Badania geochronologiczne K - Ar skał magmowych północno-wschodniego obrzeżenia Górnośląskiego Zagłębia Węglowego

Abstract

Metodą wolumetryczną wykonano 15 oznaczeń K-Ar "porfirów" i granitoidów rejonu Myszkowa i Mrzygłodu (północno-wschodnie obrzeżenie GZW). Dokonano próby interpretacji poszczególnych wyników, zarówno ostatnich, jak i archiwalnych, przeliczonych według nowych stałych. Zastosowanie metody izochronowej do kompleksowego opracowania danych pozwoliło na ujednolicenie rozbieżnych wartości. Dla rozpatrywanych rodzajów skał uzyskano wynik około 300 mln lat.

K-Ar DATINGS OF IGNEOUS ROCKS FROM NE MARGIN OF THE UPPER SILESIAN COAL BASIN

North-eastern margin of the Upper Silesian Coal Basin has been studied by several authors for many years. Results of drillings made in that area were presented in numerous reports, in which they were also given various conclusions concerning origin and age of igneous rocks.

At NE margin of the basin. magmatism is varying in character from acid (porphyries and micro-granites) to basic (lamprophyries and diabases). Igneous rocks are found among strata ranging in age from the Precambrian to Carboniferous (Silurian in the case of the studied Myszków-Mrzygłód area) so their Stratigraphic position and mutual relations have been the subject of discussions. Table 1 shows some earlier views on age of igneous rocks from NE margin of the basin. According to the majority of authors, the rocks are related to one of three following Variscan phases: Sudetic, Erzgebirgian, or Asturian.

The paper presents results of datings of igneous rocks from the above mentioned area, made using volumetric modification of the K-Ar method. Age calculations were made following the P. Bussiere et al. (1968) tables, and recalculations -following B. Dalrymple (1979) and using newer constants l_x = 0.581 • 10^-10/yr. Tables 2 and 3 show the datings obtained by J. Lis and H. Sylwestrzak (1976, 1978) - Table 2, and the latest ones - Table 3. The former represent the whole rock or, sometimes, biotite ages obtained for material from three regions: Myszków-Mrzygłód, Zawiercie. and Pilica, and the latter - biotite ages for material from the first of the regions.

The data available for regions other than the Myszków - Mrzygłód seem insufficient in number so attempt to compare them was limited to that region only. The samples covered by the datings came from the boreholes PZ-1, PZ-5, A-3, A-10, and PZ-10. The studied material often displays changes typical for early stage of biotite chloritization or cooccurrence of biotite and chlorite, which results in relative lowering of apparent age values.

Figure 1 (histogram) shows distribution of the obtained datings. The values fall within the interval 310-360 myr. So wide discrepancy of ages appears difficult to explain but, however, the foltlowing interpretation may be proposed.

Igneous rocks of the two above mentioned types are of Caledonian age, heterogeneously reset during Variscan orogeny. It would follow that the K-Ar system remained unclosed and the obtained ages correspond to the time of i~ closure after the reopening. A hypothesis of Caledonian age of the granodiorite magmatism in that area was put forward in the past but it should be noted that porphyrite from Huta Stara, regarded as coeval with dacite porphyry from the Myszków-Mrzygłód region, occurs among Carboniferous rocks. This speaks against both the above hypothesis and the interpretation proposed here. That is why it appears necessary to interpret the obtained results in a different way.

Figure 2 shows experimental data plotted in a system: weight content of argonium versus weight content of potassium, and with reference to the type of rock and its location. Both the published and newly obtained data are shown. Distribution of the experimental points appears clearly linear, with high correlation factor (r = 0.95): This suggests that it is possible to use isochrone method in interpretation of age of samples (see T. Depciuch, J. Lis, 1971).

Figures 3 - 4 show estimations of regression line for determinations of contents of potassium and argonium and their sum for porphyries and granitoids from the Myszków - Mrzygłód area.

Slope of isochrone line corresponds to age of a sample. Straight lines were estimated using the least square regression method (see also Figs. 3 -5). Results of estimations of regression line were as follows: for granitoids and porphyrcs y = 20.09 • 10^-7 x + 170 • 10^-9, for porphyries y = 22.67 • 10^-7 x + 89 • 10^-9; and for granitoids y = 23.17·• 10^-7 x + 78 •·10^-9.

This corresponds to the following ages, respectively: 272 ± 26 myr (8.5% error), 301 ± 29 myr (9.7%), and 312 ± 17 myr (5.5%). The used l_K was 0.581 • 10^-10 per year.

The obtained isochrone ages appear lower than those calculated using age formula. This is due to original excess of argonium in samples (positive section at axis y). Assuming the "detrital" argonium (i.e. that inherited by crystalline lattice from the time preceding crystallization) is not present here. The above mentioned excess of argonium may be explained as due to its entrappment in time of crystallization (e.g. in inclusions), or occlusion in mineral lattice some lime after crystallization under high-pressure conditions.

The obtained isochrone age values are close to the dating; made in Erevan and reported by T. Depciuch 298 - 303 myr (304 - 309 myr after recounting), which definitely places the question of age of igneous rocks from the Myszkw - Mrzygłód region somewhat higher in the time scale, towards the Late Carboniferous.