⁴⁰Ar/³⁹Ar age constraints on the formation of fluid-rich quartz veins from the NW Rhenohercynian zone (Rursee area, Germany)

Abstract. The late Palaeozoic Variscan orogeny (~350 Ma) dictates a significant part of the subsurface geology in north-western and central Europe. Our focus is particularly on veining that occurred in metamorphosed sedimentary rocks that are affected by this orogeny. Vein minerals serve as repositories for documenting the origin of subsurface fluid flows and dynamics, and dating them provides crucial insight into the timing of orogenic and possible reactivation events. The Rursee area (Rhenish Massif, Germany) that is part of the Variscan foreland zone on the Avalonia micro-continent represents a key locality for studying Variscan quartz vein formation. Based on structural grounds, the two different groups/types of Rursee quartz veins have been linked with the early stages of Variscan, but their absolute ages are still unknown.

The aim of this study is to date these quartz veins using the ⁴⁰Ar/³⁹Ar stepwise crushing method based on the radioactive decay of ⁴⁰K dissolved in high salinity fluid inclusions (FIs). We obtained Jurassic to Cretaceous ages, and the isotopic analysis of argon gases revealed that the fluid-rich quartz fractions release ³⁹Ar in two distinct phases. Regardless of quartz veins FIs salinity, stepwise crushing provides apparent K/Cl >1. Electron Probe Micro Analyser data confirm the presence of the K (³⁹Ar) in the K-bearing mineral inclusions (e.g., sericite, mica, and chlorite) and in microcracks and possibly in the crystal lattice of quartz.

K-bearing mineral inclusions and/or crystal lattice of quartz, which form in the Variscan-origin vein fractures, provide a plausible explanation for the young apparent isotopic ages. The presence of the quartz sub-grains may suggest that obtained ages are likely to reflect post-Variscan reactivation-recrystallisation due to tectonic activity or its cooling moment during the Jurassic-Cretaceous period rather than the original Variscan vein formation.

This study emphasizes the complexities of isotopic dating of FIs, as well as the importance of careful interpretation of such data, especially in cases where different K-bearing mineral inclusions and/or radiogenic argon from crystal lattice obscure the initial FIs signal.