Provenance and alteration of granite gravels in Rotliegend beds from the northwestern Thuringian Forest: Results of petrography, geochemistry and zircon investigations

In this paper petrographical, geochemical and zircon morphological characte ristics of granite gravels from different sediment beds of the northwestern Thuringian Forest are investigated. These are compared with granites from the Ruhla Crystalline Complex, assumed to be the gravel suppliers. The stud y shows that an assignment of gravels to granites of the Ruhla Crystalline Complex is impossible using basic petrographical and mineral chemical metho des, as well as major element geochemistry. In contrast, zircon morphology spectra in combination with Pb-207/Pb-206 Single zircon ages prove that bou lders of the Eisenach Formation are genetically related to the Ruhla Granit e. A similar assessment can be made using Zr and Nb contents. The geochemic al signature of the gravels and boulders clearly indicate a hydrothermal al teration, characterized by the loss of Ca and Mg. This alteration took plac e after deposition of the gravels in the Rotliegend basins.