MULTIPLE P-T PATHS FOR ECLOGITES FROM THE BULGARIAN RHODOPE MASSIF

Two eclogite samples from the Bulgarian Rhodope massif were investigated by petrographic and thermobarometric methods to determine conditions of equilibration and P-T paths. P-T conditions were calculated using multiple, independent, and fully buffered reactions and the thermocalc software. In the first eclogite sample, garnet grains contain an inclusion assemblage of quartz, epidote, amphibole, sphene, plagioclase, and rare rutile toward the rims. The chemistry of both the inclusions and the garnet indicates this is a pro-grade assemblage trapped during garnet growth. This assemblage records conditions in the epidote-amphibolite facies at ~570 °C and ~7.5 kbar. Matrix clinopyroxene and amphibole have core compositions of omphacite and magnesiowinchite, respectively, and peak eclogite facies conditions are calculated at ~625 °C and ≥12-14 kbar. Retrogression through the amphibolite facies produced amphibole-plagioclase and clinopyroxene-plagioclase symplectites. The data indicate a P-T path that begins with a high-grade metamorphic assemblage that subsequently increases significantly in pressure with little accompanying increase in temperature. This suggests rapid tectonic burial to attain the eclogite facies conditions. The second eclogite sample is kyanite-bearing and strongly retrogressed. Profiles of garnet cores suggest prograde growth through the amphibolite facies but do not allow rigorous determination of P-T conditions. Garnet rims and matrix kyanite grains contain an eclogite facies assemblage of omphacite, quartz, barroisite, kyanite, epidote, and rutile. Peak conditions of eclogite facies metamorphism are estimated as ~800 °C and 17-18 kbar. This sample records much higher eclogite facies conditions, both in temperature and pressure, than the previous sample. Retrogression occurred through the granulite facies based upon the presence of sapphirine in symplectites around kyanite. Thermobarometry of the symplectite assemblage yields pressures of ~10-13 kbar and temperatures greater than ~700 °C. There is no evidence present in these samples that would indicate UHP conditions were ever attained. The distinctive conditions of metamorphism between these two eclogite samples suggest differing tectonic environments and probably different ages of formation.