Petrogenesis of the Xiarihamu Nibearing layered mafic-ultramafic intrusion, East Kunlun: Implications for its extensional island arc environment.

The Xiarihamu No. I intrusion which bears super-large nickel deposit located in Early Paleozoic island arc zone in the middle part of the western section of the eastern Kunlun orogenic belt. The weighted average Pb-206/U-238 age is 439 Ma, which can be considered as the crystallization age of the intrusion. The intrusion is mainly composed of peridotite, pyroxenite and gabbro-norite with pervasive cumulatic texture and cumulatic stratification, suggesting a sill-like layered intrusion. Magma evolution processes are mainly controlled by disequilibrium fractional crystallization. The crystallization sequence of minerals is spine/olivine -> orthopyroxene -> clinopyroxene -> clinopyroxene + plagioclase -> brown colored hornblende. Rocks from the intrusion are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE) and depleted in Nb, Ta, Ti and P, consistent with the geochemical features of rocks from island arc environment. Nd-Sr isotopeic compositions suggest an enriched mantle origin with an EM II. Xiarihamu No. 111 and No. I intrusions belong to the ophiolite suite, suggestting a lithospheric fracture zone in the island arc belt in middle part of the eastern Kunlun. Regional tectonic evolution history and geophysical field suggested that middle part of eastern Kunlun orogenic belt was an extensional island-arc belt. Therefore, we conclude that extensional island arc environment is the fifth favorable tectonic setting for the formation of nickel deposits in the world and Middle Paleozoic is the third important period for the formation of nickel deposits in China.