Petrografia zmetamorfizowanych piaskowców jednostki Świerzawy w Górach Kaczawskich

Abstract

Skały omawiane w niniejszej pracy tworzą serię o miąższości około 100 - 150 m, występującą wśród zieleńców kambru. Dotychczas uważano, że mają one genezę wulkaniczną. O osadowym pochodzeniu ba danych skał świadczą ich cechy megaskopowe i mikro skopowe. Do pierwszych zaliczyć można obecność warstwowania polegającego na frakcjonalnym ułożeniu ziarn w obrębie poszczególnych ławic oraz warstwowania skośnego i form przypominających ripplemarki. W obrazie mikroskopowym stwierdza się obecność detrytycznych ziarn kwarcu i skalenia, a także drobno ziarnistych okruchów skalnych i ziarn silnie zserycytyzowanych. Mimo dość silnej rekrystalizacji metamorficznej (facja zieleńcowa), opierając się na obserwacjach mikroskopowych, można odtworzyć przybliżoną historię zmetamorfizowanych piaskowców. Przed metamorfozą badane skały były średnio ziarnistymi piaskowcami. Ilość pierwotnej matrix była przypuszczalnie niewielka a ziarno nieźle wysortowane. W wyniku zmian diagenetycznych w pierwotnym osadzie powstało spoiwo skaleniowe, kwarcowe i być może węglanowe. W czasie metamorfozy wskutek rekrystalizacji kierunkowej części minerałów blaszkowych i kwarcu utworzyła się w skale foliacja. Na okres metamorfozy przypada też blasteza wtórnego mikroklinu, który obficie występuje w odmianach piaskowców o najgrubszym ziarnie. Mikroklin ten obrasta detrytyczne ziarna skalenia potasowego, często zawierające już diagenetyczne obwódki regeneracyjne, a także tworzy samodzielne subautomorficzne blasty. W późniejszym okresie metamorfozy skały przeszły albityzację, a ostatnim etapem ich rozwoju była kaolinizacja, obejmująca przede wszystkim skalenie. Na podstawie składu mineralnego i chemicznego omawianych skał można przypuszczać, że przed metamorfozą były to piaskowce skaleniowe i arkozowe.Petrography of metasandstones of the Świerzawa Unit in the Kaczawa Mts. (Góry Kaczawskie)Abstract The paper deals with the rocks forming one of the old-Palaeozoic epimetamorphic series of the Świerzawa Unit distinguished in the southern trunk of the Kaczawa Mts. by Teisseyre (1956). The series under discussion reaches the thickness of 100 - 150 m and outcrops amidst the Cambrian greenstones (fig. 1). At first the rocks in question were believed to be quartzites (Gruerich, fide Zimmermann 1941). Next it was Zimmermann (1932) who described them as being keratophyres. Block (1938) and Ansilewski (1954) in much the same way pointed to their volcanic origin. However, Smulikowski and Teisseyre (in Teisseyre 1957) doubted their primary lava nature, considering tuff nature of the discussed rocks to be more probable. On the basis of field and microscopical evidence presented in this article the author states that the „keratophyres” of the Świerzawa Unit should be regarded as the metamorphosed sandstones. When examined with a naked eye the investigated rocks resemble sandstones (pi. I, 1-4; pl. II, 3). Several lithological varieties can be distinguished. They differ from each other in their hue, grain coarseness, and degree of foliation development. The typical variety is characterized by light colouration, distinct plate structure, and poorly developed foliation planes. Layer thickness is usually of several cm. In the village of Janówek (exposures no. 2 and 3 in fig. 1) several exceptionally massive layers of the metasandstones occur. Those rocks display coarse structure and some times fractional order of grain distribution. In the metasandstones coming from the vicinity of the village of Wojcieszów iron-bearing bands are abundant marking the primary sedimentary bedding (pi. II, 4). The presence of the latter allows to observe a pattern of the tectonic deformation which affected the discussed rocks (pi. II.2, 5, 6). Metasandstones have been interbedded subsidiarily with phyllitic rocks. The phyllites have been recognized to form thin intercalations which have separated from each other the layers characterized by coarser grain structures. Only in Janówek the phyllites appear to be more abundant and significant. Due to frequent interbedding with the metasandstones they form together a fine-laminated rock at the distance of two meters (pi. II, 1). When examined under the microscope the metasandstones display usually a presence of fine-grained groundmass composed mainly of sericite as well as insignificant amount of quartz. Larger grains of quartz, potash felspar, fine-grained quartzo-felspathic fragments reminding sometimes granophyre, and more or less sericitized grains occur against the background. Zircone and apatite can be encountered occasionally. Rarely carbonate and clear, often automorphic albite may be present. Sometimes the rocks are enriched in iron ores, leucoxene, epidote and pyrite. The phyllites consist almost exclusively od white mica. Besides the fine fragments of quartz, potash felspar as well as iron oxides can be encountered. There are several mineral constituents to built up the metasandstones. They are as follows: well preserved detrital grains, badly preserved cement developed during diagenetic processes, and products of metamorphic recrystallization. The author has established the presumable succession of transformations which affected the primary sediment. It was recognized that in the course of sedimentation, the sand composed of quartz grains, potash felspar, most likely plagioclases and quartzofelspathic fragments, was de posited. These compounds were accompanied by heavy minerals represented by zircone (pi. V, 6) and apatite. The average diameter of quartz or felspar grains reaches the value of 0,1 -0,2 mm (tab. 2). Relics of the primary sand deposit happened to be revealed in the thin sections. Thus it may be assumed that the sediment was that of a well-sorted type and an amount of protomatrix (Dickinson 1970) did not exceed several percent of the rock volume. Some fine-grained varieties have displayed an increase in a matrix content. During the process of diagenesis the cement composed of authigenic minerals was developed. Potash felspar - most likely adularia - has been believed to have crystallized at first (cf. Baskin 1956). The mineral was developed as the overgrowths around the detrital grains of potash felspar (pi. Ill, 4; pi. IV, 3 - 6), (cf. Govinda Rajulu, Nagaraja 1967; Heald 1950; 1956). Some detrital grains of the investigated potash felspar have been recognized to be slightly corroded (pi. IV, 5, 6). This allows to assume that the partial dissolution of the detrital felspar had been effected prior to the inception of the felspar rims. The former process affected mainly the finest grains. The quartz cement has been thought to have developed in the course of diagenesis after the authigenic felspar had crystallized (pi. Ill, 1, 3, 4). Most probably the metasandstones also to some extent were cemented by carbonates. However the carbonates yielded to a complete recrystallization due to metamorphism. Sericitization of nearly all the detrital plagioclase grains (pi. V, 5) is thought to have taken place at the period of late diagenesis. The process might be associated with an afflux of K-ions (Borkowska in Borkowska, Smulikowski 1973). After the sericitized grains had been deformed, due to incremental pressure the pseudomatrix was developed (Dickinson 1970). It was probably the same pressure which effected the inception of sutured contacts occurring between detrital quartz grains and between the diagenetic overgrowths of quartz (pi. Ill, 1; pi. IV, 1). Penetrative foliation paralleling the axial planes to folds was due to metamorphism (pi. II, 2, 5, 6). Certain parts of the quartz grains and flaky minerals yielded to the directional recrystallization (pi. Ill, 2), and metamorphic differentiation. Sericite laminae happened to be created in a spatial agreement with the foliation planes, which might be discordant to the sedimentary bedding. It has been recognized that within the coarser-grained layers the microcline crystals kept growing along the foliation surfaces. The microcline was developed either as independent crystals or rimmed the detrital grains of potash felspar. The rims have been considered as the second generation of overgrowths (pi. IV, 4 - 6; pi. V, 1). Sericite has usually been absent from the microcline-rich rock varieties. Therefore it is suggested that the felspar was developed at the expense of mica. Simultaneous afflux of K-ions may be assumed. Further on the rocks were subjected to fairly strong albitization. It seems to be possible that the detrital plagioclases were the source of sodium needed for inception of secondary albite. All the aforesaid felspars are replaced by carbonates (pi. V, 2-4). The process has been recognized as the youngest one. Due to presumably greater admixture of Na20 and CaO in the detrital potash felspar, it yielded to the carbonatization easier than the secondary microcline (cf. Heald 1956). Occasional kaolinitization of potash felspar or even phengite and carbonates took place in the last phase of the metasandstone development. High content of detrital felspar (cf. tab. 1 - average 18,4%) allows to assume that the questioned rocks have reached the composition of felspathic sandstones or even arkoses. The comparison of the result of the chemical analysis of the rock from Janówek with that of the typical arkose seems to support the above conclusion (tab. 3). Due to presumably low amount of protomatrix the primary sandstones are considered to have been the arenites. The coarse-grained varieties have still preserved features characteristic of the sedimentary rocks. The features have been nearly completely obscured in the finest-grained varieties composed now mainly of sericite (cf. Kicula, Wieser 1970): Both the mineral composition and texture of the metasandstones of the Świerzawa Unit differ from those displayed by the old-Palaeozoic metasandstones described by Baranowski (1973) from the northern part of the Kaczawa Mts. and by Łącka (1972) from the Cambro-Silurian in the vicinity of Wleń. The author is indebted to Prof. H. Teisseyre, Doc. H. Dziedzicowa, Mgr T. Morawski, Dr A. Nowakowski as well as colleagues from the Geological Institute of Polish Academy of Sciences for kindly helping in her investigations.