Petrology of the cretaceous anorogenic Gross Spitzkoppe granite stock, Namibia

Abstract

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The Gross Spitzkoppe granite stock (GSS) is one of the 124-137 Ma high-level anorogenic granitic-basic-peralkaline-carbonatitic intrusions of western Namibia. These intrusions are genetically related to mantle upwelling (Tristan plume) and continental rifting that led to the opening of the southern Atlantic. The rounded epizonal 30 km2 granitic stock is more than 300 million years younger than the surrounding Damara metasediments and granites. The GSS is approximately coeval with but slightly younger than the extension related, mainly NNE-trending mafic and felsic dike swarm that transects the Damara crust. The synplutonic diabase dikes and magmatic mafic-intermediate enclaves with quartz and alkali feldspar xenocrysts within the granites indicate bimodal character of magmatism

The GSS consists of three zonally arranged granite units: medium-grained biotite granite at the margins, coarse-grained biotite granite, and porphyritic granite at the centre of the stock. Late-stage silicic dikes and pegmatites cut the granites of the stock

Against the country rocks the stock has a 5 m wide layered aplite-pegmatite zone (Stockscheider), which is characterized by rhythmic variation in grain size, grain morphology and mineral composition. The layering is interpreted to result from fluctuations in vapor pressure and diffusion-controlled oscillatory nucleation, as well as rapid growth from undercooled melts. Homogenization temperatures for coeval halite-bearing H2O (±CO2) and H2O-CO2 fluid inclusions from topaz and quartz of the marginal Stockscheider suggest that these fluids were trapped at a maximum of about 480°C and lkb pressure

All the granites are highly evolved metaluminous to peraluminous topaz-bearing monzogranites that contain very iron-rich and fluorine-rich biotite (siderophyllite-annite). Common accessory minerals include fluorite, magnetite, zircon, monazite, thorite, ilmenite, columbite, and niobian rutile

The granites and felsic dikes of the bimodal dike swarm have clear A-type affinity, characterized by high content of high-field-strength-elements (HFSE) and rare-earth-elements (REE). As a result of extensive fractional crystallization the granites are enriched in F, Rb, Si, Ga, Ta, and Nb, and are impoverished in Ca, Ti, P, Mg, Eu, Sr, and Ba

The GSS granites have relatively uniform neodymium isotope composition with initial eNd 125 m) ranging from -5. 6 to -6. 4. Strontium isotopes are more varied and have initial 87Sr87Sr (125Ma) values ranging from 0. 710 to 0. 716. GSS granites are interpreted to have formed by low degree partial melting (10-20 percent) of a felsic granulitic source rock. The source rock is thought to have remained in the lower to mid crust after the extraction of the late to post-orogenic Damara granites