Abstract

The weakly porphyritic, mesocratic to melanocratic brown and grey Stolzenfels Enderbite, charno-enderbite and granodiorite crops out as a northwest-trending, 13 km long elongate pluton that crosses the Orange River border between Namibia and South Africa. It is a member of the late- to post-tectonic Komsberg Suite (~1125 to 1105 Ma) that intruded the pre-tectonic gneisses (~1230 to 1150 Ma) in the Kakamas Domain of the Mesoproterozoic Namaqua-Natal Province. The Stolzenfels pluton shows significant, but generally gradational, variations in texture, mineralogy and degree of deformation. It is most commonly weakly K-feldspar porphyritic or coarse-grained equigranular but can, rarely, be strongly porphyritic. Quartz, plagioclase, antiperthite, hornblende and biotite are the predominant minerals whereas orthopyroxene is present in minor amounts in the charnockitic varieties. Most of the pluton is only weakly deformed but a penetrative Namaqua foliation is observed along the eastern margin. Variably oriented younger discrete shear zones and quartz-filled faults cross-cut the pluton. U-Pb zircon dating of the Stolzenfels Enderbite yielded a concordia age of 1115 ± 5 Ma. Despite the textural and mineralogical variations, the Stolzenfels pluton has a very consistent major and trace element geochemistry. It is has an intermediate composition (mean SiO2: 60.5 ± 1.4 wt. %) and is strongly metaluminous (mean ASI: 0.78) which, together with the biotite-hornblende ± orthopyroxene mineralogy and the abundance of mafic xenoliths, suggests the source magma was produced by partial melting of older igneous rocks that have not undergone any significant amount of chemical weathering. The εNd and TDM values of -0.69 and 1602 Ma are typical of the Komsberg Suite and point to a significant contribution of older crustal material to the source magma.

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