Geospeedometry and Time Scales of High-Pressure Metamorphism
Titel:
Geospeedometry and Time Scales of High-Pressure Metamorphism
Auteur:
Perchuk, A. Philippot, P.
Verschenen in:
International geology review
Paginering:
Jaargang 42 (2000) nr. 3 pagina's 207-223
Jaar:
2000-03
Inhoud:
Kinetic theory allows the calculation of a time scale for metamorphic events 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) experienced different metamorphic P-T histories and display different types of zoning patterns, which allowed testing of a variety of geospeedometric procedures. In all cases, the preservation of sharp compositional gradients and hence 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 several hundreds of °C/m.y. and several cm/yr, respectively. These extreme exhumation and cooling rates apply for rocks buried to a depth greater than, for example, 20 km, thus arguing for the existence of contrasted velocity fields for eclogitic block exhumation from deep versus shallow levels of the lithosphere.
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