Late-Orogenic Juvenile Magmatism of the Mesoproterozoic Namaqua Metamorphic Province, South Africa, and Relationships to Granulite-Facies REE-Th and Iron Oxide Mineralizations

Abstract

Abstract The Bushmanland Subprovince of the Mesoproterozoic Namaqua-Natal orogenic belt in southern Africa hosts numerous occurrences of monazite-magnetite-(biotite-apatite-sulfide)-bearing veins and granitoid dykes, including the Steenkampskraal vein system, which is one of the highest-grade REE-Th ore deposits in the world. Here, we provide whole-rock geochemical data along with zircon and monazite U-Pb dates and monazite Sm-Nd isotope analyses of these REE-Th-rich veins and granitoid dykes. The U-Pb geochronology indicates that the monazite-rich veins and granitoid dykes formed between 1050 and 1010 Ma, contemporaneously with late-Namaqua granulite-facies metamorphism. They are also coeval with the Koperberg Suite mafic magmas at 1060–1020 Ma and the late stage of a major event of A-type granitoid magmatism that occurred from 1100 to 1030 Ma (i.e. the Spektakel Suite). Similar to the mafic intrusive rocks from the Koperberg Suite, monazite-rich veins and granitoid dykes, located in the southern part of the Bushmanland Subprovince, have more radiogenic Nd isotopic compositions (ɛNd(t) ∼ −1 to zero) than equivalent dykes and veins to the north (ɛNd(t) ∼ −12 to −6). Mafic rocks of the Koperberg Suite reach Th and La concentrations of >400 ppm that significantly exceed those of other rock types from the region, except for the monazite-rich veins and granitoid dykes, which suggests a genetic link between these rocks. Within veins and granitoid dykes, monazite, biotite and magnetite are commonly anhedral and occur interstitially between the felsic minerals; they are, thus, late crystallizing phases. The whole-rock REE-Th concentrations of the granitoid dykes increase with Fe-Mg contents. Therefore, their incompatible element enrichment is not linked to assimilation-fractional crystallization processes. The Nd isotopic signature as well as Fe-Mg- and REE-Th-rich character of the Koperberg Suite and monazite-rich granitoid dykes might reflect partial melting of lithospheric mantle domains, metasomatized during previous Namaqua subduction events, and the mixing of mantle-derived melts with REE-Th-rich metamorphic fluids during their ascent through the crust. We propose that the monazite-magnetite vein mineralizations represent Fe-P-rich immiscible liquids that exsolved from mantle-derived magmas with compositions similar to the most mafic and monazite-rich granitoid dykes. Within this petrogenetic model, conjugate silicate-rich immiscible liquids formed the more felsic granitoid dykes characterized by lower modal abundances of biotite, magnetite and monazite. Although they do not reach similarly high REE-Th concentrations, other A-type granitoids from the region, represented by the Spektakel Suite, also share geochemical affinities with mafic igneous rocks from the Koperberg Suite; they may have originated by melting of underplated equivalents of these late-orogenic mafic rocks.