The Donkerhuk batholith, Namibia: A giant S-type granite emplaced in the mid crust, in a fore-arc setting

The mainly S-type Donkerhuk batholith intruded the accretionary prism in the Pan-African Damara Belt of Namibia. The batholith is elongate parallel to the regional SW–NE structural grain and the Okahandja Lineament Zone, which forms the accretionary prism backstop and the magma feeder zone. Over 5000 km³ of granitic magmas were emplaced as thousands of sheets, over perhaps 20 myr, with sheet orientations determined by the regional stress field and wall-rock anisotropies. Some of the magma source rocks are inferred to have been cordierite-bearing, suggesting an upper pressure limit of 600 MPa (6 kbar). Calculated phase relations suggest magma emplacement at 450 MPa (4.5 kbar), corroborated by pseudosection modelling of the phase assemblage in a migmatitic wall rock. The magmas were initially highly H₂O undersaturated and at around 850°C, indicating fluid-absent conditions in the source rocks. This suggests intraplating of mantle magmas to provide heat and that the granitic magmas ascended only to the mid crust. The late-tectonic emplacement in a forearc shares similarities with younger and similarly large batholiths such as the Cretaceous Kodiak batholith in Alaska, but the internal architecture and age structure of the batholiths differ markedly, suggesting that different processes can trigger voluminous near-trench plutonism.