A reappraisal of K-Ar and new U-Pb age data for felsic rocks in the vicinity of the Kraków-Lubliniec Fault Zone (southern Poland)

Abstract

New U-Pb zircon SHRIMP determinations from magmatic silicic rocks adjacent to the NE part of the Upper Silesia Coal Basin (USB) are provided and compared with earlier data obtained by K-Ar dating. The zircon samples studied came from the same boreholes and depth intervals previously sampled for K-Ar datings. Recalculations of the earlier K-Ar values for biotites yielded isochron  ages in the range of 297–309 Ma. The zircon U-Pb SHRIMP values gave more precise ages between 300.1 ±1.2 Ma and 292.6 ±1.3 Ma (1σ) consistent with U-Pb and Re-Os determinations. In two cases, the recalculated results of K-Ar dates were nearly identical with those obtained from the SHRIMP isotopic analyses. In this context, the granitoids and porphyries of the USB area, once assigned to the Carboniferous-Permian based on the older K-Ar studies, actually are of the same age as other felsic igneous rocks from the vicinity of the Kraków-Lubliniec Fault Zone, with a higher dating precision, however. The new data underline the importance of Late Carboniferous- Early Permian felsic magmatism at the SW margin of the Trans-European Suture Zone in southern Poland. In the zircon populations dated, besides the dominant Late Paleozoic ages, there are some inherited zircon cores, likely remnant detrital grains from a sediment component in the source rocks, that reveal much older ages which range from ~2051 to 569 Ma.