U‒Pb ages and Hf isotopic composition of zircons in Austrian last glacial loess: constraints on heavy mineral sources and sediment transport pathways

Újvári, Gábor and Klötzli, Urs (2015) U‒Pb ages and Hf isotopic composition of zircons in Austrian last glacial loess: constraints on heavy mineral sources and sediment transport pathways. International Journal of Earth Sciences = Geologische Rundschau, 104 (5). pp. 1365-1385. ISSN 1437-3254, ESSN: 1437-3262

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Abstract

Loess sediments in Austria deposited ca. 30‒20 ka ago yield different zircon age signatures for samples collected around Krems (SE Bohemian Massif; samples K23 and S1) and Wels (half-way between the Bohemian Massif and the Eastern Alps; sample A16). CL imaging reveals both old, multi-stage zircons with complex growth histories and inherited cores, and young, first cycle magmatic zircons. Paleoproterozoic ages between 2200 and 1800 Ma (K23 and S1), an age gap of 1800- 1000 Ma for S1 and abundant Cadomian grains indicate NW African/North Gondwanan derivation of these zircons. Also A16 yields ages between 630-600 Ma that can be attributed to 'Pan-African' orogenic processes. Significant differences are seen for the <500 Ma part of the age spectra with major age peaks at 493-494 Ma and 344-335 Ma (K23 and S1), and 477 and 287 Ma (A16). All three samples show negative initial ɛHf signatures (‒25 to ‒10, except one grain with +9.4) implying zircon crystallization from magmas derived by recycling of older continental crust. Hf isotopic compositions of 330-320 Ma old zircons from S1 and K23 preclude a derivation from Bavarian Forest granites and intermediate granitoids. Rather all the data suggest strong contributions of eroded local rocks (South Bohemian pluton, Gföhl unit) to loess material at the SE edge of the Bohemian Massif (K23 and S1), and sourcing of zircons from sediment donor regions in the Eastern Alps for loess at Wels (A16). We tentatively infer primary fluvial transport and secondary aeolian reworking and re-deposition of detritus from western/southwestern directions. Finally, our data highlight that loess zircon ages are fundamentally influenced by fluvial transport, its directions, the interplay of sediment donor regions through the mixing of detritus and zircon fertility of rocks, rather than paleo-wind directions.

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