Emplacement of viscous mushes in the Jinchuan ultramafic intrusion, western China

The 6000 x 350 m Jinchuan ultramafic intrusion occurs along the SW edge of the Sino-Korean platform, and consists of olivine-orthopyroxene-chromian spinel cumulates with orthopyroxene, clinopyroxene, plagioclase and phlogopite as interstitial phases; base-metal sulphides constitute approximately 5 wt % of the rock. The rocks are overprinted by low-grade metamorpism, hydrothermal alteration and near-surface oxidation. The highly complex and variable lithological structures, limited variation of Fo content of olivine and grain sizes suggest that the intrusion was formed by injection of high-yield-strength viscous crystal mushes with variable amounts of sulphide liquid and basaltic silicate melt into planar fractures or faults. During mush emplacement, flow differentiation controlled the proximal, central part of the intrusion, resulting in subconcentric lithological zoning, whereas chaotic flow prevailed in the distal E and W parts of the intrusion, resulting in complex banding and interfingering. The observed sequence of crystallization suggests that the crystal mushes were formed in a staging chamber at a depth of 4-9 km; here olivine-poor magma occurred at the top and olivine-rich mushes towards the base in response to gravitational settling. The stratified liquid-crystal mushes were then squeezed out into planar fractures or faults at a higher level to form a series of mafic-ultramafic intrusions, possibly induced by the collapse of the chamber roof.