Paleoproterozoic structural evolution of the southern segment of the Jiao-Liao-Ji Belt, North China Craton

Abstract

The Paleoproterozoic Jiao-Liao-Ji Belt separates the Eastern Block of the North China Craton into two small sub-blocks: the northern Longgang and the southern Rangrim blocks. However, it still remains unknown or controversial about the subduction polarity, collisional deformation and kinematics between two sub-blocks. The southern segment of the belt consists of the Paleoproterozoic Fenzishan and Jingshan groups, and Paleoproterozoic high pressure mafic granulites and serpentinites blocks which are located in the Jiaodong Complex. All of which are separated from the Jiaodong Complex of Neoarchean TTG gneisses by STZ1 ductile shear zones. Structural analysis in this study indicates that most of the rocks in all the units of the southern segment of the Jiao-Liao-Ji Belt underwent three distinct episodes of folding (D1 to D3) and two stage of ductile thrust shearing (STZ1 coeval to D1 and D2, STZ2 between D2 and D3). The D1 deformation formed penetrative axial planar foliations (S1), bedding-parallel ductile shear zone, mineral stretching lineations (L1), and rarely preserved small isoclinal D1 folds in the Jingshan and Fenzishan groups. In the Jingshan Group, however, penetrative deformational transposition resulted in stacking of sedimentary compositional layers which are separated by bedding-parallel ductile shear zones (STZ1) at a period of about 1956Ma to 1914Ma. The kinematic indicators of STZ1 in the Jingshan Group with resultant prograde peak metamorphism up to granulite facies grade and the Fenzishan Group with peak metamorphism up to amphibolite facies grade indicate NW-directed compression. D2 resulted in crustal thickening with retrograded medium pressure granulite facies grade at about 1914–1893Ma. The D2 deformation produced NW-verging asymmetric and recumbent folds, interpreted to have resulted from basement-involved thicken-skin structures. The Jiaodong Complex was also involved into the development of WNW-verging asymmetric tight folds associated with D2 in the Jingshan and the Fenzishan groups. Ongoing collision led to the development of transpressional ductile shearing (STZ2), forming the transpressional Taipingzhuang dextral ductile shear zone between the Jingshan Group and the southern Archean Complex and the transpressional Tading-Xiadian sinistral ductile shear zone between the Jingshan Group and the northern Archean Complex. All three lithotectonic units were superposed during the late D3 deformation with amphibolite facies metamorphism. The D3 deformation developed WNW-trending open to tight upright folds at about 1893–1875Ma. The structural pattern resulting from superimposition of D2 and D3 is a composite synform in the Fenzishan and Jingshan groups. The structural events of D1 and STZ1, and D2 and STZ2 deformation were possibly responsible for fast syn-collisional exhumation of the high pressure mafic granulites. The structural patterns and deformational history of the Fenzishan and Jingshan groups suggest a southeastward-directed oblique subduction beneath the northwestern margin of the Rangrim Block, and that the final scissor-shaped closure of the rift led to collision between the two blocks to form the coherent basement of the Eastern Block of the North China Craton.