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Kimberlite Occurrence and Origin: A Basis for Conceptual Models in Exploration

expected, occurs perpendicular to the leucosomemelanosome interface, whereas any external introduction of material along the strike of the teucosome becomes fairly uniformly distributed through the leucosome. Ashworth and McLellan, in Chapter 5, argue that important information on migmatization processes may be gained from textures and an understanding of processes involved in the development of particular textures in spite of frequent subsequent modification of primary migmatization textures by post-migmatization events. Textural characteristics which have figured in the migmatite debate include grain size (pegmatitic versus aplitic), features regarded as resulting from crystallization of a melt (idiomorphic zoning in plagioclase feldspar, replacement textures such as myrmekite), grain shape, grain orientation, and grain contact relations (in which both Ashworth and McLelland themselves, together with Dougan, have built upon the early work of Kretz). Touret and Olsen, in Chapter 8, which might have been placed better before the regional examples of migmatite terrains, consider what might be gained from a study of fluid inclusions, a field pioneered in high-grade metamorphic rocks by Touret during the past fifteen years. It is axiomatic that some kind of fluid is necessary for the generation of migmatites, whatever the actual process of migmatization might have been, a fact reflected in the increased abundance of inclusions in the leucosome minerals, and in spite of the argument about whether or not the fluid preserved in the inclusions represents a sample of the fluid in equilibrium with the leucosome at the time of formation. In any case, the fluid in inclusions represents the only samples we have of the fluids associated in general terms with migmatites (although the methods of thermodynamics may allow the calculation of the fluid composition which must have been in equilibrium with the solid phases). The debate continues, however, over the source of the fluids now preserved as inclusions, especially the CO2 in leucosomes and the latestage H20 in leucosomes. This topic relates to the current controversy in metamorphic geology over the origin of granulite facies mineral assemblages and variably LILE-depleted bulk rock chemistries partial melting versus CO2-flushing. Finally, in Chapters 6 and 7, Traey and Barr consider the development of migmatites in New England and the Moines, respectively; both of which are polyorogenic and polygenetic migmatite terrains. The New England region has become well known to metamorphic geologists during the last ten years through the work of Robinson and his co-workers and prominent among these has been Tracy who has written this comprehensive and instructive review. In contrast, Barr's work in the Moines brings fresh views to neglected migmatites in a classic part of British Isles geology. Unfortunately, the regional examples are restricted to these two migmatite terrains. Chapters on Scandinavian migmatites (where the great controversy originally started through the works of Sederholm and Holmquist, the famous 'arterite' versus 'veinite' debate) and Australian examples (e.g. Broken Hill or Arunta) would have been welcome and instructive. In conclusion, this is an extremely useful book as an up-to-date review of and introduction to the subject of migmatites which is reasonably priced, especially considering that the market will be limited to final year undergraduate students and post-graduate research workers with an interest in petrology.