Journal of Petrology Advance Access originally published online on December 13, 2006
Journal of Petrology 2007 48(3):563-587; doi:10.1093/petrology/egl072
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Infiltration Metasomatism of Cumulates by Intrusive Magma Replenishment: the Wavy Horizon, Isle of Rum, Scotland
1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
2Institute of Theoretical Geophysics, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK
3BP Institute (Bullard Laboratories), Madingley Road, Cambridge CB3 0EZ, UK
RECEIVED NOVEMBER 25, 2005; ACCEPTED NOVEMBER 10, 2006
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Abstract |
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The Eastern Layered Intrusion of the Rum Layered Suite comprises paired peridotite and allivalite (troctolite and gabbro) layers forming 16 macro-rhythmic units. Whereas the majority of these macro-units are believed to have formed by a process of crystal–liquid differentiation involving successive accumulation of crystals from multiple picritic replenishments of the chamber, the Unit 9 peridotite is interpreted as a layer-parallel picrite intrusion. Closely correlated with this discontinuous peridotite body is a distinctive feature generally known as the Wavy Horizon, which divides the overlying allivalite into a lower troctolite and an upper gabbro along a well-defined undulating contact. We propose that the Wavy Horizon is a metasomatic feature formed consequent to the removal of clinopyroxene from an original gabbroic mush. Foundering of the mush into the picritic sill resulted in the replacement of the original interstitial liquid by one saturated only in olivine (± plagioclase). Progressive through-flow of this liquid resulted in the stripping out of clinopyroxene from the lower parts of the allivalite. We interpret the Wavy Horizon as a reaction front, representing the point at which the invading liquid became saturated in clinopyroxene. The distinctive pyroxene-enriched zone immediately above the Wavy Horizon could have formed when mixing of the interstitial liquids on either side of the reaction front formed a supercooled liquid oversaturated in pyroxene, as a result of the curvature of the olivine–plagioclase–clinopyroxene cotectic. The presence of many such approximately layer-parallel features, defined by differences in pyroxene content, in the Eastern Layered Intrusion of Rum suggests that such an infiltration–reaction process was not unique to Unit 9.
KEY WORDS: cumulate; infiltration metasomatism; Rum; Eastern Layered Intrusion
*Corresponding author. E-mail: marian{at}esc.cam.ac.uk
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