Application of in-situ U-Pb-He double-dating on detrital zircons – an example of Alpine sediments from the Inn river and its tributaries

In-situ U-Pb-He double-dating of detrital zircon provides information regarding formation and cooling ages of source rocks. Its advantage over conventional dating methods is the unique ability to obtain a high quantity of precise single zircon data with constraints for both low- and high-temperature chronology. In provenance studies the information collected by double-dating can be used to reconstruct more complex tectonic settings and geologic evolution of individual zircons. Here, it is used to distinguish the components and tentative mixing ratios of sediments from multi-source catchments with the aim of assigning them to their respective sources. The European Alps provide a great natural laboratory to test the capabilities of the method. Considering their well-studied and complex tectonic evolution, i.e. the combination of Proterozoic to Tertiary magmatism and high-grade metamorphism with spatially strongly contrasting Late Mesozoic to Neogene exhumation, none of the conventional chronological tools can draw a nearly as accurate picture. Detrital zircons from modern river sands taken in the catchments of the Alpine Inn and its tributaries were dated: Ages derived from the Zillertal (dewatering mostly the Tauern window) and the Ötztal (incising the Austroalpine basement nappes) correspond to published data and can explain the complex age distribution of zircons collected near the end of the Alpine Inn. Double-dating is extraordinarily well suited to decipher such in-depth information, especially in complex orogens where different structural units experienced different overprinting of isotopic systems, leading to high spatial contrasts in low- and high-temperature chronological data.