Provenance of <scp>P</scp>alaeo‐<scp>R</scp>hine sediments from zircon thermochronology, geochemistry, <scp>U</scp>/<scp>P</scp>b dating and heavy mineral assemblages

Abstract

Abstract Sediments deposited in the L ate C enozoic basins of the C entral E uropean R ift S ystem, including the U pper R hine G raben ( URG ) and the L ower R hine E mbayment ( LRE ), document the drastic extension of the R hine's catchment towards the C entral A lps in the L ate P liocene by distinct heavy mineral assemblages. This outstanding change in principal sediment sources should be accompanied by a change towards distinctly younger (i.e. Tertiary) detrital mineral cooling ages. Therefore, it provides a particularly well‐suited framework to explore the thermochronological provenance record in relation to heavy mineral assemblages. In this multi‐proxy approach we (i) exploit and elaborate detrital zircon ( U – T h)/ H e thermochronology ( ZH e) for sediment provenance surveys, (ii) document shortcomings if only a single geochronological method is employed, and (iii) obtain tighter constraints on the sources of P aleo‐ R hine sediments. Our results are based on P liocene and P leistocene sediment samples from the northern URG (drill core L udwigshafen P 36) and the LRE (lignite mine H ambach). In a L ate P liocene URG sample, V ariscan and P ermo‐ T riassic cooling ages dominate the age spectra of the ZH e and Z ircon fission track ( ZFT ) thermochronometers. The youngest ages are L ate C retaceous and these zircons show rare earth element signatures that suggest derivation from hydrothermally affected basement rocks of the URG margins. In contrast, a L ower P leistocene URG sample contains significant T ertiary age components that unequivocally indicate A lpine sources. This cardinal difference coincides well with a significant change in the heavy mineral assemblage. The extension of the catchment of the R hine towards the C entral A lps is considered to occur no earlier than the latest P liocene (i.e. after ~3.0 Ma). Despite strongly contrasting heavy mineral compositions, the P liocene and P leistocene samples from the LRE show largely similar ZH e and ZFT age distributions dominated by P ermo‐ T riassic and V ariscan ages. Admixture of zircon‐dominated, but overall heavy mineral‐poor sediment derived from local drainages of the R henish M assif likely explains this apparent contradiction in sediment provenance proxies. Tertiary cooling ages occur in both Pliocene and Pleistocene LRE samples. Zircon Th/U ratios and U/Pb ages reveal that the young age component in L ate P liocene sediments from the LRE is not derived from the A lps but from O ligocene trachytic members of the C entral E uropean volcanic centres of the V ogelsberg, W esterwald, and/or S iebengebirge. The integration of ZH e and ZFT techniques with zircon geochemistry and U/Pb geochronology adds the respective advantages of each method and allows for a very detailed picture of detrital zircon provenance.