Timing and processes of late Mesozoic mid-lower-crustal extension in continental NE Asia and implications for the tectonic setting of the destruction of the North China Craton: Mainly constrained by zircon U–Pb ages from metamorphic core complexes

Late Mesozoic large-scale crustal extension occurred across the northeastern section of continental Asia, and metamorphic core complexes (MCCs) are the major features in this region. This paper presents new zircon U–Pb and Ar–Ar ages of biotite and hornblende for pre- or syn- (mylonitized) and post-kinematic (unmylonitized) intrusions from these MCCs, and attempts to determine the timing of initiation, duration and termination of the mid-lower-crustal ductile extension by integrating with geochronological data from previous studies. These MCCs have similar 40Ar/39Ar biotite and hornblende ages (130 to 110 Ma, mainly from 130 to 120 Ma), which are interpreted as the cooling ages and represent the timing of the late-stage formation of the MCCs. Zircon U–Pb ages for syn- to post-kinematic intrusions from some MCCs suggest that the mid-lower-crustal extension probably initiated at ca. 150–145 Ma with a peak occurring 145–130 Ma and continued to 130–120 Ma, which is older than that suggested by 40Ar/39Ar ages by 10–20 Ma. The early ductile extension most likely initiated in Mongolia and northern China during the Late Jurassic (150–145 Ma) at a deeper level, then propagated southwards (140–130 Ma), and finally developed throughout the entire continental NE Asia (130–120 Ma). Based on the timing and processes of the extension, a geodynamic model and tectonic setting of the destruction of the North China Craton (NCC) are proposed.