Petrography, Mineralogy, and Metamorphic Evolution of an Eclogite from the Island of Margarita, Venezuela

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Abstract
The petrography and mineralogy of a coarse-grained eclogite lens from the La Rinconada Group of northeastern Margarita Island, Venezuela, reflect the metamorphic evolution of the rock. Omphacite + garnet + paragonite + barroisite + epidote + rutile/ilmenite + quartz + pyrite + apatite represents the stable assemblage marking the culmination of a single metamorphic episode. Electron microprobe analyses of the major minerals indicate an affinity with Smulikowski's low-temperature ‘ophiolitic’ type. Schreinemakers relationships among the mineral phases allow the observed complex reaction relations to be explained as late-stage depressurization effects. The crystallization of eclogite instead of amphibole gneiss, as in the chemically identical country rock, must be related to a lower αH2O in the aqueous fluid phase attending metamorphism. It is inferred from mineralogical stability data that the ‘culmination assemblage’ was stable at approximately 450–525 °C and 11.5–13.5 kb load pressure, and that depressurization to less than 5 kb occurred at slightly increasing metamorphic temperatures.

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