Tracing Alpine sediment sources through laser ablation U–Pb dating and Hf-isotopes of detrital zircons



Alpine provenance studies based on conventional methods such as sandstone framework grain and heavy mineral analyses are now enhanced by improved techniques in laser ablation inductively coupled plasma mass spectrometry detrital zircon analysis. Although the conventional methods appear to have reached their limits of resolution in palaeogeographic problems, laser ablation inductively coupled plasma mass spectrometry U–Pb dating of detrital zircons adds the time dimension to the provenance analysis. Hafnium-isotope ratios measured on dated zircons give further information on the origin of the magmas in which the detrital zircons have grown. This study reports detrital zircon U–Pb dating and Hf-isotope results from sandstone formations related to rifting, drifting and subduction settings at different stages of the Alpine Tethys development. This study is a first evaluation of the correlation between U–Pb age and isotopic features of detrital zircons aimed at describing source terranes in different palaeogeographic domains in the Alpine Tethys area. Pan-African/Cadomian (Ediacaran–Ordovician), Variscan (Middle Devonian–Carboniferous) and Post-Variscan (Permian) detrital zircon populations are present in nearly all palaeogeographic settings, but in varying amounts. Single Mesoproterozoic and Palaeoproterozoic detrital zircons are found as minor populations. When comparing the northern and southern margins of the Alpine Tethys, the southern margin detrital sources are characterized mostly by a decreased occurrence or by the absence of Silurian–Devonian zircons. A major distinction between northern (Helvetic, North and Middle Penninic domains) and southern (Austro-Alpine and South Alpine domains) detrital sources is the occurrence of Triassic zircons at the southern Alpine Tethys margin during rifting and subduction stage sedimentation. Hafnium-isotope ratios measured on uppermost Permian–Triassic zircons from the South Alpine domain suggest a continental crust derivation of the hosting magmas, as expected in a continental rift environment. In the late stage of Alpine convergence (Late Cretaceous–Palaeogene), the Permian–Triassic zircons are reworked into basins situated on the northern Alpine Tethys margin.