Zircon U-Pb dating and Hf isotopic studies on the Beishan complex in the southern Beishan orogenic belt

Beishan orogenic belt is an important part of the Central Asian Orogenic Belt (CAOB). A large number of high-grade metamorphic rocks are widely distributed in several Beishan orogenic tectonic units, which were considered as Precambrian basements and referred to the 'Beishan complex'. However, their formation ages and tectonic attributions and whether they experienced metamorphism process associated with the orogeny are still unclear. Here, we present zircon U-Pb age and Hf isotopic compositions of Shibandun amphibolite, Baidunzi gneiss and felsic veins from the Shibanshan block, which is located in the southern margin of the Beishan orogenic belt. The results show that both Shibandun amphibolite and Baidunzi gneiss show similar similar to 880Ma protolith ages and similar to 295Ma metamorphic ages. The formation age of Baidunzi felsic veins is consistent with the metamorphic age of the gneiss. Furthermore, Baidunzi gneiss and felsic veins also recorded a later metamorphism age of 270 similar to 280Ma. The protolith crystallized zircons of Shibandun amphibolite and Baidunzi gneiss exhibit varied initial Hf-176/Hf-177 ratios (0.282063 to 0.282291), epsilon(Hf)(t) values (-6.3 to 2.2) and t(DM2) model ages (1.62Ga to 2.14Ga), indicating that their protolith may be originated from a Paleoproterozoic crust with the addition of juvenile crust. Evidently, the Archean crustal basements would not occur in the southern Beishan orogenic belt, namely that the microcontinents in the orogen would not belong to the Dunhuang Block as previously considered. The metamorphic zircons of Shibandun amphibolite and Baidunzi gneiss have significantly higher initial Hf-176/Hf-177 ratios than their protolith counterparts, suggesting that they were formed by new zircon overgrowths in partial melts, rather than recrystallized from the protolith zircons. Therefore, the similar to 295Ma metamorphic event may represents an anatexis event, which is also supported by the simultaneous felsic veins occurred in the Baidunzi gneiss. We suggest that the similar to 295Ma anatexis events may be formed in a post-collisional extensional setting.