| 1. |
- Claesson, Stefan, 1950-, et al.
(författare)
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The oldest crust in the Ukrainian Shield - Eoarchaean U-Pb ages and Hf-Nd constraints from enderbites and metasediments
- 2014
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Ingår i: Continent Formation Through Time. - London : The Geological Society Publishing House.
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Bokkapitel (refereegranskat)abstract
- The oldest crust in the Ukrainian Shield occurs in the Podolian and Azov domains which both include Eoarchaeanarchaean components. U-Pb age data for Dniestr-Bug enderbites, Podolian Domain, indicate these are ca. 3.75 Ga old, and Lu-Hf isotope date indicate extraction from chondritic to mildly isotopically depleted sources with εHf up to ca. +2. Nd model ages support their Eoarchaeanarchaean age, while model ages for Dniestr-Bug metasedimentary gneisses indicate that these also include younger crustal material. Most of the Hf-age data for metasedimentary zircon from the Soroki greenstone belt, Azov Domain, reflects Eoarchaeanarchaean primary crustal sources with chondritic to mildly depleted Hf isotope signatures at 3.75 Ga. A minor portion is derived from Mesoarchaeanarchaean crust with a depleted εHf signature of ca. +4 at 3.1 Ga. U-Pb zircon ages from Fedorivka greenstone belt metasediments are consistent with the Soroki age data, but also include a 2.7‒2.9 Ga component. Nd whole rock model ages provide support for a younger crustal component in the latter. Both domains have been subject to Neoarchaeanarchaean, ca. 2.8 Ga, and Palaeoproterozoic, ca. 2.0 Ga metamorphism. The spatial distribution indicates that the Podolian and Azov domains have evolved independently of each other before the amalgamation of the Ukrainian Shield.
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| 2. |
- Högdahl, Karin, et al.
(författare)
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Reactive monazite and robust zircon growth in diatexitesand leucogranites from a hot, slowly cooled orogen : implicationsfor the Palaeoproterozoic tectonic evolution of the central Fennoscandian Shield, Sweden
- 2012
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 163:1, s. 167-188
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Tidskriftsartikel (refereegranskat)abstract
- Monazite in melt-producing, poly-metamorphic terranes can grow, dissolve or reprecipitate at different stages during orogenic evolution particularly in hot, slowly cooling orogens such as the Svecofennian. Owing to the high heat flow in such orogens, small variations in pressure, temperature or deformation intensity may promote a mineral reaction. Monazite in diatexites and leucogranites from two Svecofennian domains yields older, coeval and younger U–Pb SIMS and EMP ages than zircon from the same rock. As zircon precipitated during the melt-bearing stage, its U–Pb ages reflect the timing of peak metamorphism, which is associated with partial melting and leucogranite formation. In one of the domains, the Granite and Diatexite Belt, zircon ages range between 1.87 and 1.86 Ga, whereas monazite yields two distinct double peaks at 1.87–1.86 and 1.82–1.80 Ga. The younger double peak is related to monazite growth or reprecipitation during subsolidus conditions associated with deformation along late-orogenic shear zones. Magmatic monazite in leucogranite records systematic variations in composition and age during growth that can be directly linked to Th/U ratios and preferential growth sites of zircon, reflecting the transition from melt to melt crystallisation of the magma. In the adjacent Ljusdal Domain, peak metamorphism in amphibolite facies occurred at 1.83–1.82 Ga as given by both zircon and monazite chronology. Pre-partial melting, 1.85 Ga contact metamorphic monazite is preserved, in spite of the high-grade overprint. By combining structural analysis, petrography and monazite and zircon geochronology, a metamorphic terrane boundary has been identified. It is concluded that the boundary formed by crustal shortening accommodated by major thrusting.
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| 3. |
- Majka, Jaroslaw, et al.
(författare)
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Multiple monazite growth in the Areskutan migmatite: evidence for a polymetamorphic Late Ordovician to Late Silurian evolution in the Seve Nappe Complex of west-central Jamtland, Sweden
- 2012
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Ingår i: Journal of Geosciences. - : Czech Geological Society. - 1802-6222 .- 1803-1943. ; 57:1, s. 3-23
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Tidskriftsartikel (refereegranskat)abstract
- Monazite from granulite-facies rocks of the angstrom reskutan Nappe in the Scandinavian Caledonides (Seve Nappe Complex, Sweden) was dated using in-situ U-Th-total Pb chemical geochronology (CHIME). Multi-spot analyses of a non-sheared migmatite neosome yielded an age of 439 +/- 3 Ma, whereas a sheared migmatite gave 433 +/- 3 Ma (2 sigma). Although the obtained dates are rather similar, a continuous array of single dates from c. 400 Ma to c. 500 Ma suggests possibly a more complex monazite age pattern in the studied rocks. The grouping and recalculation of the obtained results in respect to Y-Th-U systematics and microtextural context allowed distinguishing several different populations of monazite grains/growth zones. In the migmatite neosome, low-Th and low-Y domains dated at 455 +/- 11 Ma are considered to have grown under high-grade sub-solidus conditions, most likely during a progressive burial metamorphic event. The monazites with higher Th and lower Y yielded an age of 439 +/- 4 Ma marking the subsequent partial melting event caused by decompression. The youngest (423 +/- 13 Ma) Y-enriched monazite reveals features of fluid-assisted growth and is interpreted to date the emplacement of the Areskutan onto the Lower Seve Nappe. In the sheared migmatite, the high-Th and low-U (high Th/U) monazite with variable Y contents yielded an age of 438 +/- 4 Ma, which is interpreted to date the partial melting event. Relatively U-rich rims on some of the monazite grains again reveal features of fluid-assisted growth, and thus their age of 424 +/- 6 Ma is interpreted as timing of the nappes emplacement. These results call, however, for further more precise, isotopic (preferably ion microprobe) dating of monazite in the studied rocks.
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| 4. |
- Be'eri-Shlevin, Yaron, et al.
(författare)
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Provenance of Neoproterozoic sediments in the Sarv nappes (Middle Allochthon) of the Scandinavian Caledonides : LA-ICP-MS and SIMS U-Pb dating of detrital zircons
- 2011
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 187:1-2, s. 181-200
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Tidskriftsartikel (refereegranskat)abstract
- We present U-Pb age data for detrital zircons from dike-intruded Neoproterozoic sedimentary rocks of the Caledonian Middle Allochthon in central Sweden and Norway. Detrital zircons from 11 samples from the Sarv, Saetra and upper Leksdal nappes (informally referred to as the Sarv nappes) are clustered within ca. 0.9-1.75 Ga, but display a bimodal distribution with major ca. 1.45-1.75 Ga and ca. 0.9-1.2 Ga components. An apparent increase of younger (0.9-1.2 Ga) components to the northwest reflects varying source terranes. Detrital zircons from an additional sample from the lower part of the Leksdal Nappe, of uncertain affiliation to the Sarv has a prominent 1.75-1.85 Ga component supporting previous suggestions that this part of the nappe belonged to a more proximal basin. Comparison of the Sarv age probability patterns with data from basement windows and basement slices within the Middle Allochthon in central Sweden and Norway supports the derivation of the sediments from the attenuated Baltican continental crust on which they were presumably deposited. Similar comparisons suggest that derivation from the southern segment of the Fennoscandian Shield or from eastern segments of Laurentia is less likely, mostly because they include also older components. We infer that the ca. 200 km wide belt of attenuated Baltican continental crust included northern extensions of Mesoproterozoic to early Neoproterozoic terranes exposed in the southern part of the Fennoscandian Shield and the easternmost part of Laurentia, which at ca. 900 Ma were still adjacent. Pre-1.75 Ga terranes of the Fennoscandian Shield were probably isolated from the Sarv distal basin(s) by intracratonic basins and uplifted margins associated with early development of this extended continental crust. The significantly older ages in the lower part of the Leksdal Nappe and its inferred more proximal position support this model. The proposed northern extension of Mesoproterozoic-early Neoproterozoic terranes can explain in a simpler way the occurrence of such detritus in many Caledonide-Appalachian allochthons exposed at the margins of the North Atlantic, with no need to infer large displacement along the axis of the Caledonide Orogen or to postulate selective transport of Grenville-age material from the south over large distances.One of our Sarv samples located at the Norwegian coast revealed Caledonian reworking at ca. 395 Ma. This age agrees with ages of late-tectonic amphibolite-facies metamorphism and pegmatite intrusion recorded in this part of the Caledonides.
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