The geology of a portion of southern Damaraland, South West Africa, with particular reference to the petrogenesis of the Salem granite

Abstract

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Mapping of an area 6 500sq. km in extent and situated north west of Omaruru, South West Africa, has revealed the following sequence of rocks, listed from oldest to youngest: (i) Pre-Nosib gneiss, (ii) Late-Nosib to early-Damara Naauwpoort Formation alkaline rhyolites which are more than 6 OOO m thick and are characterised by very high K, high Ba, low Sr and a Rb deficiency (K/Rb=376). Petrogenesis is discussed in the light of structural setting, volume of outpourings and chemistry, (iii) Damara Group metasediments consisting basically of 100 to 1 100 m of carbonate overlain by almost l0 000 m of schist (Khomas Formation), (iv) Mulden Group schists, (v) the porphyritic Salem granite which has a border facies of monzonite in the south east, (vi) the medium-grained Sorris-Sorris granite, intrusive mainly into the Salem granite, (vii) Karroo System sediments and sills and dykes of dolerite and alkaline felspar porphyry, (viii) a previously undescribed intra- or post-Karroo stock of alkali granite and granite porphyry (Otjihorongo granite) with associated ring dykes and cone sheets of quartz porphyry. The central granite stock is very highly differentiated (K/Rb=77) in relation to the outermost cone sheets (K/Rb=185), (ix) the post-Karroo Okonjeje Igneous Complex

Rocks of the Naauwpoort Formation and the Damara Group and their ultrametamorphic derivatives underlie most of the area and have thus been studied in considerable detail. Nomenclature has been revised and a purely lithostratigraphic terminology used. The evolution of the Nosib/Damara eugeosyncline is discussed in the light of detailed stratigraphic and structural investigations. Under the influence of regional metamorphism which increased in grade towards the south the Naauwpoort rhyolites became recrystallised, coarser grained and eventually remobilised. The resulting magma crystallised to form the Sorris-Sorris granite. Talc, tremolite, diopside, forsterite and monticellite appear successively in the Damara carbonates, and Mg-rich chlorite, biotite, and locally cordierite, sillimanite and almandine in the schist. Intrusion of the Salem granite into low grade regions produced a conspicuous thermal aureole. Zones of thermal metamorphism merge with those of regional metamorphism. Assemblages indicate pressures of about 5 kb in the south and 3, 5 kb nearly 20 km farther north

Extensive melting of the Khomas Formation schist at the peak of metamorphism produced a great deal of granitic magma. Within this magma refractory hornblende, biotite and plagioclase settled, accumulating on the floor of the chamber. Progressive crystallisation in situ produced the rocks of the Salem granite suite. These form a differentiated sequence which grades from diorite on the outer edges through monzonite into the main body of the Salem granite (adamellite). The gradational sequence has been examined by analysing 22 whole rock samples and 36 separates of biotite and hornblende for major elements and for Rb, Ba, Sr, Ni, Zr, Nb and Y