Orthopyroxene (Opx) and augite (Aug) in metasedimentary and meta-igneous gneisses in a supracrustal succession at Fyfe Hills, Enderby Land, Antarctica, exhibit intimate exsolution intergrowths. In magnesian assemblages, exsolution lamellae occur only on 100 as a result of exsolution of Opx from Aug and Aug from Opx. In more Fe-rich assemblages, exsolution lamellae occur on both 001 and 100 in Opx. The 001 lamellae indicate exsolution involving pigeonite (Pgt) subsequently “inverted” to Opx and Aug. In intermediate assemblages, Opx-Aug intergrowths are blebby rather than lamellar, a feature attributed to the decomposition of primary metamorphic Pgt in the three-phase field of Opx-Pgt-Aug. The textural relations and compositional data suggest the following phase relations at peak conditions: Opx-Aug (for
XFeOpx<0.43
), Opx-Aug-Pgt (for
XFeOpx=0.50.43
), Opx-Pgt (for
XFeOpx>0.5
), Aug-Pgt (for
XFeAug>0.48
), where XFe = Fe2+/(Fe2+ + Mg). Application of Lind-sley’s (1983, American Mineralogist, 68, 477-493) phase diagram at the relevant pressure (9 kbar) implies a peak metamorphic temperature of at least 1000°C.
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