Garnet-chloritoid-kyanite assemblages: eclogite facies indicators of subduction constraints in orogenic belts

The assemblage garnet-chloritoid-kyanite is shown to be quite common in high-pressure eclogite facies metapelites from orogenic belts around the world, and occurs over a narrowly restricted range of temperature similar to 550-600 degrees C, between 20 and 25 kbar. This assemblage is favoured particularly by large Al(2)O(3):K(2)O ratios allowing the development of kyanite in addition to garnet and chloritoid. Additionally, ferric iron and manganese also help stabilize chloritoid in this assemblage. Pseudosections for several bulk compositions illustrate these high-pressure assemblages, and a new thermodynamic model for white mica to include calcium and ferric iron was required to complete the calculations. It is extraordinary that so many orogenic eclogite facies rocks, both mafic eclogites sensu stricto as well as metapelites with the above assemblage, all yield temperatures within the range of 520-600 degrees C and peak pressures similar to 23 +/- 3 kbar. Subduction of oceanic crust and its entrained associated sedimentary material must involve the top of the slab, where mafic and pelitic rocks may easily coexist, passing through these P-T conditions, such that rocks, if they proceed to further depths, are generally not returned to the surface. This, together with the tightly constrained range in peak temperatures which such eclogites experience, suggests thermal weakening being a major control on the depths at which crustal material is decoupled from the downgoing slab.