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. A nonconformity refers to a hiatal surface locatedbetween metamorphic or igneous rocks and overlying sedimentary or volcanicrocks. These surfaces are key features with respect to understanding the relations amongclimate, lithosphere and tectonic movements during ancient times. In thisstudy, the petrological, mineralogical and geochemical characteristics ofVariscan basement rock as well as its overlying Permian volcano-sedimentarysuccession from a drill core in the Sprendlinger Horst, Germany, are analyzedby means of polarization microscopy, and environmental scanning electronmicroscope, X-Ray diffraction, X-ray fluorescence and inductively coupledplasma mass spectrometry analyses. In the gabbroic diorite of the basement,the intensity of micro- and macro-fractures increases towards the top,indicating an intense physical weathering. The overlying Permian volcanicrock is a basaltic andesite that shows less intense physical weatheringcompared with the gabbroic diorite. In both segments, secondary minerals aredominated by illite and a mixed-layer phase of illite and smectite (I–S). Thecorrected chemical index of alteration (CIA) and the plagioclase index ofalteration (PIA) indicate an intermediate to unweathered degree in thegabbroic diorite and an extreme to unweathered degree in the basalticandesite. The τ values for both basaltic andesite and gabbroic dioriteindicate an abnormal enrichment of K, Rb and Cs that cannot be observed inthe overlying Permian sedimentary rocks. Accompanying minerals such asadularia suggest subsequent overprint by (K-rich) fluids during burialdiagenesis which promoted the conversion from smectite to illite. Theoverall order of element depletion in both basaltic andesite and gabbroicdiorite during the weathering process is as follows: large-ion lithophileelements (LILEs) &gt; rare earth elements (REEs) &gt; high-field-strength elements (HFSEs). Concerning the REEs, heavy rare earth elements(HREEs) are less depleted than light rare earth elements (LREEs). Our studyshows that features of supergene physical and chemical paleo-weathering arewell conserved at the post-Variscan nonconformity despite hypogenealteration. Both can be distinguished by characteristic minerals andgeochemical indices. Based on these results, a new workflow to eliminatedistractions for paleoclimate evaluation and evolution is developed.

solid earth

Rock alteration at the post-Variscan nonconformity: implications for Carboniferous–Permian surface weathering versus burial diagenesis and paleoclimate evaluation