40Ar/39Ar dating of cataclastic K-feldspar: A new approach for establishing the chronology of brittle deformation

Constraining the timing of brittle deformation, such as in high-angle normal or strike-slip faulting, is technically challenging. In this study we present a new approach of age determinations from cataclastic K-feldspars of brittle faults developed in rocks of the Tan-Lu fault zone and from Louzidian and Liaonan metamorphic core complexes in NE China. The attempt is based on field investigations, microstructural observations, back-scattered electron (BSE) imaging, electron-microprobe analyses, X-ray diffraction, scanning electron microscope (SEM) results, and 40Ar/39Ar step-heating of K-feldspar separated from brittle fault zones of cataclastic mylonite, granite and gneiss. SEM and BSE images of individual K-feldspar crystals reveal distinct grain deformation features, including grainscale faults (fractures and microfractures) that occur parallel or perpendicular to each other. The 40Ar/39Ar dating results provide well-defined plateau spectra for the cataclastic K-feldspar grains in the Tan-Lu fault zone (74.5 +/- 1.3 Ma), and of brecciated gneisses and mylonites that formed at depth in the Louzidian and Liaonan metamorphic core complexes (similar to 120-129 Ma and 116 +/- 2 Ma). These data suggest complete resetting of the Ar isotope system of the cataclastic K-feldspars, allowing to constrain the timing of their deformation.