Selective crustal contamination and decoupling of lithophile and chalcophile element isotopes in sulfide-bearing mafic intrusions: An example from the Jingbulake Intrusion, Xinjiang, NW China

Abstract

The Jingbulake mafic-ultramafic intrusion in the South Tianshan orogenic belt, Xinjiang, NW China, is a zoned intrusive body composed of gabbro-diorite, olivine gabbro and wehrlite, locally intruded by pyroxenite, within a major W-E structure. Both olivine gabbro and wehrlite contain disseminated Ni-Cu sulfides, whereas pyroxenite hosts an ore body containing massive, net-textured and disseminated sulfide ores. Both silicate rocks and sulfide ores from the Jingbulake intrusion have low Pd/Ir ratios (10-63). The silicate rocks contain Ni-poor olivine (generally 30% crustal contamination is required to explain the radiogenic Os isotopic composition if average upper crust is adopted as the crustal contaminant, or 5%-25% if sulfide-rich crustal rock is used as the crustal contaminant. However only 5-10% crustal contamination is required to explain the Nd and Sr isotopes. We propose that the decoupling of Os from Sr-Nd isotopes in the silicate rocks was due to selective crustal contamination. Addition of external sulfur from a crustal source may be the key factor triggering sulfide saturation. The silicate magma that underwent sulfide extraction before emplacement to shallow depth preserved a crustal Os isotope signature. Continued reaction of sulfide melts with new pulses of mantle-derived magma increased the Os content and decreased the 187Os/ 188Os(i) ratios, effectively masking the crustal Os contribution in the sulfides. © 2011 Elsevier B.V.link_to_subscribed_fulltex