Geology and emplacement controls of the Stinkbank granite in the south Central Zone of the Pan-African Damara Belt, Namibia

[摘要] ENGLISH SUMMARY: Regional mapping of the Stinkbank granite in the south Central Zone of the Damara Belt, focused on an area of ca. 150 km2 in the SW parts of the granite along well exposed sections of the Khan River to the SW of the town of Usakos. The granite forms part of the regionally widespread granite suite of Salem-type granites that intruded at ca. 550-540 Ma into amphibolite-facies rocks of the Damara Supergroup. The Stinkbank granite is intrusive into the regional-scale, NE-trending D2 Kransberg syncline, cored by schists of the Kuiseb Formation and surrounded by marble units of the Karibib Formation.The granite comprises three major lithotypes that have intruded in sequence. The earliest granites are represented by biotite-rich megacrystic granites, followed by leucocratic megacrystic granites and a final stage of voluminous, garnetiferous and tourmaline-bearing, medium-grained leucogranites. Contact relationships with the wall rocks are, for the most part, concordant, documenting the largely sheet-like geometry of the granites. Internal contacts between different granite phases are well preserved and indicate that the granites have intruded as shallowly-dipping, largely concordant sheets. Intrusive contact relationships and petrographic and geochemical characteristics indicate that each of the three major granite phases represented a distinct emplacement pulse. Successive emplacement of the granite sheets point to the assembly of the Stinkbank granite from the top down, with younger sheets intruding structurally below earlier emplaced granite sheets. This has created a pseudostratigraphy within the Stinkbank granite. The mapping of the internal contacts shows that the granite sheets were progressively folded into NE-trending, upright folds, parallel to D2 folds in the surrounding wall rocks. NE-trending magmatic and solid-state foliations in all granite phases are axial planar to the folds and underline the syntectonic emplacement of the Stinkbank granite during the D2 NW-SE subhorizontal shortening.Based on the intrusive relationships and the progressive deformation of granite phases, an intrusive sequence can be developed for the Stinkbank granite. The earliest granite phases were emplaced during the onset of the regional D2 deformation, parallel to the subhorizontal bedding. Continued granite sheeting led to the vertical growth of the sheet-like granite and bending of the wall rocks above the inflating granite sheets, leading to the laccolithic geometry of the SW partsiiiof the Stinkbank granite. Progressive deformation, folding of the granite sheets and fold amplification resulted in fold interference patterns in the SW parts of the Stinkbank granite.The Stinkbank granite represents a mid-crustal granite with well-preserved granite sheeting that was assembled during regional deformation. Granite sheeting and progressive deformation illustrate the interplay between (1) regional strains, and (2) the orientation and presence of pre-existing wall-rock anisotropies (bedding) and their significance for the magmatic assembly and progressive deformation of the granite.

[发布机构] Stellenbosch University