U-Pb geochronology of tectonothermal events related to the Rodinia and Gondwana supercontinents -observations from Antarctica and Baltica

Sammanfattning: This thesis presents data from two areas, Dronning Maud Land, Antarctica, and the Scandinavian Caledonides, Baltica. In addition, a comparative geochronological study is presented. The bedrock of Dronning Maud Land (DML) is dominated by migmatitic gneisses and post-tectonic intrusions. A migmatitic gneiss at Jutulsessen nunataks, Gjelsvikfjella, has a protolith age of 1163 ± 6 Ma and a migmatisation age of 504 ± 6 Ma. The ages of post-tectonic rocks range from 486 ± 6 Ma to 521 ± 4 Ma. The youngest age represents intrusion of granitic/pegmatitic dykes and fluids. Sm-Nd model ages for the Mesoproterozoic migmatitic gneiss in Jutulsessen vary between 1390 and 1770 Ma while the post-tectonic rocks have model ages between 970 and 2200 Ma. The age of the protolith and the model ages are in accordance with a Mesoproterozoic correlation between Dronning Maud Land and the Natal Province in South Africa made by earlier authors. Consequently, it is suggested that the investigated area in DML was a part of the Kalahari (Kaapvaal) Carton since Mesoproterozoic time. The regional migmatisation and the post-tectonic intrusions are all in accordance with a post-collision collapse. It is suggested that the post-collision event is caused by the collision between the Kalahari craton and the combined block of East Antarctica and Australia, and that it completed the assembly of Gondwana. By investigating the Seve-Kalak Nappe Complex in the Scandinavian Caledonides, Baltica, it is possible to reconstruct the pre-Caledonian margin of Baltoscandia. The intrusion age of a granite–gabbro complex, hosted by mainly sedimentary migmatites and augen gneisses, has been determined to 845 ± 14 Ma. Analyses from zircon cores indicate Paleoproterozoic (1778 ± 11 Ma) and Archaean protolith ages. Bimodal magmatism of this age is not found within the Baltic shield but the age pattern allows correlations within the Seve-Kalak Nappe Complex 300km to the north. It is suggested that the bimodal magmatic complex intruded into a sedimentary basin in a continental rift setting, but that rifting was not successful. When analysing U-rich zircons from DML with the SIMS technique, reversed discordant analyses are often obtained. In order to reveal the character of these zircons the very same zircon crystals were analysed by ID-TIMS. The results show that the reverse discordance is an artefact of the SIMS analytical procedure, probably caused by a difference in the sputtering and ionizing behaviour of U-rich zircons versus that of the U-poor zircon standard. In such cases it is thus important to put more emphasis of the 207Pb/206Pb ages, which are not dependent on these differences.