Wiek bezwzględny skał polskich (Katalog oznaczeń geochronologicznych)

Jan Burchart


Absolute ages of rocks from Poland (A catalogue of geochronological determinations)

The isotopic methods in geochronology are not much in use in Poland. There exists in the country only one small laboratory (at the Geological Institute of Poland, in Warsaw) working in this field and equipped for K-Ar method. Some work on fission-track dating is being carried on at the Academy of Mining and Metallurgy in Cracow and at the Polish Academy of Sciences in Warsaw but no age determinations have been published so far. On the other hand the need for isotopic dating is well understood particularly for areas built of metamorphic and igneous rocks (the Sudetes, the Tatra Mountains, the crystalline basement of the Carpathian foreland, the crystalline basement of north-eastern Poland). This is why the specimens of Polish rocks were willingly given to be studied in geochronological laboratories abroad, mainly in Soviet Union. Some work was also done by the Polish geologists during, their stay at foreign laboratories (United States, France). As a result the age data though relatively small in number are scattered in many publications, some of them little known in this country. Therefore it seemed necessary to scan the possible sources, references, bibliographies and to compile a catalogue listing all the available results so far obtained. The material presented in the following is very uneven as far as sampling, analytical methods and even reliability are concerned. No selection of the results has been made but the reader is warned that many of the figures reported represent apparent ages only. The Polish text gives a short discussion of some factors controlling the credibility of the results: analytical methods, proper mineral separation specially important in the cases of polygenetic rocks, geological setting, the degree of weathering, and the number of determinations for a given rock series. Papers by Afanasiev, Borisevich, Shanin & Sheina (1963) and Afanasiev, Borisevich & Shanin (1962) are referred to, in which the cases are reported of apparent age increase as much as 80% due to hypergenic leaching of potassium from micas. The present author is very sceptical about the meaning of the K-Ar whole rock determinations on clastic sedimentary rocks because of their inhomogeneity (detritic minerals, syndepositional phases, and the effect of diagenetic alterations). As a proof the paper by Krylov, Lisitsyn & Silin, 1961 is cited in which recent deep sea deposits are shown to have yielded the ages of 0 to 690 m.y. (only two „zero ages” out of 51 specimens examined).

THE CRYSTALLINE BASEMENT OF POLISH LOWLANDS The age determinations are listed in Tab. 1. With the exception of the Ełk syenite the values are within reasonable limits. Unfortunately almost all the ages were obtained by K-Ar method on biotite only, and also two Rb/Sr ages were oibtained from biotite. The consistency of the results indicates that the ages represent the last event which was strong enough to eradicate any passible proceeding ages. One may expect future U-Th-Pfo determinations on zircons and whole rock Rb-Sr analyses to yield some deeper insight into the earlier history of the metamoiphic rocks. The data reported for the neighbouring areas of the Soviet Union territory (the Grodno region) are 1557 m.y. for the gabro from Moridno and 1390 m.y. for gneisses from Lososno and Glebovichi (Ger1ing, Pap, Morozova, Ovchinnikova, 1962) — the values recalculated to the „American constants” . There is much douibt about the meaning of the age obtained for the syenite from Ełk. According to Ryka (1963) the biotite analysed by K. Przewłocki (No. 40 in Tab. 1) was taken from a fissure-filling. The values reported by I. Kardymowicz were obtained by Kuznetsov using the birefrigence dispersion method the reliability of which is not commonly accepted. The ages reported in Tab. 2 cover the time span of 837 to 307 m.y. (excluding the diabases). The age pattern is illegible. Unfortunately only one determination was carried on a pure separated mineral (No. 48), and all the rest either on whole rock samples or some polymineral concentrates. The values reported seem nothing more than apparent ages and their significance is disputable. According to Pożaryski and Tomczyk (1968) the rocks encountered by borings at Kochanovka and Rudki in Western Ukraine close to the Polish border have yielded ages of 634—678 m.y. (recalculated according to the „American constants” ). The series is correlated with the metamorphic rocks of Podborze and Neczajna (Nos. 51, 52). The ages obtained by Semenenko (1962) and Semenenko et al. (1963) for the schists of the Rachov massif in the Eastern Carpathians are 605, 564, 562 and 173 m.y., and for the phyllites of the basement of the Carpathian fore-deep are 673, 653, and 634 m.y. (all the values recalculated according to the „American constants” ).

The results are listed in Tab. 3. A part of the rocks examined represents the fossil-dated Lower Palaeozoic deposits but some belong to the series variously referred to as the Eocambrian, Ryphean or Sinian (Znosko, 1961). As shown by pétrographie examinations (Kieżel 1965, Lendzion 1969) the samples from Bazów, Tłuszcz, and Radzymin are clastic unmetamorphosed sedimentary rocks in which not only detritic feldspars but also rock fragments occur. It does not seem justified to assume a complete loss of detrital argon in the course of diagenetic processes and therefore the „ages” reported may be no more than weighted means of the ages of the components, the component ages having been possibly affected by weathering and diagenetic processes.

K/Ar as well as Rb/Sr ages are available and are relatively numerous as for such a small area. The data listed in Tab. 4 indicate that the last major stage of the rock-forming processes in the crystalline core of the Tatra Mountains occurred in Carboniferous time. Some older, probably Silurian stage of metamorphic evolution is implied by Nos. 67, 68, and 112. The age of 158 m.y. (No. 113) yielded by a whole rock sample of a diaphtoritic chloride schist from the Chochołowska Valley and looked upon by the authors of the K-Ar determination (Sedletskii et al., 1966) as a proof of Jurassic metamorphism is inconsistent with the geological data (the schists occur within the crystalline core which according to the structural relationships is pre-Lower Triassic in age; the fossil-dated Triassic and Jurassic deposits of the Tatra Mountains show no sign of metamorphic changes). According to the present author the value is rather a resultant of a probably Carboniferous age and the Late Cretaceous diaphtoritic alterations. So far no age data have been reported which would record the Alpineage orogeny.

The data are given in Tab. 5. Several determinations of rocks from the Czechoslovakian part of the Sudetes close to the state boundary are also included. The Lusatia mica hornfelses from Eastern Germany (Watznauer 1964, Vattsnauer et al. 1966) have yielded the ages of 431 and 420 m.y. (recalculated to the „American constants”), which are interpreted by N, P. Semenenko as evidences of an Ordovician metamorphism, and by A. Watznauer as a resultant of a Ryphean age of original greywackes and a Carboniferous metamorphic event. The Watznauer’s view is supported by petrographic evidence of co-occurrence of two generations of micas in the rooks dated. Ages higher than those recording the Carboniferous events have been reported from several localities of Lusatia and Saxony (e.g. Schurmann 1960, Watznauer 1966, Jager & Watznauer 1969). The data available for the rocks of the Polish part of the Sudetes almost exclusively concern the age of the granitic massifs. The meaning of the two determinations of the age of the rocks from the Izerskie Mountains (Nos. 119 and 120) and of the Radzimowice phyllite (No. 121) seems disputable: what is the geological meaning of a K/Ar age of a whole rock sample of a „secondarily albitized chlorite-actinolite amphibolite” ? So far geochronology in Poland has not contributed to solve the most difficult problems of the metamorphic evolution of the Sudetes. Because of the polymetamorphic character of the majority of the crystalline rocks of this region one may expect that any future age work will often result in apparent ages only (relict and mixed ages), and that their interpretation by no means will be simple or easy. The determinations of micas (Nos. 156 and 157) from the metamorphic complex of Lądek and Snieżnik carried out at the Geochronological Laboratory of IGEM AN USSR in Moscow resulted in Devonian ages though the rocks belong to the series commonly regarded as Precambrian. Even if they are true ages they record only the last stage of blastesis and not the main period of metamorphic recrystallization. Nos. 158 and 159 represent the tuffs occurring in the Cracow region and regarded as Early Permian in age. The values reported in the tables express the isotopic ratios conventionally recalculated to „ages” . Only some of them are true mineral ages. The present author compiled the naked data stripping them of the interpretations. It seems worth to mention that some of the interpretations offered in the source paipers are inconsistent with each other. If one accepts the idea of the complete removal of argon from the detrital minerals and rook fragments in greywaokes during their diagenetic alterations it seems hardly possible to assume that micas in gneisses can preserve some pre-metamorphic „detrital” argon. The former view concerns the Lower Palaeozoic deposits of the Świętokrzyskie Mountains, the latter one is the J. Borucki’s (1965) interpretation of the ages of the Rzeszotary metamorphic rocks.

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