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Occurrence of volcaniclastic rocks at Obora locality near Boskovice (Letovice Formation, Boskovice Graben)


Jakub Jirásek, Dalibor Matýsek, Martin Sivek, Karel Martínek

Geoscience Research Reports 52, 2019, pages 53–61
Map sheets: Boskovice (24-14)

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Published online: 15 April 2019

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The occurrence of volcaniclastic rocks was confirmed for the first time within the Letovice Formation (Asselian to Artinskian?) of the Boskovice Graben. Volcaniclastics are reported from the Obora locality near Boskovice, ca. 29 km N of Brno. Volcaniclastic admixture was found in samples collected in excavations in sections Obora I and II of the locality (GPS N 49° 27.567’ E 016° 36.143’, Fig. 1) that is outstanding for large number of Permian insects, and also other fossils like discosauriscid amphibians, actinopterygian fish, xenacanthid sharks, pseudestheriid conchostracans, and remarkable ichnoassemblages. Conifers and pteridosperms were also reported. Fifteen samples representing different lithologies (Fig. 2) were taken from the studied sections. Mineralogical composition (Table 1) shows the presence of smectite (assumed montmorillonite according to the PXRD and EDS results) in all samples, with maximum content of 33 wt.% in sample No. 4b. The majority (3/4 of all samples) of investigated samples also contain kaolinite (up to 13 wt.%). Quartz was identified in all samples, and its content varies between 4 to 18 wt.%. Minerals of the feldspar group are also ubiquitous. Most common is albite (5-34 wt.%) and then more basic plagioclase (3-13 wt.%), while K-feldspar (5-15 wt.%) was found to occur only at the base of the profile. Calcite is also very common. Limonite, several zircon populations (Fig. 4a,b), Ti-rich magnetite (Fig. 4c), prismatic apatite (Fig. 4d), and almandine garnet were identified in heavy mineral fraction of sample No. 4. Magnetic susceptibility varies between 1 and 442. 10?6 SI, which confirms the presence of ferromagnetic accessory minerals. Sample No. 4 (Fig. 3), considered to be almost purely of volcaniclastic origin, shows affinity to andesitic rocks, judging from both the TAS diagram after Middlemost (1994), and Zr/Ti vs. Nb/Y diagram after Pearce (1996). The REE distribution pattern (Fig. 5, Table 2) shows also a relationship with andesitic or basaltoid rocks. The Th/ Yb vs Ta/Yb ratios in discriminant diagram (after Gorton ? Schandl 2000) are on the boundary indicating a geotectonic environment of intraplate volcanism and volcanites originating in active continental margin. Also interesting is a relatively high content of As (25 ppm), Zr (129 ppm), and Th/U ratio 6.3/7.9 ppm (Table 2). There is a strong evidence for volcaniclastic origin reflected in the composition of clay components (probably altered glass) combined with the well-developed crystals of apatite and zircon. Sample No. 4 contains minimum terrigenous admixture (the lowest content of quartz, the highest content of feldspars), and can be considered to be a true tuff. Other samples from the studied sections are to a various degree “contaminated” by volcanic material coming from adjacent areas, and re-deposited in the basin lake, being actually tuffites (bentonites). This is the first reported occurrence of such rocks in the Boskovice Graben. These rocks indicate the late volcanic activity in the Variscan orogenic belt. It is believed that the deposition of volcanic ash might have affected the basin environment and its close surroundings biota that resulted in extraordinary rich fossil fauna. The tuff containing layer, due to its content of volcanogenic zircon and apatite, seems to be suitable for U-Pb dating.


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