Role of fluids in the exhumation of the Southern Marginal Zone of the Limpopo Complex, South Africa

Abstract

The Limpopo Complex (LC) of southern Africa displays structural, metamorphic, and geochronological evidence for a complex exhumation-controlled evolutionary history that followed after burial and attainment of peak metamorphic conditions (P>9kbar, T>900°C) in the root zone (Central Zone, CZ) of the orogen before ∼2.72Ga. The Southern Marginal Zone (SMZ) of the LC, in addition, provides evidence for thrust-controlled emplacement of a hot granulite facies nappe onto the adjacent low-grade granite greenstone terrane of the northern Kaapvaal Craton (NKVC) in the interval ∼2.69–2.62Ga. A tectono-metamorphic scenario in which initial near vertical channeling (exhumation) at ∼2.69Ga was transferred into horizontal channeling (thrusting) at ∼2.67–2.64Ga is interpreted to have commenced with in situ partial melting of crustal rocks in the root zone of the orogen (CZ). This resulted in formation of melt-weakened crust (migmatized gneisses) that with continuous shortening led to vertical transfer of lower crustal material, accompanied by fluid-fluxed decompression melting that resulted in production of large volumes of leucocratic granitic material. This material detached to form buoyant diapirs that accompanied the emplacement at ∼2.65Ga of mega sheath folds at the middle crustal level throughout the entire CZ of the LC. This early stage of near-vertical exhumation is reflected (i) by the fact that linear elements of sheath folds and shear zones plunge consistently at steep angles to the SW throughout the entire CZ, and (ii) by decompression-cooling (DC) segments of P–T paths constructed for metapelitic rocks. DC paths in the far northern part of the SMZ that are associated with the steep north-dipping portion of the Hout River Shear Zone (HRSZ) traversed from P∼8.5kbar, T∼850°C to P∼6.5kbar, T=720°C. Early near-vertical exhumation transferred into horizontal channel flow that controlled emplacement at the middle crustal level of the granulite facies nappe (SMZ). Horizontal channeling (thrusting) is recorded in metapelitic granulite by near-isobaric cooling (P∼6kbar, T∼700–550°C) segments of earlier DC P–T paths that are associated with the shallow north-dipping portion of the HRSZ. Devolitilization of underthrusted greenschist due to thrusting hot granulite over cool greenschist released large volumes of high temperature aqueous fluids from the underthrusted rocks that constantly infiltrated the overriding granulite during southwards migration of the nappe. Immiscible CO2-rich and brine fluids thus derived persistently infiltrated into and reacted with cooling granulite and triggered the main pulse of anatexis at P>7.5kbar, T>750°C. Similar fluids established a regional retrograde isograd and associated zone of 4500km2 retrograde hydrated granulite at P∼6kbar, T=600–630°C located in the hanging wall of the shallow north-dipping HRSZ that bounds the SMZ in the south. Shear zone-hosted metasomatic alteration including formation of lode-gold deposits occurred at T=600–630°C, P∼6kbar in the zone of rehydrated granulite, and at higher P–T conditions in the granulite zone. The granulite facies SMZ of the LC thus preserves clear evidence for a direct link between persistent fluid flux and fluid-rock interaction as an integral component of a conveyor-like tectonic mechanism driven by vertical tectonics that operated from the core region of the orogen (CZ). This mechanism explains the important link between the thrust-controlled emplacement of the SMZ onto the NKVC and deep crustal fluid flow.