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The past half-century has seen an explosion in the breadth and depth of studies of metamorphic terranes and of the processes that shaped them. These developments have come from a number of different disciplines and have culminated in an unprecedented understanding of the phase equilibria of natural systems, the mechanisms and rates of metamorphic processes, the relationship between lithospheric tectonics and metamorphism, and the evolution of Earth’s crust and lithospheric mantle. Experimental petrologists have experienced a golden age of systematic investigations of metamorphic mineral stabilities and reactions. This work has provided the basis for the quantification of the pressure-temperature (P-T) conditions associated with various metamorphic facies and eventually led to the development of internally consistent databases of thermodynamic data on nearly all important crustal minerals. In parallel, the development of the thermodynamic theory of multicomponent, multiphase complex systems underpinned development of the major methods of quantitative phase equilibrium analysis and P-T estimation used today: geothermobarometry, petrogenetic grids, and, most recently, isochemical phase diagrams.

New analytical capabilities, in particular, the development of the electron microprobe, played an enabling role by providing the means of analyzing small volumes of materials in different textural settings in intact rock samples. It is now understood that most (if not all) metamorphic minerals are chemically zoned, and that this zoning contains a record of sometimes complex metamorphic histories involving more than one period of metamorphism. The combination of careful field studies, detailed petrographic analysis, application of diverse methods of chemical analysis, and phase equilibrium modeling has resulted in at least a first-order understanding of the meta-morphic evolution of nearly every metamorphic belt on the planet. Additionally, the behavior of fluids in the crust, although still not fully understood, has been examined experimentally, petrographically, chemically, isotopically, and via geodynamic modeling, leading to recognition of the central role fluids play in metamorphic processes.

The plate-tectonic revolution placed metamorphic facies types into a dynamic new context—one in which the peak metamorphic conditions were seen as the interplay of thermal perturbations due to tectonic and magmatic processes, thermal relaxation via conduction, and exhumation rates. The discovery of coesite and diamond from demonstrably supracrustal rocks revealed an extent of recycling between Earth’s surface and the mantle that was previously unimagined. Numerous ultrahigh-pressure and ultrahigh-temperature terranes have now been recognized, forcing a reconsideration of the diversity and vigor of lithospheric processes that permit the formation and exhumation of rocks showing such extreme metamorphic conditions.

Revolutions in the field of geochronology have had a fundamental impact on metamorphic studies in that it is now possible to obtain ages from different parts of agezoned minerals, thereby permitting absolute time scales to be constructed for the rates of metamorphic recrystallization and their tectonic driving forces. Application of diffusion and kinetic theory is providing additional insights into the time scales of metamorphic recrystallization, which are emerging to be shorter than previously suspected.

Many unanswered questions remain. As thermodynamic and kinetic theory evolves and new analytical methods emerge, augmenting the fundamental contributions of fieldwork and petrography, metamorphic petrology will continue to provide unique and irreplaceable insights into Earth processes and evolution.

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