Fluid-rock interaction during high-grade metamorphism: Instructive examples from the Southern Marginal Zone of the Limpopo Complex, South Africa

The Southern Marginal Zone of the Limpopo Complex documents strong evidence that CO₂-rich (XCO2fluid=0.7-0.9, XH2Ofluid=0.1-0.3) and brine fluids of greatly reduced water activity interacted with cooling metapelitic granulite during the thrust-controlled emplacement at 2.69-2.62Ga onto the granite-greenstone terrain of the northern Kaapvaal Craton. Interaction of cooling metapelitic granulite with CO₂-rich fluids at T<600-630°C and P<~6kbar is recognized by the presence of a regional retrograde Opx-out/Ath-in isograd and an associated zone of retrograde hydrated granulites that occupies ~4500km² of retrogressed crust located in the hanging wall section of the shallow north-dipping Hout River Shear Zone that bounds the Southern Marginal Zone in the south. On the other hand, brine fluids are considered to have triggered the main pulse of anatexis that resulted in production of large volumes of granodioritic-trondhjemitic melts that intruded and started to interact with metapelitic granulite in the deep crust at T>~900°C, P>~7.5kbar. Interaction of hot melt with metapelitic granulite continued until final emplacement in the middle crust (P=~6kbar, T=~630°C). Brine fluids also initiated shear zone-hosted metasomatism of quartzo-feldspathic gneisses at T between ~600 and ~900°C and amphibolite-facies lode-gold mineralization. Available data implicate devolatilization of underthrusted greenstone material as the dominant deep crustal source for infiltrating CO₂-rich and brine fluids.