Porfir w dolinie Czernki

Stanisław Siedlecki, Tadeusz Wieser


A porphyry in the Czernka Valley

Geological part. By S. Siedlecki. During an excursion of the Geological Department of the Jagiellonian University of Cracow a porphyry has been found in a quarry of Carboniferous Limestone. In the northern part of the village Czerna in the northeastern slope of the valley there are a few limestone quarries. The largest, marked on the sketch (fig. 5) with the number 5, exposes grey and creamy-grey limestones of the Visé stage. ]. Jaros z (3) attributes them to the zone "e", for which the index fossil is to be Productus striatus Fischer. The main wall of the quarry, ca 25 m high and about 100 m long trends in the direction approximative NW-SE. It shows a few interesting features, of which the occurence of a porphyric rock is the most interesting. In this wall, red karst phenomena call attention, which origin may be at first connected with the presence of the porphyry, suggesting a possible hydrothermal metasomatism of Carboniferous Limestones. Therefore we have examined more closely the proximity of the porphyry. The fig. 1 is a sketch of the main wall of the quarry. Bedding of limestone (on the figure marked - I) is here not very distinct. The strike and dip is approximatively constant, ca 100° SSW 350 in average, but there are small deviations from these values amounting [to a few degrees (Jarosz gives 102° — 106° S 39°). Bedding is disturbed by very well developed joints and abundant fissures. Fig. 2 and 3-b. Fine-grained sandstone, yellow with argillaceous cement. Fig. 2 and 3-c. Breccia with argillaceous (yellow) ortargillaceous and ferruginous (red) matrix, cementing limestone fragments, mostly rounded but rather subangular. Limestone fragments compact or fine crystalline resemble Devonian or Carboniferous Limestones of Dębnik. We have not found any fossils in them (c1 - red breccia, c2 - yellow breccia). Fig. 3-d Red sandy :shale, composed of argillaceous matter with hematite and small fragments of limestones. Fig. 3-e Red sandy shale, argillaceous with hematite. Fig. 3-f. A block of grey fine-cristalline limestone (Carboniferous Limestone ?). These deposits pass one into another. The presence of clastic elements (sand and limestone fragments) suggests that the pillar is a filled karst pocket, formed in the Carboniferous Limestone. It seems that the pocket after it had been filled up, has not undergone any tectonic change of its position, its axis being still nearly vertical. This karst pocket has been formed on a joint plane, parallel to NS axis of the near Dębnik anticline (Fig. 1 and 2-II). Another distinct karst form can be seen in a relict pocket, marked as a red spot in the middle of the right (SE) part of the quarry wall (Fig. 1-B and Fig. 4-B). It is filled by a conglomerate consisting of pebbles of compact, dark, finecrystalline limestone (Devonian or Carboniferous?). The dimensions and outlook of these pebbles are very much the same as in the pocket previously described. Pebbles are rounded, with weathered surface, cemented with red clayeysandy matrix coloured by hematite. The conglomerate forms a flat block, ca 1,7 m high, 1,2 m wide and 0.35 m thick. It lies on a fissure of the type II. On the walls of the fissure above the conglomerate, distinct traces of leaching by water circulation, are still visible. To the left from the conglomerate in a distance of about 1,5 m there is a nest of yellow-pink sandy limestone and siliceous marl (Fig. 1 and 4-C). These rocks have been examined in thin slides, treated with in thin slides, treated with. The composition of the sandy limestone is as follows: Quartz sand 29,6 % Argillaceous matter and siliceous silt 19,2 % Part soluble in HCl (mostly CaCO(3)) 51,2 % The composition of siliceous marl: Argillaceous matter 14,5 % Siliceous silt 75,5 % Part soluble in HC! 10,0 % These rocks are evidently of clastic origin. They also fill a karst pocket, the direct connetion of which with a vertical fracture is not visible to-day, but the surrounding rocks have been removed by exploatation of the quarry. Thin slides and elutriated samples of these karst deposits do not contain any contact minerals. Therefore we do not assume that there is any relation between the porphyry and the origin of the discussed deposits. In a few places within the quarry there are zones of cavernous limestones, partly leached by the water and intensively coloured yellow and brown by iron compounds (Fig 1-D). They are mostly developed along bedding surfaces and indicate a vivid water circulation in limestones. Within a not very distinct fracture a small mass of porphyry appears. It lies 1 m below the red karst pocket (B). The porphyry lies close to the walls of the fracture, partly separated from them by a crystalline aggregate of reddishyellow- white calcite (Fig. 1 and 4-E). The porphyry is of light-grey-creamy colour, so similar to the limestone, that it is difficult to notice the porphyry. The limestone does not show any changes in the contact with porphyry. It is possible that a narrow contact zone existed, but has been washed out and removed by water. Instead of it the calcite aggregate could have been formed. The exposed part of the porphyry is rounded and elongated in the direction of the fracture. The dimensions are 70 X 20 cm. The porphyry is compact and hard and the weathering has not influenced the compactness of the rock. It shows slickenslides; the rock, when struck with hammer, divides itself in fragments along polished slickenslides. Below and above the porphyry, within the same fracture one can see traces of the weathered porphyry covered with calcite. Small dimensions of the porphyry, its contact with calcite within the fracture, traces of tectonic pressure and the pro ximity of karst pocket suggest a few possibilities of interpretation: 1. It is not entirely impossible, that the porphyry is a large boulder, which penetrated into the fracture, enlarged by karst solution. The position of the porphyry boulder would be the same as of the limestone pebbles in the karst pockets. Against such an interpretation there are the following arguments: a) The porphyry is situated in a fracture of different direction than the karst pockets. b) No such material similar to sands, red clays and limestone fragments filling the karst pockets, is associated with the porphyry. On the other hand we did not find any magmatic material in the karst pockets. 2. Therefore it may be accepted that the porphyry occurs in the form an apophysis, which is situated „in situ“ in the fracture. The fracture after the consolidation of the intrusion has been enlarged and filled with secondary calcite. It may be accepted that this fracture was revived at a later orogenic stage and the porphyric vein was crashed with the simultaneous infiltration of water and mineral solutions. 3. It is also possible that the porphyry forms a tectonic sheet detached from the vein and partly displaced from the original position of the vein. Should this interpretation be accepted, it must be assumed, that the vein is situated near the occurence of the porphyry. A complete explanation of this problem may be brought by the removal of limestones which surround the porphyry outcrop. To-day the most probable opinion is, that the porphyry occurs in situ or is only slightly displaced from the original position. The position of this new occurence of porphyry in relation to other known porphyries of the Cracow district, is shown in the sketch map (Fig. 5). Outcrops of these are numerated in the map, as they have succesively been discovered. 1. Porphyry sheet at Miękinia, described in detail by Rozen (4) and Bolewski (2) 2. Porphyry in the Szklarka valley (Szajnocha, Siemiradzki, Rozen etc., 3. Porphyry above Dubie, mentioned by Rutkowski (6). so far not described. 4. Porphyry at Siedlec, described by Bolewski (1, 2). 5. Porphyry at Czernka valley. This list shows, that porphyries are frequent in the proximity of the Dębnik anticline. This is also supplemented by the observations of Rome r (5, p, 111) and Zaręczny (7 p. 63) of porphyry fragments on the western slope of the Bartlowa Mt. (between Miękinia and Krzeszowice) and blocks of white porphyries observed by Pa now (1, p. 3) between Dubie and Szklary. The age of the porphyry of Czerna can only be defined as younger than the Lower Carboniferous. Well advanced changes in the mineral composition of the rock make difficult a more precise determination of its magmatic relation to other similar rocks. One can infer from the diagram (Fig. 7) that it lies within an uniform series of magmatic rocks of the Cracow district, and although it occupies an extreme position in this series, it is closely related to them, and may be treated as a vein supplied from one magmatic source. The porphyry from Czernka is markedly enriched in silica and* impoverished in compounds which easily could be leached out, as potassium, sodium, calcium and others. From the geological point of view it should be underlined that in the Cracow district, porphyries occuring in veins, differ in their mineral and chemical composition from the porphyries occuring in large extrusive sheets (Miękinia, Zalas). It seems that smaller bodies of porphyry have easier undergone magmatic segregation, autohydratation and weathering, than large bodies. They formed a peripheric zone of intrusion, exposed to the action of the neighbouring rocks and apomagmatic hydrothermal processes. All these factors could cause a different outlook of this rocks as compared with closely related normal porphyry, thus creating an interesting variability of porphyries in the district of Cracow. Petrolosical part. by T. Wieser. Kaolinized porphyry. Macroscopic descript ion. The rock on fresh fracture is white or light-grey, a little yellow, owing to iron hydroxides. It resembles an acid kaolinized igneous rock, what is also confirmed by its smell, characteristic for argillaceous substance. Well visible phenocrysts (mostly of quartz) and compact groundmass, indicate that it is an acid extrusive rock. Microscopic description. In thin slides the rock makes an impression of a heterogenic lava breccia which elements with regard to direction are quite orderless (ataxite structure). But angular fragments, of which the rock is composed, embedded in the groundmass do not differ from the groundmas, except by more compact aphanitic texture. This features, i. e. autogenic composition of fragments,indicate that the rock is a volcanicfriction brecci a (Vulkanische Reibungsbreccie) and not an ataxite in which the fragments are heterogenic. The variability of the development of grains of extratelluric phase in acid extrusive rocks is common, especially in the lateral zones of igneous rocks as the described case. The fragments imbedded in the matrix owing to their light colour and apparently homogenous texture resemble phenocrysts, and therefore the rock in its megascopic outlook is similar to nevadite type of porphyries with abundant phenocrysts. But the proportion of phenocrysts of feldspar, quartz and biotite to the groundmass is as 20,2:79,8 what corresponds to the proportion typical for porphyry. The groundmass is xenomorphic-granular aggregate of microfelsitic origin; its structure is fluidal, sphaerolitic at places; these structures are preserved as relict in spite of the recrystallization of the matrix. Grains of quartz and feldspars, or strictly speaking of kaolinite-sericite aggregates with admixture of chalcedony (occuring in minute sphaerolites) can be discriminated, or present a cryptomeric aggregate of constituents difficult to determine even with greatest magnifications. Among the phenocrysts the most numerous are those of quartz (70^ of all ohenocrysts). Quartz phenocrysts are irregular, angular, indented or rounded with not numerous traces of piramidal walls. Magmatic resorption is visible. Quartz grains contain few rhombic glass inclusions 0,02 mm in diameter, situated in groups or ranks (fig. 6) and belts of very small liquid or gaseous inclusions, very rare inclusions of apatite 0,03 mm long and occasional inclusions of iron ores, rutile and fresh biotite of yellow-brown pleochroism. The diameter of quartz grains amounts to 3 mm. Phenocrysts of feldspars are less numerous. Because of well advanced decomposition it is not possible to determine, neither their type, nor their chemical composition. The decomposition has rendered invisible the cleavage plains and twinning. Feldspars are changed into aggregates of allotriomorphic small grains (ca. 0,06 mm) of secondary, strongly kaolinized and sericitized feldspar, contaminated with a small amount of iron oxides and hydroxides. Phenocrysts contours are xenomorphic, irregular and contours are so little distinct that there exists no boundary between phenocrysts and groundmass. The diameter of feldspar phenocrysts attains 4 mm; qualitatively they occupy 26,6% of surface in thin slide. Biotite forming hardly more than 3,6 % vol. of the rock, has also been decomposed; it is possible to observe all stages of its decomposition. Fresh biotite occur as minute (about 0,01 mm in diameter) inclusions in quartz. They possess strong pleochroism. X — colour = less to light-yellow, Y = Z — brown with red tint. In the transitional stage of composition the biotite has the pleochroism of chlorite (X = Y — grass-green, Z — colourless) and is filled with poikilitic net of iron oxides, which hydrated, cause yellow colour. Iron oxides inclusions are arranged along the direction of perfect cleavage (001), Occasionally rutile needles appear. In the final stage of decomposition biotite becomes similar to muscovite- or talc, loses pleochroism and its iron ore inclusions. Refringence indices for the last stage of decomposition vary within the limits (after immersion method) nmax (X,Z) = ca. 1,572 ajid nmjn (X) = ca. 1,568. The resulting birefringence = 0,004, was also confirmed by the comparison of interference colours. These data and convergence pictures allow to infer, that the examined mineral is neither muscovite nor talc, but a biotite which, while preserving structure and partly optical orientation (negative and nearly uniaxial) has undergone a baueritisation process. 1 The described product, i. e. the pseudomorphous bauerite after biotite adsorbs readily water, swells in the direction perpendicular to cleavage planes, and becomes isotropic. With this decomposition of biotite its bleaching is strictly connected and is caused by the leaching of the chemical constituents of biotite, esp. of iron oxides. Original and decomposed biotite possesses idiomorphic forms, its plates are often bent and split owing to protoclasis. The diameter of biotite plates amounts to 2 mm. Chemi cal analysi s . Owing to the kindness of Prof. A. Bolewski an analysis has been carried out by E. Gorlich in the Inst, of Miner, and Petrography of the School of Mines in Cracow. Its results, re-calculated molecular proportions and mineral normative composition (after C. I. P. W.) is given below. To compare the composition of this rock with commagmatic lavas in the Cracov district, normative minerals have been calculated in mol. proportions, according to the principles of C. I. P. W. system. For graphical presentation of the results a diagram (fig. 7) is constructed, on the horizontal axis being marked weight percents of silica, on the verticalpercents of mineral norms. 1. Quartz porphyry of Miękinia (an. Z. Rozen) 2. Quartz porphyry of Zalas (an. Z. Rozen) 3. Diabase of Niedźwiedzia Góra (an. Z. Rozen) 4 Diabase of Niedźwiedzia Góra (an. Z. Rozen) 5. Melaphyre of Alwernia (an. Z. Rozen) 6. Melaphyre of Regulice (an. Z. Rozen) 7. Quartz porphyry, weathered, of Siedlec (an. A. Bolewski) 8. Quartz porphyry, weathered, of Siedlec (an. A. Bolewski) 9. Quartz porphyry, weathered, of Siedlec (an. A. Bolewski) 10. Quartz porphyry, weathered of Miękinia (an A. Bolewski) 11. Quartz porphyry, weathered of Miękinia (an A. Bolewski) 12. Quartz porphyry, weathered, of Zalas (an. Z. Rozen) 13. Quartz porphyry, weathered, of Miękinia (an. Z. Rozen) 14. Potash-trachite, weathered, of Miękinia (an. A. Bolewski) 15. Quartz porphyry, weathered, of Czernka valley (an. E. Gorlich) On the diagram (fig. 7.) the extreme position of the Czerna porphyry is striking. This position is characterized by a large percent of norms Q and C, i. e. SiO(2) and AI(2)O(3), present in surplus in the rock, what is characteristic for rocks rich in silica and alumina (oversaturated rocks, peraluminous group of Shand). These data are in a complete agreement with microscopic observations of the rock, showing the abundance of free silica (quartz and, chalcedony) kaolinite and other argillaceous matter. At the same time the porphyry is poor in per cent of norms corresponding to femic minerals (Hy, Mt, Hm), what depends partly on the originally small content of these constituents, and secondary leaching. In connection with the silification and kaolinization process there is a considerable loss of Or, Ab and An norms (alkalis and CaO), which seem to be comprised in sericite or adsorbed, by argillaceous matter. Conclusions . On the ground of the outlook of groundmass, and autigenic angular fragments embedded in the groundmass of the crushed quartz grain and bent and split biotite (bauerite) plates, which all are features of protoclasis, I feel inclined to term the examined rock as a „volcanic - friction breccia“. The decomposed state of the rock, especially of feldspars, groundmass and biotite, allows also to name the rock as „argillaceous porphyry“ („Tonsteinporphyr“ of Rosenbusch). The changes observed in,chemical composition of the rock, baueritization of biotite, kaolinization and partial sericitization of feldspars are usually due to the action of hydrothermal solutions (autohydratation). In the case of the Czerna porphyry the changes seem to be due to the action of post-eruptive waters containing ions Cl, SO(4) and HCO(3).

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