Towards identifying the origin of metamorphic components in Austrian loess: insights from detrital rutile chemistry, thermometry and U e Pb geochronology

Újvári, Gábor and Klötzli, Urs and Kiraly, Franz and Ntaflos, Theodoros (2013) Towards identifying the origin of metamorphic components in Austrian loess: insights from detrital rutile chemistry, thermometry and U e Pb geochronology. Quaternary Science Reviews, 75. pp. 132-142. ISSN 02773791 (Submitted)

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Trace element (Cr, Nb, Zr) geochemistry and U-Pb geochronology of detrital rutile grains provide constraints on the nature of potential metamorphic sources of loess in Austria. Cr and Nb compositions reveal rutile sources with a predominance of metapelitic lithologies for all three loess samples. By contrast, Zr-in-rutile geothermometry indicates that formation temperatures for loess rutiles around Krems (at the Bohemian Massif=BM) proved to be ca. 100-125 °C higher (median T(°C) = 717 and 737 °C for two samples from Stratzing and Krems) than those at Wels (close to the Eastern Alps=EA; median T(°C) = 612 °C), implying that rutiles in loess at Wels were derived from Alpine lower amphibolite-grade rocks, while those in loess around Krems from rocks formed under upper amphibolite to granulite-grade conditions. Metamorphic temperatures recorded in rutiles from loess at the BM closely match the ‘two-reaction’ thermobarometry estimates of the last high-T overprint (7 to 11 kbar and 700 to 800 °C) of rocks making up the Varied series and Gföhl units around Krems. This event dated at 340 to 350 Ma is believed to be captured by the detrital rutile U-Pb data as well (e.g. 349±21 and 352±32 Ma). All these data demonstrate the significance of proximal BM sources (Varied series and Gföhl unit drained by the Danube) in loess formation around Krems. Fluvial transport is believed to have played a substantial role in entraining and accumulating rutiles for subsequent aeolian deflation for loess near Wels. While the ultimate sources of these grains are likely to be in the Eastern Alps, the recycling of these rutiles from the flysch zone of the EA cannot be ruled out. Despite the uncertainties of Cr-Nb-based source discriminations in some cases and the potential decoupling of Zr- and U-Pb systematics recognized in rutiles, this study demonstrates that rutiles may provide unique details on the nature and metamorphic history of their parent rocks, if used with caution and by knowing its limitations under certain conditions. It is anticipated that, beyond zircon, rutile geochemistry will become a powerful provenance tracer in loess studies too. This seems to be particularly important to gain a better understanding on the erosion and rutile fertility of different metamorphic terrains and their contributions to loess material, issues that have not been adequately addressed thus far.

Item Type: Article
Subjects: Q Science / természettudomány > QE Geology / földtudományok
Q Science / természettudomány > QE Geology / földtudományok > QE02 Geochemistry / geokémia
Depositing User: Dr. Gábor Újvári
Date Deposited: 11 Dec 2013 14:41
Last Modified: 11 Dec 2013 14:41

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