The early Cretaceous Koegel Fontein intrusive complex is situated near the Atlantic coast in South Africa, about 350 km northwest of Cape Town. The complex comprises felsic units of granite and syenite with compositionally related dykes, and a single intrusive plug of diorite. Existing zircon U-Pb ages of 144 ± 2 Ma for the syenite and 133.9 ± 1.3 Ma for the granite suggest that the emplacement of the complex took place over a period of about 10 My. This study provides additional and independent ages of the Koegel Fontein complex by 40Ar/39Ar dating to confirm the onset and duration of magmatism and better define the sequence of igneous units that comprise it.
New laser step-heating 40Ar/39Ar ages on plagioclase and biotite from the main intrusive units in the complex are presented here, including samples previously dated by U-Pb dating. The 40Ar/39Ar ages for the granite and syenite units (131.1 ± 0.9 Ma and 143.3 ± 0.9, respectively) are in good agreement with the zircon U-Pb ages. Other units not previously dated include the Rooivleitjie alkaline granite (150.7 ± 0.6 Ma), two quartz-porphyry dykes (143.0 ± 0.9 and 139.4 ± 1.7 Ma) and the Zout Rivier diorite plug (133.0 ± 1.0 Ma). The new results confirm an early onset of magmatism at Koegel Fontein relative to that of the Etendeka Province some 1000 km to the north, which is consistent with the regional south-to-north propagation of South Atlantic rifting. The youngest 40Ar/39Ar ages at Koegel Fontein (134 to 131 Ma, Rietpoort Granite and 133 Ma, Zout Rivier diorite) correspond to the age of the first magnetic seafloor-spreading anomaly offshore, and we suggest that the longevity of Koegel Fontein magmatism relates to a superposition of pre-drift magmatism onshore and spreading-related magmatism as continental separation began.