Apatite fission track and (U-Th)/He thermochronology from the Archean Tanzania Craton: Contributions to cooling histories of Tanzanian basement rocks

Borehole and surface samples from the Archean Tanzania Craton were analysed for apatite fission track (AFT) and (U-Th)/He data with the aim of deciphering cooling histories of the basement rocks. Fission track dates from borehole and outcrop samples are Carboniferous–Permian (345 ± 33.3 Ma to 271 ± 31.7 Ma) whereas (U-Th)/He dates are Carboniferous–Triassic (336 ± 45.8 Ma to 213 ± 29 Ma) for outcrop grains and are consistently younger than corresponding AFT dates. Single grain (U-Th)/He dates from the borehole are likely to be flawed by excessive helium implantation due to their very low effective uranium contents, radiation damage and grain sizes. All AFT and (U-Th)/He dates are significantly younger than the stratigraphic ages of their host rocks, implying that the samples have experienced Phanerozoic elevated paleo-temperatures. Considerations of the data indicate removal of up to 9 km overburden since the Palaeozoic.Thermal modelling reveals a protracted rapid cooling event commencing during the early Carboniferous (ca. 350 Ma) at rates of 46 m/Ma ending in the Triassic (ca. 220 Ma). The model also suggests minor cooling during the Cretaceous of the samples to surface temperatures. The suggested later cooling event remains to be tested. The major cooling phase during the Carboniferous is interpreted to be associated with compressional tectonics during the Variscan Orogeny sensu far field induced stresses. Coeval sedimentation in the Karoo basins in the region suggests that most of the cooling of cratonic rocks during the Carboniferous was associated with denudation.