts of the area.

Locate the boundaries of the rock-mass to be worked on. Note the approximate layout of outcrops of grossly different rock-types. Note, for each one, the extent and form of exposure (stream gulleys, hill-top crags, river beds), and whether the good exposures display useful information (minerals, textures, structures, veins, etc.).

Note which characters are likely to define mappable units (Section 3.2.1). Note whether the size of convenient units for description is likely to coincide with that of mappable formations, or of members of them or of groups (Section 3.2.5).

List any peculiarities which may be useful but are too specific or too local for general instructions about their treatment to have been made either during geological training or in a handbook such as this. Colour, jointing, weathering style and vegetation may correlate with rock-type (but may also vary from place to place, showing dependence on topography).

Note any minerals, geometric relationships, etc. which are likely to be troublesome and try to sort out what they are. Remember when planning your field programme that they may need extra time for study.

Also note non-geological matters, such as roads, public transport, shelter, places of refreshment, etc., which can be as important as geology in determining how fieldwork is conducted.

2.1.3 Planning a timetable

A good geological description does not demand that equal time be spent on equal geographical area, or equal area of rock exposure. It may not even result from concentrating on those rocks with greatest information content. It may be necessary to spend time searching in rocks where information is scarce. Try to work out as early as possible which localities are likely to be worth spending time on, particularly for seeking out and displaying key relationships for regional synthesis and correlation. These may include the following.

However, do not neglect descriptions of individual rock-types. Without descriptions of all the rocks being correlated, correlation is a waste of time. Remember, also, that bedded, layered or banded sequences may contain information in the nature of the banding (cyclic variation, tectonic repetition or inversion, etc.). This may not be apparent until time has been spent, for example, in constructing a log.