Trace-Element and Sr-Nd Isotopic Geochemistry of the PGE-Bearing Hongge Layered Intrusion, Southwestern China

Abstract

The Fe-V-Ti-PGE-bearing Hongge layered intrusion in the Pan-Xi area of Sichuan Province in southwestern China consists, from the base upward, of zones of olivine clinopyroxenite (Cycle I), clinopyroxenite (Cycles II, III) and gabbro (Cycle IV). Abrupt reversals of major- and trace-element values at the boundaries of individual units suggest that new, more primitive magma was injected into the resident liquids in each cyclic unit. The new magmas are interpreted to have originated from a mantle plume, and were subjected to different degrees of contamination by continental lithospheric mantle and the upper crust. The homogeneous, decoupled Sr-Nd isotopes and cyclic variations of major and trace elements imply that each cyclic unit crystallized from a magma that was thoroughly mixed before crystallization. Assimilation of wallrocks accounted for the PGE mineralization in Cycle I. Mixing between a primitive and an evolved magma resulted in the formation of the PGE-enriched layer in Cycle II and the magnetite layers in Cycles II and III.