Introduction to Geology. Volume 2, Earth History. Part I, Early Stages of Earth History

Early Stages of Earth History 6.4 Possible environmf-nts of greenstone bdt formation 104 6.5 Transvaal region 106 6.6 Bushveld igneous complex 108 6.7 Archaean crato•.1 of Rhodesia Ill 6.8 Relationships 'Jf successive mobile belts in central Africa 116 6.9 Precambrian inlier of the Hoggar showing some of the major dislocations 121 7.1 Main geological units of peninsular India 12S 7.2 Relationships of the Iron ore and Singbhum provinces in the north-east of the Indian craton 129 7.3 Relationships of metamorphic isograds and tectonic patterns in southern India 131 8.1 Main geological units of Australia 137 8.2 Main Precambrian units and main Phanerozoic basins of deposition in Australia 138 8.3 Stages in the mid-Proterozoic evolution of the Mount lsa region 149 8.4 Dated Proterozoic succession (Carpentarian) in northern Australia IS3 9.1 Main geological units of South America ISS 9.2 Guyana shield 160 9.3 Brazilian shield 161 9.4 Main geological units of Antarctica 166 10.1 Important geological events in each continent 172-3 I 0.2 Tectonic provinces stabilised in Archaean and early Proterozoic times, with basic dyke swarms 180 10.3 Precambrian polar-wandering for North America 181 10.4 Compositions of Precambrian banded iron formations compared with those of other types of sedimentary iron ore 188 l O.S Changes in the earth's atmosphere in relation to the evolution of life 189 List of Plates Part of an early dyke swarm penetrating Pre-Ketilidian gneisses in south-east Greenland II Part of the dyke swarm shown in Plate I, modified by deformation during or after emplacement III Pillow-lavas of the Onverwacht Group IV The Hamersley Range, Western Australia The Royal Air Force (Crown Copyright) Plates I and II The Editor,Nature, London Figure 4.6 The Geological Record Table 1.1. RADIOMETRIC METHODS OF AGE-DETERMINATION I. Uranium and thorium methods 2 38 U -+ 2 o • Pb + 8 • He 23sU-+2o1Pb+?•He 2 3 2 Th .... 2 o 8 Pb + 6 • He 5 Application: dating of uranium minerals and also of minerals such as zircon and monazite containing small amounts of U or Th. Zircon ages often refer to the date of formation of a rock, since zircon is resistant to change. The concordia method, which depends on determining isotopic ratios in a group of related samples, may give a reliable date of formation. ## Common lead methods Application: to galenas, dates time of separation of lead from sources containing U and Th and may therefore give dates much older than time of emplacement of galena; to whole rocks containing lead as a trace element. ## Potassium-argon methods (K-Ar) 4 °K-+ 40 A Application: a versatile method applicable to many igneous and metamorphic rocks and to sediments carrying glauconite, illite etc. K-feldspars and micas provide suitable material and minerals and whole;ocks low in K can also often be dated by this method. Low apparent ages may result from argon loss due to late thermal events: anomalously high ages from the accumulation of excess argon in certain minerals. An 'age-spectrum' which may identify several events is obtainable by special techniques. 4. Rubidium-strontium methods (Rb-Sr) 8 7 Rb-+ • 7 Sr Application: a versatile method, since Rb is a widely distributed trace element in alkali-feldspars and micas. The isochron method, which depends on determination of the ratios 8 7 Rb: 8 6 Sr and 8 7 Sr: 8 6 Sr in a number of genetically related rocks and minerals may indicate date of formation of igneous rocks with complex histories. ## I South-east Sweden The 'Gothide belt' (Fig. 2.2) is made up principally of large granitic or gneissose complexes between which lie outcrops of quartzites, mica-schists, marbles and basic and acid volcanics. Some of these supracrustal outcrops have been equated with the Jotnian formation (seep. 38), but dating of granitic intrusives in them Front Matter....Pages i-xii The Geological Record....Pages 1-15 European Shield-Areas....Pages 16-39 North Atlantic Shield-Areas....Pages 40-61 Precambrian of the North American Craton....Pages 62-88 Precambrian of Asiatic Laurasia....Pages 89-96 The African Cratons....Pages 97-123 The Indian Craton....Pages 124-135 The Australian Craton....Pages 136-153 The Cratons of South America and Antarctica....Pages 154-169 Problems of the Precambrian Record....Pages 170-191 Back Matter....Pages 192-221