Trzeciorzędowe wulkany Graczy na Dolnym Śląsku i ich założenia uskokowe

Krzysztof Birkenmajer


Terciary volcanoes of Gracze, Lower Silesia, and inferred fault pattern

The paper describes geological structure of Tertiary melabasanites at Gracze, Lower Silesia, in the eastern extremity of the Bohemo-Silesian arc of Cenozoic Central European basaltic province (Figs. 1—3). The main exposures dealt with here are those at Gracze, main quarry (Figs. 4—6 ), Rutki (Figs. 7, 8 ), Gracze-,,Ameryka” (Figs. 9, 10) and Radoszowice (Figs. 11, 12). The Tertiary volcanics are here represented by melabasanite plugs, lava flows and pyroclastic rocks with fragments of Upper Cretaceous rocks. The country rocks are represented by marly clays and shales of Lower Senonian (Coniacian) age (see Alexandrowicz & Birkenmajer, 1973). In the absence of reliable radiometric (К-Ar) dating it has not been possible to determine the exact geological age of the volcanic activity at Gracze. The volcanism was here younger than the Coniacian fossiliferous sediments, but older than the Neogene fresh-water sediments (Miocene?, Pliocene?) which are known from the vicinity. All volcanic rocks at Gracze belong to the same palaeomagnetic epoch of normal polarity (N). Their palaeomagnetic pole (mean of 129 measurements) is located at 61.3°N— 121.7°E, which agrees with Tertiary palaeomagnetic poles of Europe (see Birkenmajer et al., 1973). Three stages of volcanic activity have been distinguished at Gracze. The first stage is represented by lava flows alternating with pyroclastic breccias, with fragments of Upper Cretaceous rocks. As follows from relatively small scatter of NRM directions the dips of the lava flows — 20—45°SE are primary; these allow us to reconstruct the form of a stratified cone (Fig. 14). The lava poured out of SW-NE-directed fault fissures transverse to the Odra fault (Fig. 13). The second stage of volcanic activity resulted in tephra eruptions along the same, SW-NE-directed fault fissures; this stage is represented by pyroclastic breccia with fragments of Upper Cretaceous rocks, which forms a N-S-directed vent cutting through the lava flows and pyroclastics of the first stage. In the third stage the already formed vents have been filled up with melabasanite lava; the resultant plugs cut through volcanic rocks of the first and second stages. The plugs are elongated SW-NE (Fig. 13) along the lines of the already mentioned faults transverse to the Odra fault. The NW-SE-directed Odra fault is supposed to be a Laramide structure (Obere, 1972). The SW-NE-directed faults displacing the Odra fault recognized in the present paper at Gracze, are thought to be coeval with Tertiary volcanic activity of the area. As follows from investigations of magnetic fraction of the Gracze melabasanites, the Curie points are 250—350°C for the single-phase titanomagnetite. This suggests the depth of the magmatic chamber of the Gracze volcanoes at 30—46 km, i.e. slightly below the Moho discontinuity (see Birkenmajer et al., 1973) which in the area of the Odra fault occurs at a depth of 30—32 km (Guterch et al., 1973).

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