P,T paths of natural eclogites during metamorphism — a record of subduction

The distribution of iron and magnesium between coexisting garnet and clinopyroxene is expressed by the distribution coefficient K_D^ga-cpx. This coefficient has been experimentally determined as a function of temperature and pressure, and is used to determine the temperature of equilibration of natural eclogites.

The presence of relict zoning in both garnet and clinopyroxene in low to medium temperature eclogites permits evaluation of the P,T path followed by these rocks during prograde metamorphism.

The average P,T path for eclogites of blueschist terranes (type C eclogites) is suggested to be 40°C/Kb during prograde metamorphism. The Tasmanian eclogite (type B) records crystallization along a different P,T path (lower pressure at given temperature) from those of the type C eclogites. Eclogites from the migmatite-gneiss terrances of Poland and Norway do not preserve evidence of prograde metamorphism in mineral zoning but indicate higher temperatures at the metamorphic maximum; pressures were similar to those attained by type C eclogites.