Table Of Contentgranites
Granites
Petrology, Structure,
Geological Setting, and Metallogeny
Anne Nédélec and Jean-Luc Bouchez
translated and updated with the participation of
Peter Bowden
1
1
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Original Edition: Pe´trologie des granites – Structure – Cadre ge´ologique
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Acknowledgements
Many people have contributed to the successful completion of this book.
They include:
• Christiane Cavaré, a Paganini in drawings and layouts, who patiently and kindly
helped the authors to complete every page, never complaining about the successive
changes made to diagrams and text, up to manuscript completion.
• Our laboratory (Laboratoire des Mécanismes et Transferts en Géologie, that became
Géosciences Environnement Toulouse) and our university where we had all facilities
to aid our teaching and research during more than twenty years.
• Our students from France, Brazil, Burkina Faso, Madagascar, Cameroon, Iran,
Belgium, Portugal, Spain, UK . . . , who gave us so much and helped us to progress.
• Our colleagues, from the Petrophysics team, in particular Gérard Gleizes for his
boundless energy and Philippe Olivier for his picky kindness.
• Our first reviewers, Professor Adolphe Nicolas (Montpellier University) and Pro-
fessor Hervé Martin (Clermont-Ferrand University), for their expertise, enthusi-
asm and long-standing friendship.
• Jean-François Moyen and Patrick Thommen for their friendly support.
• Our former publisher, who confidently gave us credit for the French edition Pétrol-
ogie des granites: structure, cadre géologique, under the heading of Patrick de Wever,
Professor at the Museum National d’Histoire Naturelle of Paris.
• Peter Bowden, for the highly valuable assistance brought to this book, with so many
suggestions in both Science and the English language through systematic changes
toward eliminating our gallicisms. Your culture and energy helped us to be accepted
by a prestigious publisher, Oxford University Press.
• Sonke Adlung, our commissioning editor at OUP, for his encouragements.
Toulouse, March 2014
Anne Nédélec
Jean-Luc Bouchez
Contents
1 What is a granite? 1
1.1 Classification of granites 1
Box 1.1 Granite and granit 4
1.2 Petrography 6
1.2.1 Texture 6
1.2.2 Mineralogy 6
1.3 Granites: rocks representative of the continental crust 8
1.3.1 Structure of the continental crust 8
1.3.2 Composition of the continental crust 9
1.4 Granites and related rocks 10
1.4.1 Concept of magmatic series 10
1.4.2 Granite types and magmatic series 11
1.5 Conclusions 11
2 Origin of granitic magmas 12
2.1 Field data: migmatites and granites 12
Box 2.1 From neptunism to modern magmatism 12
2.2 Experimental data 15
2.2.1 Early experiments 15
2.2.2 The importance of water 18
2.2.3 Melting experiments with hydrous phases 20
2.3 Fertility of crustal protoliths and melt compositions 25
2.4 Tracing the sources with isotopes 26
2.4.1 The Rb–Sr pair and the 87Sr/86Sr isotopic tracer 26
2.4.2 The Sm–Nd pair and the 143Nd/144Nd isotopic tracer 28
2.4.3 Coupling Sr and Nd tracers 30
2.4.4 The 18O/16O ratio 31
2.4.5 Hf isotopes in zircon 32
Box 2.2 Oxygen isotope palaeothermometry 33
2.5 Crustal vs. mantle contributions 34
2.6 Conclusions 39
3 Segregation of granitic melts 40
3.1 Viscosity of granitic melts 40
3.2 Melt behaviour during partial melting 41
Box 3.1 Viscosity as a function of composition, temperature and water content 42
viii Contents
3.3 Progressive partial melting experiments 43
3.3.1 Melt location in a static aggregate 44
3.3.2 Dynamic melting of a granite 45
3.3.3 Dynamic partial melting of an amphibolite 46
3.4 Melt distribution in migmatites 47
3.4.1 Melt segregation rate 48
3.4.2 Role of the melting reaction 51
3.4.3 Role of gravity-driven compaction 51
3.4.4 Fracturing assisted by melt pressure 51
Box 3.2 Filter-press mechanism: a dynamic compaction mode 52
3.5 Migmatites in the field 53
3.5.1 Layering in migmatites 54
3.5.2 Other migmatitic structures 55
3.6 Conclusions 57
4 Genesis of hybrid granitoids: mingling and mixing 58
4.1 Field observations 58
4.2 Influence of viscosity on magma mingling or mixing 59
Box 4.1 Viscosity of a suspension: Einstein’s model 60
4.3 Characteristics of hybrid rocks 62
4.3.1 Mineralogy 62
4.3.2 Major and trace element chemistry 63
4.3.3 Isotopic signatures 65
4.4 Processes of magma mingling and mixing 66
4.4.1 Element interdiffusion in silicate liquids 66
4.4.2 Transfer of granitic liquid into mafic magma enclaves 67
4.4.3 Dispersion of mafic magma into a granitic magma host 67
4.5 Where do hybrid magmas form? 69
4.6 Conclusions 72
5 Transport of granitic magma 73
5.1 Diapirism 74
5.2 Transport by dykes 77
5.2.1 Analysis and orders of magnitude 77
5.2.2 Transfer from the source 79
5.2.3 Role of magma pressure 79
5.2.4 Fracturing the crust 80
5.2.5 Magma ascent rate 83
5.3 Field data 84
5.4 Conclusions 86
6 Emplacement and shape of granite plutons 87
6.1 Cessation of magma ascent 87
Box 6.1 Emplacement in an extensional context: a simple exercise of mechanics 88
Contents ix
6.2 Meeting dilatant sites 90
6.2.1 Fracturing in extensional environments 90
6.2.2 Fracturing in compressive environments 94
6.3 Granite emplacement in the ductile crust 98
6.3.1 Sills and sheet-like granites 98
6.3.2 Migmatitic domes 100
6.4 Emplacement depth 101
6.5 Three-dimensional shapes of plutons 102
6.5.1 Bouguer anomaly map 102
6.5.2 Residual anomaly and density contrast 104
6.5.3 Modelling pluton depth and floor shape 104
6.5.4 Pluton shapes 105
6.6 Passive versus forced emplacement 107
6.7 Conclusions 108
7 Thermomechanical aspects in the country rocks around
granite plutons 109
7.1 Conductive heat transfer 109
Box 7.1 Conductive heat transfer: the virtue of non-dimensional variables 110
7.2 Convective heat transfer 113
7.3 Diachronic metamorphism and rheological changes at the contact 114
7.4 Parageneses of contact metamorphism 116
7.4.1 Pelitic rocks 116
7.4.2 Carbonate rocks 118
7.5 Thermomechanical aspects at the crustal scale 120
7.5.1 Regional contact metamorphism 120
7.5.2 Crustal decoupling and HT-LP metamorphism 124
7.6 Conclusions 126
8 Crystallization of granitic magmas 127
8.1 General considerations on nucleation and crystal growth 127
8.1.1 Nucleation 127
8.1.2 Crystal growth 128
8.1.3 Crystal size distributions (CSDs) 131
8.1.4 Evolution of crystallinity with time and its rheological
consequences 134
8.2 Order of crystallization of minerals 137
8.2.1 Textural observations 137
8.2.2 Influence of magma composition on the order
of crystallization 138
8.3 Fractional crystallization and magmatic differentiation 141
8.3.1 History of magmatic differentiation 141
8.3.2 Evidence of fractional crystallization 142
Box 8.1 Compatible and incompatible elements 145
