Tab. 2: U-Pb ion-microprobe ages from Taymyr peninsula

We present the initial results of a U-Th-Pb zircon ion-microprobe investigation on samples from the Central Belt of Taimyr, in order to constrain its tectono-magmatic evolution. The zircon samples are from a deformed twomica granite (Faddey Massif), deformed metamorphosed gabbroic dike entrained as pods and lenses within metamorphosed tholeiitic basalts of the Kunar-Mod volcanic suite (Klyaz'ma River region), a metamorphosed rhyolite of the same volcanic suite overlying the basic metavolcanic rocks, as well as an undeformed dolerite dike which intrudes the metamorphosed Kunar-Mod basic volcanic rocks. Preliminary results on zircons from the two-mica granite suggest a crystallization age of ~630 Ma for this rock, with inheritance from assimilated crust 840 Ma to 1.1 Ga in age. In the Klyaz'ma River region, zircons from the meta-rhyolite yield a concordant age of -630 Ma. Zircons from the entrained metagabbroic dikes have so far yielded an age of -615 Ma (1 grain), as well as Archean ages (5 grains, concordant at 2.6-2.8 Ga). It seems likely that the Archean grains represent assimilation of older crustal material. Zircons from the post-tectonic dolerite dike have a bimodal age distribution. A well-defined younger age of 281 ±9 Ma is interpreted to represent the crystallization age of the dike, while older, concordant ages of 2.6-2.9 Ga likely represent assimilation of Archean crust (Siberian craton at depth). Several important conclusions can be drawn from the data. (1) The mafic and felsic lithologies of the Kunar-Mod volcanic suite are genetically related and should be the same age. Ages of-630 Ma (meta-rhyolite) and -615 Ma (metagabbroic dikes representing the latest stage of mafic magmatism associated the Kunar-Mod suite) suggest that these lithologies may be the same age, but more data are required to confirm this hypothesis. (2) The 630 Ma two-mica granite is similar in age to the time of high-grade metamorphism, suggesting that syntectonic granite emplacement accompanied obduction of the accretionary Central Belt to the Siberian craton. (3) An Early Permian age is well defined for the undeformed dolerite dike. Dolerite dikes occur across the whole of Taimyr, but are deformed to the south. If related, this single magmatic event pre-dates Permo-Triassic Siberian trap magmatism. Furthermore, it suggests that deformation was localized to southeastern Taimyr.

Clay mineralogy of the Cenozoic Pagodroma Group, East Antarctica

The Cenozoic Pagodroma Group in the northern Prince Charles Mountains, East Antarctica, is a glaciomarine succession of fjordal character, comprising four uplifted formations of different ages. The composition of the <2 µm fraction of sediments of the Pagodroma Group was analysed in order to help identify source areas, past weathering conditions and glacial regimes. Both clay and non-clay minerals have been quantified. The assemblage of the upper Oligocene to lower Miocene Mount Johnston Formation is characterised by the dominance of illite and intermediate concentrations of chlorite. Similar to that assemblage is the clay mineral suite of the middle Miocene Fisher Bench Formation, where illite and chlorite together account for 95% of the clay minerals. The middle to upper Miocene Battye Glacier Formation is the only formation with significant and persistent smectite concentrations, although illite is still dominant. The kaolinite concentration is also high and is even higher than that of chlorite. The clay fraction of the upper Pliocene to lower Pleistocene Bardin Bluffs Formation is characterised by maximum kaolinite concentrations and relatively low illite and chlorite concentrations. The bulk of the clay fraction in each formation can be explained by the physical weathering and erosion of a nearby source under glacial conditions. In the case of Mount Johnston Formation and Fisher Bench Formation this source may be situated in the metavolcanic and gneissic rocks of Fisher Massif. The sediments of the Bardin Bluffs Formation indicate a local source within the Amery Oasis, where Proterozoic granitoid rocks and gneisses, and Permo-Triassic fluvial rocks of the Amery Group are exposed. These results suggest a strong local imprint on the glacial sediments as northwards flowing ice eroded the bedrock in these areas. The origin of the clay fraction of the Battye Glacier Formation is a matter of debate. The smectite and kaolinite content most easily can be explained by erosion of sources largely hidden beneath the ice upstream. Less likely, these clay minerals reflect climatic conditions that were much warmer and wetter than today, facilitating chemical weathering.