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

Abstract. A substantial part of the subsurface geology in northwestern and central Europe is defined by the late Palaeozoic Variscan Orogeny (∼ 350 Ma). Our focus is mainly on veining in anchimetamorphic sedimentary rocks affected by this orogeny. Mineral veins serve as repositories for documenting the origin of subsurface fluid flows and dynamics, and dating them may provide crucial insight into the timing of orogenic and possible reactivation events. The Rursee area (Rhenish Massif, Germany), part of the Variscan foreland zone on the Avalonia microcontinent, represents a key locality for studying Variscan quartz vein formation. Based on structural grounds, two 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. We obtained Jurassic to Cretaceous ages, and isotopic analysis of the argon gases revealed that the fluid-rich quartz fractions release 39Ar in two distinct phases. Regardless of the salinity of fluid inclusions in quartz veins, stepwise crushing provides apparent K / Cl > 1. Electron probe micro-analyser data confirm the presence of K (39Ar) in the K-bearing mineral inclusions (e.g. sericite, white mica, and chlorite) and microcracks and possibly in the crystal lattice of quartz. Secondary fluid inclusions or K-bearing mineral inclusions and/or the crystal lattice of quartz, which formed subsequently in the Variscan vein fractures, provide a plausible explanation for the young apparent isotopic ages. Deformation-induced quartz sub-grains may suggest that obtained maximum apparent ages are likely to reflect post-Variscan fluid-assisted reactivation–recrystallization due to tectonic activity or its cooling moment during the Jurassic–Cretaceous period rather than the original Variscan vein formation.