The Saxonian Granulite Complex (SGC), situated at the northern margin of the Bohemian Massif, already became the type locality for the rock type "granulite" in the beginning of the last century. However, there are different opinions about the geological evolution from the SGC until today.
In this study we have tried to contribut to a better understanding of the geological development of the SGC. In the center of our investigations are the following main aims: Dating the age of the last high pressure metamorphism; what can we say about the relations between granulites and granites in the SGC; are there hints to other and older metamorphisms or other geological events in the SGC; and is it possible to find informations about the geological history of the precursor rocks?
In order to find the youngest geological event in the SGC, at first we have dated the granite of Mittweida, employing the conventional multigrain U/Pb zircon method. In accordance with an Rb/Sr-isochrone age from this rock by Haase et al. (1987) the upper intercept age was 351 ± 15 Ma. But obviously the zircons from this granite are not without exception crystallied by the intrusion of the granite into the surroundig granulite, and so the age perhaps is a mixing age and may be a few Ma older than the true age of the intrusion. This suspicion is confirmed by single zircon evaporation measurements from this granite and also from other granites from the SGC which result in a range of ages between about 330 Ma and 470 Ma (17 zircons). Two ages, exactly apart from each other - around 350 Ma and around 405 Ma - were founded by the single grain evaporation method on one zircon-grain from the granite of Mittweida by different evaporation steps. So it is assumed, that this zircons often are consisting of different old parts, that they contain inherited zircon material - and that zircons which are descended from the same rock do not have to yield the same age. The youngest obtained ages on this zircons from various granites are considered as the time of intrusion of the granites in the SGC, and this is about 330 Ma.
The obtained ages from zircons from the cordierit-gneis employing the single grain evaporation ranges between 320- and 365 Ma (5 zircons). It was observed, that from this rock sample the spherical zircon grains yields the younger- and the longish grains yields the older ages.
Youngest ages from zircons from granulits from the SGC are around 340 Ma, obtained by the vapor digestion techniques. Here also often the younger ages are descended from the more rounded zircon grains. These ages are in agreement with the 339 Ma-ages by Quadt (1993) for other granulites from the SGC. - Measurements employing the conventional multigrain U/Pb zircon method leads to 207Pb/206Pb-ages between 365- and 418 Ma, and CL-investigations show the inhomogeneous "interior" of this zircons. The age of the precursor rocks maybe characterised by the vapor digestion results and multigrain results older than 400 Ma.
Also the U/Pb-ages from monazites were determined, from a leucogranulite, from a granite and from a cordierit-gneis. Although the total obtained 207Pb/206Pb-ages vary between 282 ± 13 Ma (leucogranulite) and 357 ± 7 Ma (cordierit-gneis), each monazite sample from each investigated rock yielded nearly the same 207Pb/235U-age - about 315 Ma. If this age ist interpreted as "cooling-age", the cooling of the SGC has to be initially very fast, because the intrusion of the granites in the complex was obviously only a few Ma after high grade metamorphism.
Sm/Nd isotope measurements on garnet, clinopyroxene and whole rock from a garnet pyroxenite from the SGC yielded Sm/Nd isochron ages of 322 ± 11- and 345 ± 11 Ma. U-Pb ages of ca. 320 - 340 Ma, using the HF-vapor digestion techniques, were obtained on zircons from the same rock. This indicates, that both - garnet pyroxenites and the granulites - cooled at the same time. -
The single zircon evaporation measurements were carried out in the Institute of Mineralogy at the Freiberg University of Mining and Technology, all the other measurements at the MPI, Department of Geochemistry, in Mainz.
Haase, G., Gerstenberger, H. & Werner, C.-D., ZfI-Mitt. 133, 89-98 (1987).
Quadt, A.v., Geol Rundsch 82, 516-530 (1993).