Not for commercisal uses
Age and mineralogy of supergene uranium minerals — Tools to unravel geomorphological and palaeohydrological processes in granitic terrains
(Bohemian Massif, SE Germany)
H.G. Dill a, A. Gerdes b, B. Weber c
a Institute of Geosciences, Gem-Materials Research and Economic Geology, Johannes-Gutenberg-University, Mainz D-55099 Mainz, Becherweg 21, Germany
b Goethe-University Frankfurt, Institute of Geosciences, Petrology and Geochemistry, Altenhoeferallee 1, D-60438 Frankfurt am Main, Germany
c Bürgermeister-Knorr Str. 8 D-92637 Weiden i.d.OPf., Germany
a b s t r a c t
Uranyl phosphates (torbernite, autunite, uranocircite, saleeite) and hydrated uranyl silicates (normal and betauranophane) found in various erosion levels and structures in the Late Variscan granites at the western edge of the Bohemian Massif, Germany, were the target of mineralogical investigations and age dating, using conventional and more advanced techniques such as Laser-Ablation-Inductive-Coupled-Plasma Mass Spectrometry (LA-ICP-MS). Supergene U minerals have an edge over other rock-forming minerals for such studies, because of their inherent ‘clock’ and their swift response to chemical and physical environmental changes on different scales. Uraniferous phoscretes and silcretes, can be used to characterize the alkalinity/acidity of meteoric/per descensum fluids and to constrain the redox conditions during geomorphic processes. This study aimsto decipher the geomorphological and palaeohydrological regime that granitic rocks of the Central European
Variscides (Moldanubian and Saxothuringian zones) went through during the Neogene and Quaternary in the
foreland of the rising Alpinemobile fold belt. The study provides anamendment to the current sub-division of the regolith by introducing the term “hydraulith”, made up of percolation and infiltration zones, for the supergene alteration zone in granitic terrains. It undercuts the regolith at the brink of the phreatic to vadose hydrological zones. Based upon the present geomorphological and mineralogical studies a four-stagemodel is proposed for the evolution of the landscape in a granitic terrain which might also be applicable to other regions of the European
Variscides, considering the hydrological facies changes along with paleocurrent and paleoslope in the basement and the development of the fluvial drainage system in the foreland. Stage I (Umineralizationinthe infiltration zone) is a mirror image of the relic granitic landscapewith high-altitude divides and alluvial–fluvial terraces. Its characteristic features are preserved in the uplifted hinterland of a peneplain which in this case is tilted towards a lacustrine basin. Stage II (Umineralization in the infiltration zone, regolith and
saprock) includes two sub process, planation and exposure, resultant in the exposure of inselbergs and quartz ridges in front of the hinterland (stage I). Stages III and IV(Umineralization in percolation zone and saprock) are controlled by the base level lowering in the foreland. Rapid incision caused pinnacle-like tors and large granitic land forms to form, whereas a slow-down of fluvial incision favored its destruction and the development of weathering pits of different kinds. A full blown cycle of planation and incision lasted for approx. 10 Ma, a stagewhich covers planation and exposure, resulting in the formation of domal structures which lasted for as much as 2 Ma. Climate is an important factor but themost important factors for the geomorphological processes shaping the granitic landscape in the study area are uplift and erosion. The study area is locatedwithin the stress field of an ancient Variscan craton (Mesoeurope) and a highly mobile Alpine fold belt (Neoeurope). The rate of vertical displacement in the mobile parts of the crust had a long-distance effect also on the granitic terrains in the rigid parts of the crust.
ساحة النقاش