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The petrogenesis of the Alberta Complex within the Rehoboth Basement Inlier of Namibia

Geological Survey of Namibia, P.O. Box 2168, Windhoek, 9000, Namibia, e-mail: tom{at}mme.gov.na

Arnd Brandenburg

2 DESY Theory Group, D-22603 Hamburg, Germany, e-mail: Arnd.Brandenburg{at}desy.de

The Alberta Complex is the most prominent member of a mafic to ultramafic igneous suite within the Rehoboth Basement Inlier/Namibia, being intrusive into sediments and mafic volcanics of the Paleoproterozoic Elim Formations. Subsequent amphibolite grade metamorphism of pre-Kibaran age obscured the original mineral assemblage. Two main units have been distinguished in the past within the Alberta Complex. (1) A thick pile of various amphibolites represents original norite, hyperite, gabbro and anorthosite and constitutes the Layered Sequence. It has been subdivided into four zones, thought to represent individual magmatic pulses. (2) Later stage magmatism comprises pegmatite veins as well as pyroxenite, harzburgite and dunite, emplaced in plugs and a cone sheet.

In contrast to field observations, geochemical profiles of compatible elements across the complex indicate more complex layering and subdivision of the Layered Sequence into at least seven units. Incompatible trace elements are largely decoupled from compatible elements, which probably is the result of variable amount of interstitial liquid within cumulate phase. However, the shape of REE patterns has been controlled chiefly by fractionation of clinopyroxene and plagioclase. Finally, decoupling of hydrophile LILE from both compatible and incompatible elements may result from hydrothermal overprint or from redistribution during metamorphism.

Gabbroic rocks from the Marginal Zone of the Alberta Compex display affinity to mafic volcanic rocks of the Elim Fm. Modeling based on in situ fractional crystallisation or, alternatively, mixing of evolved liquid with cumulate supports this hypothesis. In contrast, for samples within the magma chamber, only REE could be matched closely by these models, while modeled incompatible trace elements commonly deviate from the measured data. Additional complexity of local magma chamber probably best explains these deviations.

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