Abstract

Coronas of orthopyroxene, amphibole and spinel, and occasional garnet occur between olivine and plagioclase in olivine gabbros and troctolites of the Hadlington gabbroic complex. Microprobe analyses show that changes in olivine composition are mirrored in the composition of the corona minerals. Textural evidence indicates that corona formation was sub-solidus and Mg–Fe partitioning between ortho- and clinopyroxene suggests temperatures below 850 °C. Calculations of earlier models for corona formation based on equal volume replacement, or the simple addition of silica or alumina, fail to yield a satisfactory chemical balance. Olivine and plagioclase, with the addition of water, can supply the material needed for corona formation only if a migrating reaction boundary and change in olivine composition are considered.

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