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Archean crustal evolution
Book Reviews
It belongs in all major geology libraries. Those who want to read volume 2, on the Phanerozoic geology of the state, will be pleased to know that was recently published. MALCOLM WALTER (Macquarie) SSD10301-9268(95)00060-7
Archean Crustal Evolution. K.C. Condie (Editor). Developments in Precambrian Geology, Vol. 11, Elsevier, Amsterdam, 1994, 532 pp., hard cover, US$162.75, ISBN 0-444-81621-6.
In editing a book such as this the selection of the topics to be covered and the way in which they are linked together is always largely a personal affair. It is likely, therefore, that some will be disappointed about the topics chosen. The volume comprises an Introduction and twelve rather mixed chapters which describe details of specific lithological groups, aspects of construction and longevity of greenstone belts, grey gneiss complexes and granites, metamorphism and a study of the Archaean atmosphere. Whilst the details and summaries contained within each Chapter appear up to date, and indeed many of the ideas are very stimulating, it is not obvious what rationale is behind the overall organisation of the book. To me, beginning with a subject such as komatiites, even though they clearly have an important place in the Archaean, does not appear a logical starting point for a book entitled Archean Crustal Evolution. What seems to be missing is a thread that links the chapters together and in this respect, given that the Developments in Precambrian Geology series has relatively recently published books on some of the chapter topics, perhaps an opportunity may have been missed. One of the keys to understanding Archaean crustal evolution has been the ability to obtain more precise dates and the utilisation of these to dissect a given terrain. Some contribution examining the ability to more tightly constrain the ages of units, which has great significance for any models utilised, would have certainly been useful. Most importantly, as there are now rocks that are essentially 4000 Ma old (the Acasta gneisses) and mineral
299
grains back to ~ 4200 Ma, it seems very odd in a book like this that the earliest records of sialic crust have not been given specific attention. A passing mention in a small part of a chapter is really not good enough. Whilst there are references to tectonic models in individual contributions it is strange that there is no specific contribution on the debate over possible tectonic models and the evidence for their application. The Introduction contains several points that seem incorrect. For example, it is not true to suggest that anorthosites have only recently been recognised as a very important part of most Archaean high-grade terrains. This may be deduced from the substantial reference list involving the authors of Chapter 8 and also the Advisory Editor. Also, several of the references in this section are incorrect or missing and there are some odd typing errors, which suggest that proof reading could have been better. The contributions by themselves are what makes the book worthwhile as most of the subject areas have been moving rapidly over the past few years. Since their discovery komatiites have had several contributions written about them and Chapter 1, by Arndt, is another step down the road to a better understanding of this enigmatic and difficult group of rocks. The chapter outlines the many changes in thinking on the origin of these rocks that have taken place over the years. A clear summary of the problems of utilising komatiites for modelling possible Archaean mantle reservoirs is given. Importantly, a link back through some of the early literature describing the rocks is also provided. The treatment of volcanic patterns by Thurston (Chapter 2) demonstrates the different types of volcanism that can be established throughout the many well-exposed volcanic belts, as exemplified by those in Canada and Australia. Utilising this information evidence concerning the nature of the Archaean upper crust has been established. The similarity of many volcanic processes to those of today, requiring similar volumes of liquid, are indicated and the implications for the Archaean are discussed. The editor has contributed Chapter 3 on the place of greenstones through time. Given the controversy that has arisen over the true stratigraphic sequence of the well-preserved Barberton Mountain Land, one wonders what the value is of tabulating and indeed utilising 'measured
300
Book Reviews
stratigraphic sequences' from other, less well-preserved assemblages. In Chapter 4 Lowe produces another of his very readable summaries which are useful for those not working directly in the field but wishing to remain up to date with rapid developments and consequent re-interpretations of classic areas. The data sets are dominated by information from the Kaapvaal and Pilbara cratons which is perfectly understandable because of their excellent states of preservation. By utilising the field data from different sedimentary associations constraints are placed upon the environments in which many of the greenstone belts accumulated. Chapter 5 is a complementary discussion of syn-rift and stable shelf sedimentary rocks by Eriksson and Fedo. That techniques of modern sedimentology have been successfully applied to Archaean sequences is encouraging and again shows what can be established with careful investigations. The extensive summary of the geochemical evidence for the origin of grey gneisses by Martin, Chapter 6, concludes with the hypothesis that subduction of young, hot oceanic crust in a continental margin setting, as exemplified by southern Chile, explains many of the characteristics of the voluminous calc-alkaline grey gneiss complexes. Granite genesis is also of importance to understanding crustal evolution, as it can be taken to show something of the maturity of continental crust. Chapter 7 by Sylvester comprises a detailed comparison of the geochemistry of Archaean and Phanerozoic granites. To have a unifying theory for the genesis of Archaean granites seems to me to be a little pointless and the concept that TTG's and granites form very close in time seems a little dubious as a main summary point. It is perhaps timely that there is a summary of Archaean anorthosites and Chapter 8 provides a neat entry to the recently published book Anorthosites, by Ashwat. The distinctive igneous textures in these rocks are one of the amazements in Archaean geology. After the many different hypotheses for the origin of these rocks it is at last refreshing to read the summary provided and begin to understand the true setting of at least some of the intrusive complexes. Metamorphism in the Archaean is treated in Chapter 9, by Percival, which concentrates mainly
on granulite facies conditions. The improvements in geothermometers and barometers in conjunction with fluid activities are summarised and the implications for geothermal gradients--concluded to be higher. However, the importance of obtaining accurate ages of the rocks involved is emphasised. The place of Chapter 10, which reviews the Archaean and early Proterozoic evolution of the Siberian craton does not seem to fit the pattern of the rest of the book, particularly as the geology of the former USSR has recently been treated in Volume 9 in this series. However, given the difficulty of obtaining up to date information about this vast region, it can, perhaps, be argued that it warrants inclusion. For wider appeal, and as a better demonstration of how modern research has been carried out, it surely would have been more useful to have had a summary of one of the high-grade terrains where research is better advanced and there is a more detailed understanding of the ages of the rocks, such as those from Australia, Canada or Antarctica. In this respect the chapter could have been complementary to Chapter 2, by Thurston. The place of mineralisation is obviously an important aspect and the chapter by Groves and Barley provides an extremely good overview of the application of modern studies to Archaean mineral provinces. The final chapter, by Des Marais, discusses the composition and evolution of the Archaean atmosphere. Whilst this subject is rather intangible, given the ease with which its composition can be changed, some clues have been left behind in the poorly preserved rock record. This contribution at least gives the non-expert a chance of understanding the different concepts and limitations of data interpretation. The book is basically as up to date as one can expect, though for one which is attempting to summarise what we know about the development of the Archaean, to my mind, there are some surprising omissions. Some of the chapters in the book include an almost exhaustive catalogue of summaries of different locations, a type of treatment that is inevitably going to lead to inaccuracies in the way the data are reported, and so it is here. In summary, the book will be on any Archaean research worker's reference list, but, the cost of the book may well limit the number that will purchase it. Thus the
Book Reviews
purchase will again fall on the diminishing resources of institutional libraries. CLARK R.L. FRIEND(Oxford) SSDI
0301-9268(95)00061-5
Geology of Western Gondwana (2000-500 Ma). Pan-African-Brasiliano Aggregation of South America and Africa. Roland Trompette. A.A. Balkema, Rotterdam, 1994, 350 pp., Dfl. 185.50, ISBN 90-54-10-1652. Paleogeographic and geodynamic reconstructions of Western Gondwana for the 2000-500 Ma period are biased since they "often rely on limited data because the access to information is difficult for both Africa and South America" the author states in the Introduction. This gap is now filled and any geologist either involved in research or in teaching will appreciate this up-to-date book by Roland Trompette. Indeed, the author has been actively involved in research of Precambrian geology on both sides of the South Atlantic for 30 years. It is not surprising that he did not hesitate to describe several geological provinces and fold belts spanning across the Atlantic Ocean. The main geological and petrological features of the five cratons involved in the Gondwana supercontinent are described in Part 1. Recent advances in the understanding of the Amazonian craton with its long-lived cratonisation ending about 1350 Ma ago emphasise the differences with the West African Craton stabilized around 2000 Ma. However, this review does not report on the model of juvenile crust and cratonization of former oceanic plateaus recently proposed on isotopic grounds for the Birimian of West Africa. Late-Middle Proterozoic cover sequences of the five cratons, with which the author is especially familiar, and metasedimentary sequences of adjacent fold belts are then described with many details of their lithostratigraphy. Extremely low subsidence rates of about 4 m / M a on the West African Craton, approaching those of the Russian platform, are deduced from the available geochronological data. There follows a synthetic view on the Bambui, the
301
deformed cover of the S~o Francisco Craton and on the West-Congo Supergroup. Trompette introduces Part 2, Pan-African-Brasiliano fold belts, with a historical review of the evolution of ideas concerning the significance of Pan-African tectono-thermal events and the identification of possible suture zones. The odish concept of 'geosynclinal' fold belts is reassessed; the author proposes to distinguish 'geosynclinal'-type and 'vestigial' Pan-African fold belts. He therefore has chosen, amongst many others, to portray the general Pan-African geodynamic evolution through a model of intracratonic subduction, or limited oceanic subduction, as proposed fourteen years ago for the disputable Damara example. The main features of the seven major Pan-African-Brasiliano fold belts, each presented with schematic structural maps and cross-sections, are clearly described in Chapters 5 to 11. In these 120 pages, the reader will find the basic data concerning the nature of rock sequences, the main structures and the geochronological data. The major new and personal contribution of Trompette, especially with reference to Brazil, deals with lithostratigraphic evolution and correlations which are presented and summarised in new synthetic tables and cross-sections. In this way, more data are presented from the external zones, or 'marginal fold belts', than for their internal zones, for which, however, most of the references to petrological, geochemical or kinematic aspects are given. Recent advances on kinematics are however sometimes underestimated; for example, thrusting directions indicated on maps assume displacements mostly perpendicular to orogenic fronts that do not rely on kinematic data. The internal, high-grade metamorphic domains of the Pan-African-Brasiliano belt in northeast Brazil and Nigeria-Hoggar, which may be regarded as a Precambrian analogue to the Tibetan plateau with its mega shear zone pattern generated by lateral escape of the indenter Indian plate, are presented separately in Chapter 12. The author has disconnected these large domains of "polycyclic basement with few relics of Pan-African metasediments" from the Trans-Saharan Pan-African-Brasiliano mega fold belt. Indeed, how should the high-grade metamorphic inner zones of the Variscan belt of western Europe be disconnected from the outer zones of the
It belongs in all major geology libraries. Those who want to read volume 2, on the Phanerozoic geology of the state, will be pleased to know that was recently published. MALCOLM WALTER (Macquarie) SSD10301-9268(95)00060-7
Archean Crustal Evolution. K.C. Condie (Editor). Developments in Precambrian Geology, Vol. 11, Elsevier, Amsterdam, 1994, 532 pp., hard cover, US$162.75, ISBN 0-444-81621-6.
In editing a book such as this the selection of the topics to be covered and the way in which they are linked together is always largely a personal affair. It is likely, therefore, that some will be disappointed about the topics chosen. The volume comprises an Introduction and twelve rather mixed chapters which describe details of specific lithological groups, aspects of construction and longevity of greenstone belts, grey gneiss complexes and granites, metamorphism and a study of the Archaean atmosphere. Whilst the details and summaries contained within each Chapter appear up to date, and indeed many of the ideas are very stimulating, it is not obvious what rationale is behind the overall organisation of the book. To me, beginning with a subject such as komatiites, even though they clearly have an important place in the Archaean, does not appear a logical starting point for a book entitled Archean Crustal Evolution. What seems to be missing is a thread that links the chapters together and in this respect, given that the Developments in Precambrian Geology series has relatively recently published books on some of the chapter topics, perhaps an opportunity may have been missed. One of the keys to understanding Archaean crustal evolution has been the ability to obtain more precise dates and the utilisation of these to dissect a given terrain. Some contribution examining the ability to more tightly constrain the ages of units, which has great significance for any models utilised, would have certainly been useful. Most importantly, as there are now rocks that are essentially 4000 Ma old (the Acasta gneisses) and mineral
299
grains back to ~ 4200 Ma, it seems very odd in a book like this that the earliest records of sialic crust have not been given specific attention. A passing mention in a small part of a chapter is really not good enough. Whilst there are references to tectonic models in individual contributions it is strange that there is no specific contribution on the debate over possible tectonic models and the evidence for their application. The Introduction contains several points that seem incorrect. For example, it is not true to suggest that anorthosites have only recently been recognised as a very important part of most Archaean high-grade terrains. This may be deduced from the substantial reference list involving the authors of Chapter 8 and also the Advisory Editor. Also, several of the references in this section are incorrect or missing and there are some odd typing errors, which suggest that proof reading could have been better. The contributions by themselves are what makes the book worthwhile as most of the subject areas have been moving rapidly over the past few years. Since their discovery komatiites have had several contributions written about them and Chapter 1, by Arndt, is another step down the road to a better understanding of this enigmatic and difficult group of rocks. The chapter outlines the many changes in thinking on the origin of these rocks that have taken place over the years. A clear summary of the problems of utilising komatiites for modelling possible Archaean mantle reservoirs is given. Importantly, a link back through some of the early literature describing the rocks is also provided. The treatment of volcanic patterns by Thurston (Chapter 2) demonstrates the different types of volcanism that can be established throughout the many well-exposed volcanic belts, as exemplified by those in Canada and Australia. Utilising this information evidence concerning the nature of the Archaean upper crust has been established. The similarity of many volcanic processes to those of today, requiring similar volumes of liquid, are indicated and the implications for the Archaean are discussed. The editor has contributed Chapter 3 on the place of greenstones through time. Given the controversy that has arisen over the true stratigraphic sequence of the well-preserved Barberton Mountain Land, one wonders what the value is of tabulating and indeed utilising 'measured
300
Book Reviews
stratigraphic sequences' from other, less well-preserved assemblages. In Chapter 4 Lowe produces another of his very readable summaries which are useful for those not working directly in the field but wishing to remain up to date with rapid developments and consequent re-interpretations of classic areas. The data sets are dominated by information from the Kaapvaal and Pilbara cratons which is perfectly understandable because of their excellent states of preservation. By utilising the field data from different sedimentary associations constraints are placed upon the environments in which many of the greenstone belts accumulated. Chapter 5 is a complementary discussion of syn-rift and stable shelf sedimentary rocks by Eriksson and Fedo. That techniques of modern sedimentology have been successfully applied to Archaean sequences is encouraging and again shows what can be established with careful investigations. The extensive summary of the geochemical evidence for the origin of grey gneisses by Martin, Chapter 6, concludes with the hypothesis that subduction of young, hot oceanic crust in a continental margin setting, as exemplified by southern Chile, explains many of the characteristics of the voluminous calc-alkaline grey gneiss complexes. Granite genesis is also of importance to understanding crustal evolution, as it can be taken to show something of the maturity of continental crust. Chapter 7 by Sylvester comprises a detailed comparison of the geochemistry of Archaean and Phanerozoic granites. To have a unifying theory for the genesis of Archaean granites seems to me to be a little pointless and the concept that TTG's and granites form very close in time seems a little dubious as a main summary point. It is perhaps timely that there is a summary of Archaean anorthosites and Chapter 8 provides a neat entry to the recently published book Anorthosites, by Ashwat. The distinctive igneous textures in these rocks are one of the amazements in Archaean geology. After the many different hypotheses for the origin of these rocks it is at last refreshing to read the summary provided and begin to understand the true setting of at least some of the intrusive complexes. Metamorphism in the Archaean is treated in Chapter 9, by Percival, which concentrates mainly
on granulite facies conditions. The improvements in geothermometers and barometers in conjunction with fluid activities are summarised and the implications for geothermal gradients--concluded to be higher. However, the importance of obtaining accurate ages of the rocks involved is emphasised. The place of Chapter 10, which reviews the Archaean and early Proterozoic evolution of the Siberian craton does not seem to fit the pattern of the rest of the book, particularly as the geology of the former USSR has recently been treated in Volume 9 in this series. However, given the difficulty of obtaining up to date information about this vast region, it can, perhaps, be argued that it warrants inclusion. For wider appeal, and as a better demonstration of how modern research has been carried out, it surely would have been more useful to have had a summary of one of the high-grade terrains where research is better advanced and there is a more detailed understanding of the ages of the rocks, such as those from Australia, Canada or Antarctica. In this respect the chapter could have been complementary to Chapter 2, by Thurston. The place of mineralisation is obviously an important aspect and the chapter by Groves and Barley provides an extremely good overview of the application of modern studies to Archaean mineral provinces. The final chapter, by Des Marais, discusses the composition and evolution of the Archaean atmosphere. Whilst this subject is rather intangible, given the ease with which its composition can be changed, some clues have been left behind in the poorly preserved rock record. This contribution at least gives the non-expert a chance of understanding the different concepts and limitations of data interpretation. The book is basically as up to date as one can expect, though for one which is attempting to summarise what we know about the development of the Archaean, to my mind, there are some surprising omissions. Some of the chapters in the book include an almost exhaustive catalogue of summaries of different locations, a type of treatment that is inevitably going to lead to inaccuracies in the way the data are reported, and so it is here. In summary, the book will be on any Archaean research worker's reference list, but, the cost of the book may well limit the number that will purchase it. Thus the
Book Reviews
purchase will again fall on the diminishing resources of institutional libraries. CLARK R.L. FRIEND(Oxford) SSDI
0301-9268(95)00061-5
Geology of Western Gondwana (2000-500 Ma). Pan-African-Brasiliano Aggregation of South America and Africa. Roland Trompette. A.A. Balkema, Rotterdam, 1994, 350 pp., Dfl. 185.50, ISBN 90-54-10-1652. Paleogeographic and geodynamic reconstructions of Western Gondwana for the 2000-500 Ma period are biased since they "often rely on limited data because the access to information is difficult for both Africa and South America" the author states in the Introduction. This gap is now filled and any geologist either involved in research or in teaching will appreciate this up-to-date book by Roland Trompette. Indeed, the author has been actively involved in research of Precambrian geology on both sides of the South Atlantic for 30 years. It is not surprising that he did not hesitate to describe several geological provinces and fold belts spanning across the Atlantic Ocean. The main geological and petrological features of the five cratons involved in the Gondwana supercontinent are described in Part 1. Recent advances in the understanding of the Amazonian craton with its long-lived cratonisation ending about 1350 Ma ago emphasise the differences with the West African Craton stabilized around 2000 Ma. However, this review does not report on the model of juvenile crust and cratonization of former oceanic plateaus recently proposed on isotopic grounds for the Birimian of West Africa. Late-Middle Proterozoic cover sequences of the five cratons, with which the author is especially familiar, and metasedimentary sequences of adjacent fold belts are then described with many details of their lithostratigraphy. Extremely low subsidence rates of about 4 m / M a on the West African Craton, approaching those of the Russian platform, are deduced from the available geochronological data. There follows a synthetic view on the Bambui, the
301
deformed cover of the S~o Francisco Craton and on the West-Congo Supergroup. Trompette introduces Part 2, Pan-African-Brasiliano fold belts, with a historical review of the evolution of ideas concerning the significance of Pan-African tectono-thermal events and the identification of possible suture zones. The odish concept of 'geosynclinal' fold belts is reassessed; the author proposes to distinguish 'geosynclinal'-type and 'vestigial' Pan-African fold belts. He therefore has chosen, amongst many others, to portray the general Pan-African geodynamic evolution through a model of intracratonic subduction, or limited oceanic subduction, as proposed fourteen years ago for the disputable Damara example. The main features of the seven major Pan-African-Brasiliano fold belts, each presented with schematic structural maps and cross-sections, are clearly described in Chapters 5 to 11. In these 120 pages, the reader will find the basic data concerning the nature of rock sequences, the main structures and the geochronological data. The major new and personal contribution of Trompette, especially with reference to Brazil, deals with lithostratigraphic evolution and correlations which are presented and summarised in new synthetic tables and cross-sections. In this way, more data are presented from the external zones, or 'marginal fold belts', than for their internal zones, for which, however, most of the references to petrological, geochemical or kinematic aspects are given. Recent advances on kinematics are however sometimes underestimated; for example, thrusting directions indicated on maps assume displacements mostly perpendicular to orogenic fronts that do not rely on kinematic data. The internal, high-grade metamorphic domains of the Pan-African-Brasiliano belt in northeast Brazil and Nigeria-Hoggar, which may be regarded as a Precambrian analogue to the Tibetan plateau with its mega shear zone pattern generated by lateral escape of the indenter Indian plate, are presented separately in Chapter 12. The author has disconnected these large domains of "polycyclic basement with few relics of Pan-African metasediments" from the Trans-Saharan Pan-African-Brasiliano mega fold belt. Indeed, how should the high-grade metamorphic inner zones of the Variscan belt of western Europe be disconnected from the outer zones of the