Spatial Constraints on Magma Chamber Replenishment Events from Textural Observations of Cumulates: the Rum Layered Intrusion, Scotland

The Eastern Layered Series of the Rum Layered Suite is formedof 16 macro-units each comprising a lower peridotite and anoverlying feldspar-rich layer (the local term is allivalite).The origin of the peridotite layers is unresolved, with twocontrasting models. The earlier of the two is based on repeatedreplenishment of an open-system magma chamber with depositionof fractionated material on the chamber floor. The second isbased on the early formation of a troctolitic complex, whichis then repeatedly intruded by sills of replenishing picriticmagma to form the peridotite horizons. The lack of resolutionof this fundamental problem is a consequence of the relianceof previously published studies on field observations. I presentevidence to show that the clinopyroxene in the allivalites preservesinformation about the distribution of the last melt to solidify,permitting determination of not only the extent of super-solidustextural equilibration but also the sub-solidus history of theallivalite horizons. Comparison of profiles of clinopyroxene–plagioclase–plagioclasemedian dihedral angle across allivalite units demonstrates thatit is possible to distinguish between those that were intrudedby later picrite sills and those adjacent to peridotite horizonsformed by replenishment and subsequent deposition of fractionatedcrystals above the pre-existing pile. In the region studied,only the main peridotite body of Unit 9 was intruded into apre-existing allivalitic mush. KEY WORDS: Rum Layered Intrusion; chamber replenishment; dihedral angles; cumulates