P-T CONDITIONS OF SYMPLECTITE FORMATION IN THE ECLOGITES FROM THE WESTERN GNEISS REGION (NORWAY)

Symplectitic intergrowths of sodic plagioclase + diopside ± amphibole that replace omphacite during retrogression are commonly found in eclogites. In the Western Gneiss Region of Norway, basaltic eclogites are found as meter- to kilometer-size lenses embedded within the surrounding gneiss, and many of these display symplectites. Six eclogites samples coming from four locations, representing different P-T peak eclogite conditions (low-, high-, and ultrahigh-pressure) were selected and X-ray maps of major element were acquired by EMPA. These maps were processed with XMapTools software to individualize the different mineral phases present in the lamellae, and to estimate the P-T conditions of symplectite formation. In some samples, symplectites are only a narrow layer at the edge of two omphacite crystals, whereas some others are so well-developed that they completely replace omphacite, and a rim of green amphibole + plagioclase is found around garnet grains in these samples. Most of the well-developed symplectites display lamellae of amphibole (magnesio-hornblende, edenite, actinolite) together with diopside and plagioclase. In the samples where garnet is also destabilized, the amphibole present in the garnet rim is chemically different than the one present in the lamellae (e.g., edenite in the symplectite and magnesio-hastingsite in the rim of garnet). The P-T estimates for the four locations show similar features: symplectite formation is initiated immediately after eclogite facies, when P decreases and plagioclase becomes stable, (i.e., 15-20 kbar and 650-750°C, depending on the pressure of the peak eclogite). It continues towards amphibolite facies with decreasing P and T. Garnet destabilization seems to be coeval with the last stage of symplectite formation, just before the rocks are stabilized in the amphibolite facies (5-9 kbar and 500-640°C). To conclude, symplectites record the decompression path of eclogites during the transition between eclogite and amphibolite facies.