Geochronologia intruzji waryscyjskich

Abstract

Dokonano przeglądu oznaczeń izotopowych intruzji waryscyjskich na obszarze Sudetów (w obrębie granic Polski) na tIe datowań Masywu Czeskiego, a następnie plutonów Europy Zachodniej. Waryscyjskie granitoidy można podzielić na synorogeniczne i późno- lub postorogeniczne.

GEOCHRONOLOGY OF THE VARISCAN INTRUSIONS

Since the last 25 years have brought numerous radiometric determinations of the European Variscides it seems to be impossible to present all of them in form of a short paper. The aim of the paper is. therefore, [0 show the majority of the Polish data on the phone of the Bohemian Massif ODes with further implications to the Western Europe.

Several items should be discussed before an interpretation of the age data. namely: -what is the value obtained in the laboratory, - what do we date exactly. and -with which reproducibility do we get an age information for mineral or rock.

In short - experimental results represent not an age but radiometric measurements which are further calculated into the apparent age values of the radioactive system. The age itself is not "absolute" - it is a completely relative value which might be properly understood only in relation to geological and geochemical processes.

Geochronological data of the Variscides are presented starting from the granitoids of the Polish part of the Sudetes. These rocks have been dated with K - At method (Tab. 3, Fig. 2) and in a few cases only - by Rb-Sr and FT ones (Tab. 4). Two or three distinct maxima of age might be ob· served corresponding to the values of: 280, 310 and 330 my.

The Sudetes represent a marginal part of the Bohemian Massif. The K - Ar data are, therefore, quoted for the whole complex (Tabs. 5, 6). The granitoids of the central part of the Massif show the oldest age (about 340 my) whereas granites of both central and marginal zones seem to be younger (about 300 my).

The Bohemian Massif in its turn is in Europe not separated as it concerns its Variscan activity and metamorphism (Fig. 4). It belongs to the numeral plutons which extend from the Iberian Peninsula to the Middle Europe. Fig. 5 shows the European Variscides on the ground of the Caledonides as well as Tab. 7 presents some radiometric Rb - Sr data of the Bohemian Massif and other complexes of Western Europe.

Basing on the geochronological data it might be concluded that Variscan granitoids might be divided into synorogenic (320 - 370 my or 340 - 360 my according to the different authors) and late - or postorogenic ones (250 - 310 my or 290 - 300 my, respectively). The low grade metamorphic maxima occurred possibly about 330 ± 15 and 300 - 310 my ago. Polish granitoids belong to the second group showing, however, some regional variation not discussed in paper.