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Michael Waitzinger and Fritz Finger

Abstract

Complexly zoned microcrystals of uraninite were encountered in orthogneiss from the central Tauern Window in Austria (K1 gneiss, Felbertal scheelite mine) and analysed in-situ for U, Th and Pb with state-of-the-art FE-SEM/EDX techniques. A three times finer spatial resolution was achieved using an acceleration voltage of 8 kV, compared to the classic 15–20 kV set-up of U–Th–total Pb electron microprobe dating. The lower voltage allows a spheroid of material with a diameter of only 0.3 µm to be selectively analysed. Careful tests on three uraninite reference materials show that the low-voltage method yields sufficient precision and accuracy for U–Th–total Pb uraninite dating, with errors on individual spot ages in the order of 10–30 Ma. By means of this innovative analysis technique, small-scale age zoning patterns could be resolved and dated in the uraninite microcrystals from the orthogneiss. Based on microstructures observed in backscattered electron images we interpret that an older uraninite generation in the rock, with a late Permian formation age (~260 Ma), was recycled two times through a coupled dissolution–reprecipitation process at ~210 Ma and at ~30 Ma. The younger dissolution–reprecipitation phase at ~30 Ma coincides with the Alpine regional metamorphism (lower amphibolite facies). The two older ages (~210 Ma and ~260 Ma) have been previously recognized in rocks from the Tauern Window by uraninite dating, but it is the first time here that both are recorded in the same rock and even the same uraninite grain. The present study shows that recrystallized accessory uraninite can provide a sensitive geological “hard disk” where several discrete thermal events of an area are stored. In addition, our work attests that the mineral uraninite has an unexpected geochronological robustness, even on the microcrystal scale.

Open access

Fritz Finger, Axel Gerdes, Miloš René and Gudrun Riegler

The Saxo-Danubian Granite Belt: magmatic response to post-collisional delamination of mantle lithosphere below the southwestern sector of the Bohemian Massif (Variscan orogen)

On the basis of the synchronicity of geochronological data and the similarity of granite types, it is proposed that the mid-Carboniferous Fichtelgebirge/Erzgebirge Batholith in the Saxothuringian Zone of the central European Variscan Fold Belt and the South Bohemian Batholith in the Moldanubian Zone (including the intervening Oberpfalz and Bavarian Forest granite areas) belong to one coherent and cogenetic, ca. 400 km long plutonic megastructure. Unlike older (syn-collisional) plutonic structures in the Bohemian Massif, this Saxo-Danubian Granite Belt (nov. nom.) has developed discordant to the Devonian/Early Carboniferous collision-related tectonic architecture of the Bohemian Massif. It is argued that the Saxo-Danubian Granite Belt formed in response to a post-collisional detachment of lithospheric mantle below the south-western sector of the Bohemian Massif.

Open access

Vanja Biševac, Erwin Krenn, Fritz Finger, Borna Lužar-Oberiter and Dražen Balen

Abstract

Monazite age dating, detrital heavy mineral content and whole-rock geochemistry provided insight into the provenance, depositional history and paleogeological setting of the Radlovac Complex very low- to low-grade metasedimentary rocks (South Tisia, Slavonian Mountains, Croatia). Electron microprobe based Th-U-Pb dating of detrital monazite indicates a Variscan age of the protolith (330 ± 10 Ma). The detrital heavy mineral assemblages of representative metasedimentary rocks are dominated by apatite, zircon, tourmaline and rutile accompanied by minor quantity of epidote/zoisite, monazite and titanite. Judging from the heavy mineral assemblage, felsic igneous rocks served as the source material. This is consistent with the major and trace element spectrum of studied metasedimentary rocks characterized by high concentration of Th, high L + MREEs and high ratios of La/Sc, Th/Sc, La/Co, Th/Co and Th/Cr. The occurrence of magmatic monazite, zircon and xenotime and the absence of metamorphic heavy minerals suggest that granitoids, migmatites and migmatitic gneisses served as one major source for the metapsammites. Such rock types are commonly exposed in the Papuk Complex of the older surrounding complexes, while the Psunj Complex also contains metamorphic rocks. This is in good correlation with the monazite ages presented here which fits better with ages of Papuk Complex representative rocks than with those of the Psunj Complex known from the literature. Overall, data show that the Radlovac Complex represents the detritus of the local Variscan crust characterized by granitoid bodies, migmatites and migmatitic gneisses typical for the Papuk Complex.