New geochemical results indicate a non-alpine provenance for the Alpine Spectrum (epidote, garnet, hornblende) in quaternary Upper Rhine sediment

Abstract

The heavy mineral assemblage of late Pleistocene to Modern Rhine river sediment in the Upper Rhine Graben is dominated by garnet, green amphibole and epidote. This so-called Alpine Spectrum has been taken to indicate an exclusive derivation from the Alps and has hitherto been investigated with optical methods only. We present the first single-grain geochemical data set of garnet and amphibole from the upper Rhine river and some of its main tributaries. We use the new data to test the alleged Alpine provenance in the Rhine.Our results show that the provenance of garnet, mostly pyrope-rich almandine is grain-size dependent. Particularly the pyrope content indicates that the 0.063–0.25 mm size fraction has an Alpine provenance whereas 0.25–0.5 mm grains derived from the Black Forest and the Vosges, which flank the Upper Rhine Graben. They mainly comprise of Paleozoic metamorphic and igneous rocks supplying a suite of heavy minerals similar to the Alpine Spectrum to the Rhine river. Amphibole, mostly Mg-hornblende, has an increasing TiO2 content from the south to the north of the graben, which also indicates an input from the graben shoulders. The new data indicate that the so-called Alpine Spectrum of heavy minerals in Rhine river sediment of the upper Rhine derived to significant degrees from non-Alpine sources. Furthermore, our results indicate a relatively uniform provenance for the Rhine River system during the past 1 Ma.