REGIONAL METAMORPHISM 409 that are characterized by index minerals not present at lower grades ‘Mineral-appearance’ isograds were mapped by drawing lines between locations where each index mineral first appeared It later became common practice to replace these classic mineral isograds by ‘reaction’ isograds related to a particular metamorphic reaction and to use the equilibrium assemblage of that reaction as a zone boundary The reaction isograd is defined by assemblages of reactant(s) on one side, of product(s) on the other side, and, if present, the equilibrium assemblage on the isograd line (surface) It should be emphasized that in metamorphic petrology, an isograd is not exactly a line (surface) of constant temperature (and pressure) Various aspects of metamorphic reactions should be considered (1) In cases in which minerals with variable chemical compositions (solid solutions) participate in the isograd reaction, the position of the isograd in pressure– temperature (P–T) space may depend on mineral compositions (and the bulk chemical composition of the rock) (2) The P–T position of reactions producing or consuming volatiles (H2O, CO2) may also depend strongly on the composition of the metamorphic fluid The reaction kinetics may vary on a local or regional scale in metamorphic terranes, especially at the lower metamorphic grades These aspects imply that mapped reaction isograds will only approximately reflect lines (surfaces) under identical peak temperature (and pressure) conditions Following the pioneering studies of Goldschmidt in southern Norway, in 1920, Pentii Eskola developed the concept of metamorphic facies Studying mafic metamorphic rocks (mostly basaltic protoliths) in southern Finland, he recognized that ‘‘in any rock or metamorphic formation which has arrived at a chemical equilibrium through metamorphism at constant temperature and pressure conditions, the mineral composition is controlled only by the chemical composition’’ This concept has now been validated worldwide in innumerable regional field studies demonstrating that many rock compositions develop mineral assemblages typical of the physical conditions of formation For a certain bulk composition, the mineral assemblages formed at these different metamorphic grades are then related by mineral reactions Experimental determinations of the relative stabilities of metamorphic minerals and assemblages, followed by thermodynamic calculation of such phase equilibria, have allowed an approximate quantification of the pressure and temperature conditions of each metamorphic facies The metamorphic facies diagram shown in Figure differs in some details from that proposed by Eskola, who named his metamorphic facies after Figure Pressure temperature diagram, showing principal metamorphic facies; approximate boundaries are represented by thick grey lines Thin black lines designate the stability fields of the three Al2SiO5 polymorphs (And, andalusite; Ky, kyanite; Sil, sillimanite) Reproduced with permission from Spear FS (1993) Metamorphic Phase Equilibria and Pressure Temperature Time Paths Washington, DC: Mineralogical Society of America metamorphosed mafic rocks Nowadays, differences in facies classification exist mainly in the transition from greenschist facies to amphibolite facies at higher pressure; some authors prefer, and others not, to introduce an epidote–amphibolite facies (Figure 1) In addition, the classification of the facies below the greenschist facies is summarized by several authors as ‘subgreenschist facies’ In any case, the borders between different facies are not sharp, and the classification should be used in a broad sense It is schematic, but there is consensus on the temperature and pressure limits for each facies The boundaries between the facies represent the P–T conditions at which key minerals or mineral assemblages appear or disappear Extensive petrological studies were carried out in metamorphic terranes around the world during the twentieth century These studies revealed that distinct metamorphic facies series (high, medium, and low pressure and very low grade) correlating with specific geotectonic settings could broadly be distinguished Facies of High Pressure High-pressure rocks of the blueschist and/or eclogite facies were metamorphosed at a low geothermal gradient, i.e., small increase in temperature with