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- Söderlund, Pia, et al.
(författare)
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40Ar/39Ar biotite and hornblende geochronology from the Oskarshamn area, SE Sweden: Discerning multiple Proterozoic tectonothermal events
- 2008
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Ingår i: Geological Magazine. - 0016-7568. ; 145:6, s. 790-799
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Tidskriftsartikel (refereegranskat)abstract
- Twelve 40Ar-39Ar biotite and four hornblende age determinations have been carried out to constrain the cooling history of the Proterozoic bedrock near Oskarshamn, SE Sweden; an area identified as a possible site for long-term nuclear waste storage. The bedrock hosts 1.80 Ga granites, granodiorites, quartz monzodiorites and diorites/gabbros of the Transscandinavian Igneous Belt and two 1.45-1.44 Ga granite intrusions, the Götemar and Uthammar plutons. Biotite was selected from three surface samples, representing both the older rocks and the younger granites, and from three cored boreholes at nine different depth levels. Hornblende was extracted from samples at the top and bottom of one borehole and at two sub-surface levels of another borehole. Three age groups were distinguished: ≥ 1.62 Ga, 1.51-1.47 Ga and 1.43-1.42 Ga. In the first group, two hornblende analyses yielded ages of 1799±4 Ma and 1773±13 Ma, which indicate initial fast cooling after emplacement of the 1.80 Ga rocks of the Transscandinavian Igneous Belt. Two biotite ages of 1618±7 Ma and 1621±3 Ma are interpreted to date final cooling, through 300 °C, after the youngest suite (1.67 Ga) of the Transscandinavian Igneous Belt in southcentral Sweden. Seven biotite ages, in the range 1.51-1.47 Ga, are enigmatic to interpret but largely coincide with the cessation of widespread rapakivi magmatism in Fennoscandia and the initiation of the Danopolonian event. The 1.44-1.42 Ga hornblende and biotite ages reflect cooling after thermal heating from the 1.45-1.44 Ga Götemar and Uthammar plutons. Later events thermally affected the bedrock in the Oskarshamn area as indicated by a poorly defined biotite age of 928±6 Ma, and other disturbed 40Ar-39Ar ages of samples bordering a complex deformation zone.
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| 3. |
- Söderlund, Pia
(författare)
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40Ar/39Ar, AFT and (U-Th)/He thermochronologic implications for the low-temperature geological evolution in SE Sweden
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Oskarshamn and Forsmark areas in the Fennoscandian Shield, SE Sweden, have been chosen as potential sites for hosting highly radioactive nuclear waste. To evaluate their respective suitability, the geological history of the bedrock in these two areas has been investigated. This study has focused on the thermal evolution, from c. 500 °C to c. 50 °C. 40Ar-39Ar geochronology on hornblende, muscovite and biotite, was applied to determine cooling from c. 500 °C to c. 300 °C, whereas apatite fission-track (AFT) and (U-Th)/He thermochronology were used to determine the thermal evolution between c. 120 °C and c. 50 °C. The crystalline bedrock in the Forsmark area consists of c. 1.89-1.85 Ga meta-igneous rocks that form part of the Svecofennian orogen. 40Ar-39Ar muscovite ages indicate cooling through c. 350 °C between 1.76 and 1.71 Ga. Biotite ages from surface samples suggest that the present erosion surface cooled below c. 300 °C at 1.73-1.66 Ga. The results show that the area has remained at temperatures below 300 °C since c. 1.7 Ga. The cooling of the Forsmark area reflects either slow cooling after the tectonothermal activity during the Svecofennian orogeny, or uplift in response to far-field effects of 1.7 Ga orogenic activity further to the west (or a combination of these processes). The 1.80 Ga rocks at Oskarshamn belong to the Transscandinavian Igneous Belt (TIB). Younger, 1.45 Ga granites and c. 0.95 Ga dolerite dykes are also present in the area. 40Ar-39Ar hornblende ages indicate initial rapid cooling down to c. 500 °C after the emplacement of TIB rocks. Subsequent cooling through c. 300 °C initially occurred at 1.6 Ga (40Ar-39Ar biotite ages). A 1.51-1-47 Ga 40Ar-39Ar biotite age group reflects either incomplete resetting by younger granitic intrusions in the area or thermal activity related to either the youngest manifestations of rapakivi intrusions or to the initiation of the Danopolonian event in the south. A 1.43-1.42 Ga biotite age group represents cooling after intrusion of the c. 1.45 Ga granites. The (U-Th)/He and AFT data were obtained from borehole and surface samples. Although the uncorrected (U-Th)/He and the AFT borehole ages at Oskarshamn are identical, and similar age/depth trends from the two thermochronometers are present at Forsmark, the (U-Th)/He ages are older (or AFT ages younger) than expected with respect to the closure temperatures of the two systems. The systematic age shift may be controlled by α-recoil damage of the (U-Th)/He system in U-rich apatite, possibly in combination with radiation-enhanced track annealing in the AFT system, and by invalid FT-correction of the (U-Th)/He system due to a heterogeneous distribution of U in apatite. Thermal modelling of the AFT ages reveals similar thermal histories in the two areas. Complete track annealing indicates that the present ground surface was situated at a depth of >4 km prior to c. 200 Ma. Subsequent uplift started between Late Palaeozoic and Early Mesozoic time and waned c. 100 Ma ago. Uplift may have been related to transtensional tectonics in southernmost Sweden and/or extensive volcanic activity to the south and west. After the modification in the uplift rate, continued exhumation was considerably slower.
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| 4. |
- Söderlund, Pia, et al.
(författare)
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Biotite and muscovite 40Ar/39Ar geochronological constraints on the post-Svecofennian tectonothermal evolution, Forsmark site, central Sweden
- 2009
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Ingår i: International Journal of Earth Sciences. - : Springer Science and Business Media LLC. - 1437-3254 .- 1437-3262. ; 98:8, s. 1835-1851
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Tidskriftsartikel (refereegranskat)abstract
- In order to characterize the post-Svecofennian tectonothermal evolution of the Fennoscandian Shield, 40Ar-39Ar biotite and some 40Ar-39Ar muscovite geochronological data are reported from a total of 30 surface outcrop and 1000 m long borehole samples at Forsmark, central Sweden. The thirteen surface samples were collected across three branches of a major WNW to NW trending system of deformation zones, whereas the boreholes were drilled within a tectonic lens, in between two of these zones. The 40Ar-39Ar biotite ages indicate that the present erosion surface, in central Sweden, cooled below 300 °C at 1.73-1.66 Ga, and that the rocks could respond to stress by brittle deformation between 1.8 and 1.7 Ga. The variation in surface ages is suggested to be due to faulting along the large WNW to NW trending deformation zones, following the establishment of a sub-Cambrian peneplain. The minor variation of ages within a single crustal block may be due to disturbance along ENE to NNE trending fracture zones. Possible cooling paths, derived from 40Ar-39Ar hornblende, muscovite and biotite ages, were calculated for the time interval from 1.80 to 1.66 Ga, when the area cooled from 500 °C to 300 °C. Between 1.70 and 1.64 Ga, uplift rates of c. 22 m/m.y. were calculated from borehole 40Ar-39Ar biotite data. Tectonothermal histories, inferred from the combined cooling and uplift rates, are related to simple slow cooling after the Svecofennian orogeny, to crustal movement in response to far-field effects of c. 1.7 Ga orogenic activity further to the west or to a combination of these possibilities.
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| 5. |
- Söderlund, Pia, et al.
(författare)
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Extending the time range of apatite (U-Th)/He thermochronometry in slowly cooled terranes: Palaeozoic to Cenozoic exhumation history of southeast Sweden
- 2005
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Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 1385-013X .- 0012-821X. ; 239:3-4, s. 266-275
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Tidskriftsartikel (refereegranskat)abstract
- The use of (U-Th)/He thermochronometry in old slowly cooled terranes has long been debated. Hence, studies of natural setting are needed to verify the expected diffusivity behaviour of helium. Our results demonstrate that the (U-Th)/He method can be applicable to samples older than 200 Ma, and that available helium diffusion models can reproduce the results obtained from the age-vs.-depth trends. In addition, the results shed some light on the cross-validation with the apatite fission-track thermochronometer. (U-Th)/He thermochronometry on apatites has been applied to samples taken from drill cores in the Precambrian basement in southeast Sweden to unravel the time of exhumation of the uppermost 1-3 km of the crust. Much of the Phanerozoic exhumation history in Sweden is poorly constrained due to a large hiatus in the sediment record. Results from the drill cores yield He ages decreasing with increasing depth from similar to 270 Ma at the surface down to similar to 120 Ma at 1700 in depth. The age-vs.-depth trend shows an inflection point at similar to 1400 in depth, which coincides with the upper boundary of the present Helium Partial Retention Zone. A period of slow exhumation (similar to 17 m/my), probably linked to the isostatic uplift of the Caledonian foreland basin, is revealed by the upper section of the trend in the data. The results demonstrate that the area has not been reheated since the mid-Permian. This argues against the existence of a thick Mesozoic sedimentary cover in this region, which, if present at all, cannot have exceeded 100 in in thickness. Furthermore, the age-vs.-depth trend below the inflection point allows dating the cessation of exhumation at approximately 100 Ma. The area has had no significant amount of sedimentation, erosion or uplift since this time.
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