Albitization and oxidation of Variscan granitoid rocks related to the post-Variscan paleosurface in the Sudetes (Bohemian Massif, SW Poland)
Résumé
The reddened granitoid facies in the basement of the Polish Sudetes exhibits two categories of alteration spatially arranged
with respect to fractures: (1) saussuritization and sericitization within light-colored facies in the interior of fracture-bounded
blocks and (2) albitization and hematization in reddened facies occurring adjacent to fracture walls. These alterations are
associated with the chloritization of primary ferromagnesian minerals and the development of secondary minerals such as
quartz, K-feldspar, apatite, prehnite, calcite, and titanite. We link these parageneses and the observed zonation to a unique
alteration event consisting of an interplay of chemical reactions of variable spatial extent. The complete albitization of the
feldspars (plagioclase and K-feldspar) adjacent to fractures points to a significant supply of Na for albite neoformation and
the availability of oxygen to form the associated hematite. The dating of the iron oxides by paleomagnetism and the second-
ary monazite associated with the albitized facies by U-Th-Pbtotal unambiguously indicates their post-Variscan ages. In this
context, the alterations are related to a regionally widespread post-Variscan paleosurface. The weathering profiles formed
in phreatic groundwater environments downgradient of highlands that provided a hydrological head. Sodium supply likely
originated from gigantic salt playas that characterized Permian and Triassic lowlands. Weathering was interrupted by the
Mesozoic transgression. The recognition of these weathering profiles over extensive parts of the Variscan Belt provides the
basis for reconstructing the post-Variscan paleosurface and constraining Triassic and post-Triassic geodynamics, including
erosion rates and geomorphological evolution of the Paleozoic massifs in Europe.