Evidence for metamorphic and igneous charnockites in the Southern Marginal Zone of the Limpopo Belt

Abstract

Metamorphic and igneous charnoenderbite and enderbite can be distinguished on the basis of field relations and bulk rock geochemistry in the Southern Marginal Zone of the Limpopo Belt. Termed here metamorphic charnockite and igneous charnockite respectively, both varieties are petrographically similar. They are massive, homogeneous rocks which formed after the D 1, fabric forming event associated with the Limpopo Orogeny and they at least experienced the entire retrograde portion of that Orogeny. The metamorphic charnockite probably developed first during peak metamorphism (M 1) and is often closely associated with a banded, orthopyroxene-bearing quartzo-feldspathic gneiss (the Baviaanskloof Gneiss). The metamorphic charnockite-Baviaanskloof Gneiss relations in the field are those of quiescent obliteration of foliation, and the charnockite is often characterized by the presence of relict banding. The chemical composition of the metamorphic charnockite is identical with that of the associated Baviaanskloof Gneiss. The igneous charnockite, locally retaining igneous textures, is characterized by intrusive relationships with the Baviaanskloof Gneiss and in most cases by chemical compositions that are distinct from those of the older unit. Available geochronological data indicate that the metamorphic charnockite, or its protolith, formed at or after ∼ 2715 Ma ago, significantly before emplacement of the igneous charnockite of the Matok Complex at ∼ 2671 Ma. Emplacement of igneous charnoenderbite and enderbite in the rest of the Southern Marginal Zone and in the Matok Complex was not synchronous but occurred respectively during and after peak metamorphism (M 1). Metamorphic charnockite with preserved relict banding probably formed by in situ biotite dehydration to orthopyroxene in the presence of a fluid phase with locally different activity of CO 2. H 2O dilution through extraction of a granitic melt is considered to be unfeasable because of similar bulk chemistry. The igneous charnockite crystallized from calc-alkaline magmas derived by partial melting of lower crustal rocks, similar to some phases of the Baviaanskloof Gneiss.