No abstract

The Jinchuan ultramafic intrusion in western China has an elongate surface expression of about 6000 by 350 meters. It is one of several mafic-ultramafic intrusions found in the fault-bounded Longshoushan uplifted terrane, of Proterozoic age, located along the southwestern edge of the Sino-Korea platform. Three boreholes from the western, central and eastern parts of the intrusion were sampled and studied. The rocks from the Jinchuan intrusion are olivine-orthopyroxene-chromian spinel cumulates with orthopyroxene, clinopyroxene, plagioclase and phlogopite as interstitial phases. On average, base-metal sulfides (pyrrhotite, pentlandite and chalcopyrite) constitute 5 wt% of the rocks. The samples are overprinted by low-grade metamorphism, hydrothermal alteration and near-surface oxidation. Minor chromian spinel and orthopyroxene preceded olivine in the crystallization sequence. Olivine composition varies between Fo 79 and Fo 85. This variation is negatively correlated with the amounts of trapped silicate liquid in the rocks and is ascribed to the effects of subliquidus and metamorphic re-equilibration. The complex and highly variable lithological structures, limited variation in Fo in the olivine and grain sizes suggest that the intrusion was formed by injection of high-yield-strength viscous crystal-mushes containing variable amounts of sulfide liquid and silicate melt of basaltic composition into planar fractures or faults. During mush emplacement at Jinchuan, flow differentiation controlled the proximal, central part of the intrusion, resulting in subconcentric lithological zonation in this regime, whereas chaotic flow prevailed in the distal, eastern and western parts of the intrusion, resulting in complex lithological banding and interfingering. The original emplacement and flow structures did not collapse because of the high viscosity and high yield-strength of the crystal mushes, strong enough to withhold the gravitational forces imposed on the mushes after flow eased. The observed sequence of crystallization in the rocks suggests that the crystal mushes were formed in a staging chamber at a depth between 4 and 9 km. In this staging chamber, olivine-poor magma occurred at the top and olivine-rich mushes containing minor orthopyroxene and chromian spinel, and variable amounts of silicate and sulfide liquids, were concentrated toward the base in response to gravitational settling. The stratified liquid-crystal materials were then squeezed out of the staging chamber into planar fracture or fault systems at higher levels to form a series of mafic-ultramafic intrusions, possibly induced by the collapse of the chamber roof. The roof collapse was probably triggered by regional extension. Early extraction of the crystal-poor magma from the top of the staging chamber formed the sulfide-poor mafic intrusions in the vicinity of the Jinchuan intrusion. Subsequent extraction of the sulfidebearing crystal mushes from the bottom of the staging chamber formed the Jinchuan intrusion. The overall high sulfide-silicate ratio in...