regional metamorphism

regional metamorphism In the most widespread examples of metamorphism, metamorphic rocks occur through an extensive terrane, and the metamorphism is not linked to a specific focus such as an igneous intrusion. Instead it reflects elevated temperature and pressure values resulting from burial within the crust. Regional metamorphism is almost invariably accompanied by penetrative deformation, giving rise to folding and the development of pervasive fabrics: cleavage at very low grades where the aligned phyllosilicate grains are too small to be individually visible; schistosity at higher grades where the grains are coarser. It is now recognized that virtually all but the lowest-grade regionally metamorphosed terrains have experienced multiple events of folding and fabric development. Petrographic analysis of rock fabrics in thin section makes it possible to determine the relative timing of the growth of metamorphic minerals and the development of structural fabrics. Modern work in metamorphic petrology uses such information in two ways: as a basis for applying single crystal dating techniques to date specific episodes in the metamorphic evolution of a terrain, and as a way of relating particular deformation events to a particular tectonic setting through knowledge of pressure–temperature (P–T) conditions, and hence the depth and the heat-flow regime, when the appropriate minerals equilibrated (see also metamorphism, metamorphic facies, and metamorphic rocks).

Some very low-grade metamorphic belts have experienced burial metamorphism, a form of regional metamorphism in which new mineral growth is accompanied only by open folding, and pervasive fabrics are not developed. Well-preserved fossils occur in the zeolite facies Triassic rocks of Southland, New Zealand, where Coombs first described the phenomenon in the 1950s. The temperatures and pressures during burial metamorphism are more typical of those in deep sedimentary basins, and the distinction between diagenesis and burial metamorphism is primarily in the rock-types affected. Under the same conditions, sediments composed mostly of volcanic material are much more reactive than normal continental clastic sediments rich in quartz. The changes in siliceous sediments will be confined to the cement, which will recrystallize, but the igneous components in the sediments of volcanic origin will break down and new hydrous phases, such as zeolites, will develop.

Metamorphic facies series. It was inherent in P. E. Eskola's original definition of metamorphic facies (see metamorphism, metamorphic facies and metamorphic rocks) that mineral assemblages could be indicative of different pressures and temperatures of metamorphism, but the implications of this for the recognition of thermal gradients in the past were fully appreciated only by Akiho Miyashiro in 1961. He recognized that many metamorphic belts of different ages around the world exhibited one or other of the following sequences of metamorphic grade in going from low to high grade:prehnite–pumpellyite → blueschist → greenschist or amphibolite
greenschist → amphibolite → granulite


Miyashiro used the term facies series for these characteristic associations of facies and pointed out that they distinguished contrasting baric (pressure-) types of metamorphism; in the first instance high pressures were attained at relatively low temperatures, indicative of a low heat-flow setting, whereas in the second instance temperatures rose rapidly at shallow levels, indicating high heat flow. The classical metamorphic zones known from the work of George Barrow are representative of an intermediate pressure association corresponding approximately to a normal crustal thermal gradient. Miyashiro also believed that belts of contrasting facies series and of the same age commonly occurred together. He called this association paired metamorphic belts. With the advent of plate-tectonic theory, Miyashiro was quick to recognize the tectonic significance of the different facies series. The low heat flow associated with high-pressure metamorphism and the development of blueschists is indicative of a subduction zone setting; the elevated heat flow and associated magmatism associated with low-pressure metamorphism is usually indicative of the roots of a volcanic arc.

Modern work, while building on Miyashiro's recognition of distinct facies series, has emphasized the distinction between the apparent progression of metamorphic conditions that is deduced from studying the conditions of formation of a sequence of zones across a metamorphic belt, and the actual history of changing pressure and temperature experienced by a particular rock. The trend of P–T conditions indicated by the peak metamorphic conditions of each zone is termed the metamorphic field gradient, while the history of an individual rock is known as the P–T–t (pressure–temperature–time) path. Perhaps the classic example of paired metamorphic belts, described by Miyashiro, comes from south-western Japan, where the late Mesozoic high-pressure Sanbagawa belt is flanked to the north-west by the low-pressure Ryoke belt, of similar age. Miyashiro's original interpretation of these as representing subduction zone and arc root settings respectively interprets them as a snapshot in time in the development of the Pacific margin, whereas more recent work recognizes the rocks of each as having experienced a prolonged metamorphic evolution.

Bruce W. D. Yardley

Bibliography

England, P. C. and and Richardson, S. W. (1977) The influence of erosion on the mineral facies of rocks from different environments. Journal of the Geological Society, 134, 201–13.
Miyashiro, A. (1961) Evolution of metamorphic belts. Journal of Petrology, 2, 277–311.
Miyashiro, A. (1972) Pressure and temperature conditions and tectonic significance of regional and ocean floor metamorphism Tectonophysics, 13, 141–59.
Treloar, P. J. and O'Brien, P. J. (eds) (1998) What drives metamorphism and metamorphic reactions? Geological Society Special Publication No. 138.