Investigation of metamorphic conditions and mineralogical changes at green-schist to amphibolite facies transition zone using phase diagrams and compositions of the mineral: a case study from the Kordestan province metabasites

In this paper, metamorphic conditions (T, P and fluid composition) in greenschist to amphibolite facies transition zone are investigated using textural evidence, compositions of the minerals and phase diagrams. Metabasits from the south of Kordestan province were metamorphosed from sub-greenschist to upper amphibolite facies. Field observations and petrography studies show that there are some rocks containing mineral assemblage hornblende + plagioclase + epidote + quartz formed in conditions intermediate between greenschist and amphibolite facies. Electron microprobe analyses and scanning electron microscopy images reveal that the amphiboles have chemical zoning with actinolite cores and hornblende rims and these chemical changes are discontinued. The plagioclases are commonly oligoclases but there are some remnant albites. The epidotes are enriched in Fe with low clinozoisite contents. Geothermobarometery calculations using the amphibole compositions obtained 423 oC and 2 kbar for the cores and 543 oC and 4 kbar for the rims. The calculated phase diagram based on whole-rock composition of a sample indicates that it was formed in 450 to 550 oC and 2.1 to 6.8 kbar that fall in the field of epidote-amphibolite facies condition. The O2 and CO2 vs. T variation phase diagrams show that O2 had a significant role formation of the mineral assemblage but the fluid contained very low CO2. Chemical discontinuities in the amphiboles and the plagioclases with different compositions demonstrate that the studied rocks formed in disequilibrium conditions