Origin of composite Ni-Cu sulfide- and Fe-Ti oxide-bearing mafic-ultramafic intrusions in the Central Asian Orogenic Belt, China: Role of externally derived crustal organic carbon in controlling mineralization of post-collisional mafic magmatism

Abstract

Composite Ni-Cu sulfide- and Fe-Ti oxide-bearing mafic-ultramafic intrusions have been discovered in convergent margin settings worldwide, but their origins remain enigmatic. Permian Kebu mafic-ultramafic intrusion in the Central Asian Orogenic Belt hosts both Ni-Cu sulfide and Fe-Ti oxide mineralization and formed in a post-collisional, Andean-style arc setting. The Ni-Cu sulfide- and Fe-Ti oxide-bearing rocks in the intrusion have similarly arc-like trace element patterns on the primitive mantle−normalized trace element diagram and negative εNd(t) values (−16 to −5), which indicate that the parental magma of the intrusion may have been derived from the enriched, metasomatized lithospheric mantle. The magma oxygen fugacity (fO2) was calculated to be FMQ+1.3∼FMQ+2.0, which can be attributed to the evolution of hydrated parental magma derived from the metasomatized lithospheric mantle. Also, the rocks all have negative δ13C values (−28.5‰ to −20‰), which indicates incorporation of external-derived crustal organic carbon into magmas. However, the Ni-Cu sulfide-bearing rocks contain more high-temperature carbon than the Fe-Ti oxide-bearing rocks. Modeling results based on alphaMELTS show that the magmas forming the Ni-Cu sulfide-bearing rocks may have assimilated more organic carbon than those that formed the Fe-Ti oxide-bearing rocks. Based on the estimated amounts of organic carbon in the two types of rocks, we deduced that initially oxidized magma pulses may have been reduced in variable degrees, leading to the formation of composite Ni-Cu sulfide and Fe-Ti oxide mineralization in one intrusive body. Mafic magmas in subduction-related, convergent margin settings are commonly highly oxidized due to metasomatism of the mantle wedge by slab-derived fluids/melts, and this may explain why the mafic-ultramafic intrusions associated with composite Ni-Cu sulfide and Fe-Ti oxide mineralization often occur in convergent margin settings.