| 11. |
- 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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| 12. |
- 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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| 13. |
- 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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| 14. |
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| 15. |
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| 16. |
- Singh, S., et al.
(författare)
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2.0 Ga Granite of the Lower Package of the Higher Himalayan Crystallines, Maglad Khad, Sutlej Valley, Himachal Pradesh
- 2006
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Ingår i: Journal of the Geological Society of India. - 0016-7622 .- 0974-6889. ; 67:3, s. 295-300
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Tidskriftsartikel (refereegranskat)abstract
- Along the Sutlej valley, the lower package of the Higher Himalayan Crystallines (HHC) exposes a small concordant to discordant intrusive grey granite-The Maglad Khad Granite, within garnet mica schist/banded gneiss of the Jeori Formation. This body is fine grained and foliated along the margins, whereas the central part is relatively undeformed. This body along with aplites and pegmatites intrudes the country rock during early to syn-D-1 deformation. This is later affected by the most pervasive D-2-deformation producing gneissosity within the granite. U-Pb dating of zircons by conventional isotopic dilution technique yield an upper intercept age of 2068 +/- 5 Ma (2 sigma) from 6 zircon-fractions with MSWD=0.93, constraining the age of crystallization in the basal parts of the HHC during Early Proterozoic as well as the constraining pre-Himalayan fabric development.
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| 17. |
- Hode Vuorinen, Jaana, 1974-
(författare)
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The Alnö alkaline and carbonatitic complex, east central Sweden - a petrogenetic study
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Alnö complex on the central Swedish east coast is composed of a main composite intrusion (the main intrusion) and four smaller satellite intrusions (Söråker, Sälskär, Långharsholmen and Båräng) distributed around the main intrusion on Alnö Island and on the mainland north of the island. The major rock types exposed within the complex are melilitolite, pyroxenite, ijolite series (melteigite-ijolite-urtite), nepheline syenite, carbonatite and alnöite dykes. Melilitolite is only exposed within the Söråker intrusion. The intrusive sequence is melilitolite → pyroxenite → ijolite series → nepheline syenite → carbonatite → alnöite.Mineralogical, whole rock geochemical and radiogenic isotope (Nd-Sr-Pb) studies of exposed rocks from the Alnö alkaline complex, east central Sweden, were performed in order to investigate the genetic relationships between the diverse rock-types, and to evaluate the contributions from mantle and crustal components in the genesis of the complex. Most analysed samples fall within the depleted quadrant in a eNd-eSr diagram, similar to carbonatites and alkaline silicate rocks from other complexes, indicating derivation of parental magma(s) from a source that had experienced time-integrated depletion in LIL elements. Contamination by local crust is indicated by Sr and Pb isotope data, but is geographically restricted to samples collected from the outer parts of the main intrusion and from satellite intrusions. This localized contamination is attributed to selective hydrothermal element leaching of surrounding bedrock during fenitization. Nd- and Sr-isotope data separates the carbonatites into two groups (group I and II), each related to a specific set of silicate rock types. The overlap of group II carbonatites with ijolite and nepheline syenite could indicate a common origin through liquid immiscibility but this hypothesis cannot be confirmed by trace element data because initial concentrations are obscured by fractionation processes. Interestingly, results from AFC-modelling suggest that production of ijolite residual magma requires addition of a small volume (2.4 %) of carbonatite component to the parental magma, whereas formation of nepheline syenite residuals requires removal of an almost equal amount of carbonatite (1.5 %) to yield a statistically significant result. AFC-modelling further suggests that the various silicate rock types exposed within the complex are related to the same parental olivine-melilitite magma through crystal fractionation of olivine, melilite, clinopyroxene, nepheline, Ti-andradite and minor phases. These results agree with compositional trends exhibited by clinopyroxene and Ti-andradite from the silicate rocks of the main intrusion, which suggests co-genesis of pyroxenite, ijolite series rocks and nepheline syenite. Production of ijolite-like residual liquids can be achieved by <40% fractionation whereas production of nepheline syenite residuals requires >80% fractionation.An investigation of the origin of silicate minerals in carbonatites suggest that most silicate minerals observed in the carbonatites on Alnö Island are derived from surrounding wall-rock and/or produced through corrosive interaction between carbonatite liquid and assimilated phases. This leads to ambiguities when addressing the possible genetic link between carbonatites and associated silicate rocks as occurrences of identical “liquidus” phases in inferred immiscible liquids may not actually be such.
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| 18. |
- Sultan, Lena, et al.
(författare)
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Proterozoic and Archaean ages of detrital zircon from the Palaeoproterozoic Västervik Basin, SE Sweden: Implications for provenance and timing of deposition
- 2005
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Ingår i: Gff. - 1103-5897. ; 127:Part 1, s. 17-24
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Tidskriftsartikel (refereegranskat)abstract
- Ages of detrital zircons, derived from Palaeoproterozoic metasedimentary rocks from Finland and Sweden are poorly represented in the presently exposed crust in the Baltic Shield. This study reports U-Pb ages of detrital zircons from the Svecofennian Vastervik Basin. 41 spots from 31 zircon crystals were dated using U-Pb geochronology at the NORDSIM ion microprobe in Stockholm. Most analyses are concordant and the zircon grains commonly display well-developed magmatic oscillatory zoning. The ages documented are: &SIM; 3.64 Ga, 3.03-2.95 Ga, 2.72-2.69 Ga, 2.12-1.87 Ga and 1.84 Ga. 75% of the grains are Palaeoproterozoic and 25% are Archaean. Ages gained from Proterozoic metasediments in Sweden, Finland, Svalbard, Greenland and Great Britain also report a large proportion of &SIM; 2.1-1.9 Ga and a smaller proportion of Archaean zircons with ages around 2.7 and 3 Ga. These age groups probably represent major crust forming events. The here presented results provide an estimate of the time of deposition in the Vastervik Basin for the time interval of 1882-1850 Ma, constrained by two concordant zircon analyses of 1872 &PLUSMN; 24 and 1870 &PLUSMN; 12 Ma, and the newly presented 1859 &PLUSMN; 9 Ma age for the Loftahammar granite that intrudes the metasedimentary succession in the north. The young detrital zircon age of 1837 &PLUSMN; 22 Ma might suggest that parts of the basin may be younger. As input of detrital grains may occur from several sources simultaneously (e.g. by rivers and by tidal currents from a marine source), the detrital grains were sampled from different depositional environments. Main fluvial sediment transport in the Vastervik Basin was from present north whereas the tidal sediment transport was from the present south. The age groups documented in the Vastervik Basin are poorly represented in the presently exposed crust in the Baltic Shield, but are represented in Sarmatia.
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