Petrogenesis of the anorthosite-chromitite association: crystal-chemical and petrological insights from the Rum Layered Suite, NW Scotland

Thirteen Cr-bearing spinels from major horizons of magma replenishment in the open-system Rum Layered Suite have been analysed by X-ray single crystal diffraction and electron microprobe analyses. On the basis of the structural parameters and the chemistry of these spinels the so-called Rum trend, in which Al-content increases at the expense of Cr and Fe3+, has been easily recognised. In addition, natural spinels with Fe3+ content similar to synthetic spinels on the MgCr2O4–MgFe2O4 join have been analysed for the first time. Layers of chromitite, anorthosite and peridotite situated within several cm of one another have yielded different intracrystalline exchange temperatures using an intercrystalline spinel-olivine thermometer. The Rum anorthosite Cr-spinels are interpreted as having crystallised within the cumulus pile following rejuvenation of the crystal mush. Their low Al-content is a function of simultaneous plagioclase crystallisation, reducing the amount of Al3+ present for the Cr-spinel. By contrast, Cr-spinels in well-known Archean anorthosites (e.g. Ujaragssuit nunat and Fiskenæsset, western Greenland) and Sittampundi (southern India) are very aluminous in composition, attributed to crystallisation of Cr-spinel from high-alumina basalts in lower crustal magma chambers and linked to the control exerted by plagioclase crystallisation on Al content of the melt, in the absence of clinopyroxene crystallisation. The compositional differences between the Rum anorthosite Cr-spinels and the Fiskenæsset and Sittampundi Cr-spinels suggest that postcumulus reaction of Cr-spinel and melt to low (800–900°C) temperatures, as invoked for the Rum crystals, may not have been as important a process in the Archean anorthosites.