Crustal recycling in the Damara Belt, Namibia, and interaction of the Congo and Kalahari Cratons - Evidence from zircon U-Pb, Hf and O isotopes

Abstract

Zircon U-Pb, Hf- and O-isotopic results from granites and basement rocks from the southwestern Central Zone of the Damara Belt (Namibia) are used to decipher the process of remobilization during collision in this Pan African (ca. 580 to 500 Ma) orogeny between the Congo and Kalahari cratons. The lack of juvenile Damaran magmatism highlights that little new crust was generated and, consequently, that there was little or no oceanic subduction preceding collision. Basement rocks from the Damara Central Zone have Archaean model ages, consistent with them being a southerly extension of the Congo Craton; however, Damaran granites formed in response to the collision tend to have model ages younger than those for the Congo Craton. It is argued that this indicates reworking of younger crust belonging to the leading edge of the Kalahari Craton and that the Congo Craton basement experienced only limited mid- to lower-crustal heating. We therefore invoke initial underthrusting of the Kalahari Craton below the Congo Craton during the collision of these two cratons, leading to mantle lithosphere eclogitization and delamination, and heating of the mafic lower crust of the Kalahari Craton. This produced the mafic to granitic Goas Suite at 560 to 540 Ma. Subsequent lithospheric thinning further heated the underthrust Kalahari Craton material, resulting in granulite-facies LPHT metamorphism and widespread granitoid magmatism at 520 to 510 Ma. This is consistent with geophysical transects of the orogen showing that the lithosphere below the Damara Belt is thinned, in contrast to the Congo Craton which has a thick, cold, lithospheric root and a lower geothermal gradient. This model is also consistent with petrological evidence which shows no evidence for deep burial of the Central Zone, and a near-isobaric heating path to granulite facies.