Late Palaeozoic Convergence and Accretion in the Southern Margin of Central Asian Orogenic Belt: Constraints From the Carboniferous‐Permian Magmatism in the Zhangfangshan Area

The Central Asia Orogenic Belt (CAOB), the largest accretionary orogenic belt in the world, has a complex history. Carboniferous–Permian magmatism in the Zhangfangshan area of China is the key to understanding the accretion orogeny and reconstructing the orogenic belt structure in the southern Beishan Orogenic Belt, which is a part of the southern margin of the CAOB. The Zhangfangshan complex is exposed in the southeast of the Beishan area and includes plagiogranite, quartz diorite, siliceous rocks, limestone, and metamorphic sandstones. We obtained weighted mean zircon 206 Pb/ 238 U ages of 351 ± 2 Ma and 338 ± 3 Ma from plagiogranite and quartz diorite in the complex, respectively. The plagiogranite yields clear negative Nb–Ta anomalies on a trace element spider diagram and ε Hf ( t ) values of +14.8 to +20.1, characteristic of depleted mantle. These results indicate that the Zhangfangshan ophiolite was formed during the early Mississippian. The quartz diorites have similar trace element and Hf isotopic compositions to O‐type adakites and high‐Mg diorites, implying the partial melting of subducted oceanic crust. The undeformed granodiorite porphyrite and diabase that intrude the Zhangfangshan complex yield mean 206 Pb/ 238 U ages of 270 ± 1 Ma and 250 ± 1 Ma, respectively, suggesting that the ocean basin represented by the Zhangfangshan ophiolite was closed before ~ 270 Ma. The diorite–granodiorite porphyrites have low Mg# (42.8–46.1) and positive ε Hf ( t ) values (+4.7 to +10.0), suggesting that they were generated from juvenile crust. The diabases have geochemical characteristics similar to the diorite porphyrite and have evolved ε Hf ( t ) values (−0.5 to +11.7) that may be related to crust–mantle mixing in their source. Based on our results and previous findings, we reveal the Late Palaeozoic evolutionary process in the southern Beishan orogenic belt.