Abstract
Anorthosite genesis in the Proterozoic Namaqua mobile belt of South Africa encompasses a range of intrusive histories, including massif-type as in the occurrences of Plat Sjambok and possibly Koperberg Suite and layered-type as at Oranjekom. Different compositional ranges at all levels of analytical resolution (mineralogical, geochemical and isotopical) are revealed. There is a spectrum from orthopyroxene dominance and extremely high LREE (Koperberg Suite), to clinopyroxene-horn blende dominance with normal REE patterns (Oranjekom, Plat Sjambok) and low iron (Oranjekom). Isotopic data for the various complexes (RbSr, SmNd), indicate differences in magma evolution. The Oranjekom Complex represents a depleted mantle origin with low to moderate contamination ( ϵ Sr = +6 to +9; ϵ Nd = +7 to +9), while the Plat Sjambok Complex reflects a depleted mantle origin with a noticeable contribution from a crustal component ( ϵ Sr = +40 to +150; ϵ Nd = −5 to +1). In contrast, the Koperberg Suite ( ϵ Sr = +300, ϵ Nd = −9), if derived from the mantle, would require mixing between a highly enriched mantle source and an extremely enriched crustal contaminant (high ϵ Sr = +700 to +800; ϵ Nd ∼ − 2), resulting in an unusual isotopic trend with a positive slope. The Waterkop pluton that has in the past been regarded as related to the Plat Sjambok Complex, represents a quite different syenitic magma from a slightly enriched mantle source with very little contamination and is not a cogenetic with the latter. Collectively, the analytical data support original magma generation from different mantle sources, followed by various intrusive histories ranging from rapid transit between source and emplacement in the upper crust (Oranjekom), to prolonged travel and differentiation involving considerable crustal contamination (Koperberg Suite). The fact that the different intrusive types occur in different suspect terranes of the mobile belt may reflect originally contrasting tectonic settings during a late-Proterozoic continental collision that played a role during the assembly of Rodinia.
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