Phase relations of calc-silicate assemblages in the Auerbach marble, Odenwald Crystalline Complex, Germany

In the Odenwald Crystalline Complex, calc-silicate rocks are concentrated a t the margins of the marble layer of Auerbach. They were presumably formed by metasomatic exchange between the calcite marble and the neighbouring gra nodioritic and quartz-dioritic intrusives. The investigated samples contain the characteristic mineral assemblages:,garnet+clinopyroxene+epidote/clino zoisite+calcite+quartz+/-titanite (1) and wollastonite+clinopyroxene+garnet +calcite+/-quartz+/-epidote/clinozoisite+/-titanite (2). Microprobe analyse s revealed the following compositional ranges: garnet grs(40-98)adr(2-55)al m(<5.5)sps(<5.5)pyp(<1); clinopyroxene di(46-88)hed(9-47)joh(0-5)cats(0-6); epidote/clinozoisite ps(20-80) Different phase diagrams were calculated in the system CaO-MgO-Al2O3-TiO2-SiO2-CO2-H2O (CMATSCH) to decipher the P-T-X -CO2 parameters of metamorphism: isobaric T-X-CO2 sections and a P-T projec tion with mixed volatiles. The phase diagrams illustrate that the observed assemblages can only form in the presence of an H2O-rich volatile phase. Th e assemblages are stable over a large range of temperatures, from 580 <degr ees>C to <400 <degrees>C (at 4 kbar) and at X-CO2 values of less than 0.055 (at 4 kbar). Higher temperatures can be inferred from reaction textures wh ich indicate that gamet+plagioclase (T > 580 degreesC, at 4 kbar) and wolla stonite+plagioclase (T > 660 degreesC, at 4 kbar) coexisted during an early metamorphic stage. A minimum pressure of 3.5 kbar can be inferred for the early high-temperature stage. Furthermore, on the basis of the calculated p hase diagrams, combined with modal abundances in thin sections, it is possi ble to evaluate fluid behaviour; in the investigated specimens, infiltratio n of fluids from an external reservoir occurred. A minimum fluid : rock rat io of 3.6:1 can be estimated.