K-AR DATING OF A MESOZOIC HYDROTHERMAL ACTIVITY IN CARBONIFEROUS TO TRIASSIC CLAY-MINERALS OF NORTHERN SWITZERLAND

Clay fractions of 0.2 mu m up to 2-6 mu m size from Upper Triassic (Ke uper series at Frick) and Upper Carboniferous sediments (Stephanian st rata at Weiach, NAGRA drillhole), both in northern Switzerland, were a nalyzed for their K-Ar ages in order to trace their post-depositional history and to date a hypothetical hydrothermal overprint. The latter had to be assumed because of published K-Ar and Rb-Sr data that are by up to 200 million years younger than the age of deposition in sedimen ts or metamorphism in crystalline rocks, respectively, of the same reg ion. The K-Ar ages from an Upper Triassic marl, a sandstone and a shal e are all younger than their depositional age. They do not indicate an y hydrothermal overprint, but most likely indicate diffusive Ar loss f rom poorly organized sheet silicates. Siltstones from the Weiach boreh ole (Upper Carboniferous from the North Swiss Permocarboniferous Troug h, NPT) reflect old detrital components and a post-depositional Ar los s. Three tuff samples interlayered with the siltstones define an age o f 183 +/- 5 Ma for a penetrative hydrothermal overprint causing comple te illitization of the tuffs and extensive cementation of secondary po re space of the siltstones, partly by replacement of precursor clays ( kaolinite). The three siltstone samples were differentially overprinte d by the fluids, according to their depth in the borehole: The deepest sample shows a nearly size-independent age distribution with a mean v alue around the proposed age of hydrothermal activity of 183 Ma. Sampl es higher up in the column are more strongly dominated by their detrit al components, exhibiting the typical correlation between apparent age and grain size. Hump-shape type age patterns characterize samples tha t contain abundant aggregates of small sized particles in their coarse size fractions. A model is proposed, which infers fluid migration in the deeper part of the NPT (1700 to 2050 meters) along subhorizontal s hear zones, causing highly variable illite/kaolinite ratios depending on the fluid/rock ratio. These fluids may have introduced enough addit ional heat to create an impact on thermal indicators used for basin mo delling, such as porosity, illite/smectite ratios, fluid inclusions an d the degree of maturation of the organic material (vitrinite reflexio n).