The European continental crust through detrital zircons from modern rivers: Testing representativity of detrital zircon U-Pb geochronology

With the aim of testing the representativity of detrital zircon U-Pb datasets, we combine new and published data from 31 large river systems in Europe and compare this information with the well-known geology of the continent. Detrital zircon ages range from Archean to Cenozoic and age clusters can be linked to different orogenic cycles and the formation of supercontinents thus representing all relevant geological events in Europe at the continental scale. The Variscan Orogen is the largest episode of crustal reworking in Europe and consequently Variscan detrital zircons occur in all rivers. Other important age clusters are the Alpine 40-25 Ma, post-Alpine 10-0.2 Ma, the Caledonian 490-400 Ma, and the Avalonian-Cadomian 650-540 Ma. Detrital zircons of 1170-930 Ma and 1700-1400 Ma are significant in Scandinavia, as well as 2900-2500 Ma and ca. 1850 Ma in east Europe. Although the observed distribution of U-Pb detrital zircon ages is qualitatively representative of the geology of Europe at the continental scale, it is not always the case at the basin scale. Importantly, quantitative representativity for both, basin and continental scales, is not achieved. Variable zircon fertility and the sand generation potential of different lithologies highly bias the U-Pb age distributions. For instance, the magma poor Alpine orogen is relevant in minor age clusters in the Po and Danube rivers, but not in rivers draining to the west and north of the Alps, such as the Rhone and Rhine. Similarly, detrital zircons in the range of 10-0.2 Ma occur only in sediments from the Garonne and Loire rivers, although the Cenozoic igneous provinces are widespread in Europe. Detrital zircons from the post-Variscan Permian rocks were mostly found in the Po and Glomma rivers, where Permian granitoids are more abundant than volcanics of this age. Other natural causes of bias, such as transport and recycling can be identified based on the analyses of size and shape of detrital zircons. In the case of west Europe, our data shows that U-Pb detrital zircon patterns and zircon shape are mainly affected by recycling. Moreover, we tested corrections to the U-Pb age distribution for each river basin based on simple geomorphological parameters obtained from Digital Elevation Models. This attempt to overcome the overrepresentation of certain source regions resulted in a considerable change in age peak proportions towards a better quantitative representativity. However, full representativity seems unachievable, even in studies of modern sedimentary systems with known geology.

Automated summary

This study aims to test the representativity of detrital zircon U-Pb datasets by combining new and published data from 31 large river systems in Europe. The results show that although these data qualitatively represent the geological events in Europe at the continental scale, this is not always the case at the basin scale. The study finds that variations in zircon fertility and sand generation potential greatly bias the U-Pb age distributions.