CHAPTERMetamorphicRocks and Metamorphism: An OverviewFundamental Questions Considered inThis Chapter1. What is the nature of subsolidus changes in the fabric and composition of rocks that occur during metamorphism?2. What transfers of energy and matter in geologic systems bring about metamorphism?3. In what geologic settings does metamorphism occur, and why?4. What role do metamorphic rocks play in our understanding of crustal evolution, global plate tectonics, and how the Earth works?INTRODUCTIONIn the dynamic Earth, changes in geologic systems resulting from transfers and transformations of energy and from movement of rock, magma, and fluids are continually occurring, chiefly near margins of lithospheric plates. States of thermodynamic equilibrium are perturbed, causing rock systems to seek new, lower energy, more stable states by adjustment of their fabric and composition. These equilibrating adjustments that take place in the solid state at elevated temperatures are called metamorphism. Equilibration processes and the kinetic factors that control their rates vary widely, depending on the fabric and composition of the parent rock, the 7and Pof the evolving rock system, the composition of fluids in it, and the prevailing state of stress. Changing geologic conditions in metamorphic systems range between those where buried sediment is cemented and lithified in the sedimentary process known as diagenesis at relatively low T and P, up to where rock partially melts (Figure 14.1). Simply put, during metamorphism, rocks are hot enough to recrystallize but not hot enough to mek.Many sedimentary and some magmatic rock-forming systems can be directly observed. However, our study of metamorphic rock-fbrming systems can never enjoy this benefit because of their concealment well beneath the surface of the Earth, where P and T are elevated. Consequently, equilibrating metamorphic processes can generally only be understood from observations made of their rock products long after the metamorphism has terminated, combined with logic and inferences from necessarily simplified thermodynamic models and laboratory experiments.Like magmatism, metamorphism involves transfer of heat and mass. Consequently, many concepts developed in preceding chapters apply equally well to metamorphic systems as to magmatic ones. Examples include concepts of thermodynamics and kinetics (Chapter 3), phase equilibria (Chapter 5), diffusive transport of atoms and heat (Chapter 6), and stress-deformation relationships and rheologic behavior (Chapter 8).Nonetheless, there are significant contrasts between magmatic and metamorphic systems. Magmatic behavior is dominated by a melt, or silicate liquid, that contains dissolved volatiles and interacts with crystalline phases. In contrast, subsolidus metamorphic systems lack a melt but usually have a volatile fluid that interacts chemically, physically, and thermally with the(b)14.6 Prograde thermal metamorphism of diabase under essentially hydrostatic stress conditions, (a) Weakly metamorphosed diabase (metadiabase) or greenstone. Relict ophitic fabric (Figures 7.15 and 7.22) is well preserved because recrystallization has not produced large enough grains to obliterate original grain boundaries. Magmatic pyroxenes have been replaced by aggregates of randomly oriented, chemically zoned actinolites. Embedded within the actinolite aggregate arc patches of fine-grained chlorite and tiny epidotes. Ti-bcaring oxides have partially reacted with mobile Ca and Si to produce rims of sphene (titanite). Original lath-shaped calcic plagioclases (labradorite) are partly replaced by aggregates of minute epidote (high relief) and white mica grains; remaining plagioclase is more albitic. Local large calcice grains represent relict amygdules, (b) Greenstone or fine-grained amphibolite. An isotropic aggregate of the same minerals comprising (a), except here grains formed by solid-state growth are larger with simpler, cleaner outlines. The original magmatic fabric is virtually obliterated; only vaguely defined rectangular areas of untwinned albite and epidote aggregates suggest the former existence of magmatic plagioclases, (c) Amphibolite. Coarser, well developed granoblastic fabric. All vestiges of the original magmatic fiibric have been erased, (d) Granoblascic plagioclase-pyroxene granofek. Mineral phases in this high-grade rock have compositions of a near-solidus gabbro but the texture is clearly metamorphic (compare Figure 7.16a).T under highprogressive metamorphism at increasingCrystallization Yielding New Minerals and New Fabrics. Subsolidus equilibration generally yields new mineral phases as well as new imposed fabrics, both stable under new conditions. This is well illustrated by thewater fugacities of an ophitic diabase in Figure 14.6. In the initial stages of subsolidus crystallization at low temperatures, primary magmatic labradorites arc14.7 Over