The Messina Layered Intrusion, Limpopo Belt, South Africa: An example of in-situ contamination of an Archean anorthosite complex by continental crust

Abstract

The Messina Layered Intrusion was emplaced prior to 3.0 Ga into supracrustal rocks presently comprising the Central Zone of the Limpopo Belt. Analyses of relict cumulate plagioclase megacrysts in the Intrusion and its tectonic setting show that it was emplaced as a thick body of batholithic dimensions, composed of varying proportions of bytownite (An 77) and hypersthene. The magma giving rise to the Intrustion was heterogeneous, characterized by positive ϵ Sr values, neutral to negative ϵ Nd values and high 207Pb/ 204Pb ratios relative to 206Pb/ 204Pb. It also was light REE enriched. These characteristics probably originated by the interaction of a magma derived from a depleted mantle source with crustal material similar in composition to the rocks of the enclosing Beit Bridge Group and Sand River Gneisses. As a result, most of the characteristics of the Intrusion are closer to those of Proterozoic massif-type anorthosite complexes than to those of Archean complexes. The Messina Layered Intrustion was metamorphosed and deformed prior to 3.0 Ga, when its composition was changed to quartz-hornblende-plagioclase gneisses by the introduction of water. Pb, Sr and Rb were also added to most of these rocks, probably derived from the host rocks but the primary isotopic characteristics of relict plagioclase megacrysts and of partially assimilated xenoliths of metapelite were preserved. More recently the Intrusion was metamorphosed under granulite facies conditions at ∼ 2.0 Ga, prior to the joining of the rocks of the Central Zone with the Zimbabwe and Kaapvaal cratons. The emplacement, metamorphism and deformation of the Intrusion had nothing to do with early evolutionary events on those cratons.