Early Precambrian Basic Magmatism

Basic magmatic rocks make up approximately three-quarters of the crust ofthe present day Earth. Because we can observe and study the volcanic products of present day tectonic regimes comprehensively, we can shed light on ancient tectono-magmatic provinces, and thereby deduce the petrogenesis and evolution of the oldest basic rocks. This is the primary objective of this book. The book was conceived in order to provide a comprehensive review of the basic rocks produced during the first half of the Precambrian, i.e. the Archaean and early Proterozoic, to about 1.8 Ga years ago. Two major questions are addressed. First, what basic magmas were generated during the early Precambrian: were these magmas globally uniform, and to what extent were prevailing tectonic controls and compo­ sitions analogous to those of the present day? Clearly, this can be answered only by bringing together fundamental information about all relevant basic magmatic events. Second, is there any systematic temporal variation in the nature of basic suites, and what implications might such variations have on our interpretations of early Earth history? Are there important differences between early Archaean, late Archaean, Proterozoic and modern basic magmatic suites? The book uses two approaches to address these questions. Early chapters examine the fundamental characteristics of these basic rocks, whilst later chapters assess regional distribution and development by providing an overview of each major early Precambrian craton.

1 Introduction: basic magmatism and crustal evolution.- 1.1 Early thermal and magmatic history of the Earth.- 1.2 Early Precambrian oceans and greenstone belts.- 1.3 Continental crust and the major basic intrusions.- I General Aspects.- 2 Archaean basic magmas.- 2.1 Distribution of Archaean basic volcanic suites.- 2.1.1 Greenstone belts.- 2.1.2 High-grade gneiss terranes.- 2.2 Komatiites.- 2.3 Tholeiitic basalts.- 2.4 Komatiitic basalts.- 2.5 Petrogenesis.- 2.5.1 Komatiites.- 2.5.2 Komatiitic basalts.- 2.5.3 Tholeiitic basalts.- 2.5.4 Archaean ‘oceanic’ basic magmatism.- 2.6 The Archaean mantle.- 2.6.1 Mantle evolution.- 2.6.2 Mantle heterogeneity.- 2.7 Phanerozoic analogues of Archaean mafic volcanism.- 2.7.1 Komatiites.- 2.7.2 Komatiitic basalts.- 2.7.3 Tholeiitic basalts.- 2.8 Concluding remarks.- 3 Geochemical characteristics of Precambrian basaltic greenstones.- 3.1 Introduction.- 3.2 Classification of greenstone basalts.- 3.3 Geochemical characteristics.- 3.4 Precambrian mantle sources.- 3.5 Discussion.- 3.5.1 Selective preservation of lithological assemblages.- 3.5.2 Early and late Archaean greenstone basalts.- 3.5.3 The Archaean–Proterozoic boundary.- 3.6 Conclusions.- 4 Early Precambrian layered intrusions.- 4.1 Introduction: intrusion and crystallisation mechanisms.- 4.2 The Bush veld Complex.- 4.2.1 Regional setting.- 4.2.2 Intrusion form.- 4.2.3 Lithology.- 4.2.4 Mineralogy.- 4.2.5 Parental magma.- 4.3 The Stillwater Complex.- 4.3.1 Regional setting.- 4.3.2 Intrusion form.- 4.3.3 Lithology.- 4.3.4 Mineralogy.- 4.3.5 Parental magma.- 4.4 Dyke-like layered intrusions.- 4.4.1 Great Dyke of Zimbabwe.- 4.4.2 Finnish layered intrusions.- 4.4.3 Jimberlana intrusion.- 4.5 Archaean anorthosite complexes.- 4.6 Other layered intrusions.- 4.6.1 Kaapvaal craton.- 4.6.2 Yilgarn Block.- 4.6.3 Pilbara Block.- 4.6.4 Superior Province.- 4.7 Tectonic setting and petrogenesis.- 4.8 Concluding remarks.- 5 Noritic magmatism.- 5.1 Introduction: high-Mg basic rock types.- 5.2 Mineralogy.- 5.3 Geochemistry.- 5.4 Petrogenesis.- 5.4.1 Komatiites, SHMB, ‘U’-type magmas, norites and boninites.- 5.4.2 Chemical mobility and alteration.- 5.4.3 Fractional crystallisation.- 5.4.4 Degree of partial melting.- 5.4.5 Magma mixing.- 5.4.6 Crustal contamination.- 5.4.7 Mantle metasomatism.- 5.4.8 KREEP: a lunar analogue?.- 5.5 Concluding remarks.- 6 Mantle evolution.- 6.1 Introduction.- 6.2 Mantle processes.- 6.2.1 Convection.- 6.2.2 The lithosphere.- 6.2.3 Geoid anomalies and convection scales.- 6.2.4 Magmatism.- 6.3 Thermal evolution of the mantle.- 6.3.1 Geological evidence.- 6.3.2 The thermal state: convection models.- 6.3.3 Initial thermal state and ‘magma oceans’?.- 6.4 Melting in the early Precambrian mantle.- 6.4.1 Melt production at higher temperatures.- 6.4.2 Heat loss by melting.- 6.4.3 Komatiites.- 6.5 Chemical evolution of the early mantle.- 6.5.1 Chemical reservoirs.- 6.5.2 The growth of geochemical resevoirs.- 6.5.3 Mantle chemistry: primitive mantle and crustal depletion.- 6.5.4 The timing of crustal growth.- 6.5.5 U–Th–Pb systematics: crustal or lower mantle recycling?.- 6.5.6 Crust formation.- 6.6 Conclusions.- 7 Lunar magmatism.- 7.1 Introduction.- 7.2 Lunar highland rocks.- 7.2.1 Anorthosites.- 7.2.2 Mg-suite.- 7.2.3 Petrogenesis.- 7.3 Mare basalts.- 7.3.1 Petrography and mineralogy.- 7.3.2 Volcanic glasses.- 7.3.3 Age of mare basalts.- 7.3.4 Origin of mare basalts.- 7.4 KREEP basalts.- 7.5 Comparison with terrestrial magmatism.- 7.5.1 Anorthosites.- 7.5.2 Basalts.- 7.6 Concluding remarks.- 8 Mineralisation associated with early Precambrian basic magmatism.- 8.1 Introduction.- 8.2 Nickel mineralisation: Archaean volcanic suites.- 8.2.1 The komatiite–sulphide association.- 8.2.2 Typical greenstone belts.- 8.2.3 Kambalda and Scotia deposits.- 8.2.4 Summary and genetic models.- 8.3 Gold mineralisation in volcanic suites.- 8.3.1 Gold production from basic rocks.- 8.3.2 Lode deposits.- 8.3.3 Crustal setting.- 8.3.4 Role of basic magmatism.- 8.3.5 Gold and tholeiitic versus calc-alkaline magmatism: an Archaean enigma?.- 8.4 Chromite mineralisation: major layered intrusions.- 8.4.1 Stratigraphy.- 8.4.2 Textures.- 8.4.3 Geochemistry.- 8.4.4 Petrogenesis.- 8.5 Platinum group elements (PGE): layered complexes.- 8.5.1 Reef characteristics.- 8.5.2 Geochemistry.- 8.5.3 Petrogenesis.- 8.6 Closing remarks.- II Regional Syntheses.- 9 Early Precambrian basic rocks of the USA.- 9.1 Introduction.- 9.2 The Minnesota–Wisconsin region.- 9.2.1 Early Archaean suites.- 9.2.2 Late Archaean basic metavolcanics.- 9.2.3 Early Proterozoic rocks.- 9.2.4 Tectonic implications.- 9.3 Wyoming Province: Archaean metavolcanic rocks.- 9.3.1 Early Archaean.- 9.3.2 Late Archaean.- 9.4 Wyoming Province: intrusive rocks.- 9.4.1 Southern Idaho, northern Utah and Nevada.- 9.4.2 Black Hills, South Dakota.- 9.4.3 Southern Wyoming and northern Colorado.- 9.4.4 Stillwater Complex and minor intrusions, Beartooth Mountains.- 9.4.5 Tobacco Root and neighbouring mountains, southwestern Montana.- 9.4.6 Bighorn Mountains.- 9.4.7 Teton Mountains.- 9.4.8 Wind River Range.- 9.4.9 Owl Creek Mountains.- 9.4.10 Granite, Seminoe, Ferris, Casper and northern Laramie Mountains.- 9.4.11 Summary of basic intrusive rocks, Wyoming Province.- 9.5 Early to mid-Proterozoic metavolcanic belts of western central USA.- 10 Early Precambrian basic rocks of the Canadian Shield.- 10.1 Introduction.- 10.2 Superior Province.- 10.3 Granite–greenstone terranes.- 10.3.1 Platform sequences.- 10.3.2 Mafic plain volcanism.- 10.3.3 Arc volcanism.- 10.3.4 Pull-apart basins.- 10.3.5 Late-stage mafic magmatism.- 10.4 Mafic–ultramafic intrusions.- 10.4.1 Platform terranes.- 10.4.2 Arc terranes.- 10.5 Slave Province.- 10.6 Rae Province.- 10.7 Post-tectonic dykes.- 10.8 Summary and conclusions.- 11 Early Precambrian basic rocks of Greenland and Scotland.- 11.1 Introduction.- 11.2 Age and distribution of the basic rocks.- 11.2.1 Archaean suites.- 11.2.2 Proterozoic units.- 11.3 Early Archaean volcanism: the Isua–Akilia association.- 11.4 Mid-Archaean basic magmatism.- 11.4.1 Ameralik dykes.- 11.4.2 Metavolcanic suites.- 11.4.3 Fiskenaesset-type gabbro–anorthosite complexes.- 11.4.4 ‘Intra-Nûk’dykes.- 11.4.5 Archaean basic rocks of Scotland.- 11.5 Early Proterozoic basic magmatism.- 11.5.1 Ketilidian, Nagssugtoqidian and Rinkian volcanics, Greenland.- 11.5.2 Early Proterozoic basic dyke swarms of Greenland.- 11.5.3 Loch Maree Group metavolcanics, northwest Scotland.- 11.5.4 Early Proterozoic intrusive basic rocks of Scotland.- 11.6 Basic magmatic evolution and crustal growth in the North Atlantic craton.- 12 Early Precambrian basic rocks of the Baltic Shield.- 12.1 Introduction.- 12.2 The Archaean domain.- 12.3 Lapponian Supergroup (earliest Proterozoic).- 12.4 Early Proterozoic layered gabbroic complexes.- 12.5 Supracrustal belts marginal to the Lapland Granulite Complex.- 12.6 Karelian Supergroup (early Proterozoic).- 12.7 Marginal zone of the Svecofennian Domain.- 12.7.1 Karelidic Schist Belt.- 12.7.2 Early Proterozoic ophiolitic (?) complexes.- 12.8 Basic rocks of the Svecofennian Domain.- 12.8.1 Northern Province.- 12.8.2 Southern Province.- 12.8.3 Central Province.- 12.9 Syn- and post-orogenic intrusions.- 12.10 Concluding remarks.- 13 Early Precambrian basic rocks of China.- 13.1 Introduction.- 13.2 Distribution, metamorphism and age.- 13.2.1 Eastern Hebei Province: the Qianxi Group.- 13.2.2 Shandong Province: the Taishan Complex.- 13.2.3 Northeastern China (Manchuria): the Anshan Group.- 13.2.4 Inner Mongolia (Yingshan region): the Jining and Wulashan Groups.- 13.2.5 Shanxi (Taihang–Wutai–Luliang region): the Fuping and Wutai Groups.- 13.2.6 Henan Province: the Dengfeng and Taihua Groups.- 13.3 Geochemistry and petrogenesis.- 13.3.1 Bulk-rock chemistry and trace element differentiation.- 13.3.2 REE geochemistry.- 13.3.3 Nb anomalies.- 13.4 Implications for early continental evolution.- 13.4.1 Nd-Sr isotopic constraints.- 13.4.2 Amphibolite–TTG gneiss relationships.- 13.5 Conclusions.- 14 Early Precambrian basic rocks of Australia.- 14.1 Introduction.- 14.2 Distribution and stratigraphy.- 14.2.1 Archaean rocks.- 14.2.2 Proterozoic mobile belts and dyke swarms.- 14.3 Diversity of basic magmatism.- 14.3.1 Komatiites.- 14.3.2 Tholeiites.- 14.3.3 Siliceous high-Mg basalts (SHMB) and high-Mg basalts (HMB).- 14.3.4 Calc-alkaline mafic rocks.- 14.4 Magmatic and thermal evolution.- 14.4.1 Earliest Archaean (4.2–3.8 Ga).- 14.4.2 Early to mid-Archaean (3.6–2.9 Ga).- 14.4.3 Late Archaean (2.9–2.5 Ga).- 14.4.4 Early Proterozoic (2.7–1.8 Ga).- 14.5 Petrogenetic synthesis.- 15 Early Precambrian basic rocks of India.- 15.1 Introduction.- 15.2 The Dharwar craton.- 15.2.1 Greenstone belts.- 15.2.2 High-grade ultrabasic–basic complexes.- 15.2.3 Anorthosite complexes.- 15.2.4 High-grade basic lenses.- 15.2.5 Dykes.- 15.3 The Singhbhum craton.- 15.4 The Aravalli craton.- 15.5 Discussion.- 15.5.1 Geochemical variation of basic rocks with metamorphic grade.- 15.5.2 Early Precambrian basic magmatism and the evolution of the Indian Shield.- 16 Early Precambrian basic rocks of Africa.- 16.1 Introduction.- 16.2 Distribution of Archaean rocks in Africa.- 16.3 North and West Africa.- 16.3.1 Uweinat Inlier.- 16.3.2 Reguibat Rise.- 16.3.3 Guinea Rise.- 16.3.4 Greenstone belts of Sierra Leone.- 16.4 Equatorial Africa.- 16.4.1 Zaire granite–greenstone terrane.- 16.4.2 Tanzania granite–greenstone terrane.- 16.4.3 Greenstone belts of Equatorial Africa.- 16.5 Southern Africa.- 16.5.1 Zimbabwe.- 16.5.2 Kaapvaal Craton.- 16.5.3 Barberton greenstone belt.- 16.5.4 Pongola Supergroup.- 16.5.5 Ventersdorp Supergroup.- 16.6 Tectonic settings.- 16.7 Final comment.- 17 Early Precambrian basic rocks of South America.- 17.1 Introduction.- 17.2 Archaean basic volcanism.- 17.2.1 Imataca Complex, Venezuela.- 17.2.2 Bimodal volcanism, Serra dos Carajás.- 17.3 Archaean greenstone belts.- 17.3.1 Crixás greenstone belt.- 17.3.2 Rio das Velhas greenstone belt.- 17.3.3 Piumhi greenstone belt.- 17.3.4 Morro do Ferro greenstone belt.- 17.3.5 Greenstone belts of the southeastern Guaporé Shield.- 17.4 Proterozoic greenstone belts.- 17.4.1 Mazaruni Group, Guyana.- 17.4.2 Paramacca Series, French Guiana.- 17.4.3 Vila Nova and Serra do Navio belts, Amapá, Brazil.- 17.4.4 Rio Itapicuru greenstone belt.- 17.5 Early Precambrian mafic–ultramafic plutonic complexes.- 17.5.1 Cana Brava complex.- 17.5.2 Niquelândia complex.- 17.5.3 Barro Alto complex.- 17.5.4 Americano do Brasil layered complex.- 17.5.5 Caraiba complex.- 17.5.6 Rio do Jacare sill.- 17.5.7 Jacurici River Valley and Campo Formoso complexes.- 17.5.8 Mafic intrusive complexes of the Guiana Shield.- 17.6 Proterozoic dyke swarms and sills.- 17.6.1 Avanavero Suite.- 17.6.2 Mafic dyke swarms.- 17.7 Conclusions.- References.