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

Abstract

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-orthoproxenecordierite-bearing assemblages using THERMOCALC THERMOCALC is consistent with peak S3 conditions of 6.0-6.5 kbar and 850-900°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 6 kbar. Sparse garnet nucleii formed at 760°C, along with concentrated leucosome development and preferentially partitioned HREE. Further heating to 800°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 withinsample 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.

Some Remarks on Melting and Extreme Metamorphism of Crustal Rocks

“…The development of garnet as a peritectic product of biotitebreakdown melting exerts a strong control on the rare earth element (REE) chemistry of the melt (Brown 2007b). Furthermore the presence of garnet exerts a strong control on the REE chemistry of phases that may grow in equilibrium with it during cooling and on those phases that may replace it along the retrograde P-T evolution (Rubatto 2002;Hermann and Rubatto 2003;Clarke et al 2007). …”

Origines des enrichissements en biotite dans les granites de type S :

“…1) De nombreuses études ont déjà été faites sur cette zone et ont mis en évidence la présence d'hétérogénéités et d'enrichissement en minéraux ferromagnésiens dans le granite de Wuluma (Clarke et aL 9 2007;Collins et al, 1989;Lafrance et al, 1995;Sawyer et aL, 1999) (voir section 1.2.2. ).…”

Monazite-Fluorapatite Characteristics as Evidence for Interplay Between 2.04 Ga Fluid-Induced Dehydration and Melting of the Sand River Gneiss, Limpopo Complex, South Africa

“…In either case, the volume proportion of melt is small. Usually the formation of orthopyroxene in smallvolume, melt-bearing migmatitic gneisses is considered as products of fluid-absent dehydration melting (e.g., Waters, 1988;Clarke et al, 2007). Fluid induced melting involving H 2 O-rich fluids is less likely in such cases, as it will produce voluminous melt (e.g., Prince et al, 2001).…”

“…The pyroxene-bearing assemblages are commonly surrounded by felsic melt in a patchy irregular distribution (e.g., Clarke et al, 2007) or along the center of felsic melt-bearing veins (e.g., Rajesh et al, 2013). In either case, the volume proportion of melt is small.…”