The peridotite plugs of northern Rum

Abstract

Synopsis Minor intrusions of peridotite occur as plugs and dyke-like bodies within Torridonian country rocks to the north of the Rum Central Complex. These are collectively termed the satellitic plugs to distinguish them from three dyke-like apophyses (tongues) of peridotite which are contiguous with the main layered intrusion (intrusive peridotite plugs also cut the layered sequence locally). The satellitic peridotite plugs are characterized by their lithological uniformity, compared with the cumulates in the central complex. The only exception is the West Sgaorishal plug, which has a less mafic marginal facies, and also displays regular, but subtle, fine-scale matrix banding. This generally dips inwards at moderate angles from the near-vertical margins of the intrusion, and is locally colloform. The layering reflects relatively minor changes in the proportions of interstitial plagioclase and clinopyroxene. Mineral compositions, notably olivine (Fo 88–84 ), are similar to those in the main cumulate succession. The tongues are petrologically similar to the satellitic plugs, and one of them displays the same type of fine-scale layering. The satellitic plugs (and tongues) are thought to represent conduits emanating from the main subvolcanic magma chamber, that were filled with olivine-rich crystal mush displaced from the unconsolidated cumulus pile during major subsidence events, and lubricated by interstitial basaltic magma. The distinctive layering is attributed to a self-regulating oscillatory process affecting the nucleation of plagioclase and clinopyroxene in the intercumulus liquid; perhaps during unusually tranquil conditions.