Uwagi o cieszyńskiej prowincji magmowej

Abstract

Comments on the Cieszyn Magmatic Province (West Carpathian Flysch) Polish geologists working in the West Carpathian Flysch of the Cieszyn nappe used to name teschenite any igneous intercalation within those strata, notwithstanding its composition-and texture. This habit cannot be approved by petrologists. The general rock name teschenite was first proposed in 1861 by a geologist Hohenegger after the German spelling („Teschen”) of the Polish name of the town Cieszyn. Shortly after (1866), however, Tschermak separated therefrom under the name picnte melanocratic olivine-rich rocks, confining the name teschenite to olivine-free granular, light- and black-spotted rocks. Rosenbusch (1887) stated justly teschenites as varieties of essexites and theralites in which feldspathoids are represented mainly by analcite, but unfortunately he broadened the term picrite over all melanocratic and olivine-rich basaltoid lavas. Similar rocks have been later encountered in many other regions of the world and described under the restricted meaning of the term teschenite, though accompanied by different dike rocks of variable composition, frequently of lamprophyric facies. It seems inadmissible from the pétrographie point of view to call all igneous rocks of the Cieszyn nappe teschenites irrespective of their texture and composition as has been a common practice of many Polish geologists. That is why I proposed (1930) a general classification scheme of igneous intrusions in the whole Cieszyn nappe of both Polish and Czech territories. It was perhaps somewhat too complicated and detailed for the geological practice and now I would like to propose its simplification in the following manner: A) Hypabyssal Intrusions I. Granular mesotype rocks of plutonite-like texture, distinctly silica- undersaturated, belonging to essexite- or theralite groups, but containing analcite as the main feldspathoid. a) Teschenites. Analcite for the most part primary filling abundant interstices between the feldspars, which are often partly corroded by analcite, but predominantly well preserved. These rocks may be subdivided into theralitic teschenites (plagioclase alone), essexitic teschenites (plagioclase prevailing over alkali feldspar) and monzonitic teschenites (alkali feldspar accompanied by natrolite pseudomorphs after nepheline prevailing over plagioclase). More voluminous teschenite bodies may comprise locally leucocratic veins, patches or schlieren enriched in alkali feldspar, analcite and natrolitized nepheline, acquiring the mineral composition of alkali feldspar-or analcite syenites. b) Bekinkinites (colour index > 50) and lugarites (c. i. < 50). Feldspars nearly totally replaced by analcite leaving at the most small corroded feldspar relics. Primary and secondary analcite mixed to a more or less clouded groundmass. According to the modern plutonic rocks classification this kind of rocks belongs to the foidolite group, although the chemical composition — except the higher degree of hydratation — is much the same as that of teschenites. II. Diabases, much less common in the whole region than the teschenites, display ophitic — granular texture with haphazard disposition of plagioclase laths, common augite and chlorite aggregates filling the angular interstices between them. Analcite is of accessory occurrence only, the silica deficiency being quite negligible and the rocks belonging to the ordinary gabbro group. Analcitization is never observed, very common instead is carbonatization of ferromagnesian minerals. III. Monchiquites, very common in the whole area in thinner sills from 10 cm to a few meters thick, or ,as endocontact border-facies of thicker teschenite- and bekinkinite intrusions. The rocks are dark grey to greenish black, aphanitic or finely porphyritic. Small phenocrysts of titanaugite, lamprobolite, sometimes biotite or olivine and microlites of the same mafic minerals are embedded in a cryptocrystalline more or less turbid basis of glassy appearance, but approaching analcite in chemical composition. This basis may approach that of wholly analcitized bekinkinites, but in the latter corroded relics of bigger feldspars usually appear, while in the monchiquites only tiny lath-shaped feldspar microlites non affected by analcitization may be sometimes recognized. It seems obvious that the smaller size and rapid cooling of monchiquite sills forced the felsic residua of hydrous magma to solidify into a glass- -like mixture of feldspar and analcite. The rocks of this kind may be divided into olivine-bearing monchiquites ’’sensu stricto”, olivine-free augite-lamprobolite fourchites and biotite-rich ouachitites. Abundant plagioclase laths in the rock groundmass denote camptonites which are, however, of rather exceptional occurrence in the whole region. IV. Picrites are melanocratic dike rocks with olivine accumulating up to 40—50 percent of rock volume, titanaugite, lamprobolite and biotite jointly not exceeding 40 volume per cent. Felsic cryptocrystalline basis makes up several per cent of volume. Typical picrites are rare in the whole region and restricted to its Czech part, but there exist all possible transitions between them and olivine rich monchiquites. B) Melabasaltoid volcanites These rocks are amply developed in the Moravian part of the region, poorly, however, in the Silesian part and in its eastern continuation, where they have been found only in allochtonous situation. ”In situ” they represent mainly submarine lavas and pyroclastics, sometimes, resembling pillow-lavas with variolitic incrustation of individual pillows, more rarely flat layers covered by younger sediments. What concerns their composition they represent either olivine-rich feldspar-free ankaratrites, or olivine-rich plagioclase-bearing melabasalts (oceanites), rarely olivine-poor augite melabasalts (ankaTamites). The lava of those volcanites was probably similar and genetically closely connected with the magma having formed intrusions of teschenites, bekinkinites, monchiquites and picrites. There is, however, no reason to reckon them directly among the teschenites, as was frequently done in the geological literature of the West Carpathians. Intrusive rocks of the Cieszyn Magmatic Province are conspicuous for their variability of mineralogical as well as chemical composition which may be explained by the differentiation of their magma in various stages of its origination and development. I suppose three main stages of their evolution. The last and best perceptible stage of differentiation was realized during the intrusion of the essexite- or theralite magma strongly enriched in water vapour in between the lower Cretaceous strata. At the very contact this magma solidified to a nearly aphanitic rock of monchiquite type, usually olivine-, sometimes biotite-bearing. Further from the contact toward the centre of the sill olivine disappeared, titanaugite and lamprobolite prisms increased, felsic minerals i.e. feldspars,, analcite, frequently also nepheline accumulated giving rise to various teschenites. In the more voluminous, lense-like swelled intrusions those felsic minerals accumulated locally into light patches or veins of syenite-like composition. Such differentiation may be explained by the assumption that early crystallizing dark minerals agglutinated and concentrated in quickly chilled parts along the contact of the intrusion, while in its central parts the magma flowed further with gradually increasing augite and hornblende crystals suspended in more and more felsic liquid. It happened not infrequently that the lamprobolite prisms embedded in the felsic groundmass were broken, which points to protoclastic magma movements. At the beginning of the intrusion the magma was probably hot (700— 900° C), but it was cooled rather quickly and the coarse grain of titanaugite and lamprobolite in the central parts of bigger sills could be achieved only owing to a rapid increase of water vapour pressure in the residual liquid. Many microscopic observations denote that after the mafic minerals and lime-rich plagioclase were separated same pause in the crystallization process occurred, the residual liquid was strongly enriched in alkalies and got the properties of a pneumatolitic-hydrothermal medium. Titanaugite was partly resor'bed with formation oif aegyring-aueite and sphene, lamprobolite was bordered with dark green ferrihastingsite, alkali feldspar got the properties of low temperature soda orthoclase. Stronger accumulation of hydrothermal residues caused intense analcitization of feldspars and transformation of teschenites to bekinkinites and lugarites. Another stage of differentiation, preceding the magma intrusion in between the Lower Cretaceous strata occurred probably in a magma reservoir — possibly several reservoirs — many tens of kilometers south of actual teschenite outcrops, somewhere under the roots of the Cieszyn Flysch nappe. Several chemically different magma types were produced in this stage: scarcely silica-deficient gabbroid magmas yielding diabases and their carbonatization products, distinctly silica-deficient essexite and theralite magmas supplying teschenites; bekinkinites and monchiquites, strongly mafic magmas furnishing picrite sills and chemically similar melanocratic basaltoids in mostly submarine extrusions. There may, however, hardly be any doubt about close genetic connection of all those rock groups and about their affiliation to the same magmatic province. Up to present we have quite insufficient information about this stage of magma differentiation. Still more mysterious appears the most primitive stage of magma generation and primary differentiation from the peridotitic Upper Mantle matter. Various conjectures might be made about continental plate movements connected with the foundation of the Carpathian geosyncline, but nothing definite may be said upon the magma generation and its differentiation below the earth crust. Further examination of the igneous rocks of the Cieszyn nappe with the application of quite modern methods will perhaps contribute to the future elucidation of so far inexplicable facts.