Phase Equilibria of Amphibolites from the Post Pond Volcanics, Mt. Cube Quadrangle, Vermont

Amphibolites of the Post Pond Volcanics, south-west corner of the Mt. Cube Quadrangle, Vermont, are characterized by a great diversity of bulk rock types that give rise to a wide variety of low-variance mineral assemblges. Original rock types are believed to have been intrusive and extrusive volcanics, hydrothermally altered volcanics and volcanogenic sediments with or without admixtures of sedimentary detritus. Metamorphism was of staurolite-kyanite grade. Geothermometry yields a temperature of 535 ± 20 °C at pressures of 5–6 kb. Partitioning of Fe and Mg between coexisting phases is systematic, indicating a close approach to chemical equilibrium was attained. Relative enrichment of Fe/Mg is garnet > staurolite > gedrite > anthophyllite ≈ cummingtonite ≈ hornblende > biotite > chlorite > wonesite > cordierite ≈ dolomite > talc; relative enrichment in Mn/Mg is garnet > dolomite > gedrite > staurolite ≈ cummingtonite > ≈ hornblende > ≈ anthophyllite > cordierite > biotite > ≈ wonesite > ≈ chlorite > talc. between coexisting amphiboles varies as a function of bulk Fe/Mg, which is inconsistent with an ideal molecular solution model for amphiboles. Mineral assemblages are conveniently divided into carbonate + hornblende-bearing, hornblende-bearing (carbonate-absent) and hornblende-absent. The carbonate-bearing assemblages all contain hornblende + dolomite+ calcite + plagioclase (andesine and/or anorthite) + quartz with the additional phases garnet and epidote (in Fe-rich rocks) and chlorite ± cummingtonite (in magnesian rocks). Carbonate-bearing assemblages are restricted to the most calcic bulk compositions. Hornblende-bearing (carbonate absent) assemblages occur in rocks of lower CaO content than the carbonate-bearing assemblages. All of these assemblages contain hornblende + andesine ± quartz + Fe-Ti oxide (rutile in magnesian rocks and ilmenite in Fe-rich rocks). In rocks of low Al content, cummingtonite and two orthoamphiboles (gedrite and anthophyllite) are common. In addition, garnet is found in Fe-rich rocks and chlorite is found in Mg-rich rocks. Several samples were found that contain hornblende + cummingtonite + gedrite + anthophyllite ± garnet +chlorite + andesine + quartz + Fe-Ti oxide ± biotite. Aluminous assemblages contain hornblende + staurolite + garnet ± anorthite/bytownite (coexisting with andesine) ± gedrite ± biotite ± chlorite ± andesine ± quartz ± ilmenite. Hornblende-absent assemblages are restricted to Mg-rich, Ca-poor bulk compositions. These rocks contain chlorite ± cordierite ± staurolite ± talc ± gedrite ± anthophyllite ± cummingtonite ± garnet ± biotite ± rutile ± quartz ± andesine. The actual assemblage observed depends strongly on Fe/Mg, Ca/Na and Al/Al + Fe + Mg. The chemistry of these rocks can be represented, to a first approximation, by the model system SiO2−Al2O3−MgO−FeO−CaO−Na2O−H2O−CO2; graphical representation is thus achieved by projection from quartz, andesine, H2O and CO2 into the tetrahedron Fe−Ca−Mg−Al. The volumes defined by compositions of coexisting phases fill a large portion of this tetrahedron. In general, the distribution of these phase volumes is quite regular, although in detail there are a large number of phase volumes that overlap other phase volumes, especially with respect to Fe/Mg ratios. Algebraic and graphical analysis of numerous different assemblages indicate that every one of the phase volumes should shift to more magnesian compositions with decreasing μH2O. It is therefore suggested that the overlapping phase volumes are the result of different assemblages having crystallized in equilibrium with different values of μH2O or μCO2 and that the different values may have been inherited from the original H2O and CO2 content of the volcanic prototype. If true, this implies that either a fluid phase was not present during metamorphism, or that fluid flow between rocks was very restricted.