Nowe dane o paleozoicznych skałach magmowych okolic Olsztyna

Marta Juskowiak, Olech Juskowiak




Phenomena of the palaeozoic magmatism between the Mazury-Suwałki elevation and the Peri-Baltic syneclise have so far been recognized in some bore holes only. Petrographical and chemical examinations of magmatic rocks that cut the Palaeozoic sedimentary formations of various stratigraphical horizons (Table 1), observed in bore hole Olsztyn IG 2, have demonstrated that these rocks represent both diabases and albite microsyenite. The albite microsyenite, characterized by porphyry texture with bostonite groundmass, is built of albite and chlorite and of other subordinate minerals (Table 2), its chemical composition corresponding to intermediate syenites (Table 4, Fig. 2 and 3). Diabases that are found within the Silurian and Cambrian formations are differentiated, mainly as concerns their mineral composition and secondary alterations (Table 3). The Silurian schists are cut by quartz-bearing diabase of porphyry-intersertal texture, whereas the Cambrian sandstones reveal type diabase, characterized by porphyry texture with intersertal or ophitic groundmass. The diabases make small intrusions of hypabyssal type, mainly thin veins and dikes, as proved by their structural features and by the development of contact zones. The geological form of the microsyenite intrusion that occurs on the Silurian deposits, may hardly be defined univocally, since its top is strongly affected by denudation processes. A detailed definition of the individual stages of the Palaeozoic magmatism is difficult due to insufficient reconnaissance of the area considered. An increased magmatic activity was most probably related to the Ordovician and Silurian periods,since these formations reveal, in addition to numerous diabase veins, also numerous layers of bentonitized pyroclastic deposits (A. Kuźniarowa, 197l). Alcalic and peralcalic Palaeozoic magmatism, the upper boundary of which ends with the prealcalic Zechstein period, developed longer. The absolute age of the anorthoclase microsyenite (bore hole Olsztyn IG-1, depth 2025 m), determined by K-Ar method, amounts to 289,0 · 106 years, and that of the albite microsyenite (bore hole Olsztyn IG 2, depth 2174 m) – 292,0 · 106 years (T. Depciuch, 1971). According to Z. Gumowska and M. Juskowiak (1969) the anorthoclase microsyenite from the bore hole Olsztyn IG 1 represents the final products, of magma differentiation of the agpaite series, and genetically is related to the alkali syenites of the Ełk intrusion (O. Juskowiak, 1971). Emphasizing that the youngest elements found within the platform area are built of sodium magmatic rocks, O. Juskowiak and W. Ryka (1969) have demonstrated that the syenites of the Ełk intrusion and the syenites that cut the Śniardwy Lake gabbro are more potassic than the sölvsbergite from Ciechanów and the microsyenite veins from Ełk and Olsztyn.

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