Pressure—temperature—time (P—T—t) histories of erogenic belts

Abstract

Thermal modelling shows that a cycle of crustal thickening and erosion reproduces many of the characteristics of medium-pressure metamorphic terranes. In contrast, the structural and metamorphic features of high-pressure terranes suggest rapid exhumation, possibly tectonically as fault-bounded blocks. Low-pressure metamorphism requires an augmented heat supply. Such terranes are characterized by granite--gneiss domes, and evidence of crustal extension, and hence may be the result of the mechanically likely orogenic sequence of early thickening followed by extension. Whether earlier isograd sequences are extended, condensed, or reset depends upon the relative rates of deformation and thermal relaxation, and when the deformation occurs relative to the thermal peak of metamorphism. Detailed determinations of relations between deformation events and metamorphism is made difficult by the contrast between continuous metamorphic evolution and short time-span deformation events. Combined microstructural and geochronological studies, together with a consideration of the distribution of isograds will give most information on complex, polymetamorphic histories, and allow distinction between regional and local features, especially those due to differential uplift. Considerations of rates of heat flow within the crust indicate how isotherms evolve in response to tectonic events, and how isograd distribution will relate to local and regional structures. Important controls are the relative rates of deformation and thermal relaxation, and whether the deformation predates, is synchronous with, or postdates the metamorphic peak. Only tectonic events close to or after the metainorphic peak, result in deformed isograds; at the peak, in areas where deformation induces local cooling; after the peak isograd sequences may be folded and expanded or condensed by crustal thickening or thinning. As repeated Information on orogenic development during the cooling and uplift stage of metamorphism, obtained by combined geochronological, metamorphic and structural studies, suggests that uplift in a mountain belt is generally very unevenly distributed, The only indications of deformation events before the peak of metamorphism, and of how these may have transiently affected the thermal structure of the orogenic belt, will come from combined microtextural and structural studies. Microtextural studies may also be important in distinguishing between deformation events at the peak of metamorphism and those significantly post-dating it. Parameters such as the size and distribution of successive porphyroblasts, and the size of grains in a rock matrix are functions, through reaction kinetics, of heating rates deformation and fluid involvement. At present, the quantitative nature of the relations is poorly understood. Useful information may, however, be obtainable by measuring the variation of any parameter across a metamorphic terrane (see Ridley 1986). Contrasting microtextural histories, and contrasting patterns of isograd distributions in terranes showing the various facies of metamorphism, suggest indeed that the facies series are diagnostic of specific tectonic regimes. Only metamorphism in the kyanite-sillimanite facies series can be, adequately modelled as the thermal result of simply crustal thickening and subsequent erosion. Both lower-pressure and higher-pressure metamorphism require a different or more complex history. It is suggested that crustal extension is important in low-pressure metamorphic belts. The presence of igneous rocks in such an environment may be the result of extension rather than the cause of the high heat flow. There may be various causes of high-pressure metamorphism, but the consistent pattern of An end to metamorphic recrystal- lization at, the end of a major deformation event suggests that the rocks are prevented from further heating up. Tectonic uplift as fault-bounded blocks is considered to be important.