The 1.8 Ga Gladkop Suite: The youngest Palaeoproterozoic domain in the Namaqua-Natal Metamorphic Province, South Africa

Abstract

The Steinkopf Domain covers about 3500 km2 of the NW corner of the Bushmanland Subprovince of the late Mesoproterozoic (ca. 1.2–1.0 Ga) Namaqua Sector, Namaqua-Natal Province. Granitoid orthogneisses of the Gladkop Suite constitute about 90% of the exposed surface area of the domain. The Gladkop Suite comprises three distinct magmatic phases. From intrusive relationships, the oldest, Steinkopf Gneiss, comprises grey migmatitic biotite granitic to granodioritic orthogneiss with minor diorite. It tends to show transitional contacts with the compositionally homogeneous, grey biotite granitic Brandewynsbank Gneiss, but both are intruded by the leucocratic, pinkish Noenoemaasberg Gneiss. A total of ten representative samples of the three phases give statistically identical Orosirian crystallisation ages between 1810 and 1825 Ma. 31 whole-rock major and trace element geochemical analyses show that the rocks form a sub-alkaline, high-K calc-alkaline, peraluminous to weakly metaluminous series, with broadly arc-like, I-type granite characteristics. Coherent geochemical trends suggest they are co-magmatic. Radiogenic isotope data show that the rocks were, however, not generated in a juvenile arc setting, but one with a significant component of older crustal material, including possible late Neoarchean crust and cannibalisation of older, 2.0–1.9 Ga, magmatic arc crust, such as the nearby Sperrgebiet Domain and Richtersveld Magmatic Arc, respectively. The suite thus represents a third pulse of arc-related Palaeoproterozoic igneous activity marginal to the magmatism of older arcs documented to the north.The zircon data show evidence for significant zircon growth at ca. 1.75 and 1.2–1.03 Ga testifying to later metamorphism. The former marks an elusive hitherto unrecognised Statherian-Calymmian magmatic-metamorphic event, with the latter representing the effects of the main Namaqua orogenesis which imposed the main regional tectonic fabrics upon the Steinkopf Domain and was associated with thermal pulses accompanying widespread polyphase granitoid magmatism. The rocks of the Gladkop Suite are migmatised and this might be attributable to either post-crystallisation metamorphic event. Our study has helped to constrain crustal evolution in NW Namaqualand and puts forward a model for the geotectonic setting for the Steinkopf Domain within the Bushmanland Subprovince, the bulk of which is characterised by a much more cryptic Palaeoproterozoic infrastructure.