K-Ar dating of mixed-layer illite-smectite from clay fractions extracted from pyroclastic horizons was used to address the controversy about the age and mechanism of the thermal alteration of Carboniferous rocks from the Upper Silesia Coal Basin (USCB). The clay fractions were also investigated by X-ray diffraction in order to select for dating samples possibly rich in illite-smectite, and to evaluate the K-Ar dates for possible contamination by K-bearing pre-diagenetic minerals, of pyroclastic or epiclastic origin.

The K-Ar dates document intense Variscian tectonic burial illitization produced by thrusting (~290 Ma) in the SW of the basin, and the lack of intense burial illitization in the NE, which is consistent with sedimentological models of the basin. The burial illitization in its final phase (<30%S in illite-smectite) involved incorporation of measurable amounts of ammonium cation in the illite structure (substitution for K).

Advanced illitization in the NE of the basin is much younger than its tectonic inversion (uplift and erosion started in Permian), and the corresponding K-Ar dates have to be interpreted as the result of a Mesozoic thermal event, which produced widespread pervasive illitization at shallow depth. This event was dated at 175 Ma, but it may have started earlier and could have lasted longer. This conclusion is consistent with widespread evidence of a major Mesozoic thermal event all over Central Europe, produced by rifting and lithospheric thinning during the opening of the Tethys and Atlantic oceans.

This study demonstrates that smectite illitization histories may be very complex, and that the nature of the illitization mechanism results in mixed K-Ar dates encompassing pro-longed or multiple illitization histories. Dating of several grain-size fractions may help to unravel such histories.

When a calibration using data from Neogene sedimentary basins is applied, vitrinite reflectance and %S in I-S indicate similar palaeotemperatures of tectonic burial diagenesis in the USCB, but they produce very different estimates of the temperature of the thermal event.

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