Behavior of trace elements in relation to Lu-Hf and Sm-Nd geochronometers during metamorphic dehydration-hydration in the HP domain of Vardalsneset, Western Gneiss Region, Norway

摘要

The geochemical and isotopic characterization of an eclogite and the associated retrogressive amphibolite at Vardalsneset, WGR, Norwegian Caledonide was undertaken to investigate the mobility of REE and Hf and the behavior of Lu-Hf and Sm-Nd geochronometers during metamorphic dehydration/rehydration. Eclogitic garnets display a distinct core-rim chemical zoning. Thermodynamic modeling indicates that both cores (13-22kbar, 500-580°C) and rims (>16 kbar, 610-660°C) crystallized under eclogite-facies conditions. The core-rim zoning corresponds to the dehydration of the system. This petrographic disequilibrium is associated with Lu-Hf and Sm-Nd disequilibrium, which prevents dating of the eclogitic stages. At the rock scale, the incoming fluid responsible for eclogite-amphibolite retrogression brought in Sm and Nd, leached Lu, and had no influence on Hf. At the grain scale, mass balance shows that Sm and Nd were stored in clinozoisite since the first eclogitic stage, whereas Lu and Hf, which were more thoroughly redistributed among minerals during retrogression, enable the dating of the amphibolitic facies at 378 +- 17 Ma.