Kersantyt z otworu wiertniczego Wyciąże 1 koło Krakowa

Wiesław Heflik, Władysław Moryc


Kersantite from the borehole Wyciąże 1 near Kraków

During the prospecting works conducted by the petroleum industry in the Carpathian Foreland in 1965/1966, some deep boreholes were drilled near Wyciąże, about 18 km east of Kraków (fig. 1). In one of those boreholes, Wyciąże 1, a vein of lamprophyric rock has been found on the contact of the Lower Devonian and Precambrian sediments (Fig. 2), A detailed description of the geological structure of the region of Wyciąże was given by Jawor (1970). The borehole in question penetrated the following profile: 0 —395.0 m — Quaternary-f Miocene; —496.0 m — Upper Cretaceous; —782.5 im — Upper-I-Middle Jurassic; —1867.0 m — Upper 4- + Middle Devonian; —1898.0 m — Lower Devonian; — 1902.5 m — lamprophyre; —2000.0m — Precambrian. The lamprophyre found in the borehole Wyciąże 1 is a compact rock of brown-grey colour. It is made up essentially of two elements: the grundmass and phenocrysts, mainly biotite, plagicolase, apatite and hornblende. The groundmass consists of feldspars, apatite, ore minerals, single quartz grains and large concentrations of secondary minerals filling, as a rule, pseudamorphs after feldspars. Compared with the other mineral components, biotite is best preserved (Plate I), exhibiting strong pleochroism: [alfa]— brown-yellowish, [gamma] — brown-dark yellowish. In the sections perpendicular to Z axis, i.e. in (001) planes, it shows zonal structure, reflecting its successive growth. The (peripheral zones of biotite flakes are characterized by more intensive pleochroic colours. Some biotite flakes are intergrown by secondary constituents, titanite and ilmenite, and sometimes biotite is accompanied by single grains of hornblende which underwent biotitization. This process is visible, as a rule, in the external zones of rods. Fresh amphibole grains show fairly intensive pleochroism: a colourless, [beta] — pale green-yellowish, [gamma] — pale green, Megaphenocrysts of plagioclase are almost completely metamorphosed, leaving pseudomorphs made up of calcium carbonate. They contain about 55% of anorthite, which corresponds to the composition of andesine. The andesinic character of plagioclases has been also confirmed by X-ray examinations (Table 1). The relict forms after plagioclases indicate that these minerals constituted originally about 35 vol. %. Apatite appears mainly in the interstices of feldspar pseudomorphs and biotite flakes. The groundmass of the lamprophyric rodk is made up of fine xenomorphic grains of potassium feldspar, radial concentrations of desmine, intensely altered mica flakes (hydroimuscovite and biotite), and few xenomorphic quartz grains. It also contains a considerable amount of magnetite, titanite, ilmenite, orthite and chromite grains. The content of those minerals is about 3%. Dolomite, quartz and albite are the secondary minerals occurring in the rock in question. Dolomite impregnates lamprophyre or forms irregular concentrations. Quartz and albite, which are both xenomorphic, cooccur forming irregular veins or concentrations. The chemical composition of the rock is: SiO(2) — 51.15%, TiO(2) — 2.66%, Al(2)O(3) — 20.50%, Fe(2)O(3) — 5.80%, FeO — 3.00%, CaO — 2.10%, MgO — 4.60%, K(2)O — 1.20%, Na(2)O — 8.09%, H(2)O+ — 0.90%. Moreover, Zn, Pb, Cu, Co, Ga, V, Zr, Sn, Ag, Mo and Cr have been detected by spectroscopic methods. As appears from the mode of occurrence of the individual minerals, biotite, hornblende and then andesine crystallized in the lamprophyre in the staige of mesocrystallization. Apatite, magnetite and picotite were the first to crystallize. Titanite, ilmenite and quartz formed during the final phase of crystallization. The secondary processes gave rise to zeolites and carbonates and promoted albitization and silicification. According to the current opinions, the kersantite from Wyciąże is to be associated with the Carboniferous—Permian volcanic activity. It is feasible that there were in that period two or three, not necessarily coeval, zone of pre-Zechstein occurrences of lamprophyric (rocks (fig. 4): northern, along the line Daleszyce—Iwaniska—Klimontów, and southern, presumably branching, with the strike Zawiercie—Opatkowice and Zawiercie— Zabierzów—Borzęta—W yciąże. In the southern zone, Wyciąże excepted, lamiprophyres showing the prevalence of K(2)O over Na(2)O, i.e. characteristic of minette, predominate. In the northern zone, on the other hand, lamprofphyres in which Na(2)O prevails over K(2)O, i.e. corresponding to karsantites, are dominant. The uniform strike of these zones seems to be indicative of their relation with the directions of dislocations or dislocation zones utilized by the intruding magma, the source of which may have been the generally accepted common reservoir of magma. Therefore, the kersantite from Wyciąże may be genetically associated with the young Variscan granitoid intrusions of the Foreland of the West Carpathians.

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