Magmatism in Extensional Structural Settings

I. East African Rift Volcanism.- Distribution of the Main Phanerozoic African Rifts and Associated Magmatism: Introductory Notes.- 1 Introduction.- 2 The Palaeozoic African Rifts.- 3 The Mesozoic African Continental Rifts.- 3.1 Southern Africa.- 3.2 Western Africa.- 3.3 Eastern Africa.- 4 The Cainozoic African Continental Rifts.- 4.1 The East African Rift System (EARS).- 4.2 Northeastern Africa.- 4.3 West Central Africa.- 5 Summary.- Acknowledgements.- 1 The Discovery of the African Rift Geology: A Summary.- 1 Foreward.- 2 Beginnings.- 3 1880 to World War I: Geological Pioneers and Eduard Suess' Synthesis.- 4 Post-World War I: New Reviews and Old Theories.- 5 Old Problems and New Theories.- 6 Post-1939: The New Revolution Ignores Its Predecessors.- 7 References.- 2 Phanerozoic Volcanism of Southwest Uganda: A Case for Regional K and lile Enrichment of the Lithosphere Beneath a Domed and Rifted Continental Plate.- 1 Introduction.- 2 Tectonic and Geological Setting of the Major Volcanic Areas.- 2.1 The Southern Field of Bufumbira.- 2.2 The Central Fields of Bunyaruguru (+Katunga) and Katwe-Kikorongo.- 2.3 The Northern Fields of Fort Portal and Kasekere.- 2.4 Other Volcanic Occurrences.- 3 Volcanic Style and Products.- 4 The Nature of the Volcanic Activity.- 5 Petrography.- 5.1 The Host Melts.- 5.2 Ultramafic Xenoliths.- 6 Information Provided by P, T Experiments and Mineral Chemistry: Xenoliths, Xenocrysts and Phenocrysts (OBP series).- 6.1 Olivine.- 6.2 Clinopyroxene.- 6.3 Dark Mica.- 6.4 OBP Series: Summary.- 6.5 Ores.- 7 Lower Pressure and Groundmass Assemblages.- 7.1 Melilite.- 7.2 Leucite.- 7.3 Nepheline and Kalsilite.- 7.4 Clinopyroxene.- 7.5 Phlogopite with mol Na2O + K2O>Al2O3.- 7.6 Glasses.- 7.7 Carbonatite Groundmass.- 8 Bulk Rock Chemistry.- 8.1 Major and Minor Element Chemistry.- 8.2 Trace and Rare Earth Elements.- 8.3 Sr, Nd and Pb Isotope Systematics.- 9 Summary of the Key Physical, Petrographic and Chemical Features of South West Ugandan Quaternary Volcanism.- 9.1 Physical.- 9.2 Petrographic.- 9.3 Chemical.- 10 Discussion.- Acknowledgements.- 3 Source Regions for Ultrapotassic Mafic-Ultramafic Magmatism in the Southwest Uganda Region of the African Rift: Implications from Experimental Studies.- 1 Introduction.- 2 Chemistry of the Mafic-Ultramafic Ultrapotassic Rocks of the Southwest Ugandan Area.- 3 Inferences of Experimental Studies to Source Regions.- 4 Experimental Techniques.- 5 Inferred Mantle Source Regions from Experiments on Ugandite, Katungite and Mafurite.- 6 Inferred Source for Ultrapotassic Magmas from Partial Melting Experiments of a Model Clinopyroxenite Nodule.- 7 Implications of Experiments on Magmatic Processes in Continental Rift Regions.- Acknowledgements.- 4 Magmatic Evolution and Petrogenesis in the East African Rift System.- 1 Introduction.- 2 Regional Setting of Volcanism in the East African Rift.- System.- 3 Volcanic and Structural Evolution in Eastern Africa.- 3.1 Eastern Rift.- 3.2 Western Rift.- 4 Classification of Rift Magmatic Types.- 4.1 Suite 1: Ultraalkaline/Carbonatitic Rocks.- 4.2 Suite 2: Alkaline Rocks.- 4.3 Suite 3: Transitional Rocks.- 4.4 Suite 4: Tholeiitic Rocks.- 5 The Nature of the Mantle Beneath Eastern Africa: Information from Ultramafic Xenoliths.- 6 The Nature of the Mantle Beneath Eastern Africa: Inferences from Mafic Rock Trace Element Geochemistry.- 6.1 General Remarks.- 6.2 ITE Variations Related to Basalt Composition.- 6.3 ITE Variations Related to Degree of Crustal/Lithospheric Extension.- 6.4 ITE Variations at Individual Volcanic Centres and Within Given Volcanic Subprovinces: Small-Scale Mantle Heterogeneities.- 6.5 Medium-Scale Lateral Heterogeneities in the Eastern African Mantle.- 6.6 Large-Scale Lateral Heterogeneities in the Eastern African Mantle.- 7 The Nature of the Mantle Beneath Eastern Africa: Inferences from Isotope Geochemistry.- 7.1 Nd and Sr Isotopes.- 7.2 Pb Isotopes.- 7.3 Th and Ra Isotopes.- 7.4 O and C Isotopes.- 8 Evidence for Mantle Source Enrichment and Identification of Mantle End-Members.- 9 Silicic Rocks: Fractionation, Assimilation or Anatexis?.- 9.1 Crustal Anatexis.- 9.2 Assimilation.- 9.3 Crystal Fractionation.- 10 Magma Genesis and Asthenosphere-Lithosphere Dynamics..- 10.1 Magmatism and Rift Propagation.- 10.2 Magmatism and Modality of Evolution of Rift Systems.- 11 Conclusions.- Acknowledgements.- II. West African Magmatism.- West African Magmatism: Introduction.- 1 Upper Mantle Features Beneath West Africa: Relevant Trace Element Constraints.- 2 Upper Mantle Features Beneath West Africa: Inferences from Isotopic Constraints.- 3 Discussion and Conclusion.- Acknowledgements.- 5 The Mesozoic Tholeiitic Province of Northwest Africa: A Volcanotectonic Record of the Early Opening of Central Atlantic.- 1 Introduction.- 2 Geological Setting.- 2.1 Mode of Occurrence.- 2.2 Structural Outline.- 2.3 Age of the Dolerites.- 3 Petrography and Mineralogy.- 3.1 Textures.- 3.2 Mineralogy.- 3.3 Pyroxene Chemistry.- 4 Geochemistry.- 4.1 Sampling and Analytical Procedures.- 4.2 Major Elements.- 4.3 Trace Elements.- 4.4 Geochemistry and Tectonic Setting.- 5 Petrogenesis.- 5.1 Alteration.- 5.2 Low-Pressure Fractionation.- 5.3 The Origin of the Continental Tholeiites: Mantle Source, Partial Melting, Crustal Contamination.- 6 Geodynamic Implications.- 6.1 Location of the CT Mantle Source.- 6.2 Mechanism of Continental Rifting and Breakup.- Acknowledgements.- Cretaceous and Cainozoic Magmatism of the Senegal Basin (West Afcrica): A Review.- 1 Introduction.- 2 Geological Setting.- 2.1 Geodynamic Framework.- 2.2 Structure and Stratigraphy of the Senegal Basin.- 3 Cretaceous Magmatism.- 4 Tertiary Volcanism.- 4.1 Stratigraphy and Age of Volcanism.- 4.2 Types of Occurrence and Volcanic Activities.- 4.3 Petrographic Features of the Lavas.- 5 Quaternary Volcanism.- 5.1 Stratigraphy and Age of Volcanism.- 5.2 Types of Volcanic Activities.- 5.3 Petrographic Features of the Lavas.- 6 Comparisons with Other Central Atlantic Alkaline Provinces.- 7 Conclusion.- Acknowledgements.- 7 The Canary Islands: Tectonics, Magmatism and Geodynamic Framework.- 1 Introduction.- 2 Geodynamic Evolution of the Canary Islands Area.- 3 The Crust Under the Canary Islands Region.- 4 Network of Fractures and Identity of Vertical Movements in the Canary Islands Region.- 4.1 Transoceanic and Continental Fracture Zones.- 4.2 Southern Atlas Fault.- 4.3 Faults Which have been Deduced by Volcanological, Geological and Geophysical Methods.- 4.4 Characteristics of Displacements by Faults.- 4.5 Crustal Shortening Model.- 5 Petrology and Mineralogy of the Canary Islands Igneous Rocks.- 5.1 Basal Complexes.- 5.2 Old Basaltic Series.- 5.3 Recent Series.- 6 Relationship Between the Tectonics of the Area and the Genesis and the Eruption of Magmas.- 7 Conclusions.- 8 Relationships Between Cainozoic Magmatism and Upper Mantle Heterogeneities as Exemplified by the Hoggar Volcanic Area (Central Sahara, Southern Algeria).- 1 Introduction.- 2 Nature of the Upper Mantle Beneath Hoggar.- 2.1 Heterogeneities due to Magmatism.- 2.2 Heterogeneities due to Partial Melting.- 2.3 Heterogeneities due to Metasomatism.- 3 Nature of the Basaltic Lava Sources.- 4 Conclusions.- III. Ring Complexes and Related Structures.- Ring Complexes and Related Structures: Introduction.- 1 Classification of Anorogenic Granitoids.- 2 Characteristics of the Basic Rocks Emplaced in Anorogenic Ring Complexes.- 3 Genetic Relationships of Basic Igneous Rocks and Granites in Ring Complexes.- 4 Conclusion.- Acknowledgements.- 9 The Cameroon Line: A Review.- 1 Introduction.- 2 Various Interpretations of the Cameroon Line.- 3 A New Tectonic Model of the Cameroon Line.- 4 Anorogenic Ring-Complexes.- 4.1 Mount Bana.- 4.2 NdaAli.- 4.3 Ntumbaw.- 4.4 Mayo Darle.- 4.5 Guenfalabo.- 4.6 Mboutou.- 4.7 Golda Zuelva.- 4.8 Other Intrusive Anorogenic Massifs.- 4.9 Conclusion.- 5 Volcanism.- 5.1 Pagalu.- 5.2 Sao Tome.- 5.3 Principe.- 5.4 Bioko.- 5.5 Mount Etinde.- 5.6 Mount Cameroon.- 5.7 Kumba Graben.- 5.8 Rumpi Hills.- 5.9 Tombel Plain.- 5.10 Manengouba.- 5.11 Bamoun Plateau.- 5.12 Mounts Bambouto.- 5.13 Oku Massif.- 5.14 Garoua Upper Benue Valley.- 5.15 Kapsiki Plateau.- 5.16 Other Volcanic Regions in Cameroon.- 5.17 Conclusion.- 6 Discussion About the Nature of the Cameroon Line.- Acknowledgements.- 10 Palaeozoic Magmatism in the Air Massif, Niger.- 1 Introduction.- 2 History of Geological Research.- 3 Geological Setting.- 3.1 The Precambrian Basement.- 3.2 Palaeozoic Subvolcanic Ring-Structures.- 3.3 Volcanism.- 4 Structure.- 4.1 Gravity Data.- 4.2 Remote Sensing.- 4.3 Autocorrelation Analysis.- 4.4 Interpretation.- 5 Ring-Complexes of Taghouaji Type.- 5.1 The Taghouaji Ring-Complex.- 5.2 Conclusion.- 6 Ring-Complexes of Goundai Type.- 6.1 Goundai.- 6.2 Bilet.- 6.3 Relationship Between the Goundai-Type and the Uranium Deposits of Arlit.- 7 Ring Complexes of Ofoud Type.- 8 Discussion and Conclusions.- 8.1 Age Relationships.- 8.2 Hypotheses on the Origin of the Lineament.- 11 Ring-Complexes of Ofoud-Type in Air, Niger: A new Anorogenic-Type Anorthosite Association.- 1 Introduction.- 2 Field Relationships, Petrology and Mineralogy of the Complexes.- 2.1 Bous.- 2.2 Tamgak-Tchinadene.- 2.3 Ofoud.- 2.4 Meugueur-Meugueur.- 2.5 Taguei.- 2.6 Abontorok.- 2.7 Iskou.- 3 Geochemistry.- 3.1 Major Elements.- 3.2 K-Rb Data.- 3.3 REE Distributions.- 4 Strontium Isotopie Composition.- 4.1 Basic Rocks.- 4.2 Salic Rocks.- 5 Discussion and Conclusions.- 5.1 Summary of the Main Characteristics of Ofoud-Type Ring-Complexes.- 5.2 Comparison of Ofoud-Type Ring-Complexes with Other Anorthosite Occurrences.- 5.3 Comparison of the Ofoud-Type with Other Anorogenic Ring-Complexes.- Acknowledgements.- 12 The Chilwa Alkaline Igneous Province of Malawi: A Review.- 1 Introduction.- 2 The Chilwa Province.- 2.1 Age of the Province.- 2.2 Igneous Activity and Rift Tectonics.- 3 The Earliest Work on the Chilwa Alkaline Province-Dixey, Campbell Smith and Bisset.- 4 The 1950's and 1960's and the Work of the Geological Survey.- 5 Work Since 1969.- 5.1 Fenitization.- 5.2 Mineralogy.- 5.3 Petrochemistry.- 5.4 Petrogenesis.- Acknowledgements.- 13 Magmatism and Mineralization Associated with Phanerozoic Anorogenic Plutonic Complexes of the African Plate.- 1 Introduction.- 2 Oversaturated Alkaline Complexes with Granites and Syenites.- 2.1 Introduction.- 2.2 Structure and Petrology.- 2.3 Problems of Classification.- 2.4 Age and Petrological Variations of the Different Provinces.- 2.5 Major Element Chemistry of the Oversaturated Ring- Complexes.- 2.6 Ttace Element Chemistry of the Oversaturated Ring- Complexes.- 2.7 The Effect of Late-Stage Fluids.- 2.8 The Nature of the Late-Stage Fluids.- 2.9 Geochemistry of the Alteration Processes.- 2.10 Mineralization and Economic Aspects of the Over- saturated Ring-Complexes.- 3 Mixed Saturated and Undersaturated Alkaline Provinces with Carbonatites.- 3.1 Introduction.- 3.2 Structure and Petrology.- 3.3 Age and Variations of the Different Provinces.- 3.4 Major Element Chemistry of Carbonatites and Related Rocks.- 3.5 Trace Element Chemistry of Carbonatites and Related Rocks.- 3.6 The Effects of Late-Stage Fluids: Fenitization and Related Processes.- 3.7 Mineralization and Economic Aspects of Carbonatites..- 3.8 Mineralization and Economic Aspects of Oversaturated Ring-Complexes in Mixed Provinces.- 4 Undersaturated Complexes with Carbonatites.- 4.1 Introduction.- 4.2 Age and Petrological Variations of the Provinces.- 4.3 Mineralization.- 5 Conclusions.- 5.1 Introduction.- 5.2 Summary Comparisons Between the Oversaturated and Undersaturated Mineralization.- 5.3 Summary Contrasts Between the Oversaturated and Undersaturated Mineralization.- 5.4 Final Comment.- Acknowledgements.- IV. African Kimberlites.- African Kimberlites: Introduction.- 1 Introduction.- 2 Southern Africa.- 3 Central Africa.- 4 Western Africa.- 5 Eastern Africa.- 6 Conclusions.- Acknowledgements.- 14 Southern African Kimberlites and Their Xenoliths.- 1 Kimberlites.- 1.1 History and Distribution.- 1.2 Kimberlite Morphology and Petrography.- 1.3 Kimberlite Geochemistry.- 2 Peridotite Xenoliths.- 2.1 Low-Temperature Peridotites.- 2.2 High-Temperature Peridotites.- 2.3 Modally Metasomatized Peridotites.- 2.4 Pyroxenites.- 2.5 MARID Nodules.- 3 Eclogite and Granulite Xenoliths.- 3.1 Off-Craton and Craton Margin Eclogite Xenoliths.- 3.2 Central Craton Eclogite Xenoliths.- 4 Megacrysts.- 4.1 Cr-rich Megacrysts.- 4.2 The Granny Smith Diopside Association.- 4.3 Cr-poor Megacrysts.- 5 Diamonds and Their Mineral Inclusions.- 5.1 Diamond Characteristics.- 5.2 Inclusion Characteristics.- 5.3 Isotopic Model Ages.- 6 Kimberlite and Diamond Indicator Minerals.- 7 Synthesis.- Acknowledgements.- 15 The Kimberlites of Central Africa: A Review.- 1 Introduction.- 2 Geological Situation and Tectonic Setting.- 2.1 The Kimberlites of Zaire.- 2.2 The Kimberlites of Angola.- 3 Petrography and Mineralogy.- 3.1 Petrography of the Kimberlites.- 3.2 Mantle Xenoliths and Megacryst Suite.- 3.3 Diamonds.- 4 Geochemistry.- 4.1 Major Elements.- 4.2 Trace Elements.- 4.3 Sr, Nd and Pb Isotopie Composition.- 4.4 C, O and N Isotopie Compositions at Mbuji-Mayi.- 5 Conclusions.- Acknowledgements.- References (compiled by A.B. Kampunzu).- Geographical Index (compiled by R.T. Lubala).