Paleoproterozoic polyphase deformation in the Helanshan Complex: Structural and geochronological constraints on the tectonic evolution of the Khondalite Belt, North China Craton

The Khondalite Belt is widely regarded as a Paleoproterozoic collisional orogen between the Yinshan and Ordos Blocks in the Western Block of the North China Craton. However, the deformation history of the Khondalite Belt is poorly understood, and many aspects of the tectonic processes involved in the collision remain enigmatic. In this paper, we carried out detailed field-based structural analyses integrated with geochronological studies in the Helanshan Complex, one of the multiply deformed high-grade gneiss terrains in the Khondalite Belt. Three major phases of deformation (D-1-D-3) were identified in the Helanshan Complex. D-1 mainly produced S(SW)-verging recumbent to overturned intrafolial isoclinal folds F-1, originally sub-horizontal penetrative transposition foliations S-1 and SSW-plunging mineral lineations L-1. D-1 deformational fabrics occurred coevally with the granulite-facies peak metamorphism. D-2 dominantly generated steeply SE(E)-plunging asymmetrical upright open folds F-2 that successively refolded the previous D-1 structures. D-3 gave rise to a series of NE- to E-trending ductile shear zones, featured by top-to-the-(N)NW oblique thrusting with a component of sinistral strike-slip movement. LA-ICP-MS U-Pb zircon dating results of seven deformation-related samples reveal that D-1 occurred at some time from 1954 Ma to 1911 Ma, most likely at 1954-1949 Ma. D-2 successively happened in the period of 1911-1870 Ma, and D-3 had broadly taken place at similar to 1870 Ma. The D-1-D-3 deformation sequences recorded tectonic processes including the initial top-to-the-SSW thrusting and crustal thickening, subsequently followed by exhumation of the khondalites in the Paleoproterozoic. Combined with previous studies, these new structural and geochronological data indicate that the Khondalite Belt underwent a long-term (>100 Myr) orogenic history and developed polyphase deformation, high-grade metamorphism as well as magmatism in response to the NNE-SSW-directed oblique collision between the Yinshan and Ordos Blocks at similar to 1.95 Ga.