Granite Genesis: In-Situ Melting and Crustal Evolution

About this book

Granitic rocks are a major component of the continental crust and the many and complex problems of their origin that have challenged geologists over some 200 years still are presenting challenges today. Current ideas of granite formation involve lower crustal melting, segregation, ascent (as dykes or diapirs) and emplacement in the upper crust.

In this book we suggest an alternative model for the origin of granite in terms of in-situ meltingintracrustal convection that physically determines the process from partial melting of mid-upper crustal rocks to formation of a convecting magma layer. We illustrate the model using the geological, geochemical and geophysical studies from Australia, North and South America, Europe and China, and conclude that heat convection within a crustal partial melting layer is essential for formation of granite magma and that without convection, partial melting of rocks produces migmatites rather than granites. Granite is layer-like within the crust, and shape and size of granite bodies reflect the geometric relationship between an irregular upper surface of the crystallised magma layer and erosion surface. Repeated melting of the crust generates downward-younging granite sequences. Chemical and isotopic compositions of granites indicate differentiation within the magma rather than different deep sources. Of a number of proposed heat sources that can cause mid-upper crustal anatexis, large-scale crustal melting and formation of a granite magma layer is considered to be primarily related to plate convergence. A dynamic model with examples from the western Pacific continental margin in SE China and Tethys-Tibet is proposed to explain the relationship between plate convergence, granite and compressive deformation of the continental crust. Mineralisation related to granite formation, fault-block basins, formation of continental red beds and volcanism with examples from SE China, are also discussed in terms of the new model. In a final section, we suggest a new rock cycling model of the continental crust and the concept of Geochemical Fields of Elements, illustrating the unity between the microcosm and macrocosm of the natural world.

Contents

Preface Acknowledgements 1 Introduction 1.1 Rock genesis and its relationship to geosystems 1.1.1 Sedimentary rocks and continental geology 1.1.2 Basaltic rocks and plate tectonics 1.1.3 "Whence the granites" 1.2 Granites, migmatites and granite problems 1.2.1 Definitions 1.2.1.1 Granite 1.2.1.2 Migmatite: terminology and classification 1.2.2 Granite magma intrusion and its problems 2 Crustal melting: experiments and conditions 2.1 Introduction 2.2 Mineral melting 2.2.1 Topology of melting 2.2.2 Muscovite dehydration melting 2.2.3 Biotite dehydration melting 2.2.4 Hornblende dehydration melting 2.2.5 Biotite and hornblende melting in granitic rocks 2.2.6 Other hydrous minerals 2.2.7 Suprasolidus decompression dehydration reactions 2.3 Rock melting -- experimental evidence 2.3.1 Melt compositions 2.3.2 Restite compositions 2.3.3 Rock solidi 2.3.4 Melt fraction 2.3.5 Conclusion 2.4 Structure and composition of the crust 2.5 Water in the crust 2.6 Crustal heat and partial melting 2.6.1 Introduction 2.6.2 Thickened crust 2.6.3 Burial of high radiogenic rocks 2.6.4 Shear heating 2.6.5 Extension and removal of lithospheric mantle 2.6.6 Intrusion of mafic magma 2.6.7 Crustal thinning and "diapiric" decompression 3. In-situ melting and intracrustal convection: granite magma layers 3.1 Introduction 3.1.1 Geophysical evidence for crustal melting 3.1.1.1 Himalayas and Tibetan plateau 3.1.1.2 The Andes 3.1.2 P-T conditions of granite, migmatite and granulite formation 3.2 Crustal melting I: Initial melting and partial melt layer 3.2.1 Formation of a partial melt layer 3.2.2 Development of a partial melt layer in heterogeneous crust 3.3 Crustal melting II: Convection and formation of magma layer 3.3.1 Gravitational separation and formation of magma layer 3.3.2 Convection and development of magma layer 3.3.3 Upward thickening of magma layer 3.4 Compositional variation within magma layer 3.5 Magma layer, granite layer and granite bodies 3.6 MI fluctuation (remelting) and granite sequence 3.7 Conclusion 4. Geological evidence for in-situ melting origin of granite layers 4.1 Migmatite to granite 4.1.1 Thor-Odin dome, Canada 4.1.2 Broken Hill, Australia 4.1.3 Mt. Stafford, Australia 4.1.4 Trois Seigneurs massif, Pyrenees 4.1.5 Velay Dome, France 4.1.6 Coastal migmatite-granite zone, SE China 4.1.7 Cooma and Murrumbidgee, Australia 4.1.8 Optica grey gneiss, Canada 4.2 Contact metamorphism 4.3 Xenoliths and mafic enclaves 4.4 Granite layer and granite exposures 4.5 Fluctuation of MI and downward younging granite sequence 5. Differentiation of magma layer: geochemical considerations 5.1 Introduction 5.2 Compositional variation 5.3 Strontium isotopes 5.4 Oxygen isotopes 5.5 Rare earth elements 5.6 Summary 6. Mineralisation related to in-situ granite formation 6.1 Introduction 6.2 Source of ore-forming elements 6.3 Formation and evolution of ore-bearing fluid 6.4 Types of mineral deposits 6.4.1 Vein mineralisation 6.4.2 Disseminated mineralisation 6.5 Age relationships 6.6 Temperature distribution 6.7 Formation and distribution of hydrothermal mineral deposits 6.7.1 Precipitation of ore-forming elements 6.7.2 Oxygen isotope evidence 6.8 Mineralised depth horizons 6.9 Mineralisation during elevated crustal temperatures 6.10 Mineralisation during granite remelting 6.10.1 Oxidation 6.10.2 Uranium mineralisation 6.11 Patterns of element redistribution and element fields 6.12 Summary 7. Heat source for crustal magma layers: tectonic models 7.1 Introduction 7.2 Crustal temperature disturbance related to plate convergence 7.3 Subduction and granite formation: western Pacific continental margin 7.3.1 Introduction 7.3.2 Tectonic framework of SE China and granite formation 7.3.3 Tectonic model 7.3.4 Multiple melting (remelting) and granite belts 7.3.5 Summary 7.4 Continental collision and granite formation: Tethys Belt 7.4.1 Tectonic framework and granite distribution of Tibet plateau 7.4.2 Tectonic phases in relation to subduction and collision 7.4.3 Magma layers and plate convergence 7.5 Concluding statement 8. Geological effects of crystallisation of a crustal granite magma layer: SE China 8.1 Fault-block basins 8.1.1. Characteristics and distribution of Mesozoic basins 8.1.2. Basin formation 8.1.3. Origin of red beds 8.1.4. Summary 8.2. Volcanism 9. Material and element cycling of the continental crust and summary 9.1. Rock cycling of continental material 9.2. Element cycling of the continental crust 9.3. Overview References Appendix 1 Map showing provinces of SE China Appendix 2 Results of experimental rock melting

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