title: Exploring first-order hiatal surfaces: mineralogical, geochemical and petrological characteristics of the post-Variscan nonconformity in Central Europe
Creators: name: Liang, Fei
Publisher: name: UNSPECIFIED
Year: 2023

description: The post-Variscan nonconformity was formed during Permo-Carboniferous and widely distributed in Central Europe. While the tectonic evolution of the Variscan Orogeny has been well studied, research about the subsequent weathering and burial diagenesis is fairly scarce. The basement of the post-Variscan nonconformity in southwestern Germany contains different types of plutonic rock, which is partly covered by volcanic rock. The weathering profiles of both are well preserved. Compare to the modern weathering profiles, the paleo-weathering profile was overprinted by hypogene diagenesis. If it would be possible to compare quantitatively the weathering conditions with respect to the exposure time of different rocks, it would be possible to construct regional palaeoclimate and alteration models. Based on these quantitative models, predictions of rock alteration and its resulting properties are possible, which may help to assess reservoir and fluid flow characteristics. To achieve these goals the following scientific questions arise: (i) how to distinguish the supergene and hypogene alteration and minimize the errors of the weathering degree evaluation. In addition, for the weathering research about both the modern and paleo-weathering profiles, the physical weathering characteristics were less mentioned compared to the chemical weathering. Among these research, the physical weathering characteristics were only qualitatively described, which surely leads to inconvenience for the weathering research. Therefore, (ii) it is necessary to develop a method to quantify the physical weathering degrees. However, as the susceptibilities of different types of rock to both physical and chemical weathering are varied, (iii) the weathering indices need to be normalized by the susceptibility to reflect the weathering intensity, which also enables the comparison of weathering characteristics among different types of rock. (iv) Finally, previous studies focus mainly on one aspect of the weathering characteristics, a systemically model among macro/mesoscale, microscale, and mineralogical and geochemical data never accrues, therefore, a systemically model for weathering evaluation is urgently needed. To figure out the questions above, in total four drill cores with different basement lithologies and overlaid volcanic rock (gabbroic diorite and basaltic andesite, granodiorite, tonalite, and granite) from southern Germany (Sprendlinger Horst and Langenthal) were selected. In total 69 samples were collected and prepared into powder and thin section groups. The collected samples were measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS), X-ray fluorescence (XRF), and X-Ray Diffraction (XRD) for trace elements, major elements, and mineral compositions, the thin sections were also made, which are for the analysis by scanning electron microscope (SEM), backscattering electron microscope (BSE) and polarizing microscope. The proportion of primary minerals overall increases from the top to the bottom while the secondary minerals have an opposite trend. The XRD results indicate the clay minerals are dominated by illite and a mix-layer of illite and smectite (I/S) accompanied by a minority of kaolinite, vermiculite, and chlorite. Other secondary minerals are mainly composite of calcite and dolomite with a minority of metallic oxide. The depletion or enrichment degree of the elements is quantified by the τ value model, which is based on the relationship between the mobile elements and the immobile elements, such as Ti and Zr. The chemical weathering degree is quantified by the chemical index of alteration (CIA). To distinguish the supergene weathering and hypogene diagenesis for a paleo-nonconformity, a new workflow is established. The secondary clay mineral aspect is distinguished with the anhedral and euhedral forms observed under the SEM, the clay minerals with anhedral forms are attributed to supergene alteration while the euhedral forms are attributed to hypogene diagenesis. For the aspect of mineral compositions, the composition of the parent rock is considered, To compare both physical and chemical weathering intensity among different rock types, the physical weathering degree should be first quantified. To quantify the physical weathering, a method based on the ratio between the fracture area and the total area of the thin section under the polarizing microscope was developed. This method establishes a new index of physical weathering (IPW). To apply it regionally, normalization of the relevant rock strength along the weathering profile is crucial and enables the comparison of the physical intensity among different lithologies. For the normalization of chemical weathering indices, two new conceptions, the total weathering mess (TWM) and chemical weathering ability (CWA) were proposed based on the weathering rate among different mineral types. Similar to the normalization of IPW, the chemical index of alteration (CIA) is normalized by CWA to reflect chemical weathering intensity. With the results of both physical and chemical weathering intensity, the climate condition can be evaluated. With the optimized workflow for the evaluation of both physical and chemical weathering degrees along the paleo-weathering profile, the weathering characteristics among macro/mesoscale, microscale, mineralogical and geochemical characteristics along the basement of post-Variscan nonconformity are described. Based on this, a comprehensive workflow for the evaluation of weathering degree can be developed. This workflow integrated the weathering characteristics under different scales and set up relationships among different scales of these characteristics. The physical weathering was qualitatively classified at the macro/mesoscale and quantitatively at the microscale. The chemical weathering was quantified by the mineralogical and geochemical characteristics. Compare to previous studies, the physical and chemical weathering intensity are also compared among different lithologies which can be applied to evaluate the paleoclimate directly. The model and methods developed in the thesis are applied to the nonconformity in Sprendlinger Horst and Langenthal in southwestern Germany. The results show that the weathering products were overprinted by hypogene diagenesis. The hypogene diagenesis led enrichment of K, Cs Rb, and Ca. The input of K caused the transformation of smectite to illite and/or I/S. Both the inputs of K and Ca affect the evaluation of the chemical weathering degree. After correction of these two elements’ concentration, the CIA values along the surface of the nonconformity yield between 66 and 97. After being normalized by CWA, the chemical weathering intensity is between 70 and 83. The maximum physical weathering degree is between 1.5 and 18 while the physical weathering intensity is between 0.02 and 4. The higher the weathering degree the more depletion of the mobile elements will be. In addition, the transformation among clay minerals and the precipitation of minerals such as dolomite point to a temperature interval of 100 to 300 °C for the circulating fluids during burial. By the combination of the data from both fieldwork and laboratory analysis, an integrated model for the weathering evaluation is set up. In this model, the quantified chemical versus physical weathering intensity may shed a light on the research of palaeoclimate under which the nonconformity formed.