Base metal mineral segregation and Fe-Mg exchange inducing extreme compositions of olivine and chromite from the Xiadong Alaskan-type complex in the southern part of the Central Asian Orogenic Belt

Abstract

The Xiadong Alaskan-type complex shares much in common with typical Alaskan-type complexes worldwide, while showing some unique features in terms of mineral compositions. Olivine from the Xiadong dunites is characterized by extremely high Fo component of 91.7–96.7 and anomalously negative correlation of Fo with NiO, while chromite is featured by high 100×Fe3+/(Fe3++Cr+Al) (>70), high 100×Fe2+/(Fe2++Mg) (>70), high 100×Cr/(Cr+Al) (>90), low MnO (<0.6wt%) and TiO2 contents (<0.5wt%). To investigate these particular features, we conducted petrographic observation and mineral composition analyses for the Xiadong dunite. A number of Fe and/or Ni sulfides and alloys occurring as inclusions in olivine and chromite indicate that base metal mineral segregation took place prior to crystallization of olivine and chromite and probably induced Fe and Ni depletions in olivine. The FeO and MgO variations in profile analyses from chromite to adjacent olivine are compatible with Fe-Mg exchange. The diffusion mechanism of Fe from olivine to chromite and Mg from chromite to olivine may have elevated both Fo of olivine and 100×Fe2+/(Mg+Fe2+) ratio of chromite and furtherenhanced the decoupling of Fo and NiO in olivine. We thus suggest that base metal mineral segregation and Fe-Mg exchange play important roles in the extreme compositions of the Xiadong dunite. The Ni depletion of olivine and degree of Fe-Mg exchange between olivine and chromite may be used as indicators of mineralization in mafic-ultramafic intrusions.