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

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