Hornblende-plagioclase peridotite has formed in aluminous reaction zones around mafic lenses in chromite-bearing forsterite-enstatite-tremolite metaperidotite close to the contact of a gabbroic phase of the Mount Stuart Batholith. Assemblages encountered in these aluminous rocks are consistent with a theoretically-derived topology for the reactions governing the occurrence of plagioclase in peridotite, which indicates that the low-temperature limit of plagioclase peridotite in the pure system is the reaction 5 anorthite + 8 forsterite + 2 H2O = 2 tremolite + diopside + 5 spinel.

Associated pelitic hornfelses contain assemblages which indicate that the metamorphism occurred at pressures between 2 and 4 kilobars. The reaction chlorite = forsterite + enstatite + spinel + H2O, which occurs in the metaperidotite near the contact with the batholith, establishes that the temperature at the time of metamorphism reached 715 ± 25°C. Only in a small pendant enclosed within the gabbro do mafic hornfelses contain the assemblage horn-blende-clinopyroxene-orthopyroxene-plagioclase, indicative of transition to pyroxene-hornfels facies.

The reaction-zone rocks contain assemblages which are indicative of both sides of the reaction 2 forsterite + anorthite = enstatite + diopside + spinel, with the enstatite and diopside-bearing assemblages containing spinel richer in chromite than the forsterite + anorthite-bearing assemblages. Thermodynamic calculations using observed spinel compositions suggest pressures at least 2 kilobars below the pressure inferred from the pelitic assemblages. It is argued that the main cause for this discrepancy is error in the assumption of ideality in the spinel solid solution.

Hornblende in the hornblende-plagioclase peridotite is colorless and shows a systematic variation in alkali content and tetrahedral aluminum along a trend rich in the tschermakite endmember. By expressing this compositional change mathematically, diagrams are derived which show that for the hornblende compositions used, the low-pressure peridotites can be divided into three chlorite-free mineral facies. They are, in increasing grade, horn-blende-spinel peridotite, hornblende-plagioclase peridotite, and finally plagioclase lherzolite.

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