U-Pb geochronology of brittle deformation using LA-ICP-MS imaging on calcite veins

Abstract: Absolute age dating of a brittle deformation using radiometric techniques is often problematic. Unlike ductile deformation, suitable minerals with high parent/daughter and low common Pb are rare. Nevertheless, U-Pb dating of carbonate minerals has become one of the most conceivable geochronometers despite low U and high initial Pb in carbonates. Carbonates can be formed in wide range of environments and are a typical fracture-filling minerals in rocks affected by brittle deformation. U-Pb dating technique using Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS) has been utilized with some success for near a decade. However, challenges remained, and a new method (LA-ICP-MS imaging U-Pb dating) is developed recently which enables surface scan of larger analytical area to increase the possibil-ity of getting sufficient spread in U-Pb ratios. The main aim of this research project was to test the LA-ICP-MS imaging technique on fracture filling calcite minerals. Samples were collected at the Dalby and Skrylle quarries which are located along the Sorgenfrei Tornquist Zone (STZ) of Scania in southernmost Sweden. Linear surface scans were conducted on selected rectangular areas (analytical areas) of chosen samples across relatively less altered and homogeneous zones based on petrographic characterization. Age dating calculations were done based on selection and pooling of data points using certain criteria. Age dating results of calcite in the Skrylle quarry are 222 ± 46 and 213 ± 29 Ma. Dated calcite from the Dalby quarry yielded 495 ± 29, 553 ± 10, and 505 ± 9 Ma. The results show that combining petrography and LA-ICP-MS imaging U-Pb analyses is a promising approach to acquire plausible absolute ages of car-bonates and hence brittle deformation. On the assumption that the ages are accurate, and that calcite is formed at the instant of fracturing, these U-Pb ages are the first that directly dates brittle deformation along the STZ. Given that, age dating results of both localities imply timing of faulting related to STZ, it can be deduced that the STZ is not the result of single tectonic event of brittle deformation, but a pro-longed tectonic history in agreement with previous research of the STZ. However, the geological signifi-cance of the age dating results cannot be taken as unequivocal conclusive and further study is recom-mended.