Barrow (1893) introduced three important ideas that furthered understanding of metamorphic processes: (i) the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism; (ii) the concept of progressive metamorphism; and (iii) the concept of magmatic advection of heat as a possible cause of regional metamorphism. This article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the P–T–t paths of metamorphic rocks, and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts.
Understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum T and maximum P. Orogenic belts characterized by clockwise P–T paths achieved maximum P before maximum T, the metamorphic peak normally post-dated early deformation within the belt and additional heating above the ‘normal’ conductive flux has been related to the amount of overthickening. By contrast, orogenic belts characterized by counterclockwise P–T paths achieved maximum T before maximum P, the metamorphic peak normally pre-dated or was synchronous with early deformation within the belt and additional heating above the ‘normal’ conductive flux has been related to the emplacement of plutons. Techniques used to constrain portions of P–T–t paths include: the use of mineral inclusion suites in porphyroblasts and reaction textures; thermobarometry; the use of fluid inclusions; thermodynamic approaches such as the Gibbs method; radiogenic isotope dating; fission track studies; and numerical modelling. We can utilize specific mineral parageneses in suitable rocks to determine individual P–T–t paths, and a set of P–T–t paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism. Recent advances are reviewed with reference to collision metamorphism, high-temperature–low-pressure metamorphism, granulite metamorphism, and subduction zone metamorphism, and some important directions for future work are indicated.