Evidence of NeoArchean Magma Transporting Large Amounts of Water and Carbon dioxide in South Africa

Fluid-assisted interaction of peraluminous metapelites with trondhjemitic magma within the Petronella shear-zone, Limpopo Complex, South Africa

Authors:

Safonov et al

Abstract:

The principal problem concerning the evolution of Precambrian granulite complexes located between granite-greenstone cratons is their interaction with the underthrusted greenstone belts during exhumation from the lower crust. Besides evidence for pervasive and localized fluid fluxes arising from devolatilization of the greenschist and amphibolite-facies rocks in the course of their burial underneath the granulites, this interaction might also be expressed in formation of diverse magmas of granitic, trondhjemitic and granodioritic composition. We present results of a petrological, fluid inclusion and thermobarometric study of interaction between fluidized trondhjemitic magma and peraluminous metapelitic granulites associated with the regional high-grade Petronella shear zone located in the Southern Marginal Zone (SMZ) of the Limpopo Granulite Complex (South Africa). The hot (T ∼ 1000 °C) trondhjemitic magma, which, presumably, originated from partial melting of a basaltic (amphibolite) material at the base of the granulite complex or at the top of the underthrusted greenstone blocks, intruded granulites at P ∼ 8.0–9.5 kbar (24–28 km depth) at 2.667 ± 0.9 Ga during the exhumation of the SMZ. The magma heated and assimilated orthopyroxene-cordierite metapelites and dragged them to a depth of 18–20 km (6.3–6.5 kbar). The magma was heterogeneously saturated with MgO, FeO, Al2O3 by the dissolved metapelites. This process provoked crystallization of several garnet generations from the trondhjemitic melt. Various mineral assemblages included in the different generations of garnet allowed application of TWQ method combined with PERPLE_X pseudo sections to trace sub-isobaric cooling of the magma from T ∼ 900–600 °C at 5.5–6.5 kbar. The isobaric cooling also affected the associated metapelites. Fluid inclusions trapped in garnet and quartz in the trondhjemite show that the magma transported carbonic fluid with densities corresponding to the late stages of magma cooling (600–650 °C and 5.5–6.5 kbar). Carbonic fluids coexisted with aqueous-salt fluids (preserved as inclusions with salinity up to 20.58 wt.% NaCl eq.). These low water activity fluids (aH2O less than 0.3)(aH2O less than 0.3) bearing Na, K and Ca salts, being exsolved from the magma on cooling and solidification, provoked formation of complex Na-gedrite + biotite + sillimanite + quartz ± staurolite ± plagioclase-bearing assemblages after cordierite in metapelites at temperatures 630–570 °C and pressures 5.5–6.5 kbar. These data provide evidence that hot trondhjemitic melts played a critical role in the exhumation of granulites onto the adjacent granite-greenstone craton. The trondhjemite transferred heat from the lower to the middle crust and transported large volumes of external aqueous-carbonic-salt fluids that participated in the rehydration of a significant portion of the SMZ.