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- 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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| 6. |
- Sjöström, Håkan, 1945-, et al.
(författare)
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Deformation Zones in Eastern Bergslagen (Uppland-Sörmland)
- 2001
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In this report we present results from a structural investigation of deformation zones in Eastern Bergslagen. Key areas for investigation have been the NNE-SSW Österbybruk/Skyttorp Zone (ÖSZ), the N-S Gimo Zone (GZ) and the Banded Series in the southeastern Stockholm archipelago.Both the ÖSZ and the GZ is here interpreted to be splays from the Singo Shear Zone (SSZ) system where they originally enveloped tectonic lenses oriented approximately parallel to the SSZ. Solidification of a c. 1.8 Ga granite within the SSZ possibly caused a strain distribution southwards from the SSZ leading to the large scale clock-wise rotation and development of Everging footwall folds to the SSZ. The E-verging fold have steep axes and rotate both the regional foliation as well as the stretching lineation.The ÖSZ and the GZ show E-up kinematics (with a sinistral horizontal component) related to the E-vergent folding. An earlier(?) W-up pattern along the western margin of ÖSZ is here interpreted to be related to a dextral/NE-down shear along the SSZ prior to the folding. The GZ located closer to the SSZ shows generally a higher metamorphic grade than the ÖSZ. Microstructures and chlorite-bearing shearbands of the ÖSZ indicate upper greenschist facies conditions during deformation, while striped gneisses and a C-S fabric with stable hornblende and biotite along both planes indicate amphibolite facies conditions along the GZ. The Banded Series (BS) is a high-T deformation zone exposed on several islands in the Stockholm archipelago (Uto-Orno-Morto-Runmaro-Skarprunmarn). The shear zone evolved from folds and has in turn been folded by regional-scale S-folds. The kinematic pattern iscomplex due to the existence of tectonic lenses, recrystallisation and annealing, and partly due to the later folding of the BS. The tectonic lenses contains hinges of rootless folds with generally NE plunging axes parallel to a very pronounced stretching lineation indicating intense stretching during shearing. Red Kf-augens common within the BS is here interpreted to be derived from dismembered early pegmatites, while white, less common augens were derived form neosomes. These conditions indicate that the BS was temporally related to c. 1816-1821 Ma pegmatites and peak metamorphism.Additional studies of the intensely folded Värmdölandet-Ingarö-Södertörn area has revealed that an important sinistral/W-up shear occurred coeval with an eastward to southeastward hinge escape of the E-W mainland folds. A regional approximately N-S shortening has probably caused both the hinge escape as well as a conjugate, co-existing shear system with a dextral/NE-down (SSZ) and sinistral/NW-up (BS) shear system.
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| 7. |
- Persson, katarina, et al.
(författare)
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Den dynamiska jorden
- 1999
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Ingår i: Ytor på djupet (Naturvetenskapliga forskningsrådets årsbok). - : Naturvetenskapliga forskningsrådet. ; , s. 39-51
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Bokkapitel (populärvet., debatt m.m.)
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