Petrochemiczne zróżnicowanie a geneza młodopaleozoicznyoh wulkanitów Dolnego Śląska

Józef Lis, Hubert Sylwestrzak



Subsequent volcanism related to Variscan orogeny is one of the most typical features of Variscan stage in evolution of central Europe. The volcanism is characterized by occurrence of large masses of both acid (porphyry) and basic (melaphyry) volcanic rocks. The igneous phenomena were active for a long time, from the turn of the Westphalian A and B till the Late Rotliegendes. In order to explain nonuniformities in composition, recorded even in the case of small volcanic bodies, there was carried out comparative pertochemical analysis of Uppper Paleozoic volcanic rocks and granitoids from the Lower Silesia and some volcanic rocks from other parts of central Europe. The analysis was carried out with the use of H. de la Roche (1962, 1968, 1972) rectangular mineralogical-chemical diagrammes in the pattern:
Si/3 - (K + Na + 2/3Ca) : K - (Na + Ca) and Al/3 - K: Al/3 - Na (these parameters were calculated from miliatomgrams per 100 g of rock).
The results of chemical analyses analyses, given in diagrammes (Figs. 1-5), show that acid and basic volcanic rocks completly differ in features of geochemicall differentiation. Melaphyres (and Karkonosze lampprophyres), similarly as Sudetic granitoids (especially Karkonosze and Strzegom granitoidis of undoubtful igneous origin) were derived from sources characterized by a marked homogenity. Acid volcanic rocks are extremely uniform in comparison with the former, which indicates that they were formed of magma coming from various undifferentiated sources. This in accordance with the point of view of H. Stille (1939, 1950), subsequently followed by E. Bederke (1959) and A. Rittman (1960). According to that point of view, subsequent acid magma is the product of partial or complete melting of Earth crust masses. The results of the analysis inclined the authors to precise the above viewpoint. It maybe stated that acid volcanic rocks related to subsequent volcanism have originated under conditions of rather shallow local melting of sedimentary and metamorphic rocks along tectonic fracture lines in central Europe.
The results of comparison of chemical coposition of Żeleźniak porphyres and rocks of the Radzimowice Shale Series (Fig. 6) may serve as an example of genetic relationship between volcanic rocks and metamorphic rocks from their direct neighborhood. Differentation im chemical composition of this small porphyry neck is similar to that in composition of graywacke phyllites of the Radzimowice Series, markedly exceeding the admissible range for the case of differentiation of a magma reservoir.

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