Albitization and oxidation of Variscan granitoid rocks related to the post-Variscan paleosurface in the Sudetes (Bohemian Massif, SW Poland)

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 secondary monazite associated with the albitized facies by U-Th-Pb-total 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.

Automated summary

The reddened granitoid facies in the basement of the Polish Sudetes show two categories of alteration associated with fractures: (1) saussuritization and sericitization within light-colored facies in the interior of fracture-bounded blocks, and (2) albitization and hematization in reddened facies adjacent to fracture walls. These alterations are linked to the chloritization of primary ferromagnesian minerals and the development of secondary minerals. The recognition of these weathering profiles over extensive parts of the Variscan Belt provides important information for understanding the post-Variscan paleosurface and Triassic geodynamics in Europe.