Geospeedometry and time scales of high-pressure metamorphism

Kinetic theory allows the calculation of a time scale for metamorphic event s using the extent of relaxation of garnet growth zoning along a particular P-T trajectory. Eclogitic garnets from the Kokchetav Massif (Kazakhstan), the Great Caucasus (Russia), and the Yukon-Tanana terrane (Canada) experien ced different metamorphic P-T histories and display different types of zoni ng patterns, which allowed testing of a variety of geospeedometric procedur es. In all cases, the preservation of sharp compositional gradients and hen ce the limited degree of diffusive modification of garnet compositions can be explained if associated tectono-metamorphic processes were of very short duration. Results of diffusion modeling indicate rates of temperature and pressure change on the burial and/ or the exhumation path on the order of s everal hundreds of degrees C/m.y. and several cm/yr, respectively. These ex treme exhumation and cooling rates apply for rocks buried to a depth greate r than, for example, 20 km, thus arguing for the existence of contrasted ve locity fields for eclogitic block exhumation from deep versus shallow level s of the lithosphere.