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. Those surfaces are key features to understand the relations among climate, lithosphere and tectonic movements during ancient time. In this study, the petrological, mineralogical, and geochemical characteristics of Variscan basement rock and 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 which shows less intense physical weathering compared to the gabbroic diorite. In both segments, secondary minerals are dominated by illite and a mix-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 value 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 hydrothermal minerals such as adularia suggest subsequent overprint by (K-rich) hydrothermal 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 (LILE) > Rear earth elements (REE) > High Field Strength Element (HFSE). Concerning the REE, heavy rare earth elements (HREE) are less depleted than light rare earth elements (LREE). 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, with the results, a new workflow to eliminate distractions for paleoclimate evaluation and evolution is well developed.

Quantification of physical and chemical paleoweathering at the microscale: a new concept

“…The results acquired by Liang et al (2021) indicate that the weathering profile in the area was overprinted by K-metasomatism. This affects chemical weathering indices such as CIA (Fedo et al 1995;Zhou et al 2017;Fàbrega et al 2019).…”

“…Geochemical analysis including major and trace elements and C and O isotopes of the carbonate within the samples, was performed at the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Science. The detailed processes for the measurement of major and trace elements are documented in Liang et al (2021). For the measurement of C and O isotopes, the powder was first roasted in vacuum at 400 ℃ for 4 h to remove organic contaminants.…”

“…The latter contains volcanic rocks (Menning et al 2005). Previous studies assigned the Rotliegend sediment of the two drill cores GA2 and GA1 to the Moret Formation and the Langen Formation with basaltic andesite, respectively (Liang et al 2021). The weathering depth in the basaltic andesite is approximately 6 m, in the gabbroic diorite basement is approximately 6.5 m, and in the GA1 drill core, the weathering depth in the GA2 basement is approximately 8.2 m.…”

“…Three drill cores for this study that fulfil the following criteria were selected: (a) penetration through the Variscan nonconformity at a depth of at least 13 m to rule out modern weathering (Liang et al 2021), and (b) the basement rock should be different for each drilling. The selected drill cores were from north and south of the MGCZ.…”