Extending our understanding of Ultrahigh temperature crustal metamorphism

Abstract

Rapid developments in the scope and quality of internally consistent thermodynamic data sets arising from the incorporation of new experimental data and solid-solution models for minerals and melts involved in ultrahigh temperature (UHT) metamorphism now place petrologists in an excellent position to evaluate the detailed mineral assemblage evolutions of many UHT rocks and place these in quantitative frameworks using P-T diagrams and pseudosection approaches. These developments will soon be enhanced by extension of the methods to systems at high fO2, enabling a wider range of critical mineral assemblages to be considered and their UHT P-T evolutions, at present largely qualitatively constrained, to be determined quantitatively. Cordierite provides a useful monitor of the role and composition of any fluids and melts attending UHT metamorphism at <1000°C. Such fluids or melts remaining in communication with the mineral assemblages following UHT have a significant effect on preservation of the metamorphic record and may also strongly influence the ages recorded by geochronological systems, even to the extent of producing widespread post-UHT metamorphic zircon. Modelling of UHT metamorphism and P-T paths, whether IBC, ITD or hybrid ITD-IBC, in terms of specific and testable tectonic settings and processes is still problematic, with those models that can successfully duplicate the extraordinary temperatures attained in some UHT terrains generally being geologically unreasonable. Successful modelling of the thermal conditions and evolution of UHT terrains, constrained by quantitative P-T-t paths, is an important objective for future research that will inform our understanding of this intriguing aspect of the behaviour of the Earth system.