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-d iamond transitions has been firmly established from petrological studi es in at least three collisional belts. The typically disrupted struct ural settings of the UHP rocks raise important questions about the sca le and extents of UHP metamorphism and whether any present example can be interpreted as a regional terrane across which metamorphic P-T gra dients can be defined. Improvements in the precision with which UHP me tamorphism can be characterized will rely on extending experimental st udies to a greater range of chemical systems and solid solutions and o n developments in the application of microbeam techniques to experimen tal products. The role of fluids in continental crust undergoing subdu ction, important to any understanding of melt generation in convergent zones, needs to be considered further using the available UHP areas a s 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 po ssible tectonic models to explain the exhumation of UHP metamorphic ro cks and therefore must be determined in detail using all the petrologi cal 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 experi encing continued underplating is a favored mechanism for the uplift an d 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 re markable rocks become available.