Granulite facies metamorphism and subsolidus fluid‐absent reworking, Strangways Range, Arunta Block, central Australia

Abstract

AbstractOrthopyroxene‐rich quartz‐saturated granulites of the Strangways Range, Arunta Block, central Australia, record evidence of two high‐grade metamorphic events. Initial granulite facies metamorphism (M1, at c. 1.7 Ga) involved partial melting and migmatization culminating in conditions of 8.5 kbar and 850 °C. Preservation of the peak M1 mineral assemblages from these conditions indicates that most of the generated melt was lost from these rocks at or near peak metamorphic conditions. Subsequent reworking (M2, at c. 1.65 Ga) is characterized by intense deformation, the absence of partial melting and the development of orthopyroxene–sillimanite ± gedrite‐bearing mineral assemblages. Gedrite is only present in cordierite‐rich lithologies where it preferentially replaces M1 cordierite porphyroblasts. Pseudosection calculations indicate that M2 occurred at subsolidus fluid‐absent conditions (aH2o ∼ 0.2) at 6–7.5 kbar and 670–720 °C. The mineral assemblages in the reworked rocks are consistent with closed system behaviour with respect to H2O subsequent to M1 melt loss. M2 reworking was primarily driven by increased temperature from the stable geotherm reached after cooling from M1 and deformation‐induced recrystallization and re‐equilibration, rather than rehydration from an externally derived fluid. The development of the M2 assemblages is strongly dependent on the intensity of deformation, not only for promoting equilibration, but also for equalizing the volume changes that result from metamorphic reactions. Calculations suggest that the protoliths of the orthopyroxene‐rich granulites were cordierite–orthoamphibole gneisses, rather than pelites, and that the unusual bulk compositions of these rocks were inherited from the protoliths. Melt loss is insufficient to account for the genesis of these rocks from more typical pelitic compositions. In quartz‐rich gneisses, however, melt loss along the M1 prograde path was able to modify the bulk rock composition sufficiently to stabilize peak metamorphic assemblages different from those that would have otherwise developed.