Premium
P–T–X controls on phase stability and composition in LTMP metabasite rocks – a thermodynamic evaluation
Author(s) -
PHILLIPS G.,
HAND M.,
OFFLER R.
Publication year - 2010
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.2010.00874.x
Subject(s) - amphibole , epidote , actinolite , chlorite , geology , titanite , geochemistry , tremolite , mineralogy , metamorphic rock , quartz , materials science , paleontology , asbestos , metallurgy
The stability of pumpellyite + actinolite or riebeckite + epidote + hematite (with chlorite, albite, titanite, quartz and H 2 O 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 Fe 2 O 3 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 Fe 2 O 3 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 Fe 2 O 3 ‐saturated (6.16 wt%) metabasic rocks, whereas actinolite and pumpellyite will be stable in CaO‐rich (10.30 wt%) rocks. With intermediate Fe 2 O 3 (∼3.50 wt%) and CaO (∼8.30 wt%) contents, sodic amphibole, actinolite and epidote can coexist at these P–T conditions. For Fe 2 O 3 ‐saturated rocks, compositional isopleths for sodic amphibole (Al 3+ and Fe 3+ on the M2 site), epidote (Fe 3+ /Fe 3+ + Al 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.