Constraining the timing of deformation in the southwestern central zone of the Damara Belt, Namibia

Structural investigations and U-Pb sensitive high-resolution ion microprobe (SHRIMP) dating of rocks from the southwestern Central Zone of the Damara Belt, Namibia, reveal that a major SE-verging deformation event (D2) occurred at between 520 and 508 Ma

Amphibolites of the central zone: New shrimp U-Pb ages and implications for the evolution of the Damara Orogen, Namibia

New SHRIMP U-Pb zircon and titaniie data are presented for two amphibolites from the Central Zone of the Damara Orogen, Namibia. These data (the first ages for amphibolites in the Central Zone) show that amphibolites are derived from both Palaeoproterozoic (2026.9 ± 2.3 Ma-sample ACAM-1) and Pan-African (557.2 ± 7.4 Ma-sample LKR021) precursors emplaced into the Palaeoproterozoic Abbabis Complex basement in the Central Zone. The younger amphibolites are interpreted as part of a mafic to dioritic phase of magmatism in the Central Zone (the Goas Suite), and major-And trace-element geochemistry from a suite of amphibolites from the southwestern Central Zone confirms this relationship. This phase of mafic to dioritic magmatism is contemporaneous with the onset of collision between the Congo and Kalahari Cratons. It is suggested that delamination of dense mafic lower cnist and underplating of mantle-derived magmas following crustal thickening may have led to partial melting of pre-Pan-African mafic lower crust, generating this mafic to dioritic suite of rocks. Metamorphic zircon overgrowths in sample LKR021 give an age of 520 ± 6.9 Ma, consistent with recent estimates for the age of peak metamorphism in the southwestern Central Zone. Titanites from LKR021 give an age of 493.4 ± 6.4 Ma, and this is interpreted as a cooling age for the Central Zone

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

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.Luke Longridge would like to thank the GSSA’s REI fund for support towards acquiring the isotopic data presented in this paper, and to the NRF for financial support during this research

Zonation of Merensky-Style platinum-group element mineralization in Turfspruit Thick Reef Facies (Northern Limb of the Bushveld Complex)

The stratigraphy, whole-rock composition, and mineralogy from a thick undisturbed section of the Platreef at depth on Turfspruit in the northern limb of the Bushveld Complex have been examined in detail. This section has much more in common with the Critical zone than with the Platreef structure updip although the high-grade platinum-group element (PGE) reefs, located close to the boundary with the Main zone, are much thicker than any known Merensky reef facies. A key feature that marks the conversion of the Platreefstyle mineralization into the Merensky style is the regular appearance of chromite seams, which become more persistent with decreasing contamination westward and downdip. The similar zonal distribution of platinum-group minerals (PGM) in two consecutive thick PGE reefs correlates with changes in silicate compositions and vertical zonation of PGE concentrations in whole rocks and base metal sulfides. This supports orthomagmatic in situ crystallization of ore minerals during mixing of consecutive magmatic influxes. The typical Merensky-style ore assemblages of predominant Pt sulfides and alloys are developed within ~1-m-thick intervals, which correspond to the highest temperature mixing zones between two magmas, whereas the rest of the reefs host Pt-Pd bismuthotellurides and arsenides. PGE content and tenor, as well as Cu/Pd in major base metal sulfides in the Turfspruit reefs, are within the range of the typical high-grade Merensky values, suggesting the same enrichment process