2025/05/16 by James M.D. Day, James M. D. Day
Earth and Planetary Sciences · #Mineralogy and Gemology Studies
paper · doi:10.1144/jgs2025-035
openalex publication_date 2025/05/16 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
Some of the most enigmatic petrological features of layered mafic intrusions are Cr-spinel seams. Here a new Cr-spinel seam occurrence is described from the Great Eucrite layered gabbro of the Paleogene Ardnamurchan Igneous Complex, Scotland. Laterally impersistent millimetre-thick Cr-spinel seams occur within cumulate anorthosite (>90 modal%, An 66–77 ) blocks that are preserved within a gravity-slumped olivine-rich gabbro raft of the Great Eucrite. The Cr-spinel grains are coarser (>1 mm) in the seam centre, with higher Al and Cr and lower Fe 3+ than smaller peripheral grains. Cr-spinel grains can be partially or wholly enclosed by thin (<20 µm) rims of olivine within intercumulus augite. The observations support Cr-spinel seam formation in situ through reaction of percolating basaltic melt within a pre-existing plagioclase- and olivine-dominated mush. Melt interaction led to modification of cumulus plagioclase and crystallization of Cr-spinel and intercumulus clinopyroxene. The blocks of anorthosite and Cr-spinel were preserved through incorporation into an olivine-rich gabbro slurry that gravitationally slumped on the palaeo-magma chamber floor of the Great Eucrite layered intrusion. The Great Eucrite Cr-spinel seams share similarities with some Rum Eastern Layered Intrusion Cr-spinel seam occurrences and highlight the array of mechanisms for generating Cr-spinel seams in layered mafic intrusions, including those with basaltic parental melts.