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

Abstract

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 grain-scale 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 (~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.