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

Abstract

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 contain- ing 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 (Al 3+ and Fe 3+ on the M2 site), epidote (Fe 3+ ⁄ - Fe 3+ +A l 3+ ) and chlorite (Fe 2+ ⁄ Fe 2+ + 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 Al 3+ 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, Fe 2+ -Mg ratios in chlorite may be dependent on pressure. The Fe 3+ ⁄ Al and Fe 2+ ⁄ Mg ratios in epidote and chlorite are reliable thermometers in actinolite, epidote, chlorite, albite, quartz, hematite and titanite buffered assemblages.