Application of in situ 40Ar/39Ar laser probe analysis to a continental shear belt

Shear zones represent important geological features that record the long-term tectonic history and geodynamic evolution of the Earth's crust. However, their complex and cyclic deformation history poses significant challenges for accurately applying 40Ar/39Ar thermochronology and interpreting geochronological data. To overcome these hurdles, we present a comprehensive case study from the Red River Shear Zone (RRSZ) in northern Vietnam, utilizing a multifaceted approach that combines detailed microstructural analyses, advanced in situ geochronological techniques, and numerical modeling. The study focuses on a garnet-sillimanite-biotite gneiss from the Day Nui Con Voi (DNCV) metamorphic complex within the RRSZ. Intra-grain age patterns are extracted from syn-shearing matrix biotite and pre-shearing biotite inclusions within garnet porphyroblasts using in situ 40Ar/39Ar laser probe dating. Age contour maps are used to display younger age zones connecting to fracture tips within garnet porphyroblasts indicating partial loss of radiogenic argon along microstructurally controlled pathways during continuous shearing episodes. To determine the possible initiation time for argon loss, numerical diffusion modeling and fractional argon loss modeling are employed. This method is useful to illustrate how deformation directly influences the heterogeneous age pattern and creates a fast diffusion network within mineral grains. Additionally, the use of mineral inclusions allows for a detailed comparison with matrix minerals to delineate more precise deformation histories. This case study highlights the significance of integrating microstructural analyses, in situ geochronological techniques, and numerical modeling in tackling the complexities of shear zone dynamics. By overcoming these challenges, researchers gain valuable insights into the tectonic and thermal evolution of shear zones, further advancing our understanding of the Earth's geodynamic processes.