408 REGIONAL METAMORPHISM Regional Metamorphic Processes Large-scale geotectonic processes move rocks along particular paths in pressure–temperature–time (P–T–t) space During movement along this path, the rocks re-equilibrate, and when chemical equilibrium is attained (when the reaction kinetics is fast enough), the mineral assemblage and the compositions of the individual minerals will react continuously by adapting to the new physical conditions In addition to the mineralogy, the fabric of the rocks also changes, because regional metamorphism is a dynamic process typically accompanied by an anisotropic pressure field Minerals with an elongated or platy habit will commonly align to the confining stress field Mineral grains will also increase in size with increasing temperature, reducing the surface free energy (typically observed, for example, in metacarbonate rocks and quartzites) This process is termed ‘static recrystallization’ Deformation in the anisotropic pressure field, however, reduces the grain size by various processes, depending on temperature and confining pressure; this is called ‘dynamic recrystallization’ Particularly at low-level metamorphic conditions, when deformation of the rock is non-penetrative, and in fluid-deficient rocks, equilibrium is often incomplete and minerals initially present in the protolith, and minerals formed during early metamorphic stages, may locally survive metastably Such metastable persistence of minerals (e.g., as relict inclusions in newly formed minerals or as compositional zoning in minerals from core to rim) is fortunate for the petrologist in the way that it records the early geological history (the prograde path) of rocks Petrological studies of metamorphic rocks traditionally aim to estimate the peak metamorphic conditions (pressure, temperature, and composition of coexisting fluid phase) that a rock has experienced, based on the assumption that the mineral assemblage essentially documents this peak equilibrium assemblage Equilibrium among the minerals is commonly verified by textural evidence, i.e., that all minerals of the peak assemblage should be in mutual contact Chemical evidence for equilibrium can be obtained from comparison of the composition of coexisting minerals in different bulk compositions; no crossing tie lines should be present and the tie lines (e.g., of garnet– biotite) should follow a systematic pattern During the cooling and exhumation (the retrograde path), this peak metamorphic assemblage experiences only minor changes, because these processes are commonly associated with fluid-free conditions and minor deformation A retrograde reaction, i.e., the formation of lower grade, mostly hydrous minerals from higher grade ones, thus typically occurs on a local scale Examples include the replacement of garnet or biotite by chlorite or the replacement of plagioclase by calc-silicates such as (clino)zoisite, prehnite, or pumpellyite Many occurrences of retrograde hydrous and carbonate phases are spatially connected with late (brittle) deformation, implying that deformation may facilitate local fluid introduction and thus formation of retrograde minerals In virtually all regional metamorphic terranes, mineral reactions are associated with deformation Therefore, it is often possible to determine relative time relationships between growth of minerals and major deformation events characterized by development of a distinct foliation, folding, or shearing Growth of a mineral can be specified as pre-, syn-, or post-tectonic with regard to specific deformation events Relationships between mineral growth and deformation may be complex, however, because multiple or long-lasting deformation and mineral growth are the rule rather than the exception during orogenic cycles Studies of regional metamorphic rocks conveniently separate the rocks’ evolution into the three categories of prograde, peak, and retrograde stages using petrological and structural criteria The researcher should be aware, however, that this is a simplified picture and the P–T–t paths of metamorphic rocks are in general quite complicated Regional Metamorphic Zones and Facies It was recognized very early that metamorphic rocks can be classified according to different ‘intensities’ of metamorphism, or metamorphic grades The concept of gradation is not applicable to the other families of sedimentary or magmatic rocks; a magmatic rock cannot be ‘more magmatic’ than another one, whereas a metamorphic rock can be metamorphosed more intensely This concept also emphasizes that the processing of a sediment through diagenesis, then forming a metamorphic rock, is transitional, and it therefore is impossible to mark an unequivocal beginning of metamorphism (see Diagenesis, Overview) The concept of metamorphic index minerals typical of metamorphic grade was first introduced by George Barrow in the south-east Highlands of Scotland in the late nineteenth and early twentieth centuries Studying a sequence of metamorphosed clay-rich sediments, Barrow related the systematic change in mineralogy in these metapelitic rocks to changing metamorphic conditions Barrow recognized chlorite, biotite, garnet, staurolite, kyanite, and sillimanite zones as representing increasing metamorphic grades