ULTRADEEP CRUSTAL METAMORPHISM - A PROSPECTIVE VIEW

Ultradeep, or ultrahigh-pressure (UHP), metamorphism of crustal rocks at depths greater than those of the quartz-coesite and even graphite-diamond transitions has been firmly established from petrological studies in at least three collisional belts. The typically disrupted structural settings of the UHP rocks raise important questions about the scale and extents of UHP metamorphism and whether any present example can be interpreted as a regional terrane across which metamorphic P-T gradients can be defined. Improvements in the precision with which UHP metamorphism can be characterized will rely on extending experimental studies to a greater range of chemical systems and solid solutions and on developments in the application of microbeam techniques to experimental products. The role of fluids in continental crust undergoing subduction, important to any understanding of melt generation in convergent zones, needs to be considered further using the available UHP areas as natural laboratories and through high-pressure experimental studies of the solubilities of minerals in aqueous and saline fluids. pressure-temperature paths provide an essential constraint on the choice of possible tectonic models to explain the exhumation of UHP metamorphic rocks and therefore must be determined in detail using all the petrological tools available and integrated with precise geochronology focused on the. timing of mineral growth in relation to structural markers and P-T evolution. Extension of thickened crust or a crustal wedge experiencing continued underplating is a favored mechanism for the uplift and exhumation of UHP areas which will continue to be evaluated as new P-T-t data constraining the rates of exhumation and cooling of these remarkable rocks become available.