P-T-X controls on phase stability and composition in LTMP metabasite rocks - a thermodynamic evaluation

The stability of pumpellyite + actinolite or riebeckite + epidote + hematite (with chlorite, albite, titanite, quartz and H₂O in excess) mineral assemblages in LTMP metabasite rocks is strongly dependent on bulk composition. By using a thermodynamic approach (THERMOCALC), the importance of CaO and Fe₂O₃ bulk contents on the stability of these phases is illustrated using P–T and P–X phase diagrams. This approach allowed P–T conditions of ~4.0 kbar and ~260 °C to be calculated for the growth of pumpellyite + actinolite or riebeckite + epidote + hematite assemblages in rocks containing variable bulk CaO and Fe₂O₃ contents. These rocks form part of an accretionary wedge that developed along the east Australian margin during the Carboniferous–Triassic New England Orogen. P–T and P–X diagrams show that sodic amphibole, epidote and hematite will grow at these conditions in Fe₂O₃-saturated (6.16 wt%) metabasic rocks, whereas actinolite and pumpellyite will be stable in CaO-rich (10.30 wt%) rocks. With intermediate Fe₂O₃ (~3.50 wt%) and CaO (~8.30 wt%) contents, sodic amphibole, actinolite and epidote can coexist at these P–T conditions. For Fe₂O₃-saturated rocks, compositional isopleths for sodic amphibole (Al³⁺ and Fe³⁺ on the M2 site), epidote (Fe³⁺ ⁄ - Fe³⁺ + Al³⁺) and chlorite (Fe²⁺ ⁄ Fe²⁺ + Mg) were calculated to evaluate the efficiency of these cation exchanges as thermobarometers in LTMP metabasic rocks. Based on these calculations, it is shown that Al³⁺ in sodic amphibole and epidote is an excellent barometer in chlorite, albite, hematite, quartz and titanite buffered assemblages. The effectiveness of these barometers decreases with the breakdown of albite. In higher-P stability fields where albite is absent, Fe²⁺-Mg ratios in chlorite may be dependent on pressure. The Fe³⁺ ⁄ Al and Fe²⁺ ⁄Mg ratios in epidote and chlorite are reliable thermometers in actinolite, epidote, chlorite, albite, quartz, hematite and titanite buffered assemblages