Preprints
https://doi.org/10.5194/se-2020-221
https://doi.org/10.5194/se-2020-221

  25 Jan 2021

25 Jan 2021

Review status: a revised version of this preprint is currently under review for the journal SE.

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

Fei Liang1, Jun Niu1,2, Adrian Linsel1, Matthias Hinderer1, Dirk Scheuvens1, and Rainer Petschick3 Fei Liang et al.
  • 1Material and Geosciences, Institute of Applied Geosciences, Technical University of Darmstadt, Darmstadt, 64287, Germany
  • 2Faculty of Petroleum,China University of Petroleum-Beijing,Karamay Campus, Karamay, 834000, China
  • 3Faculty of Geosciences/ Geography, Goethe-University, Frankfurt, 60438, Germany

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.

Fei Liang et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on se-2020-221', Reinhard Gaupp, 02 Feb 2021
    • AC1: 'Reply on RC1', Fei Liang, 16 Mar 2021
  • RC2: 'Comment on se-2020-221', Henrik Friis, 28 Feb 2021
    • AC2: 'Reply on RC2', Fei Liang, 16 Mar 2021

Fei Liang et al.

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Short summary
In this study we conclude that, surface weathering has a prime control on the rock characteristic and also guides fluids through the system during deep burial process; The formation of the rock decomposition zone depends on rock composition, climatic conditions, and the duration of the process; The most important issue is that we provide a workflow for data reliability analysis for paleoclimate research.