Contrasting behaviour of rare earth and major elements during partial melting in granulite facies migmatites, Wuluma Hills, Arunta Block, central Australia

Low-P granulite facies metapelitic migmatites in the Wuluma Hills, Strangways Metamorphic Complex, Arunta Block, preserve evidence of polyphase deformation and migmatite formation which is of the same age of the c. 1730 Ma Wuluma granite. Mineral equilibria modelling of garnet-orthoproxene-cordierite-bearing assemblages using THERMOCALC is consistent with peak S3 conditions of 6.0-6.5 kbar and 850-900 degrees C. The growth of orthopyroxene and garnet was primarily controlled by biotite breakdown during partial melting reactions. Whereas orthopyroxene in the cordierite-biotite mesosome shows enrichment of heavy-REE (HREE) relative to medium-REE (MREE), orthopyroxene in adjacent garnet-bearing leucosome shows depletion of HREE relative to MREE. There is no appreciable difference in major element contents of minerals common to both the mesosome and leucosome. The REE variations can be satisfactorily explained by decoupling of major element and REE partitioning, in the context of appropriate phase-equilibria modelling of a prograde path at similar to 6 kbar. Sparse garnet nucleii formed at similar to 760 degrees C, along with concentrated leucosome development and preferentially partitioned HREE. Further heating to similar to 800 degrees C at constant or subtly increasing pressure conditions additionally stabilized orthopyroxene and decreased the garnet mode. Orthopyroxene in the leucosome inherited an REE pattern consequent to the partial consumption of garnet, it being distinct from the REE pattern in mesosome orthoproxene that was mostly controlled by biotite breakdown. Such within-sample variability in the enrichment of heavy REE indicates that caution needs to be exercised in the application of common elemental partitioning coefficients in spatially complex metamorphic rocks.