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Kraus, Stefan (2005): Magmatic dyke systems of the South Shetland Islands volcanic arc (West Antarctica): reflections of the geodynamic history. Dissertation, LMU München: Faculty of Geosciences
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Abstract

The Antarctic Peninsula forms part of a magmatic arc at least since Jurassic times. Magmatic dykes are essential elements of such arcs and intrude along zones of instability. In contrast to other hypabyssal intrusions and the effusive products of arc activity, dykes do not only reflect the geochemical characteristics of their magma source but also the tectonic parameters at the time of their emplacement. The South Shetland Islands form an archipelago located at the northern tip of the Antarctic Peninsula and belong to this arc. Areas of up to 100,000 m2 have been mapped at several locations of these islands, mainly on King George and Livingston Island. A structural analysis of the dykes and the host rocks was carried out, and about 250 dykes were sampled for further studies. As deduced from field relationships, on Livingston Island six different intrusive events could be distinguished, on King George Island up to seven. This subdivision into different intrusive events is also well reflected by the geochemical data. Analysis of the structural data of the dykes and their host rocks shows, that the tectonic stress field was not only very similar throughout the archipelago, but that moreover only minor changes of this stress field occurred during the time of dyke emplacement. This holds for all investigated areas in the South Shetland Islands. The geochemical data (ICP-MS) reveal, that most dykes of the South Shetland Islands belong to a calc-alkaline, arc-related suite, ranging from basalts to highly differentiated rhyolites. However, especially during early stages of intrusive activity in the respective areas, also tholeiites occur. Isotopic data (Sr, Nd, Pb) prove a strong crustal component during initial stages of magmatic activity, especially on Hurd Peninsula (Livingston Island). This crustal component decreased with time, accompanied by an increase of sedimentary input into the subduction zone. The high amount of crustal contamination during the initial stages was probably due to a still unstretched continental crust. Besides the continental crust underlying the South Shetland Islands, partial melts from the subducted sediments, fluids derived from the subducting plate and a depleted, heterogeneous mantle wedge contributed to arc magma genesis. According to Ar-Ar datings on plagioclase separates and K-Ar (WR) age determinations, dyke intrusion was restricted to the Paleocene and Eocene. The dykes started to intrude around the Cretaceous/Paleogene boundary at Livingston Island. Only around the Thanetian/Ypresian boundary, dyke intrusion commenced also further NE at Nelson and King George Island, culminating during the Lutetian at 47-45 Ma in all investigated areas. Dyke intrusion then ceased in the latter areas but still continued at Livingston Island until the Priabonian. Combining the information given by the tectonic and geochemical datasets, the time interval covered by the dykes obviously marks a period of geodynamic stability. This includes a stable geometry of the subduction zone and the corresponding parameters (subduction direction and velocity) during that time, as well as stable magma sources. The contribution of the respective sources (sediments, slab, mantle, crust) varied, but the sources themselves remained the same. Very primitive, olivine tholeiitic dykes sampled on Penguin Island as a by-product of this work yielded an unexpectedly high Ar-Ar age (Tortonian), thus questioning the onset of rifting in Bransfield Strait during the Pliocene, as believed so far.