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

• Field geology, geochemistry and geochronology were conducted for Lebaquan Complex.
• Four stages of deformation are identified in this part of the Beishan.
• D1 and D2 formed in a compression setting around 423 Ma.
• D3 and D4 formed in transpression and extension setting between 423 and 280 Ma.
• Lebaquan Complex formed in a forearc–arc setting of a northward subduction.

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 ~ 424 Ma, which constrains the depositional age of the rocks protoliths. A syn-deformation leucogranitic dyke, which underwent D2 but not D1 formed at ~ 423 Ma, and a post-deformation mafic dyke that intruded all the lithologies formed at ~ 280 Ma. These data indicate that D1 occurred in the time interval of 424–423 Ma, that D2 took place at or shortly after 423 Ma, and that D3 and D4 formed in the period of 423–280 Ma. Combined with other published data, we conclude that the Lebaquan Complex represents a subduction–accretionary complex formed in a forearc setting.