Polyphase deformation of a Paleozoic forearc–arc complex in the Beishan orogen, NW China

Abstract

The Lebaquan Complex in the central Beishan (NW China) plays a significant role in understanding the subduction–accretion–collision processes of the southern Central Asian Orogenic Belt. Four stages of deformation have been identified. The D1 deformation is characterized by a pervasive foliation (S1) defined by metamorphic layering and intrafolial isoclinally folded quartz veins (F1). The D2 deformation is marked by a crenulation cleavage (S2) mainly developed in the schists, widespread tight to open folds and asymmetric folds (F2) as well as pinch-and-swell structures (boudins). The D3 deformation is characterized by high-strain ductile shears that modified the earlier structures. Small-scale asymmetric folds, σ-type porphyroclasts and S–C fabrics in the schists, metacherts and pegmatites indicate dextral shearing in an east–west direction. The D4 deformation is represented by semi-ductile to brittle structures including open/gentle folds, kink folds and small-scale thrusts which overprinted earlier structures. The overall deformation pattern of this complex indicates a tectonic regime that changed from an earlier NS-directed compression to a later EW-directed transpression, and finally to an EW extension. Geochemical data show that the protoliths of the metasedimentary rocks were sourced from an active margin. New LA-ICP-MS zircon U–Pb dates indicate that the youngest age peak of detrital zircons from a garnet–mica–quartz schist is ~424Ma, which constrains the depositional age of the rocks protoliths. A syn-deformation leucogranitic dyke, which underwent D2 but not D1 formed at ~423Ma, and a post-deformation mafic dyke that intruded all the lithologies formed at ~280Ma. These data indicate that D1 occurred in the time interval of 424–423Ma, that D2 took place at or shortly after 423Ma, and that D3 and D4 formed in the period of 423–280Ma. Combined with other published data, we conclude that the Lebaquan Complex represents a subduction–accretionary complex formed in a forearc setting.