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- Paulsson, Oskar, et al.
(författare)
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A geochronological and geochemical study of rocks from Gjelsvikfjella, Dronning Maud Land, Antarctica - implications for Mesoproterozoic correlations and assembly of Gondwana
- 2003
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Ingår i: Precambrian Research. - 0301-9268. ; 125:1-2, s. 113-138
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Tidskriftsartikel (refereegranskat)abstract
- The Jutulsessen nunataks (72degrees00'S; 2degrees30'E), Gjelsvikfjella, Dronning Maud Land (DML), consist mainly of migmatites of two types. A heterogeneous banded amphibolite facies gneisses; and a more homogeneous part. In the more homogeneous part, partial melts form along axial planes to tight folds. Numerous pegmatitic dykes occur in both migmatites. The homogeneous part of the migmatite has a granodiorite composition. It displays the depletion of Nb-Ta typical for rocks from destructive plate margins and a strongly fractionated REE pattern, specially in LREE (La/Lu ratios varying between 500 and 800). SIMS dating of zircon from the homogeneous migmatite and two pegmatite dykes resulted in two age groups. A concordant age of 1163 +/- 6Ma is calculated from zircon crystals with no rim/core structure and from cores from structurally complex crystals. This age represents the age of the protolith of the migmatite. A Cambrian age of 504 +/- 6 Ma is obtained from zircon rims and from sector-zoned zircons. This age represent the time of migmatisation. Sm-Nd depleted mantle model ages range from 1390 to 1770 Ma and suggest that the protolith to the migmatites contained components of older crust (pre-1163 Ma). An igneous complex consisting of a syenite plug (Stabben syenite), gabbroic rocks and aptitic dykes intrudes the metamorphic complex. The syenite and the aplitic dykes are neither deformed nor migmatised or penetrated by pegmatitic dykes. These rocks have elevated LREE and LILE concentrations with an La/Lu ratio of 450 and an Nb-Ta trough. The gabbroic rocks range in composition from melagabbro to monzogabbro and host numerous pegmatitic dykes. SIMS zircon U-Pb data from the Stabben syenite give an age of 500 8 Ma. This age is regarded as the intrusive age of the Stabben syenite. By the single zircon-Pb evaporation method an age of 495 14 Ma is obtained from the aplitic dykes. Sm-Nd depleted mantle model ages between 1800 and 2220 Ma indicate that the dykes formed from a Paleoproterozoic source. A Mesoproterozoic volcanic arc setting of DML and a correlation with the Natal Province, as suggested by several authors, is supported by data in this study. The studied area has consequently been a part of the Kaapvaal/Kalahari craton since Mesoproterozoic time. The Cambrian migmatisation and the intrusions are interpreted as a result of post-collision activity related to the collision between the Kalahari craton and the combined block of Antarctica and Australia during the final assembly of Gondwana. This collision is suggested to be included in the Kuunga Orogeny introduced by Meerat and Van der Voo.
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| 2. |
- Paulsson, Oskar, et al.
(författare)
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Attempted break-up of Rodinia at 850 Ma: geochronological evidence from the Seve-Kalak Superterrane, Scandinavian Caledonides
- 2002
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Ingår i: Journal of the Geological Society. - 2041-479X. ; 159:6, s. 751-761
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Tidskriftsartikel (refereegranskat)abstract
- Lower thrust sheets of the Scandinavian Caledonides derive from the margin of Baltica, which was imbricated during Early Palaeozoic closure of oceans formed during separation of the Baltica, Laurentia and Siberia cratons. At Vistas (Kebnekaise Mts), the Seve Nappe Complex preserves rare lenses of a dolerite-intruded granite formed by anatexis during emplacement of gabbro into metasedimentary rocks. Ion microprobe U-Pb dating of prismatic zircons from the granite yielded an age of 845 +/- 14 Ma (mean square weighed deviation (MSWD) 1.15), interpreted to date magmatic crystallization. Cores of complex crystals indicate protolith ages of 1778 +/- 11 Ma (MSWD 0.86); one core yielded an Archaean age. From narrow rims, a 605 +/- 42 Ma metamorphic age is obtained, and is interpreted to reflect the emplacement of the extensive Vendian rift magmatic dolerite dyke swarm. The age pattern allows correlation with a previously dated magmatic complex within the equivalent Kalak Nappe Complex 300 km to the north. Bimodal magmatic complexes with this age pattern do not occur within the basement of the Caledonide foreland, nor elsewhere within the Baltic Shield. However, recent reinterpretations of the Knoydartian event in Scotland as rift related invites correlation with the West Highland Granite Gneiss, which intruded Moine metasedimentary rocks at c. 870 Ma. In Central Taimyr, 850-900 Ma granites of continental crustal derivation and with 1800-1900 Ma Sm-Nd model age occur associated with c. 740 Ma plagiogranites. Using recent palaeogeographical reconstructions, we interpret the evidence of 850-900 Ma magmatism, in Scandinavia, Scotland and Taimyr as bimodal rift magmatism in connected arms during an attempted break-up of Rodinia.
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| 3. |
- Paulsson, Oskar
(författare)
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U-Pb geochronology of tectonothermal events related to the Rodinia and Gondwana supercontinents -observations from Antarctica and Baltica
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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