Geochronological Investigations on Grey Gneisses from the Freiberg Region (Eastern Erzgebirge)

M. Tikhomirova Institute of Mineralogy from the Freiberg University of Mining and Technology, Freiberg, Germany

B. V. Belyatski Institute of Precambrian Geology and Geochronology, St. Petersburg, Russia

H.-J. Berger Saxonian Survey of Environment and Geology, Freiberg, Germany

E. A. Koch Saxonian Survey of Environment and Geology, Freiberg, Germany

Grey gneisses in the Eastern Erzgebirge have been long since subdivided into two subgroups. Hofmann (1965) introduced the terms "Inner gneisses" as substitution for so far used terms "Core gneisses" or "Lower Grey Gneiss Unit" and "Outer Gneisses" for "Upper Grey Gneiss Unit". The "Inner Gneisses" form a dome-like structure near Freiberg and are surrounded by "Outer Gneisses", differing by their fine-grained structure and abundant intercalations of garnet-bearing schists, muscovite gneisses, quartzites and some metacarbonates.

We carried out geochronological investigations on grey gneisses from the Inner and Outer unit and on some amphibolite bodies, forming lenses within these gneisses. We obtained internal Rb/Sr-isochrones (mica versus whole rock) from the Freiberg "Inner Gneiss", indicating ages between 330 and 320 Ma. These data has been interpreted as cooling ages, representing the final phase of Variscan metamorphism as documented already in the Western Erzgebirge (Tikhomirova et al., 1994) and in the Fichtelgebirge (Teufel, 1988; Kreuzer, 1989). This Variscan process is also reported by Sm/Nd-mineral isochrones from amphibolite bodies (Tikhomirova et al., 1995).

U/Pb analyses of "Inner grey gneisses" reveal a large amount of inherited zircons with different morphological habitus forming fan-shaped field in the U/Pb diagramm with upper intercept ages up to 2 b.y. and more. The multistage growth history of such zircons is obvious revealed in cathodoluminescenc and BSE-images. Sm/Nd model ages of 1.5 - 1.7 b.y. of these gneisses underline Lower Proterozoic provenance ages, indicating a period of major crust building at this time. Melting processes lead in some parts of the "Inner gneisses" (gnf) to formation of this S-type granodioritic orthogneiss (eNd (T=550Ma) = -4.9 to - 6.0) with growth of new longprismatic internally zoned zircons with high length to width ratios ("needle like zicons"). Our 207Pb/206Pb-evaporation ages of single zircon grains (Kober method) between 530 and 550 Ma manifest earlier obtained Cadomian crystallization ages (Bombach et al., 1989; Kröner et al., 1991; Kröner et al., 1995) of these orthogneisses.

The paragenetic origin of "Outer grey gneisses" is supported by the fact, that they do not contain new formed longprismatic zircons. U/Pb analyses yield 207Pb/206Pb-ages of discordant zircons between 950 and 2300 Ma, 207Pb/206Pb-evaporation ages of single grains are between 550 and 1900 Ma. It is interesting to note, that the younger ages between 550 and 1000 Ma are obtained from isometrically almost rounded zircon grains, the older from idiomorphic zircons. Sm/Nd model ages do not differ to those from Inner Freiberg gneisses, but eNd(T=550Ma) show slightly more negative valus (-6.4 to -7.8).


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