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Late Variscan deformation events in the Bardo Syncline revealed by biotite K-Ar dating of Ludlow-age tuffite (Holy Cross Mountains, Poland)

Wójcik Emil, Pacek Adrian, Pelc Andrzej

Geological Quarterly

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2021

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Vol. 65, No. 1

art. no.15

EN

In the Bardo Syncline (Holy Cross Mountains, Poland), two beds of tuffite (named the Niewachlów tuffites) have been discovered within greywackes of Ludlow (Silurian) age. K-Ar dating of the lower bed yielded an age of 292.8 ±4.0 Ma, while the upper bed provided an age of 341.9 ±4.3 Ma. The dates pertain to metamorphic alteration of the tuffite, related to reactivation of the Daleszyce Fault Zone. In this zone, biotite crystals present in the foliation zones were recrystallized and their K-Ar isotopic system was rejuvenated in a temperature range of ~270-300°C. The dates that were obtained define the ages of two tectono-metamorphic episodes and reflect the timing of displacements along the fault which, in turn, reflect two deformation phases during the Variscan Orogeny (Mississippian and Early Permian). Moreover the dates may be correlated with Late Variscan magmatic activity in the Holy Cross Mountains and adjacent areas. The deformation of the Daleszyce Fault Zone may be regarded as resulting from displacements of lithospheric blocks within the Trans European Suture Zone during the Late Variscan reconstruction of the Baltica palaeocontinent margin.

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K-Ar dating of bentonite diagenesis in accretionary-wedge turbidites : case study from Western Outer Carpathians

Świerczewska A., Paul Z., Banaś M., Tokarski A. K.

Annales Societatis Geologorum Poloniae

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2015

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Vol. 85, No. 1

177--185

EN

Sixteen bentonite layers of Eocene–Oligocene age were sampled at thirteen sites in accretionary-wedge turbidite sequences, in the three innermost nappes of the Western Outer Carpathians. K-Ar dating was carried out for five of these layers to obtain the maximum burial ages. All of the ages obtained are older than the stratigraphic ages of the host strata. This relationship is due to significant contamination of the bentonites with non-authigenic minerals. This contamination resulted from rapid sedimentation during synsedimentary folding, which is a common feature in accretionary wedges. It follows that the K-Ar dating of bentonite layers in the turbidite sequences of accretionary wedges should be largely restricted to the very distal facies of turbidites or to pelitic intercalations within the turbidites.

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K-Ar evidence for a Mesozoic thermal event overprinting burial diagenesis of the Carboniferous Upper Silesia Coal Basin

Banaś M., Bylina P., Środoń J.

Volumina Jurassica

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2006

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Vol. 4, no. 1

39-39

EN

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 investigated by x-ray powder diffraction (XRD) in order to select for dating the samples 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 Variscan tectonic burial illitization produced by thrusting (ca. 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 potassium). 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 unravel such histories. When a calibration using data from Neogene sedimentary basins is applied, vitrinite reflectance and % S in I-S indicate similar paleotemperatures of tectonic burial diagenesis in the USCB, but they produce very different estimates of the temperature of the thermal event.