Rock alteration at the post-Variscan nonconformity: implications for Permo-Carboniferous surface weathering versus burial diagenesis

Nonconformity surfaces are key features to understand the relations among climate, lithosphere and tectonic movements in Earth history. One of the most prominent stratigraphic surfaces in Central European is the post-Variscan nonconformity separating the metamorphic Variscan basement from non-metamorphic platform deposits starting in the Upper Carboniferous, but more widely distributed since the Permian. This study aimed to analyze and distinguish the physical and chemical processes happening during exposure of the surface as well as during subsequent burial diagenesis on a drill core located on the eastern graben shoulder of the Upper Rhine Graben. The core penetrates through a Lower Permian volcano-sedimentary succession into the crystalline basement which here consists of Cadomian plutonic rocks. Samples were analyzed across the nonconformity by means of polarization microscopy, environmental scanning electron microscope, X-Ray diffraction, X-ray fluorescence and Inductively Coupled Plasma Mass Spectrometry. In both segments, secondary minerals are dominated by illite and a mix-layer phase of illite and smectite (I/S). The corrected weathering indices indicate an intermediate to unweathered degree in the gabbroic diorite and an extreme to unweathered degree in the basaltic andesite. The τ value for both parts indicates an abnormal enrichment of K, Rb, and Cs. Accompanying minerals such as adularia suggest subsequent overprint by (K-rich) fluids during burial diagenesis which promoted the conversion from smectite to illite. Overall, a new workflow to eliminate distractions for paleoclimate evaluation and evolution has been developed. Our study shows that features of supergene physical and chemical paleo-weathering can be disentangled by a multi-proxy approach.