Mantle source and magma oxygen fugacity of Permian Ni--Cu sulfide-bearing mafic-ultramafic intrusions in the Inner Mongolia, North China: Controls on the sulfide saturation of magmas in the Andean-style continental convergent margins

Abstract

Permian Erbutu and Kebu Ni--Cu sulfide-bearing mafic-ultramafic intrusions in the central Asian orogenic belt (CAOB) formed in the Andean-style continental convergent margin adjacent to the northern margin of the North China Craton (NCC). The rocks of the two intrusions show arc-like affinity on the primitive mantle-normalized trace element patterns and have εNd(t) ranging from −8.7 to −4.2, which are interpreted to be related to derivation from the metasomatized, enriched, subcontinental lithospheric mantle (SCLM) beneath the NCC. Using the olivine-orthopyroxene-spinel oxygen barometer, the magma fO2 of the Erbutu and Kebu intrusions was calculated to be from FMQ-0.8 to FMQ + 0.2, and from FMQ + 1.3 to FMQ + 2.0, respectively. The sulfides in the rocks have δ34SCDT ranging from +5.3 to +7.5‰ for the Erbutu intrusion and from +2.0 to +4.5‰ for the Kebu intrusion, much higher than that of the MORB mantle (δ34SCDT = −1.5 to 0.6‰). The rocks of the Erbutu and Kebu intrusions have δ13C ranging from −26.1 to −20.4‰ and − 26.4 to −20.9‰, respectively, similar to that of organic carbon in the crust (δ13C = −28 to −20‰), but much lower than that of the MORB mantle (δ13C = −7 to −5‰). These may indicate the addition of externally derived sulfur (S) and organic carbon into the magmas, which would increase S contents and decrease S solubility of the magmas, respectively, triggering sulfide saturation of the magmas. It is noted that the sulfide saturation of the Erbutu intrusion was mainly related to the addition of externally derived sulfur in the magmas as sulfur is generally present as S2− species in the low-fO2 magmas. Both intrusions have trace element patterns and εNd(t) values nearly identical to those of the Permian Elitu volcanic rocks in the same region, indicating that they were likely genetically linked. The Elitu volcanic rocks show negative correlation of (Gd/Yb)N with >3.0 wt% MgO, which is interpreted as the primary magmas of the rocks may have experienced early garnet fractionation at high pressure (e.g., 0.8 to 2.0 GPa). The calculated thickness of the crust in this region is ~56 km, equivalent to ~1.7 GPa in pressure. The primary magmas of the Erbutu and Kebu intrusions were produced by partial melting of the enriched SCLM beneath the thickened crust and are likely Ni-enriched, which is also crucial to the economic Ni--Cu sulfide mineralization in the Andean-style continental convergent margin in the CAOB.