Provenance of Pleistocene Rhine River Middle Terrace sands between the Swiss-German border and Cologne based on U-Pb detrital zircon ages

Detrital zircon U-Pb age distributions derived from samples representing ancient or relatively young large-scale continental drainage networks are commonly taken to reflect the geochronological evolution of the tapped continental area. Here, we present detrital zircon U-Pb ages and associated heavy mineral data from Pleistocene Rhine River Middle Terrace sands and equivalents between the Swiss-German border and Cologne in order to test the commonly assumed Alpine provenance of the material. Samples from eight localities were analyzed for their heavy mineral assemblages. Detrital zircon U-Pb ages were determined by laser ablation inductively coupled mass spectrometry on selected samples from five locations along the Rhine River. The zircon age populations of all samples show a similar distribution, their main peaks being between 300 and 500 Ma. Minor age populations are recognized at 570 and 1,070 Ma. The 300-400 Ma maximum reflects the Variscan basement drained by or recycled into the Rhine River and its tributaries. The 400-500 Ma peak with predominantly Early Silurian ages points to Baltica or to the mid-German crystalline rise as original sources. One distinct peak at c. 570 Ma probably represents input from Cadomian terranes. The Precambrian U-Pb ages are compatible with derivation from sources in Baltica and in northern Gondwana. The heavy mineral populations of Middle Terrace sands and equivalents are characterized to a variable extend by garnet, epidote, and green hornblende. This association is often referred to as the Alpine spectrum and is considered to be indicative of an Alpine provenance. However, hornblende, epidote, and garnet are dominant heavy minerals of collisional orogens in general and may also be derived from Variscan and Caledonian units or from intermittent storage units. A remarkable feature of the detrital zircon age distribution in the Rhine River sediments from the Swiss-German border to Cologne is the absence of ages younger than 200 Ma and in particular of any ages reflecting the Alpine orogeny between c. 100 and 35 Ma. Sediments from rivers draining the equally collisional Himalaya orogen contain detrital zircons as young as 20 Ma. Our results question the assumption that Pleistocene Rhine River sediments were directly derived from the Alps. The lag time between the formation and deposition age of the youngest zircon in the studied Pleistocene Rhine River deposits is 200 Ma. Together with the absence of Alpine zircon ages, this stresses that detrital zircon age data from ancient sedimentary units found in poorly understood tectonic or paleogeographic settings need to be interpreted with great care, one could miss an entire orogenic cycle.