Fluid evolution of partially retrogressed pelitic granulite from the Southern Marginal Zone of the Neoarchean Limpopo Complex, South Africa: Evidence from phase equilibrium modelling

Abstract

Hydration of granulites and formation of amphibolite-facies mineral assemblages are common processes in retrogressed high-grade metamorphic terranes worldwide. Here, we report new petrological data for a partially hydrated pelitic granulite from the retrograde (orthopyroxene-out) isograd developed in the Southern Marginal Zone (SMZ) of the Neoarchean Limpopo Complex in South Africa, and discuss the P–T–fluid evolution based on mineral equilibrium modelling in the NCKFMASH system. The results demonstrate the stability of the peak M1 assemblage (garnet+biotite+orthopyroxene+K-feldspar+plagioclase+quartz) at 750–900°C and 8–9kbar, and relatively dry condition of aH2O [H2O activity]=0.40–0.55. M1 was followed by near-isothermal decompression to M2 (ca. 800°C, 7.7–8.0kbar, M(H2O) [molar H2O% of the rock]=0.5–2.5mol.%) at a constant aH2O condition of 0.4–0.5 as inferred from garnet+biotite+orthopyroxene+cordierite+K-feldspar+plagioclase+quartz assemblage in the sample. The formation of anthophyllite corona around orthopyroxene and kyanite+orthoamphibole+quartz intergrowth after cordierite suggest significant interaction with an aqueous fluid during retrograde metamorphism. The T-M(H2O) pseudosection constrain the first hydration event (M3a), during which anthophyllite replaced orthopyroxene at 630–730°C and M(H2O)=3.0–3.5mol.%, to have occurred after the M2 event. Further increase in M(H2O) condition (4.5–6.0mol.%) and cooling to <630°C triggered the progress of the second hydration event (M3b) and M4 event, during which kyanite/sillimanite+orthoamphibole+quartz and garnet+kyanite/sillimanite+quartz replaced cordierite. A consistent aH2O condition of 0.2–0.3 during M3 and M4 events suggests that aH2O was buffered to lower levels by progress of the hydration reactions. Such an increase in molar H2O content in the rock with the formation of hydrous mineral assemblages after anhydrous assemblages established a retrograde isograd and zone of hydrated granulite in the SMZ. This event might be related to influx of H2O-bearing fluid along the Hout River Shear Zone (HRSZ) that marks the major crustal break between the SMZ and the adjacent Kaapvaal Craton. The aqueous fluid was probably derived from devolatilization of low-grade underthrusted greenschists in the footwall section of the HRSZ by the overriding granulites during the thrust-controlled spreading of the SMZ onto the Kaapvaal craton.