Age of ferroan A-type post-tectonic granitoids of the southern part of the Keimoes Suite, Northern Cape Province, South Africa

The granitoids of the Keimoes Suite represent part of a complex group of late Mesoproterozoic intrusions of granodioritic to potassic granite composition that occupy an outcrop belt striking approximately NW–SE to the immediate west of Upington, Northern Cape Province, South Africa. The intrusions are believed to mark the contact between the Kaapvaal Craton and the Namaqua Sector of the Namaqua-Natal Province. Published geological, geochemical and geochronological knowledge of the Keimoes Suite is poor in general, with most data restricted to the central portion of the outcrop belt. In this contribution new ages and geochemical data for three granitoids of the southeastern Keimoes Suite in the Putsonderwater – Van Wykspan area, the Klip Koppies, Gemsbokbult and a member of the Kleinbegin Subsuite, are presented.All three intrusions show petrological and geochemical characteristics that agree well with literature data for the central and northern outcrop portions of the Keimoes Suite. They are weakly foliated to unfoliated, medium- to coarse-grained ferroan, alkali-calcic late- to post-tectonic A-type biotite granitoids intruded within a collisional within-plate tectonic setting. Zircon U–Pb SHRIMP ages for the intrusion of the three granitoids are within error of one another at ca. 1100 Ma. Marked differences between the three granitoids include the abundance and age of inherited zircons and Sm–Nd isotope systematics. With an εNd(t) value of 1.83, the Gemsbokbult Granite is dominantly sourced from partial melting of material with a mantle derivation with a minor crustal component. The other two intrusions, on the other hand, show a strong crustal source component, as indicated by εNd(t) values of −2.03 for the Klip Koppies Granite and −8.46 for the Kleinbegin Granite.The Keimoes Suite granitoids provide important clues as to the timing and tectonic evolution of the western margin of the Kaapvaal Craton at its junction with the Proterozoic Namaqua Sector of the Namaqua-Natal Province. A contribution to these melts from the host rocks, the ∼1.24–1.30 Ga Areachap Group volcanic belt is traced through the presence of inherited zircon cores of this age that is particularly evident in the Klip Koppies Granite. Inheritance also reflects the major tectono-magmatic events which affected the western margin of the Kaapvaal Craton during Mesoproterozoic times, with the bulk of the inheritance in the zircon populations due to peak metamorphism and partial melting caused by continental collision between the Kaapvaal Craton and the Kakamas Terrane and Bushmanland Subprovince to the west at approximately 1.18–1.2 Ga. Post 1.1 Ga ages reflect isotopic disturbance due to a later collisional event to the west at ∼1.08 Ga and subsequent exhumation of the western margin of the Kaapvaal Craton.

Research highlights
► This research investigated three biotite-bearing granitoids of the Keimoes Suite.
► The granitoids are alkali, ferroan A-type post-tectonic biotite monzogranites.
► They intruded in a within plate setting along a subduction zone suture at 1.1 Ga.
► Derivation was by melting of a crustal source with an initial mantle derivation.
► Inherited zircons broadly reflect tectonomagmatic events in the area.