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, composition of the minerals and phase diagrams. Metabasites 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 analysis and scanning electron microscopy images reveal that the amphiboles have chemical zoning with actinolite cores and hornblende rims and these chemical changes are discontinous. The plagioclases are commonly oligoclase but there is some remnant albite. The epidotes are enriched in Fe with low clinozoisite contents. Geothermobarometry calculations using the amphibole compositions represent 423 °C 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 °C 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.