40AR/39AR DATING ON BRITTLE DEFORMATION: ANOTHER NEW APPROACH

Constraining the timing of brittle deformation, such as in high-angle normal or strike-slip faulting, is technically challenging. The challenge addressed here is a consolidation of 40Ar/39Ar dating of original feldspars that were intimately subjected to cataclastic deformation, which potentially induced a complete reset of their Ar-system. Hence, we present a new approach of age determinations from cataclastic K-feldspars of brittle faults and chlorite developed in cataclastic rocks of the Tan–Lu fault zone in eastern China. The brittle fault surfaces are characterized by cataclastic rocks and fault gouges, pseudotachylite, authigenesis of chlorite, and cataclastic K-feldspar. As K-feldspar cataclasis was accompanied by chlorite formation, it suggests that the deformation occurred at a temperature range of ~ 180 and 330 °C. The attempt is based on field investigations, microstructural analyses, BSE imaging, electron-microprobe analyses, X-ray diffraction, SEM results, and 40Ar/39Ar step-heating of K-feldspar separated from cataclastic mylonite, and granite of brittle fault zones. SEM and BSE images of individual K-feldspar crystals reveal distinct intracrystalline deformation features, including grain-scale faults (fractures) that occur parallel or at right angles to each other. The 40Ar/39Ar dating results provide well-defined plateau spectra of cataclastic K-feldspar grains or sub-grains along high-angle normal or strike-slip faults in the Tan–Lu fault zone (74.5 ± 1.3 Ma), and of chlorite that formed on the fault surface (74 ± 1 Ma). These data suggest complete resetting of the Ar-system of the cataclastic K-feldspars and new growth clay mineral, allowing to date the timing of their deformation.