This book presents a detailed discussion of a few selected topics in metamorphism. Chapter I (Turner) is a historical account of the development of the facies concept. A new definition of metamorphic facies is formulated.Chapters II to IV (Fyfe and Verhoogen), each of which ends with a summary in nontechnical language, deal respectively with certain thermodynamic and kinetic aspects of metamorphic-reactions. In Chapter II, the authors consider successively the magnitude of the free energy of metamorphic reactions, methods for computing approximate values of the entropies of silicates, a definition of the pressure variables in metamorphism, the role of surface films, and structural aspects of metamorphic mineralogy. The conclusion of Chapter III, which is devoted to a study of kinetics, is that reaction rates in systems containing water are generally such as not to lead to difficulties or inconsistencies in the interpretation of metamorphic facies. The role of water in metamorphic reactions is stressed again. This leads to a closer study of systems containing water, which are discussed in Chapter IV. Some properties of water are evaluated, such as its dielectric constant and ionic product, at high pressures and temperatures. A model is set up by which thermodynamic functions for hydration-dehydration reactions can be evaluated. There follows a discussion of solubility in aqueous systems and of variations of pH under natural conditions. _ In Chapter V, Fyfe and Turner correlate field and experimental data on the stability of critical mineral assemblages in metamorphic rocks. They conclude that there are at present no major inconsistencies between field and experimental data. Metamorphic facies, defined by mineralogical and field criteria, can still be interpreted in terms of a few intensive physical variables.The role of water and heat in metamorphism is considered in Chapter VI (Fyfe and Verhoogen). Regional metamorphism is not a normal phenomenon, in the sense that it would occur wherever rocks are buried to sufficient depth. On the contrary, it appears that regional metamorphism can occur only where the heat flow is notably increased and where water, probably of juvenile origin, is abundant.Chapter VII (Turner) is a revision of individual metamorphic facies. In the light of new experimental and mineralogical data the limits of some facies and subfacies are redefined, and a few new divisions are proposed.
Extensive experiments on the deformation of rocks at temperatures from 25° to 800°C. have been performed in triaxial-test apparatus at 5 kb confining pressure, and in shearing apparatus at pressures to 20 kb. A variety of rocks, crystals, and mixtures have been tested, including peridotite, pyroxenite, basalt, granite, dolomite, marble, and quartz crystals and aggregates.Strength is reported as a function of temperature. High quartz is found to be very strong. Peridotite, pyroxenite, and granite have nearly the same strength over the entire temperature range-20 kb at 25°C. and 7 kb at 800°C. Basalt has a similar strength to 600°C., but its strength decreases rapidly at higher temperatures. Dolomite is weaker than these rocks at low temperature but at 500°-800°C. has nearly the same strength as peridotite, pyroxenite, and granite. Marble is much weaker at low temperature and decreases in strength more rapidly than the other rocks as temperature increases.The principal mechanism of deformation of diopside was found to be {100} translation. Grains of enstatite highly deformed at 500°C., 5 kb locally invert to clinoenstatite. The mechanism of deformation of olivine was not identified.Stress-strain curves of calcite single crystals change character at 500°-600°C. At 600°C.,/ translation becomes important, and at 800°C. basal glide may occur. At 800°C., twinning is little easier than r translation, which is still the easiest translation. This large change in the relative ease of translation and twinning at high vs. low temperature has important consequences in the deformation of marble at 800°C.Syntectonic recrystallization reaches a maximum at 600°C. in marble deformed at 3 per cent per minute. At a lower strain rate the maximum syntectonic recrystallization seems to occur at a lower temperature. The recrystallized grains show a pronounced orientation with the c-axes parallel to the maximum principal compressive stress. Water and CO2 do not affect this recrystallization in our experience to date. The recrystallization observed in these experiments is thought to be of the same type as that which occurs in metamorphic marmorization.Only very limited plastic deformation of quartz has been observed at 500° and at 800°C., 5 kb. The mechanism could not be determined. High preferred orientations have been developed in quartz in shearing experiments, particularly when the quartz crystallized from an amorphous phase subject to large shear stress and strain.
Classic views on the scope and subdivision of the field of rock metamorphism are reviewed and summarized, and the general basis for classification of metamorphic rocks is outlined.The zonal distribution of rocks showing different degrees of metamorphism in those metamorphic terranes that have been studied in detail is attributed to progressive variation in physical conditions controlling development of a state of internal equilibrium in rocks of varying composition in different chemical environments. The resulting equilibrium assemblages of minerals can be interpreted broadly in terms of the phase rule. This leads to the concept of metamorphic facies as a basis for classifying and interpreting metamorphic rocks. Eskola's scheme of facies is modified and subdivided insofar as this is at present justified by the data of metamorphic petrography. While metamorphism in some cases involves only insignificant change in total chemical composition of the rocks affected, in others important change results either from metasomatism or from metamorphic differentiation.The fabric of metamorphic rocks has developed by growth and/or deformation of crystalline minerals in a solid medium. Its characters therefore differ strikingly from those of typical igneous and sedimentary fabrics. The essential features of metamorphic fabrics are reviewed, and interpretation of the fabric of deformed rocks is considered in some detail.The memoir concludes with a discussion of some of the problems specially connected with regional metamorphism, namely, the causes of regional metamorphism, the phenomenon of retrogressive metamorphism, the development of migmatites, and the relation of regional metamorphism to plutonic igneous intrusion. on June 21, 2015 memoirs.gsapubs.org Downloaded from on June 21, 2015 memoirs.gsapubs.org Downloaded from CHAPTER I. SCOPE OF METAMORPHISM DEFINITIONAccording to current general usage of the term, metamorphism is the mineralogical and structural adjustment of solid rocks to physical or chemical conditions that have been imposed at depths below the surface zones of weathering and cementation, and which differ from the conditions under which the rocks in question originated. Van Hise's (1904) extension of the scope of metamorphism to include weathering and cementation has now been generally abandoned, since it so enlarged the field of metamorphism that the term became virtually synonymous with "rock alteration" and therefore had little practical value in petrology. Other writers have narrowed the field of metamorphism as defined above by excluding such rock transformations as involve addition or removal of material, i.e., changes into which metasomatism enters. It is true that in many cases of metamorphism the composition of the rock remains a,lmost unchanged throughout, and to such Eskola (1939, p. 264) has applied the term isochemical metamorphism. On the other hand addition or removal of substances of high vapor pressure, notably water and carbon dioxide, may play an essential part in rock alterations universall...
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