Petrography and geochemistry of the Gross Spitzkoppe Igneous Complex Central-Western Namibia

The Gross Spitzkoppe Igneous Complex of western central Namibia represents an anorogenic magmatic event related to the opening of the South Atlantic ocean during the late Mesozoic period. The magmatic event (Tristan mantle plume?) is characterised by bimodal epizonal emplacement of evolved melt fractions and mafic dyke swarms

The complex is composed of three subsolvus biotite monzogranites. A medium-grained biotite granite along the margin of the complex hosts mafic enclaves and Stockscheider units. This marginal granite often has a sharp contact with coarse-grained biotite granite, which is marked by schlieren bands and orbicular structures. A gradational contact between the coarse-grained biotite granite and the central porphyritic granite is often marked by small pegmatite pods and schlieren bands. Numerous late stage porphyritic and aplitic dykes cut all intrusive units of the complex, with subhorizontal aplitic dykes cutting the porphyritic granite

The granites are weakly peraluminous (A/CNK 0, 93 to 1, 09) and are characterised by high silica content (SiO2 74 to 78 wt percent), NajO + K2O (ca. 9 wt percent), F (up to 1, 16 wt percent), Rb (500 to 800 ppm) and Nb (50 to 120 ppm) as well as low MgO ( 0, 01 wt percent), CaO (0, 3 to 0, 70 wt percent) and P ( 0, 04 wt percent). The granites are strongly differentiated with high Rb/Sr (~36) and Rb/Ba (~14) ratios. Chondrite-normalised rare earth patterns show slight enrichment in LREE (La/Yb ~4), and a prominent negative Eu anomaly (Sm/Eu ~ 33) is common to all granites of the complex

Geochemically, the granites have A-type character and are probably derived from a low crustal igneous protolith. Tectonomagmatic diagrams indicate a within plate (WPG) tectonic setting related to crustal extension. The granites are composed of perthitic alkali feldspar, quartz, plagioclase (An 5-20) and extremely Fe-rich biotite (annite to siderophyllite). Accessory minerals include, fluorite, topaz, zircon, monazite, magnetite, columbite, thorite and Nb-rutile

Subsolidus reactions are common to all intrusive phases, involving exsolution, recrystallisation and alteration of some primary minerals. The transformation of perthitic alkali feldspar to microcline reflects the postmagmatic activity of volatiles, resulting in the development of secondary albite along feldspar grains. The volatile-rich nature of the residual melt is indicated by marginal pegmatites (Stockscheiders), abundant pegmatite pods and hydrothermal veins. Pressure quenching textures along the pluton's periphery are characterised by banded aplites containing dendritic alkali feldspar. These dendritic alkali feldspar nucleated on the country rock and grew towards the center of the complex, reaching lengths of about 20 cm. Undercooling textures are also visible along apical portions of Stockscheider units and along the hanging wall of subhorizontal dykes

Though the complex can be classified as tin bearing, no evidence of metal mineralisation was found, this may be attributed to the oxidising conditions of the melts

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