Channelled metasomatism in Rhum layered cumulates – evidence from late-stage veins

Abstract

AbstractLate-stage veins have been studied in the Rhum layered intrusion as part of a wider investigation into postcumulus processes – in particular, to study compositions of residual liquids and their effects on host cumulates. Studies have concentrated on Unit 10 of the Eastern Layered Series (ELS), though brief investigations have also been made in the Western Layered Series (WLS).The Unit 10 veins are generally concordant with the layering, although locally they are transgressive with dilational relationships. Two principal types can be recognized: ultrabasic and basic. Detailed microprobe studies have shown that the liquids responsible for the veins have substantially modified the compositions of the cumulus minerals in the wall-rock peridotite up to 1 cm from the vein margin.The style of veining in the WLS is more complex than in the ELS, and several generations of vein can be identified. Both normal and harrisitic cumulates have undergone localized chemical modification near to the veins. Modification occurs regardless of the vein orientation.The ELS and WLS veins are geochemically distinct from the numerous gabbro intrusions associated with the layered series, and it is concluded that they represent fractionated intercumulus liquids injected into early formed fractures in the cumulates. The observed chemical modification is explained by a model involving mixing of the intrusive vein melt with trapped intercumulus melt in the adjacent peridotite, followed by diffusion-controlled re-equilibration of the cumulus minerals. This type of highly localized compositional modification along fractures is termed channelled metasomatism.