Metamorphic Processes

This book is for senior undergraduate or postgraduate students who want an insight into some modern approaches to metamorphic petrology. Its aims are to explain, in reasonably simple, informal terms, the processes underlying (i) metamorphic reactions and (ii) the production of micro­ structures in metamorphic rocks, these currently being the things that interest me most, geologically. The first aim requires discussion of equilibrium factors, reaction kinetics and reaction mechanisms, empha­ sising both the complexity of realistic reactions and the need to combine the chemical and microstructural approaches to them. The second aim requires discussion of deformation, recovery, recrystallisation and grain growth processes, with emphasis on experiments on silicate minerals. The book concludes with a general attempt to relate chemical and physical processes in metamorphism, although it will be clear from reading earlier chapters (especially Chapter 4) that the two aspects can rarely be separated completely in detailed metamorphic studies. Petrological and experimental investigations of metamorphic reactions and microstructural development are advancing so rapidly these days that students are faced with an ever-increasing volume of information and a relatively rapid obsolescence of data. So, in this book I do not try to be comprehensive, or to present much so-called 'factual' information. Instead, I deal more with basic principles, in the hope that these will guide the student in his or her encounters with the details of specific metamorphic problems.

1 Background Discussion.- Scope of Metamorphism.- Temperatures and Pressures of Metamorphism.- Movement of Material in Metamorphism.- Mineral Assemblages (‘Pangeneses’).- Microstructures of Metamorphic Rocks.- Preferred Orientation in Metamorphic Rocks.- Metamorphic Grade.- Metamorphic Facies.- Tectonic Setting of Metamorphism.- References.- 2 Equilibrium Aspects of Metamorphic Reactions.- Driving Force for Metamorphic Reactions.- Types of Metamorphic Reactions.- Variance of Metamorphic Reactions.- Metamorphic Reactions and Resulting Assemblages.- Sliding Reactions at Isograds.- Effect of One Phase on the Stability of Another.- Criteria of Stable Coexistence of Metamorphic Minerals.- Domains of Local (Mosaic) Equilibrium.- Calculation of the Topology of Metamorphic Phase Diagrams (Schreinemakers’ Analysis).- Calculation of Slopes of Reaction Curves.- Experimental Location of Reaction Curves.- Reversal of Experimental Reactions.- Discrepancies Between Calculated and Experimentally Determined Reaction Curves.- Open Systems in Metamorphism.- Fluid Phase in Metamorphism.- Pressure of Fluid Phase.- Effect of Fluid Pressure on Reaction Curves.- Effects of Composition of Fluid Phase (Fugacities of Mixed Volatile Components) on Reaction Curves.- Effect of Oxygen Fugacity.- Coexisting Magnetite and Ilmenite.- Effect of fO2 on Hydrous Silicates Without Cations of Variable Valency.- Distribution of Elements Between Coexisting Minerals.- References.- 3 Kinetics of Metamorphic Reactions.- Diffusion in Metamorphism.- Nucleation and Growth.- Application of Reaction Kinetics to Metamorphism.- Zoning in Metamorphic Minerals.- Problems of Aragonite in Metamorphic Rocks.- Kinetics of the Reaction: Calcite + Quartz ? Wollastonite + CO2.- References.- 4 Reactions in Metamorphic Rocks.- Mechanisms of Metamorphic Reactions.- Reactions in Metapelitic Systems.- Reactions in Mafic and Ultramafic Systems.- How Relevant Are Available Experimental and Theoretical Determinations of P-T-X Equilibria?.- Retrograde Reactions.- Reactions Involving Zoned Minerals.- Reactions Involving Zoned Garnet.- Compositional Changes During Metamorphism (‘Metamorphic Differentiation’).- References.- 5 Stable Metamorphic Microstructures.- Unstable Grain Arrangements.- Stable Grain Arrangements in Isotropic Single Phase Aggregates.- Anisotropic Single Phase Aggregates.- Aggregates with More than One Phase.- Spatial and Size Distribution of Grains in Metamorphic Rocks.- Environments of Grain Adjustment in the Solid State.- References.- 6 Deformation, Recovery and Recrystallisation Processes.- Brittle Versus Ductile Deformation.- Crystal Plasticity.- Primary Modes of Intragranular Plastic Deformation.- Secondary Modes of Deformation.- Dislocations.- How Do Dislocations Assist Deformation?.- Experimental Detection of Dislocations.- Mobility of Dislocations.- Production of Dislocations.- Dislocations in Relation to Work-Hardening and -Softening.- Partial Dislocations.- Dislocations in Non-Metals.- Surface Imperfections 160 Effect of Grain Boundaries on Deformation (Intergranular Deformation).- Recovery and Recrystallisation 166 Development of Crystallographic Preferred Orientation by Recrystallisation.- Development of Preferred Orientations by Deformation.- Some Generalities.- Appendix: Methods of Detecting Slip Systems.- References.- 7 Deformation, Recovery and Recrystallisation of Some Common Silicates.- Quartz.- Microstructural Features of Deformed Quartz.- Experimental Deformation, Recovery and Recrystallisation of Quartz.- Natural Deformation, Recovery and Recrystallisation of Quartz.- Preferred Orientation.- Variation in Quartz Microfabric with Metamorphic Conditions.- Stress Indicators in Deformed Quartz.- Olivine.- Experimental Deformation, Recovery and Recrystallisation of Olivine.- Natural Deformation, Recovery and Recrystallisation of Olivine.- Preferred Orientation of Olivine.- Layer Silicates.- Deformation of Mica Single Crystals.- Deformation of Fine-Grained Layer Silicate Aggregates.- Recovery and Recrystallisation of Mica Single Crystals.- Experimental ‘Recrystallisation’ of Fine-Grained Layer Silicate Aggregates.- Preferred Orientation Mechanisms in Layer Silicates.- Systematic Study of Natural Deformation and Recrystallisation of Mica.- Plagioclase.- Experimental Deformation of Plagioclase.- Natural Deformation of Plagioclase.- Experimental Recrystallisation of Plagioclase.- Natural Recrystallisation of Plagioclase.- Generalisations.- References.- 8 Relationships between Chemical and Physical Processes in Metamorphism.- Effect of Chemical Reactions on Rock Mechanical Properties.- Effect of Differential Deformation on Mineral Assemblage.- Examples of Metamorphic Layering Formed By Preferential Removal of Minerals.- Large-Scale Metasomatism and Deformation.- Pegmatites and Deformation.- Migmatites.- Time Relationships between Deformation and Mineral Growth.- Solution Transfer.- Spinodal Decomposition?.- References.- Author Index.