THE GROSS SPITZKOPPE GRANITE STOCK OF THE CRETACEOUS DAMARALAND ANOROGENIC PROVINCE IN NAMIBIA—AN EXAMPLE OF BIMODAL MAGMATISM RELATED TO CONTINENTAL RIFTING

Cretaceous continental rifting that led to separation of Africa and South America was associated with mantle plume activity (Tristian plume) and bimodal volcanic-plutonic magmatism in southwestern Africa and Atlantic margin in South America. In Damaraland, Namibia, about 20 high-level plutons as well as numerous NW (rift direction) and NE-striking mafic and silicic dykes intruded the Neoproterozoic – Paleozoic Damara orogenic belt. The plutons are 124-137 Ma in age and granitic, mafic, alkaline or carbonatitic in composition (Milner et al., 1995).

The Gross Spitzkoppe stock (30 km²) is composed of three texturally different types of highly evolved A-type granites. The bimodal character is indicated by syn-plutonic diabase dykes and magmatic mafic enclaves containing quartz and alkali feldspar xenocrysts. Some lamprophyre and several subhorizontal aplite dykes cut the granites. The stock is rimmed by banded pegmatite–aplite stockscheider, and various schlieren structures demonstrate the magma flow mechanisms. The granites are marginally peraluminous (A/CNK 0.98 to 1.13) high-silica monzogranites characterized by high F (0.5 wt-%) and low P₂O₅ (0.01 to 0.03 wt-%). Magmatic topaz and iron-rich biotite (Fe/Fe+Mg 0.90 to 1.00) occur as minor constituents, and columbite, magnetite, monazite, and thorite are typical accessory minerals. The granites have high Ga, Rb, Nb and Ta, and low MgO, Sr and Ba. (La/Yb)_N is 4.1 to 5.2, and (Eu/Eu*)_N 0.04 to 0.07. The granites show within plate granite characteristics. Geochemical and isotopic studies suggest a crustal source, and mafic underplating is regarded as an obvious genetic model. The Cretaceous granites of Namibia provide a useful reference for granitic magmatism of intracratonic rifting environment. Many similarities with the Proterozoic rapakivi granites of Fennoscandia and other regions suggest similarities in origin.