THERMAL HISTORY OF LOWER PALAEOZOIC ROCKS FROM THE EAST EUROPEAN PLATFORM MARGIN OF POLAND BASED ON K-Ar AGE DATING AND ILLITE-SMECTITE PALAEOTHERMOMETRY

摘要

Large-scale shale gas prospecting in the Polish part of the East European Platform did not discover large reserves of this resources. The article presents new research indicating that one of the reasons for the lack of shale gas relates to the thermal history of the Lower Palaeozoic rocks. Illite-smectite palaeothermometry was used to reconstruct the history of the platform and determine the maximum temperatures to which these rocks were subjected. The age of illitisation was also constrained using the K-Ar method. This method allowed precise dating of the maximum age of thermal transformations due to the deposition of numerous pyroclastic horizons (K-bentonite) throughout the entire geological profile from the Cambrian to the Silurian. Isotopic dating was made on over 53 samples of Lower Palaeozoic bentonites and low-grade metamorphic clays. These results were supplemented by analysis of the degree of thermal (smectite to illite) transformation in the profiles of 37 deep boreholes. 11 zones could be distinguished with different tectonic histories within the Polish part of the East European Platform edge. Maximum heating occurred in this region at about 320–340 Ma, corresponding to the Early Carboniferous or the turn of the Early and Late Carboniferous, phase A of the Variscan orogeny, known as the Sudetian phase. In the southern part of study area, the maximum of thermodiagenesis is slightly younger – 270–290 Ma, which responds to the Early Permian, the Asturian phase, the last phase of the Variscan orogeny. This means that the generation of hydrocarbons occurred before significant Mesozoic exhumation of the Polish part of the East European Platform, which led to the escape of a considerable amount of the gas generated. The study also presents the results of an interlaboratory comparison of illite age dating using the K-Ar and Ar-Ar methods. The comparison was conducted to find out what realistic error should be considered when interpreting geological K-Ar dating results.