Journal of Petrology Volume 41 Number 10 Pages 1541-1543 2000
© Oxford University Press 2000
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Book Reviews
Determination of Structural Successions in Migmatites and Gneisses by A. M. Hopgood. Kluwer Academic, Dordrecht, 1999. xxiv + 346 pp. ISBN 0-412-75800-8. £99.00, US$170, NLG 285Gneiss complexes form major components of continental crust throughout the world and have great significance in investigation of mechanisms for crustal growth and evolution. To obtain this information a thorough understanding of the structure is paramount. However, gneiss complexes have proven to be one of the most difficult rock associations to understand because they are commonly complexly deformed and metamorphosed. These processes are usually polyphase and often obliterate much of the primary evidence required. For example, it was only about 1970 that it become generally accepted that gneiss complexes were largely composed of deformed, intermediate, plutonic granitoids rather than melted sediments. In 1987, it was shown that some complexes may not be one contiguous piece of crust but were sequentially constructed from different blocks with completely different evolutionary histories—the terrane concept had been introduced. Once these hypotheses are understood, correlation of structures over large areas takes on a new and completely different meaning. Further, now that geochronological methods have advanced, integrated structural and geochronological studies are becoming common. Indeed, it is now apparent that in some complexes serious errors have been made by relying on correlation by structures alone. Another recent revolution in structural geology has been in gaining an insight into the ways that ductile and plastic rocks behave. I find it incredible that a book that deals with rocks behaving in these states makes no mention of folds with sheath geometry. All of the above concepts have changed the way that gneiss complexes are now viewed and, although our current views may not be entirely correct, they still represent major advances. Not that we are necessarily very far down the road of understanding, but to me this book seems firmly rooted in the period when understanding grey gneiss complexes was in its infancy.
The book comprises 15 chapters that progress through a general description of migmatites and gneisses, explanations of the structural principles employed and examples of applications, and it ends with a summary of the methodology of application, a brief discussion of the implications and some conclusions. There is an integrated reference list and separate indices for authors, localities and subjects. The first four chapters are broadly descriptive and an introduction to the concepts to be employed. Chapter 5 deals with fold successions and deformational sequences, which is followed up in Chapter 6 by correlation of structures. The characteristics of successions are then summarized in Chapter 7 and the principles of determination of a structural succession are outlined in Chapter 8. Chapter 9 deals with field procedures and is certainly helpful in explaining what to look for and which techniques might prove useful. How to set about mapping a gneiss complex is not a subject that has been extensively covered in the literature, and as the volume brings a new slant to this, there is at least some merit here. In Chapter 10, covering structural observations, no mention is made of the way that the different rheology of successive layers can cause significant changes in the way overall units behave. Any minor warp, by definition, is placed in a different phase of deformation. In many instances, particularly where there is progressive shear, arguments against this can be made. A set of examples is given in Chapter 11, which is followed up by various summaries and a conclusion in Chapter 15.
Assessing a book like this, where there are aspects that are correct and have been well described together with aspects that have ‘strange’ interpretations, is difficult. Migmatitic rocks are controversial and, by their very nature, are difficult to describe and correlate. However, it is accepted that important evidence is held within them. A rock undergoing partial melting and subject to heterogeneous strain because of melt migration will inevitably have an apparently very complicated structural history. Much of this may be of great importance regarding how the melting progressed, but may have absolutely no relevance to either any earlier history that the precursor underwent, or to external areas that are not melting. When attempting to elucidate regional geology, some of the very detailed features are recorded, but otherwise ignored. The emphasis of the book is very much that every feature represents an important event. However, ‘importance’ must surely depend upon the level of the study being carried out. Within gneiss terrains it is clear that, in certain places, there are indeed simple, superimposed sequences of events, and good examples of this are illustrated in this volume. However, there are several examples of folds given in the book that would be good candidates for sections through sheath folds and can be argued not to have resulted from interference of two phases of folding. The theory of sheath geometry and fold development by progressive shear, even if it is wrong, is one that is widely accepted and, consequently, should have been discussed here as a possibility.
The book is mostly well illustrated with many fine examples of folds and in some cases sequences of events that can definitely be discerned. However, there are several photographs that are very hard to decipher, even with line drawings to explain what is depicted. Some of the repeat photographs show that the reproduction varies considerably. An infuriating aspect of this and many other books is to have a small photograph in which details are difficult to see with a large blank space next to it. The book is certainly provocative and is a competitor to the volume by Passchier et al. (1990) which, despite now being 10 years old, still has a modern feel to it. Given the price of this volume, I am dubious as to how many individuals will purchase it.
Clark R. L. Friend
Oxford Brookes University
Physics and Chemistry of Partially Molten Rocks edited by N. Bagdassarov, D. Laporte and A. B. Thompson. Petrology and Structural Geology, Vol. 11. Kluwer Academic, Dordrecht, 1999. xvi + 271 pp. ISBN 0-412-84720-5. £79, US$136, NLG 225
The second volume of this collection by Kluwer was published after a special symposium of the EUG ninth meeting held in Strasbourg. However, the volume is not simply a set of papers bound together, but rather a sum of eight review papers that marks the state of the art on the topic of partially molten rocks (PMR). Papers are grouped into four sections, which respectively present the rheology of PMR, the topology of partial melt, the modelling of melting processes and finally two natural examples of PMR. All sections are of similar length, and present the latest findings and developments that relate to the rheology and behaviour of PMR. All scales are examined, from the grain boundary interface to large-scale modelling, with preference given to small-scale observations and interpretations, which are of crucial importance in understanding the large-scale effects.
The volume opens with two review papers, which present the latest technical results on the rheology and viscoelasticity of PMR. This is an especially ‘hot topic’ in the sense that new analytical techniques (TEM, AEM and EELS, as presented in the last chapter by Wirth & Franz) have proven to give new insights into the nature of intergranular boundaries. Briefly, the creep experiments, presented by D. Kohlstedt and his group, show that PMR behave differently at low and high confining pressure: low confining pressure experiments are characterized by a diffusion creep (n = 1), which gives way at higher pressures to dislocation creep (n = 3–4). When melt is added, such a break in the deformation regime is also observed. However, the experiments presented are all characterized by very low melt fraction (<3% by volume).
An interesting summary of anelastic and viscoelastic behaviour of PMR is covered in the second paper, by N. Bagdassarov. This addresses the transient rheological properties of PMR, which have direct implications for the understanding of seismic wave attenuation and transient rheological responses. The response at low melt concentrations (<10%) corresponds to grain boundary sliding. At high melt fraction (>60%), the melt phase is predominant over the solid particles, which freely rotate under shear stress. Between these concentrations, a complex behaviour occurs in which the solid particles aggregate and form a weak framework that may carry stresses. This separation into three domains clearly indicates that PMR evolve from a solid-like to a suspension behaviour with a large (10–60%) domain of complex interactions. This domain is further described in the natural example of Hobson et al.
Two papers (by Faul and Laporte & Provost) form the core of the book, with an exhaustive presentation of incipient melt distribution at grain boundaries. The topic has already been widely discussed in Earth Sciences, but its geological implications are numerous. They naturally cover the first magmatic stages in igneous rocks, here represented by mantle- and crust-derived magmas, but also metamorphic transformations and core accretion. Future implications of the measurement of contact geometry between PMR relate to the chemical evolution of the melt: is textural equilibrium achieved in natural systems? Evidently the response is not simple and the role of deformation in segregating the first formed melts may provide part of the answer.
Modelling of melt segregation is examined from a numerical point of view (Schmeling), whereas its consequences on the magma chemical evolution are developed by Matile et al. One interesting point developed by the latter relates to the partitioning of heat loss during melting of surrounding crust while melt crystallizes and fractionates. Commonly, to explain crustal-derived magma, some researchers simply advocate a possible source of heat related to basic magma underplating. This popular model, however, fails to take into account the heat that underplated magma must release to assimilate the surrounding crust. Here, the authors have designed a code for fractional crystallization that examines the heat balance between magma crystallization and source rock assimilation. Two end-members of crustal rocks are selected that bracket granitic magma production between a less fertile (tonalitic) and a more fertile (pelitic) source. The largest amount of heat is lost by the mafic magma during the very first stage of fractionation, at very high temperatures (1200–1250°C). Later, fractionates can assimilate a small amount of fertile crustal rocks (30–40% equivalent mass), and assimilation is more efficient for preheated rocks. Such energy balance calculations and comparison of magmatic rates are urgently needed before models are built on the basis of poorly constrained models.
Heat is also addressed in Schmeling’s paper, but on a much wider scale. Upper-mantle convection is examined as a possible source for melt production through decompression and melt segregation through compaction. Solitary porosity waves develop that can propagate without interfering with the surroundings. The author argues that his model could explain some propagating melt instability observed for the formation of hot-spot tracks. I had some problems reading this paper. Although not being a specialist in mantle convection, I was left with the impression that the model sounds good, but the geological application is not adapted, because the model focuses on upper-mantle problems.
The two last papers are natural examples of PMR. Again, two scales of observation are presented, ranging from field observation in the Canary Islands (Hobson et al.) to microscopic (TEM) observations on xenoliths (Wirth & Franz). Two scales, two different approaches; once more the book adopts a balance between styles and I acknowledge the editorial process that was conducted. The field example deals with the melting that results from the intrusion of alternating horizons of gabbro and pyroxenite. Competition between melting as a result of the additional heat and deformation results in small-scale melt segregation and interesting rheological behaviours (ductile–ductile or brittle–ductile) depending on the rocks in contact. The paper apparently lacks more data, but often refers to a similar paper in which they may be fully found.
The second example relates to intergranular layers at mineral interfaces in xenoliths. It largely describes new transmission electron microscopy (TEM), coupled with new techniques such as electron microprobe analysis (EMP), analytical electron microscopy (AEM) or electron energy-loss spectroscopy (EELS). Those techniques allow both detailed qualitative and quantitative analysis at a very small scale. Results are very promising. In particular, the authors describe amorphous thin (
10 nm) to thick (<1000 nm) intergranular films. These may extend along one whole grain boundary, which does not fit the wetting angle criteria (see Laporte & Provost). Their composition may be different from the expected bulk melt composition, with notable enrichment in Ca. These films have a strong effect on diffusion processes, which is not yet really taken into account. They have an evident rheological role, which is not at present entirely understood. In brief, this chapter provides a very new set of data, which should have a stimulating effect on the community of workers who deal with PMR.
Naturally, some criticisms can also be made concerning this book. I have already commented on the editorial process that manifestly prevailed before publication. One drawback is the apparent time lag between the presentation of the papers and the publication date (nearly 3 years). I agree, however, that some papers are more than factual papers and present the state of the art.
A second criticism addresses the high price per page of the book. In these times of general budget cuts, I have some doubts that a book valued at US$0.5 per page would be a commercial success.
The last criticism refers to the rocks. Unfortunately, most of the presented studies relate to mantle rocks, and little attention is paid to crustal-derived PMR. We know that the mantle rheology differs significantly from that of crustal rocks. In consequence, the papers presented in the volume can stimulate ideas, or analogies, for those workers who deal with felsic PMR, but they can hardly be transposed directly to this type of rock.
Jean Louis Vigneresse
CREGU, Vandoeuvre les Nancy, France
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