Myron G. Best. Igneous and metamorphic 2003 Blackwell Science
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Myron G. Best
Brigham Young University
IGNEOUS AND
METAMORPHIC PETROLOGY
SECOND EDITION
IGNEOUS AND
METAMORPHIC PETROLOGY
To
Viv
Karl, Richard, Tyler
Karen, Jenny, Teresa, Katrina, Laura
Myron G. Best
Brigham Young University
IGNEOUS AND
METAMORPHIC PETROLOGY
SECOND EDITION
© 2003 by Blackwell Science Ltd
a Blackwell Publishing company
Editorial Offices:
350 Main Street, Malden, MA 02148-5018, USA
108 Cowley Road, Oxford OX4 1JF, UK
550 Swanston Street, Carlton South, Melbourne, Victoria 3053, Australia
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The right of Myron G. Best to be identified as the Author of this Work has been asserted in
accordance with the UK Copyright, Designs and Patents Act 1988.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval
system, or transmitted, in any form or by any means, electronic, mechanical, photocopying,
recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act
1988, without the prior permission of the publisher.
First published 2003 by Blackwell Science Ltd
Library of Congress Cataloging-in-Publication Data
Best, Myron G.
Igneous and metamorphic petrology / Myron G. Best.—2nd ed.
p. cm.
Includes bibliographical references and index.
ISBN 1-40510-588-7 (alk. paper)
1. Rocks, Igneous. 2. Rocks, Metamorphic. I. Title.
QE461 .B53 2002
552′.1—dc21
A catalogue record for this title is available from the British Library.
On the cover: Photographs of rock from the upper mantle and deep continental crust.
Beneath the lettering is a photomicrograph of peridotite viewed in cross-polarized light.
The other rock lies in an outcrop in Swaziland along the Ngwempezi River and is Archean
mafic gneiss that was probably derived from a basaltic protolith initially metamorphosed
at 3.5 Ga, making it one of the oldest rocks exposed on the African continent. The gneiss
was subjected to at least three episodes of deformation and nine intrusive events, the youngest
product being a 2.6 Ga felsic pegmatite seen here. Photograph courtesy of Cees Passchier.
Set in 10/12pt Simoncini Garamond
by Graphicraft Limited, Hong Kong
Printed and bound in
Italy by G. Canale & C. S.p.A., Turin
For further information on
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v
CONTENTS
P
REFACE XIX
C
HAPTER
1:
Overview of Fundamental Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
C
HAPTER
2:
Composition and Classification of Magmatic Rocks . . . . . . . . . . . . . . . . 16
C
HAPTER
3:
Thermodynamics and Kinetics: An Introduction . . . . . . . . . . . . . . . . . . . 51
C
HAPTER
4:
Silicate Melts and Volatile Fluids in Magma Systems . . . . . . . . . . . . . . . . 72
C
HAPTER
5:
Crystal-Melt Equilibria in Magmatic Systems . . . . . . . . . . . . . . . . . . . . . 87
C
HAPTER
6:
Chemical Dynamics of Melts and Crystals . . . . . . . . . . . . . . . . . . . . . . . 122
C
HAPTER
7:
Kinetic Paths and Fabric of Magmatic Rocks . . . . . . . . . . . . . . . . . . . . 148
C
HAPTER
8:
Physical and Thermal Dynamics of Bodies of Magma . . . . . . . . . . . . . . 183
C
HAPTER
9:
Magma Ascent and Emplacement: Field Relations of Intrusions . . . . . . 210
C
HAPTER
10:
Magma Extrusion: Field Relations of Volcanic Rock Bodies . . . . . . . . . 241
C
HAPTER
11:
Generation of Magma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283
C
HAPTER
12:
Differentiation of Magmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316
C
HAPTER
13:
Magmatic Petrotectonic Associations . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
C
HAPTER
14:
Metamorphic Rocks and Metamorphism: An Overview . . . . . . . . . . . . 404
C
HAPTER
15:
Petrography of Metamorphic Rocks: Fabric, Composition,
and Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447
C
HAPTER
16:
Metamorphic Mineral Reactions and Equilibria . . . . . . . . . . . . . . . . . . 473
C
HAPTER
17:
Evolution of Imposed Metamorphic Fabrics: Processes and Kinetics . . 520
C
HAPTER
18:
Metamorphism at Convergent Plate Margins: P–T–t Paths, Facies,
and Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 564
C
HAPTER
19:
Precambrian Rock Associations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 610
A
PPENDIX
A 657
A
PPENDIX
B 661
R
EFERENCES
C
ITED
666
G
LOSSARY
691
I
NDEX
717
vii
PREFACE
CHAPTER 1
OVERVIEW OF FUNDAMENTAL CONCEPTS
1.1 ENERGY AND THE MANTLE HEAT ENGINE . . . . . . . . . . . . 2
1.1.1 Forms of Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.2 Flow and Transformation of Energy . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.3 Heat Flow in the Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.4 Implications of Mantle Convection . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.1.5 Energy Budget of the Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2 GRAVITY, PRESSURE, AND GEOBARIC GRADIENT . . . . . . 10
1.3 ROCK-FORMING PROCESSES AS CHANGING STATES
OF GEOLOGIC SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.4 ROCK PROPERTIES AND THEIR SIGNIFICANCE . . . . . . . . 11
1.4.1 Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.4.2 Field Relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.4.3 Fabric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.5 HOW PETROLOGISTS STUDY ROCKS . . . . . . . . . . . . . . . . . 14
CHAPTER 2
COMPOSITION AND CLASSIFICATION OF MAGMATIC ROCKS
2.1 ANALYTICAL PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.1.1 Sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.1.2 Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2 MINERAL COMPOSITION OF MAGMATIC ROCKS . . . . . . . 22
2.2.1 Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.3 CHEMICAL COMPOSITION OF MAGMATIC ROCKS . . . . . 23
2.3.1 Variation Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.3.2 Continuous Spectrum of Rock Compositions . . . . . . . . . . . . . . . . 24
2.4 CLASSIFICATION OF MAGMATIC ROCKS . . . . . . . . . . . . . . 25
2.4.1 Classification Based on Fabric . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.4.2 Classification Based on Field Relations . . . . . . . . . . . . . . . . . . . . . 27
2.4.3 Classification Based on Mineralogical and
Modal Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.4.4 Classification Based on Whole-Rock Chemical Composition . . . . 30
2.4.5 Rock Suites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.4.6 Classification of Basalt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.5 TRACE ELEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.5.1 Partition Coefficients and Trace Element Compatibility . . . . . . . . 38
2.5.2 Rare Earth Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.5.3 Other Normalized Trace Element Diagrams . . . . . . . . . . . . . . . . . 42
2.6 ISOTOPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.6.1 Stable Isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
CONTENTS
viii
2.6.2 Radiogenic Isotopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
2.6.3 Cosmogenic Isotopes: Beryllium . . . . . . . . . . . . . . . . . . . . . . . . . . 47
CHAPTER 3
THERMODYNAMICS AND KINETICS: AN INTRODUCTION
3.1 WHY IS THERMODYNAMICS IMPORTANT? . . . . . . . . . . . . 51
3.2 ELEMENTARY CONCEPTS OF THERMODYNAMICS . . . . . 52
3.2.1 Thermodynamic States, Processes, and State Variables . . . . . . . . . 52
3.2.2 First Law of Thermodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.2.3 Enthalpy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.2.4 Entropy and the Second and Third Laws of Thermodynamics . . . . . 54
3.2.5 Gibbs Free Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.3 STABILITY (PHASE) DIAGRAMS . . . . . . . . . . . . . . . . . . . . . . . 56
3.3.1 Slope of the Melting Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.3.2 Determination of Phase Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.4 THERMODYNAMICS OF SOLUTIONS: SOME
BASIC CONCEPTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.4.1 Components and Mole Fractions . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.4.2 Partial Molar Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.4.3 Partial Molar Gibbs Free Energy: The Chemical Potential . . . . . . 60
3.4.4 P-T-X Phase Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.5 APPLICATION OF THERMODYNAMICS
TO SOLUTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.5.1 Fugacity and Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.5.2 Equilibrium Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.5.3 Silica Activity, Silica Buffers, and Silica Saturation . . . . . . . . . . . . 63
3.5.4 Oxygen Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.5.5 Fe-Ti Oxide Buffers: Oxygen Geobarometers
and Geothermometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.6 KINETICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.6.1 Activation Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.6.2 Overstepping and Metastable Persistence and Growth . . . . . . . . . 68
CHAPTER 4
SILICATE MELTS AND VOLATILE FLUIDS IN MAGMA SYSTEMS
4.1 NATURE OF MAGMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4.1.1 Atomic Structure of Melts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4.2 VOLATILE FLUIDS IN MELTS . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.2.1 Nature of Volatiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.2.2 Solubilities of Volatiles in Silicate Melts . . . . . . . . . . . . . . . . . . . . 76
4.2.3 Exsolution of Volatiles from a Melt . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.3 CONSEQUENCES OF FLUID EXSOLUTION
FROM MELTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.3.1 Explosive Volcanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.3.2 Global Atmosphere and Climate . . . . . . . . . . . . . . . . . . . . . . . . . . 82
4.3.3 Fumaroles, Hydrothermal Solutions, Ore Deposits, and
Geothermal Reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84