Kyanite eclogite, partially hydrated during decompression, shows conspicuous domains containing unusual phases, such as staurolite, corundum, and/or hercynite-dominant spinel. Detailed analysis of these domains indicates that they formed by prograde growth of porphyroblasts of lawsonite, kyanite, and garnet, prior to the complete conversion of igneous plagioclase to omphacite. In the evolution following the pressure peak, the chemically differentiated domains remained largely distinct from the matrix. Quantitative thermodynamic models set up for each domain allow us to analyze each domain type, in order to test (a) to what extent they behaved as closed systems, (b) whether local equilibrium was maintained during decompression, and (c) what reaction mechanisms and volumes were involved in the observed transformations. Results indicate that metasomatic effects were quite limited; local equilibration was possible to mid-crustal depths at temperatures near 700 °C; reaction paths followed involved minimal transport of Al. Both prograde and retrograde parts of the P-T path could be deduced for eclogite samples from the Southern Steep Belt of the Central Swiss Alps. A late-orogenic, prograde thermal overprint (heating spike) is confirmed.

This research was supported by the Swiss National Science Foundation (SNF).

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Last modified 15th March, 2005

Staurolite and other aluminous phases in Alpine eclogite: Analysis of domain evolution.