11:30 AM - 11:45 AM
[SGL39-10] U-Pb dating of zircons and measurement of organic carbon isotope in the sedimentary rocks from the BGB
Keywords:U-Pb Dating of Zircons, Measurement of Organic Carbon Isotope, Barberton Greenstone Belt
U-Pb dating of zircon is the most prevailing method of determining the depositional ages from the sedimentary rocks of Archean-Proterozoic in which the biostratigraphy has not been established. We collected 7 redsandstone samples from the Mapepe Formation and 5 intrusive ones from the Onverwacht Group, and were able to obtain zircon from only 1 sample each. U-Pb measurements of Zircons were conducted by LA-ICP-MS installed at the National Science Museum.
We measured two zircons; from the Mapepe Formation redsandstone and the Onverwacht Group intrusive rock. Zircons from the Mapepe Formation has 29 concordant ages and a peak in age about 2954±9Ma〜3546±13Ma. There are more than a few zircons younger than this age, and it is considered that there are two possibilities; the zircons reflect the depositional age of the area, and a possibility of Pb defect. In any case, these data are in contradiction with the previous research (Lowe D. R. et al., 1999), so that has quite a bit of room for improvement such as increasing the number of measured zircons to discuss statistically. Zircons from the Onverwacht Group has 6 concordant ages and indicates 3270±14Ma. This data is consistent with the previous research (Lowe D. R. et al., 1999)
Green shale from the Mapepe formation showed at an average of 0.72wt% of amount of organic carbon. Black shale, green shale, and jasper from the Mapepe formation showed between -30.6~-20.3‰, which suggests a kind of possibility of biological activities.