Exhalative metasediments - clues to the real nature of regional metamorphic processes?

Abstract

Regional metamorphic petrogenetic theory is currently dominated by the hypothesis that the principal grade indicator silicates and their related minerals arise by a variety of chemical reactions. It is postulated that, with rise (or fall) in regional crustal temperature and pressure, original materials become destabilized, break down, and are dissolved in the intergranular "metamorphic fluid". The resulting ions then diffuse within, and are transported by, this fluid, in due course reacting with each other to form new minerals in equilibrium with each other and the associated fluid under the new conditions and in accordance with the Phase Rule. A second mechanism, widely accepted for such as metamorphic oxide, carbonate, sulphate and sulphide minerals, but not for the principal metamorphic silicates, is that the latter form largely by in situ solid:solid transformation plus or minus SiO2 plus or minus H2O, rather than by the serial process indicated above. It is now proposed that a test of the "reaction theory" is whether or not chemical equilibrium has been established on a fine (grain) scale in materials metamorphosed to high grades. On the basis of some simple - but very precise - electron microprobe measurements on some high grade, but still finely bedded, exhalative metasedimentary rocks, it appears to fail the test.