Vermicular orthopyroxene-magnetite symplectites from the Wateranga layered mafic intrusion,Queensland, Australia

Orthopyroxene-magnetite intergrowths (symplectites), partly or completely surrounding olivine, are described from the Wateranga layered mafic intrusion, Queensland, Australia. The texture occurs in unmetamorphosed plagioclase-rich norites, olivine gabbros and troctolites in which the primary minerals are olivine (Fo_63–69) orthopyroxene (En_66–72), clinopyroxene (Wo₄₂En₄₂Fs₁₆), plagioclase (An_49–65), hornblende, ilmenite, magnetite and sulphides. Symplectites range from incipient fine grained developments around corroded olivine grains to intricately formed pseudomorphs after olivine and slow a consistent orthopyroxene/magnetite ratio. Orthopyroxene in symplectites is commonly in optical continuity with surrounding magnetite-free orthopyroxene rims. Later intercumulus hornblended has replaced orthopyroxene. There is marked chemical similarity between primary and simplectite, orthopyroxenes and magnetites. Textures similar to those described here are considered elsewhere to have formed at a late magmatic stage or by solid state reactions involving subsolidus oxidation of olivine. In the Wateranga intrusion textural relations, the chemical similarity between primary and symplectite phases, and the consistent volume proportions of magnetite and orthopyroxene in the intergrowths suggest that they developed during late magmatic crystallization.