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Structural and U–Pb isotopic study of late Archaean migmatitic gneisses of the Presvecokarelides, Lylyvaara, eastern Finland
- Erkki J. Luukkonen
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- Journal:
- Transactions of the Royal Society of Edinburgh: Earth Sciences / Volume 76 / Issue 4 / 1985
- Published online by Cambridge University Press:
- 03 November 2011, pp. 401-410
- Print publication:
- 1985
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The migmatitic gneiss complex of Lylyvaara in the eastern part of Finnish Presvecokarelides of the Baltic Shield shows evidence of a polyphase deformational and metamorphic history and of the emplacement of a number of mafic and felsic igneous intrusions at various stages during this history. Sequential structural development has been established on the bases of refolding and cross-cutting relationships. U–Pb zircon and sphene isotopic data combined with structural studies indicate that the first six deformational phases took place in late Archaean (=Presvecokarelian) times. The seventh deformational phase is constrained as being early Proterozoic (=Svecokarelian) from regional considerations.
The gneissic foliation in the dominant tonalitic to trondhjemitic palaeosome is parallel to lithological layering (So). Mostly it is composite S1–S2; only in F2 fold hinges can separate S1 and S2 be unequivocally distinguished. There, both of these fabrics, which were formed in amphibolite facies conditions of metamorphism during D1 and D2 have retained their identity despite extensive tectonic overprinting. Further tonalitic or granodioritic material was intruded during D3 or between D2 and D3. Effects of the third deformational phase (D3) ar6e expressed only locally as asymmetrical and polyclinal folds, which deform S1–S2 and F2. These folds now have a northeasterly axial trend and they show considerable variations in the style of their parasitic structures. F4 folds are common. They are dextral and asymmetrical, have NW—NNW-trending axes and show complex interference patterns with F2 and F3 folds. During D4, much aplogranitic neosome material was emplaced in NW—SE-trending movement zones, which correspond to the axial planes of F4 folds. Superimposition of F5 and F6 structures on previously formed patterns add to the structural complexity although they only result in minor modifications. Both are open and upright and locally have associated cleavages or healed fractures (S5, S6). D7 is expressed throughout the migmatitic complex as narrow NW—SE-trending shear zones which reactivate the S4 trend.
U–Pb zircon isotopic data indicate that the metamorphism associated with gneiss formation took place 2843 ± 18 Ma ago. U–Pb sphene ages of c. 2660 Ma and 2620 Ma indicate that metamorphic conditions prevailed for a very considerable time. An aplogranitic neosome related to F4 axial planes gave a 2657 ± 32 Ma U–Pb zircon age, while granodiorite and pegmatite dykes related to D6 yielded U–Pb zircon ages of c 2670 Ma and 2640 Ma, respectively.
Geochronological studies of the Bohemian massif, Czechoslovakia, and their significance in the evolution of Central Europe
- O. van Breemen, M. Aftalion, D. R. Bowes, A. Dudek, Z. Mísař, P. Povondra, S. Vrána
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- Journal:
- Transactions of the Royal Society of Edinburgh: Earth Sciences / Volume 73 / Issue 2 / 1982
- Published online by Cambridge University Press:
- 03 November 2011, pp. 89-108
- Print publication:
- 1982
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U–Pb zircon and Rb–Sr whole-rock analyses from various gneisses and plutonie rocks of the Moldanubian and Moravo-Silesian zones and the stable foreland of the Hercynian (Variscan) orogenic belt indicate that most of the crust in Central Europe was first formed during the Cadomian orogeny which straddles the Precambrian–Cambrian boundary. Zircons, however, have a memory of older ages which correspond with those of events known in Fennoscandia. The new radiometrie data are consistent with the stratigraphie record in that they do not provide any evidence for a major early Palaeozoic tectonothermal event between the Cadomian and Hercynian orogenies.
Granulites from two localities in the Moldanubian zone yield U–Pb zircon ages of 345 ± 5 Ma; discordant zircon data points indicate that the granulite facies metamorphism was not of long duration. Tectonic units containing these high grade rocks were emplaced amongst amphibolite facies rocks during an event of widespread shearing which has been dated at 341 ± 4 Ma on the basis of a lower U–Pb zircon intercept age from one of the sheared gneisses and 338 ± 3 Ma U–Pb ages from monazites. Rb–Sr muscovite ages of 331 ± 5 Ma from pegmatites axial planar to asymmetrical folds date the last stage of SE-directed simple shear. A Rb–Sr whole-rock isochron of 331 ± 4 Ma from a principal magmatic type of the Central Bohemian pluton confirms the field evidence that the large NE-trending plutons of the Moldanubian zone were emplaced during a late stage of the deformation. The strong disturbance of the U–Pb zircon isotopic system in the sheared gneisses suggests U loss while a high U/Th ratio in monazite from one of these tectonised rocks suggests the simultaneous passage of hydrothermal fluids. Thus a crustal source is indicated for the uranium deposits of the Moldanubian zone.
Critical to any plate tectonic model for the development of the Middle European Hercynides was the existence of an ocean in Early Devonian times which separated a North European continent from a South European continent(s). The northward movement of the South European continent over a shallowly-dipping subduction zone and subsequent continental collision can explain the high T–low P metamorphism and the imbricated tectonic style of the Moldanubian zone and adjacent Moravo-Silesian zone along the southeastern Hercynian foreland. The temporal separation of granulites and granites implies distinct conditions of formation and it has been suggested that the plutonism, following on from the imbrication of the Cadomian crust, was initiated by the subduction of wet oceanic sediments.