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

Abstract

Abstract. A nonconformity refers to a hiatal surface located between metamorphic or igneous rocks and overlying sedimentary or volcanic rocks. These surfaces are key features with respect to understanding the relations among climate, lithosphere and tectonic movements during ancient times. In this study, the petrological, mineralogical and geochemical characteristics of Variscan basement rock as well as its overlying Permian volcano-sedimentary succession from a drill core in the Sprendlinger Horst, Germany, are analyzed by means of polarization microscopy, and environmental scanning electron microscope, X-Ray diffraction, X-ray fluorescence and inductively coupled plasma 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 volcanic rock is a basaltic andesite that shows less intense physical weathering compared with the gabbroic diorite. In both segments, secondary minerals are dominated by illite and a mixed-layer phase of illite and smectite (I–S). The corrected chemical index of alteration (CIA) and the plagioclase index of alteration (PIA) indicate an intermediate to unweathered degree in the gabbroic diorite and an extreme to unweathered degree in the basaltic andesite. The τ values for both basaltic andesite and gabbroic diorite indicate an abnormal enrichment of K, Rb and Cs that cannot be observed in the overlying Permian sedimentary rocks. Accompanying minerals such as adularia suggest subsequent overprint by (K-rich) fluids during burial diagenesis which promoted the conversion from smectite to illite. The overall order of element depletion in both basaltic andesite and gabbroic diorite during the weathering process is as follows: large-ion lithophile elements (LILEs) > rare earth elements (REEs) > high-field-strength elements (HFSEs). Concerning the REEs, heavy rare earth elements (HREEs) are less depleted than light rare earth elements (LREEs). Our study shows that features of supergene physical and chemical paleo-weathering are well conserved at the post-Variscan nonconformity despite hypogene alteration. Both can be distinguished by characteristic minerals and geochemical indices. Based on these results, a new workflow to eliminate distractions for paleoclimate evaluation and evolution is developed.

Multiscale Characterisation of Fracture Patterns of a Crystalline Reservoir Analogue

“…In the Mainzer Berg vicinity, main directions are N015-030 • E, N060 • E, N130 • E, and N150 • E. N015-N030 • E is the primary deformation direction, indicating two possible origins for these deformation features: (1) N015-030 • E structures to the shear direction of the Otzberg fault zone (N010 • E), and (2) influence of R (N060 • E) and R' (N015-N030 • E) conjugated structures to the Messel strike-slip (N045 • E) system. Directions N130 • E and N150 • E may be associated with the role of conjugated structures of the URG shearing zone [25], which were active since the Permian (faulting and weathering [69]). These directions are also present at the regional scale, linked with the shearing phase D4 [15,18], and thus could play a role in the orientation of favourable for permeable fault zones and fracture corridors.…”

Komparasi Karakter Pengayaan Logam Pada Dua Seksi Pluton Lassi Di Wilayah Solok, Provinsi Sumatra Barat

“…Perbedaan pada dua profil yang diteliti terutama disebabkan oleh ketidaksamaan sistematika pelapukan atau batuan induknya (Dolui et al, 2016;Liang et al, 2021). Kedua profil masih berada pada iklim yang sama dan berjarak <25 km sehingga perbedaan mekanisme pelapukan diabaikan.…”

Structural Architecture and Permeability Patterns of Crystalline Reservoir Rocks in the Northern Upper Rhine Graben: Insights from Surface Analogues of the Odenwald

“…The Permian extension formed large intra-mountainous basins filled with coal and siliciclastics (e.g., Saar-Nahe, Lorraine) [61,62], and records the beginning of the basement weathering [63,64].…”