Phase Diagrams and their Application to Determine Pressure-Temperature Paths of Metamorphic Rocks
Will, Thomas M.
published: Oct 20, 1998
ArtNo. ESP154017402001, Price: 29.00 €
Several methods and their limitations to unravel the pressure-temperature (PT) paths experienced by metamorphic rocks during their tectonometamorphic evolution are discussed. Special emphasis is placed on the applicability of phase diagrams towards this aim. The advantages of phase diagrams compared to conventional geothermobarometers, especially with respect to the problem of local disequilibria in rocks, are outlined. Phase diagrams that are useful in delineating PT trajectories of metamorphic rocks are diagrams in which the mineral solid solutions change composition with variations in pressure and temperature, as they do in nature. Two types of diagrams, PT projections (or petrogenetic grids) and PT pseudosections, are discussed in detail. PT projections are diagrams that summarise the entire pressure-temperature-composition information for all possible bulk compositions in a given model system. PT pseudosections, which can be constructed from PT projections, are phase diagrams, that are drawn for a specific bulk composition, i.e. for a particular rock. After a general discussion of petrogenetic grids and certain features of complex phase diagrams, the relationship between PT projections and PT pseudosections is outlined. Finally, the applicability of phase diagrams to reconstruct detailed PT paths is demonstrated with examples from the Spessart Crystalline Complex in Germany and the Kaoko-Belt in NW-Namibia.