ECLOGITIZATION OF THE DEEP CRUST IN CONTINENT COLLISION ZONES

The p-T conditions prevailing in the roots of continent collision zones can stabilize eclogite facies rocks. Eclogitization is associated with marked changes in petrophysical properties. Density increases on average by 0.2 g/cm(3) during the granulite-eclogite facies transition. The eclogite facies rocks have densities that range from crustal values of 3.1 g/cm(3) for felsic compositions through typical upper mantle values of 3.3 g/cm(3) to values of 3.6 g/cm(3) for mafic compositions. Eclogite facies rocks containing hydrous phases are highly ductile compared to their dry granulite facies protoliths. The transformation of dry igneous rocks and granulites into eclogites depends on the availability of fluids. Under fluid-absent conditions, experimental work and field occurrences suggest that a minimum overstepping of reaction boundaries of the order of 5 kbar is possible at temperatures typical of the lower crust (600-700 degrees C). The dry non-eclogitized lower crust may deform in a brittle manner during the eclogitization process. In the unreacted protolith the presence of hydrofractures and pseudotachylytes with eclogite-facies minerals indicates a forceful process that may be associated with seismic activity. The brittle deformation of the unreacted rocks will allow fluid to reach new volumes of rock. At a stage where 30-40% of the crust is eclogitized, the crust apparently loses coherence and behaves like a newtonian fluid where blocks of granulite float in a matrix of dense eclogite. A density fractionation of this highly ductile and partially eclogitized crust, with loss of dense eclogites to the mantle, can be anticipated. The geodynamics of crustal roots depends on the fluid budget (amount and compositions) as well as on fluid transport and not only on pressure and temperature as assumed in most existing models.