The system CaO-Al2O3-Na2O-MgO-SiO2-H2O is suitable for modelling phase equilibria and mineral parageneses for low-T metamorphism of basaltic rocks; univariant reactions marking the transitions between various sub-greenschist facies are identified and some have been determined experimentally. The introduction of Fe2O3 into a model of fixed FeO/MgO ratio creates continuous reactions for facies boundaries and discontinuous reactions for invariant points. The XFe3+ isopleths for epidote are plotted to illustrate the transition from the zeolite facies, through prehnite-pumpellyite to the prehnite-actinolite facies, and T-XFe3+ relations are established for both continuous and discontinuous reactions relating such facies transitions. An isobaric Al-Ca-Fe3+ plot from the chlorite projection can be applied to low-T metamorphic rocks where two or three Ca-Al silicates occur.-R.A.H.
ASJC Scopus subject areas
- Geochemistry and Petrology