Thermal and Fluid Effects of Granitoid Intrusions on Granulite Complexes: Examples from the Southern Marginal Zone of the Limpopo Complex, South Africa

Abstract

The paper summarizes data on the petrology of granitoid intrusions in the Southern Marginal Zone (SMZ) of the Neoarchean Limpopo granulite complex, South Africa, and discusses the thermal and fluid effects of these intrusions on the granulites. The intrusions were emplaced in SMZ at 2680–2640 Ma, when the granulite complex was overthrust on the rocks of the adjacent Kaapvaal Craton. The mineral assemblages of the granitoids reflect temperatures above 900°C for the magmas that crystallized under pressures of 6–9 kbar. The granitoid magmas assimilated host rocks and were thereby enriched in MgO, FeO, and Al2O3, which was favorable for the crystallization of garnet, spinel, sillimanite, and corundum in the granitoids. Fluid inclusions in the granitoids and estimates of the fluid composition based on mineral equilibria indicate that the CO2/H2O ratio of the fluids broadly varied. Along with H2O–CO2 fluid, the magmas carried H2O–salt fluids, which penetrated the host rocks and triggered various metasomatic reactions in them. The thermal effects of the intrusions on the host granulites resulted in the development of partial melting zones that host orthopyroxene with >7 wt % Al2O3. Depending on the fluid regime and temperature, the orthopyroxene is found in equilibrium with either garnet and potassic feldspar or with biotite. The ages of the partial melting zones are comparable with those of the intrusions.