Unravelling the history of mountain belts through U-Pb and Lu-Hf zircon and 40Ar/39Ar white mica dating: the Eastern Alps

Prof. Dr. Franz Neubauer, Dept. of Environment and Biodiversity, Geology Division, Paris-Lodron University of Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria

Abstract

Radiogenic isotopes of igneous and detrital minerals from various clastic rocks of mountain belts are used to reveal tectonic and sedimentary processes, which are otherwise difficult to detect. Here, we discuss the results of U-Pb and Lu-Hf zircon systems, and ⁴⁰Ar/³⁹Ar white mica dating in the Eastern Alps. Zircon and white mica are chemically and mechanically stable and occur in magmatic, metamorphic and sedimentary rocks. During subsequent metamorphism, zircon is resistant against high temperature, >650 °C (U-Pb) and 900 °C (Lu-Hf). The Lu-Hf zircon system is used as tracer of initial magma separation from the (depleted) mantle, the U-Pb zircon system records magmatic crystallization. The ⁴⁰Ar/³⁹Ar white mica system is stable up to 400–450 °C and date either formation or cooling after high-grade metamorphism.

Application of detrital U-Pb zircon ages on two major rivers draining Eastern Alps do not record any sign of Alpine orogeny or metamorphism. Consequently U-Pb zircon studies can entirely miss the record of collisional orogeny in cool, magma-poor collision orogens. In contrast, ⁴⁰Ar/³⁹Ar white mica ages record Early and Late Alpine metamorphism but is limited to reveal the pre-orogenic history. U-Pb zircon and ⁴⁰Ar/³⁹Ar white mica yield different information in provenance studies. U-Pb zircon dating of magmatic and clastic rocks indicate intense formation of in part juvenile magmatic rocks between 630 and 230 Ma. Felsic rocks dominate the older age groups, and increasingly young mafic rocks were dated, specifically between 500 and 250 Ma. Hf isotopes record increasing juvenile input since ~630 Ma. Two different groups in respect to Mesoproterozoic de-pleted mantle ages are shown: (1) one group with a Mesoproterozoic age gap typical for Gondwana-derived units, and (2) a rare group with Mesoproterozoic ages recording a new tectonic element in the Austroalpine basement of the Alps.