The age and source of Cambrian post-orogenic magmatism in the Kuboos-Bremen Igneous Province (Southern Namibia)

Abstract

Post-orogenic complexes in Central and Southern Namibia record the amalgamation of Gondwana along the Pan-African Gariep and Damara Belts. Five complexes of the Kuboos-Bremen Igneous Province in Southern Namibia (Grootpenseiland-Marinkasquellen, Marinkas Carbonatite, Kanabeam, Young Bremen, Garub suite) have been dated with zircon U-Pb and biotite Ar-Ar, while Hf isotopes on zircon were used to characterize the source of the magmatic province. Within errors, nine granites and syenites of the Grootpenseiland-Marinkasquellen, Kanabeam and Young Bremen Complex(es) are contemporary and emplaced between 493 and 509 Ma (with typical errors of ±8 Ma). The Marinkas Carbonatite Complex yields slightly higher ages (508–536, ±10 Ma). Two lamprophyric dykes from the Garub Suite that consists mainly of carbonatite breccia pipes yield an age of 525 ± 3 Ma, contemporaneous with the Marinkas Carbonatite Complex. Negative εHf(t) indicate post-orogenic magmatism resulting from lithospheric thinning and heating of metasomatized mantle. Only one sample, a Si-undersaturated nepheline-sodalite syenite from the Kanabeam complex, has positive, close-to unmetasomatized mantle εHf(t) values, indicating a heterogeneously metasomatized (lithospheric) mantle.

Monzonite and monzodiorite samples from the Otjisazu Carbonatite Complex in Central Namibia yield U-Pb zircon ages of 521 and 550 Ma (±10 Ma), respectively, sharply contrasting a previous U-Pb titanite age of 840 Ma. The negative zircon εHf(t) in a system that evolves to carbonatites indicates crustal contamination of the mantle source but negligible crustal assimilation during magma ascent.

In addition, the southern Namibian Haruchas Stock and the Old Bremen Complex were dated. Notably, the heavily Si-undersaturated Haruchas Stock yielded an age of 896 ± 18 Ma, and is hence not akin to the Si-undersaturated rocks of the Kuboos-Bremen Suite but the northernmost extension of the Richtersveld Suite. Our U-Pb ages for the Old Bremen Complex (880–903 ± 18 Ma) confirm previous results. The positive εHf(t) of +1.3 - +7.5 of samples from the Haruchas Stock and Old Bremen Complex indicate that their source had a lesser involvement of crustal components than the Cambrian magmatism, consistent with mantle-derived alkaline magmatism within an extensional setting.