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Laterally extensive (∼2 mm thick) Cr-spinel seams in the Rum Layered Suite, NW Scotland, occur at the bases of several of the coupled peridotite–troctolite macro-rhythmic units that form the bulk of the eastern part of the intrusion. Detailed petrography, mineral chemical analyses and quantitative textural measurements of the rocks above and below two of these seams suggest that existing models for seam petrogenesis involving early crystallization and gravitational settling of Cr-spinel from a newly emplaced body of magma need to be reassessed. We argue for assimilation of troctolitic cumulate by a new influx of picrite magma at the crystal mush–magma interface, and subsequent in situ crystallization of the Cr-spinel seams. The bases of seams are characterized by Mg- and Al-rich Cr-spinel, with slightly more Fe-rich crystals toward the tops. These seams crystallized from a superheated hybrid magma generated by the initial assimilation and dissolution of the plagioclase-rich cumulate floor by the picrite. Coeval syn-magmatic deformation of the crystal mush at the unit boundary between peridotite and troctolite caused localized expulsion of Cr-spinel seed crystals several centimetres upward into the peridotite mush, resulting in the formation of overlying supra-seams as well as possibly developing a distinctive texture comprising chains of Cr-spinel around cumulus olivine crystals immediately above unit boundaries. The mineral compositional and textural evidence collected from the Rum Cr-spinel seams also indicates that they exhibit varying degrees of postcumulus chemical and textural equilibration. [ABSTRACT FROM PUBLISHER]

Abstract: Thin (~2 mm) Cr-spinel seams are present at the bases of several of the coupled peridotite–troctolite units that comprise the Rum Eastern Layered Intrusion. In some cases, ‘subsidiary’ Cr-spinel seams have also developed at between 6 and 12 cm below the unit boundaries. The subsidiary seams are thinner than the normal seams (approx. 1 mm), discontinuous, and occur within and at the base of a thin (~10 cm) layer of anorthosite, which is sandwiched between peridotite and troctolite. The troctolite contains cumulus olivine, but in the Cr-spinel seam and anorthosite, olivine is intercumulus only. Cr-spinel is scarce in the troctolite, but common in the anorthosite, where it has a different composition (more Fe-and Cr-rich) and crystal size distribution (CSD) profile to the Cr-spinel in the subsidiary seam, suggesting that it represents a different crystal population. A model involving downward infiltration of hot picrite and resulting troctolitic cumulate assimilation is developed here to explain the subsidiary seams. This is based on petrographic observation, quantitative textural measurement and mineral chemical analyses. It is suggested that as the picritic magma was emplaced, downward percolation of this melt occurred into a troctolite mush. The anorthosite represents a layer of almost completely melted troctolite, formed by growth of high-anorthite zones from the contaminated picrite onto residual cumulus plagioclase. Assimilation of large amounts of the troctolite cumulate mush forced the contaminated picrite onto the olivine-spinel cotectic, leading to Cr-spinel crystallisation. Subsequently, the Cr-spinel crystals in the anorthosite have reacted with intercumulus melt over a wider temperature interval and have gained a more Fe3+-rich composition than the subsidiary seam Mg- and Al-rich Cr-spinels. It is suggested that the separation of a small fraction of immiscible sulphide liquid in the Cr-spinel seams is the result of locally changing SiO2 and oxygen fugacity at the crystal–melt interface, during the peritectic reaction of some of the Cr-spinel to produce olivine and plagioclase. [Copyright &y& Elsevier]