Syn-collisional extension and Ni-Cu sulfide-bearing mafic magma emplacement along the Irtysh Shear Zone in the Central Asian Orogenic Belt

The Central Asian Orogenic Belt was characterized by a long-lived accretionary history from Neoproterozoic to Paleozoic, followed by a collisional phase of orogeny in the latest Paleozoic with the closure of the Paleo-Asian tion associated with mantle-derived mafic magmas in the Central Asian Orogenic Belt was relatively short-lived. Thus, the coherence between the protracted accretionary and collisional history of the Central Asian Orogenic Belt and the transient Ni-Cu sulfide-bearing mafic magmatism is difficult to understand. We investigated both Ni-Cu sulfidebearing and sulfide-barren mafic intrusions along a major suture zone of the Irtysh Shear Zone within the Central Asian Orogenic Belt to understand the origin of shortlived Ni-Cu sulfide mineralization in the context of the collision of two arc systems of the Chinese Altai and the East Junggar regions. Zircon U-Pb dating results for the Ni-Cu sulfide-bearing mafic intrusions show that they were only emplaced briefly from 290 Ma to 280 Ma after subduction terminated (ca. 312 Ma), substantially earlier than Ni-Cu sulfide-barren mafic intrusions (ca. 274-271 Ma). Zircons from all Ni-Cu sulfide-bearing/barren mafic intrusions from 6.0 to 7.4%0, higher than the typical mantle value ( 5.3%0), which indicates their derivation from a metasomatized mantle with inherited subduction signatures. The 611B values of the rocks from these intrusions range from -17.2%0 to -2.3%0, significantly lower than those of typical volcanic arcs (up to +18%0), which suggests that the metaso- matic components in the mantle source were dominated by sediment melts rather than fluids. The pressure calculated using amphi- bole thermobarometry shows that the pa- rental magmas of the Ni-Cu sulfide -bearing mafic intrusions were emplaced shallower than those of Ni-Cu sulfide -barren mafic intrusions. This is supported by the fact that the country rocks of the Ni-Cu sulfide - bearing mafic intrusions are shallow crustal lithologies of sedimentary rocks, whereas the country rocks of the barren intrusions are relatively deeper crustal lithologies of high-grade metamorphic rocks and gran- ites. Olivine from the Ni-Cu sulfide -barren intrusions has forsterite (Fo) contents rang- ing from 51.1 mol% to 76.2 mol% and Ni from 196 ppm to 1312 ppm. These values are much lower than those of the Ni-Cu sulfide -bearing intrusions (Fo = 72.6- 79.4 mol%; Ni = 1022-1925 ppm), which indicates that the parental magmas of the barren intrusions may have experienced ex- tensive olivine crystallization. Notably, the emplacement of the Ni-Cu sulfide -bearing mafic intrusions (290-280 Ma) coincidently overlapped with a transient period of oro- gen -parallel extension (ca. 295-280 Ma) in response to the collision of the Chinese Altai and East Junggar. Syn-collisional extension may lead to the rapid ascent and emplace- ment of mantle -derived mafic magmas at a shallower crustal level, preventing signifi- cant loss of Ni from olivine crystallization at depths, and thus providing metal -rich mafic magmas for potential Ni-Cu sulfide miner- alization. Our results highlight that syn-col- lisional extension is an essential geodynamic mechanism that controls the emplacement of Ni-Cu sulfide -bearing mafic intrusions in orogenic belts.