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Structures and mechanisms of dyke emplacement at a volcanic rifted margin – AMS, microfabric analyses and geochronology of mafic dyke swarms from NW Namibia.

Principal Investigators: Prof. Dr. Reinhard O. Greiling, Dr. Robert Trumbull, Dr. Agnes Kontny

GreilingRundeII Mafic dyke swarms provide a record of the orientation, mechanism, and timing of crustal extension, and of the composition and origin of the magmas emplaced in them. This project incorporates these aspects in a study of the Henties Bay - Outjo Dyke Swarm (HOD) in NW Namibia. The work builds on extensive existing geochemical datasets complemented by new field sampling and remote-sensing work in key areas identified within the coastal and inland dykes. Quantitative ages of dyke emplacement will be provided by a program of Ar-Ar dating, information on magma composition and the P-T conditions of magma storage will come from petrologic and geochemical analysis. For the structural and tectonic aspects of dyke emplacement, the main focus is application of the AMS (anisotropy of magnetic susceptibility) method backed up by careful magnetomineralogy, to determine the degree and orientation of flow fabric in the dykes and thus understand the dynamics of magma flow and intrusion within the crust, their relation to rifting and the potential role of preexisting structures. AMS will also be applied to test for post-intrusion structural overprints. These fabric studies will be complemented by microstructural work. In order to separate the effects of ferro- and paramagnetic minerals on dyke fabrics, the AMS work will be supported by AARM (anisotropy of anhysteretic remanent magnetization) determinations. Post-intrusion tilting of dykes from rifted margin evolution is to be tested by palaeomagnetic work. In the final stage of the project we will combine regional structural and geochemical data sets from the Namibian rifted margin to build a comprehensive model of dyke emplacement in the interplay of tectonics and magmatism related to continental breakup in the South Atlantic.