Variation in metamorphic style along the northern margin of the Damara Orogen, Namibia

Abstract

The northern margin of the Inland Branch of the Pan-African Damara Orogen in Namibia shows dramatic along-strike variation in metamorphic character during convergence between the Congo and Kalahari Cratons (M3 metamorphic cycle). Low-P contact metamorphism with anticlockwise P-T paths dominates in the western domains (Ugab Zone and western Northern Zone), and high-P Barrovian metamorphism with a clockwise P-T path is documented from the easternmost domain (eastern Northern Zone). The sequence of M3 mineral growth in contact aureoles shows early growth of cordierite porphyroblasts that were pseudomorphed to biotite-chlorite-muscovite at the same time as an andalusite-biolite-muscovite transposed foliation was developed in the matrix. The peak-T metamorphic assemblages and fabrics were overprinted by crenulations and retrograde chlorite-muscovite. The KFMASH P-T pseudosection for metopelites in the Ugab Zone and western Northern Zone contact aureoles indicates tight anticlockwise P-T loops through peak metamorphic conditions of 540-570°C and 2·5-3·2 kbar. These semi-quantitative P-T loops are consistent with average P-T calculations using THERMOCALC, which give pooled mean of 556 ± 26°C and 3·2 ± 0·6 kbar, indicating a high average thermal gradient of 50° C/km. In contrast, the eastern Northern Zone experienced deep burial, high-P/moderate-T Barrovian M3 metamorphism with an average thermal gradient of 21°C/km and peak metamorphic conditions of c. 635°C and 8·7 kbar. The calculated P-T pseudosection and garnet compositional isopleths in KFMASH, appropriate for the metopelite sample from this region, document a clockwise P-T path. Early plagioclase-kyanite-biotite parageneses evolved by plagioclase consumption and the growth of garnet to increasing XFe, XMg and XCa and decreasing XMn compositions, indicating steep burial with heating. The developed kyanite-garnet-biotite peak metamorphic parageneses were followed by the resorption of garnet and formation of plagioclase moats, indicating decompression, which was followed by retrogressive cooling and chlorite-muscovite growth. The clockwise P-T loop is consistent with the foreland vergent fold-thrust belt geometry in this part of the northern margin. Earlier formed (580-570 Ma) pervasive matrix foliations (M2) were overprinted by contact metamorphic parageneses (M3) in the aureoles of 530 ± 3 Ma granites in the Ugab Zone and 553-514 Ma granites in the western Northern Zone. Available geochronological data suggest that convergence between the Congo and Kalahari Cratons was essentially coeval in all parts of the northern margin, with similar ages of 535-530 Ma for the main phase of deformation in the eastern Northern Zone and Northern Platform and 538-505 Ma high-grade metamorphism of the Central Zone immediately to the south. Consequently, NNE-SSW-directed convergent deformation and associated M3 metamorphism of contrasting styles are interpreted to be broadly contemporaneous along the length of the northern margin of the Inland Branch. In the west heat transfer was dominated by conduction and externally driven by granites, whereas in the east heat transfer was dominated by advection and internally driven radiogenic heat production. The ultimate cause was along-orogen variation in crustal architecture, including thickness of the passive margin lithosphere and thickness of the overlying sedimentary succession. © Oxford University Press 2004; all rights reserved.