On limits to distances of movement of matter during regional metamorphism: An investigation of nine samples from high-grade metamorphic terranes

Abstract

Ideas since the time of Lyell (1830) on distances of movement of matter during regional metamorphism are reviewed, and it is concluded that the problem and its implications remain not fully resolved. An attempt at systematic investigation is then made by the micro-analysis of minerals and mineral groupings that appear to be particularly suitable for the purpose. The electron-microprobe analyzer is applied to determine variations in the incidence of major and trace elements in populations of like mineral grains constituting well-preserved contiguous fine beds and foliae in nine samples of metasedimentary materials from exhalative ore environments that have undergone high-grade (sillimanite) regional metamorphism. In all cases, chemical equilibrium has not been attained over distances down to small fractions of a millimeter. Given the high grades of metamorphism involved and the long times available for equilibration, it is concluded that movement of matter required to establish such equilibrium during metamorphism has been restricted to such distances, and hence to those no greater than present grain-size. It follows from this that each grain of a metamorphic mineral occupying a given domain now must have arisen from material that occupied that domain prior to metamorphism, i.e., in these cases, regional metamorphism has been isochemical. Further, evidence is presented indicating that this has not occurred through reactions among the components of ultrafine mixtures occupying such domains, but through transformation of a single material in each case. From this it is proposed that, rather than reflecting similar processes operating in different parts of a temperature–pressure spectrum, contact and regional metamorphism are dominated by two distinctly different processes of metamorphic mineral formation: chemical reaction (as long recognized) the dominant process of contact metamorphic–metasomatic mineral formation, solid–solid transformation the dominant process in regional metamorphism. On this basis, it is suggested that some present problems in regional metamorphic petrology stem from the current assumption that the dominant processes of regional metamorphism are analogous to those of contact metamorphism, where in fact they are substantially different. Clearly, extrapolation from nine samples to generality invites and requires extensive testing. It is therefore further proposed that studies of finely bedded exhalative metasediments (with their commonly sharp between-bed compositional contrasts), of the extensive mineral assemblages of many regionally metamorphosed exhalative ore deposits (especially VMS deposits and their environments), and of the exhalative metasedimentary milieu generally, may well constitute an important field for future investigation in regional metamorphic petrology.