Structural evolution and 40Ar/39Ar dating of the Zanhuang metamorphic domain in the North China Craton: constraints on Paleoproterozoic tectonothermal overprinting

The Zanhuang domain in the hinterland of the North China Craton, consists of Archaean, Paleoproterozoic, Mesoproterozoic and Palaeozoic rocks, and was previously considered a Mesozoic metamorphic core complex. Our reappraisal of regional structural history and mineral 40Ar/39Ar geochronology shows that the domain was subjected to a number of deformation events and had a heterogeneous uplift-cooling history during the Paleoproterozoic. Four primary deformation events have been recognized in the domain. The first event reflects the earliest deformation in the region, however, only closed and rootless folds are preserved. D2 was dominated by WNW-ESE shortening and top-to-ESE thrusting in a collisional environment. D3 was a divergent extensional and ductile shearing event, driven by the post-collisional extensional collapse and exhumation of the thickened crust in the domain. D4 was characterized by localized ductile shearing with the development of three NNE-trending ductile shear zones. The 40Ar/39Ar plateau ages derived from the thermochronology analyses of the biotite concentrates from the metamorphic rocks are interpreted as cooling ages through biotite 40Ar/39Ar closure temperature (300 °C) and this allows us to constrain the timing of deformation events. In conjunction with the evidence from structural analyses and previous geochronological data, we conclude that 1870 and 1870–1826 Ma define the timing of D1 and D2 events, respectively, while 1826–1793 Ma represents the age of D3. The last event (D4) occurred at 1689–1633 Ma, as constrained by 40Ar/39Ar plateau ages of biotite concentrates from the mylonites within the ductile shear zones. These data, together with K-feldspar 40Ar/39Ar geochronology, allows us to depict the cooling history of the region during the Paleoproterozoic and Mesoproterozoic. It appears that the uplift-cooling rate varies temporally and spatially in the region.