Post-collisional granite magmatism in the central Damaran (Pan-African) Orogenic Belt,western Namibia

The central Damaran (Pan-African) Orogenic Belt in western Namibia is dominated by elongated granite-gneiss domes, surrounded by Neoproterozoic Damaran cover as periclinal synclinoria, mainly composed of amphibolite-facies metasedimentary rocks. The domes consist of remnants of pre-Damaran gneissic basement and/or Damaran granitoids. There is a high strain zone along the margin between the granite-gneiss domes and the Damaran cover, interpreted in terms of a late extensional mid-crustal detachment, following oblique north-south collision of the Kalahari and Congo Cratons (ca 550 Ma). The present paper focusses on two granite-gneiss domes (Khan and Ida) in western-central Namibia, where a previously unrecognised generation of early post-collisional Damaran granitoids (ca 530 Ma old), have invaded the pre-Damaran basement and cross-cut the detachment zone. These granitoids are classified as syeno-monzogranites with peraluminous high-K calc-alkaline compositions and I/S-type signatures. Geochemical data suggest that the syeno-monzogranites were derived from a lower crustal source, tentatively ascribed to the pre-Damaran gneissic basement. The granitoids also show evidence for magma contamination through partial assimilation with dark, amphibolitic cover rocks. As a result of this hybridisation of the syeno-monzogranites, granitoids with more dioritic compositions were developed. The syeno-monzogranites were then cross-cut by Damaran pegmatitic leucogranites (ca 510 Ma). Further evidence of episodic post-collisional magmatic activity is provided by late-stage mineralisation, and by a ca 465 Ma (⁴⁰Ar-³⁹Ar) thermal overprint in the cover rocks. It is proposed that the post-collisional magmatic activity emanated from the endogenic part of a mid-crustal Damaran granitic batholith, whose cupola-like emplacement in dome structures coincided with regional uplift, and subsequent cooling before ca 465 Ma.