40Ar/39Ar, AFT and (U-Th)/He thermochronologic implications for the low-temperature geological evolution in SE Sweden

Sammanfattning: 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.