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Sm-Nd, Rb-Sr and Pb-Pb dating of silicic carbonates from the early Archaean Barberton Greenstone Belt, South Africa: Evidence for post-depositional isotopic resetting at low temperature

Abstract:

Two silicic carbonate sequences occurring in the Onverwacht Group (OG) and the overlying Fig Tree Group (FTG) of the early Archaean Barberton Greenstone Belt, South Africa, contain silicified dolomites with primary structures and textures. They occur with volcanic rocks dated by single zircons. The associated Onverwacht felsic volcanics have an age of ~3440-3416 Ma, whereas the Fig Tree volcanics are ~3225 Ma old. Therefore, the depositional age of both sequences is between 3440 and 3225 Ma. Pb-isotopic data for different parts of a single OG whole rock sample form a linear array corresponding to a Pb-Pb age of 2632±99 Ma. Sm-Nd and Rb-Sr isotopes were analysed in four whole rock samples of the FTG silicified carbonates, and form an Sm-Nd array corresponding to an age of 2612±84 Ma with initial εNd=-8.1±0.5, and an Rb-Sr array of 2743±36 Ma with initial 87Sr/86Sr=0.7013±0.0003. These whole rock samples have Rb/Sr ratios of ~1 and unusually high Rb concentrations for carbonates (~100 ppm) which derive from processes accompanying silicification. All three isotopic systems of whole rocks indicate ages of ~2.7 Ga, much younger than the depositional age of the successions. The agreement provides strong evidence that an alteration event reset all of the isotopic systems (Sm-Nd as well as Rb-Sr and U-Pb). We suggest that the ages represent the timing of silicification and sericification of the carbonates by large-scale fluid movement at an apparent temperature of ~200°C.