CLAY-MINERALS AS RECORDS OF TEMPERATURE CONDITIONS AND DURATION OF THERMAL ANOMALIES IN THE PARIS BASIN, FRANCE

Upper Triassic sandy horizons in the Paris Basin were sampled at depth s ranging from outcrop in the northeast to 2,700 m in the centre of th e basin. The smallest clay sub-fractions (<0.2 mu m) from the deepest central samples consist mainly of illite and chlorite whose K-Ar age i s similar to 190 Ma. These minerals formed at a relatively high temper ature of 220-250 degrees C, as determined from the oxygen isotope frac tionation between authigenic illite and associated quartz overgrowths, but at a burial depth of only 500 m. In a nearby drillhole that cross cuts a fault zone reactivated during the 190 Ma event, illite is less well-crystallized and has a higher delta(18)O value suggesting differe nt physical and chemical conditions of formation during the same hydro thermal episode. Two other generations of illite-smectite mixed-layer clays formed in the same Triassic horizon: one at similar to 150 Ma an d the other at similar to 80 Ma. These younger clays have higher delta (18)O values and thus may have formed at somewhat lower temperatures. The delta(18)O values of the fluids from which the different illite-sm ectite mixed-layer minerals form range from +9 to +13.5 per mil (SMOW) . An Ar diffusion code was used to estimate, on the basis of the Ar lo ss of the clay-type material, the duration of these events. The result s suggest that the duration of the hydrothermal events at 190 Ma and 1 50 Ma were rather short, <1 Ma, whereas the youngest event was protrac ted over a much longer period of similar to 37 Ma. Comparison between K-Ar ages of the different mixed-layer minerals and sedimentation rate s of the sediments since the Palaeozoic shows significant acceleration s of the rates at similar to 190-200 Ma and 150 Ma, and a less importa nt one at similar to 80 Ma. These observations provide additional evid ence that the first two events were promoted by basement-related tecto no-thermal activities. The third event is considered to be of diagenet ic origin.