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

Abstract

The Eastern Layered Series of the Rum Layered Suite is formed of 16 macro-units each comprising a lower peridotite and an overlying feldspar-rich layer (the local term is allivalite). The origin of the peridotite layers is unresolved, with two contrasting models. The earlier of the two is based on repeated replenishment of an opensystem magma chamber with deposition of fractionated material on the chamber floor. The second is based on the early formation of a troctolitic complex, which is then repeatedly intruded by sills of replenishing picritic magma to form the peridotite horizons. The lack of resolution of this fundamental problem is a consequence of the reliance of previously published studies on field observations. I present evidence to show that the clinopyroxene in the allivalites preserves information about the distribution of the last melt to solidify, permitting determination of not only the extent of supersolidus textural equilibration but also the sub-solidus history of the allivalite horizons. Comparison of profiles of clinopyroxene– plagioclase–plagioclase median dihedral angle across allivalite units demonstrates that it is possible to distinguish between those that were intruded by later picrite sills and those adjacent to peridotite horizons formed by replenishment and subsequent deposition of fractionated crystals above the pre-existing pile. In the region studied, only the main peridotite body of Unit 9 was intruded into a pre-existing allivalitic mush.