Elsevier, Quaternary Science Reviews, (75), p. 132-142
DOI: 10.1016/j.quascirev.2013.06.002
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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 (Tmedian = 717 and 737 °C for two samples from Stratzing and Krems) than those at Wels (close to the Eastern Alps = EA; Tmedian = 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 (7-11 kbar and 700-800 °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.