Infiltration Metasomatism of Cumulates by Intrusive Magma Replenishment: the Wavy Horizon, Isle of Rum, Scotland

The Eastern Layered Intrusion of the Rum Layered Suite comprisespaired peridotite and allivalite (troctolite and gabbro) layersforming 16 macro-rhythmic units. Whereas the majority of thesemacro-units are believed to have formed by a process of crystal–liquiddifferentiation involving successive accumulation of crystalsfrom multiple picritic replenishments of the chamber, the Unit9 peridotite is interpreted as a layer-parallel picrite intrusion.Closely correlated with this discontinuous peridotite body isa distinctive feature generally known as the Wavy Horizon, whichdivides the overlying allivalite into a lower troctolite andan upper gabbro along a well-defined undulating contact. Wepropose that the Wavy Horizon is a metasomatic feature formedconsequent to the removal of clinopyroxene from an originalgabbroic mush. Foundering of the mush into the picritic sillresulted in the replacement of the original interstitial liquidby one saturated only in olivine (± plagioclase). Progressivethrough-flow of this liquid resulted in the stripping out ofclinopyroxene from the lower parts of the allivalite. We interpretthe Wavy Horizon as a reaction front, representing the pointat which the invading liquid became saturated in clinopyroxene.The distinctive pyroxene-enriched zone immediately above theWavy Horizon could have formed when mixing of the interstitialliquids on either side of the reaction front formed a supercooledliquid oversaturated in pyroxene, as a result of the curvatureof the olivine–plagioclase–clinopyroxene cotectic.The presence of many such approximately layer-parallel features,defined by differences in pyroxene content, in the Eastern LayeredIntrusion of Rum suggests that such an infiltration–reactionprocess was not unique to Unit 9. KEY WORDS: cumulate; infiltration metasomatism; Rum; Eastern Layered Intrusion     

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     

Magmatic lineations inferred from anisotropy of magnetic susceptibility fabrics in Units 8, 9, and 10 of the Rum Eastern Layered Series, NW Scotland

The Eastern Layered Series of the Rum Layered Suite, NW Scotland, comprises a sequence of sixteen (30–150 m thick) cyclic units. The upper troctolite–olivine gabbro parts of each of these units exhibit small-scale modal layering and a pervasive, layer-parallel mineral lamination that is often associated with ‘soft-sediment’ deformation structures. A sporadic, macroscopic magmatic lineation measurable on mineral lamination surfaces is also observed in places. Anisotropy of magnetic susceptibility (AMS) fabrics were studied in three of these cyclic units, (8, 9, and 10) in the northern part of the Eastern Layered Series. Magnetic fabrics measured in the troctolites and gabbros yield one dominant trend in which magnetic foliations parallel magmatic layering and magnetic lineations trend NW–SE and plunge gently. Magnetic fabrics measured for two detailed traverses through Unit 10 on the northern side of Hallival also yield one dominant trend, similar to that measured elsewhere in the Eastern Layered Series. However, toward the centre of Unit 10 in each traverse, magnetic lineations sometimes plunge approximately downdip (SW) on the magnetic foliation planes. The implications of these results are discussed with reference to previous textural and fabric observations on Rum. A model is suggested in which weak linear arrangements of cumulus olivine and plagioclase crystals are developed due to slumping and soft-sediment deformation of unconsolidated crystal mushes during central sagging of the Rum Layered Suite.     

On the Use of Changes in Dihedral Angle to Decode Late-stage Textural Evolution in Cumulates

The melt-filled pore structure in the final stages of solidificationof cumulates must lie somewhere between the two end-membersof impingement (in which pore topology is controlled entirelyby the juxtaposition of growth faces of adjacent grains) andtextural equilibrium (in which pore topology is controlled bythe minimization of internal energies). The exact position betweenthese two end-members is controlled by the relative rates ofcrystal growth and textural equilibration. For samples in whichgrowth has stopped, or is very slow, textural equilibrium willprevail. A close examination of dihedral angles in natural examplesdemonstrates that these two end-member textures can be distinguished.The impingement end-member results in a population of apparentsolid–melt dihedral angles with a median of 60° anda standard deviation of 25–30°, whereas the texturallyequilibrated end-member population has a median of 28° anda standard deviation of 14°. For the specific case of cumulatesin the Rum Layered Intrusion, residual porosity in troctoliticcumulates was close to the impingement end-member, whereas thatin peridotites was close to melt-bearing textural equilibrium.Suites of glass-bearing samples from small, or frequently disturbed,magma systems show modification of initial impingement textures.These modifications may be a consequence of textural equilibrationor of diffusion-limited growth during quenching. Distinctioncan be made between these two processes by a consideration ofgrain shape. The geometry of interstitial phases in suites offully solidified cumulates from the Rum Layered Intrusion showsvariable approach to sub-solidus textural equilibrium from aninitial state inherited by pseudmorphing of the last melt. Texturalequilibration at pore corners occurs as a continuous process,with a gradual movement of the entire dihedral angle populationtowards the equilibrium final state. If the initial, pseudomorphedstate is one of disequilibrium (i.e. a melt-present impingementtexture) this change is accompanied by a reduction in the spreadof the population. If it is one of equilibrium, the change isaccompanied by an initial increase in the spread of the population,followed by a decrease. These observations demonstrate thatpreviously published models of dihedral angle change involvingthe instantaneous establishment of the equilibrium angle inthe immediate vicinity of the pore corner are incorrect. KEY WORDS: cumulate; dihedral angle; textural evolution; Rum intrusion; Kula; Santorini     

Textural Maturity of Cumulates: a Record of Chamber Filling, Liquidus Assemblage, Cooling Rate and Large-scale Convection in Mafic Layered Intrusions

Textural maturity describes the extent to which a rock has evolvedfrom the initial reaction-controlled texture towards texturalequilibrium controlled by the minimization of interfacial energy.Solidification in a magma chamber results in the formation ofan impingement texture by the random juxtaposition of planar-sidedgrains. Orthocumulates, in which the initial melt-filled poresare pseudomorphed by later-crystallizing phases, have an ophiticor intersertal texture immediately after complete solidification,which then evolves towards solid-state equilibrium by roundingof initially planar grain boundaries and an increase in themedian dihedral angle subtended at the junctions of two primocrysticgrains with the interstitial phase. The bulk of the increasein angle occurs just below the solidus temperature in kilometre-scalemafic plutons. Quantification of textural maturity via measurementof dihedral angle populations in troctolitic and gabbroic cumulatesfrom the Rum Eastern Layered Intrusion and the Skaergaard Intrusiondemonstrates that the rocks preserve a record of thermal eventsrelated to magma chamber replenishment and the onset of chamber-wideconvection. Textural maturity is also a function of the liquidusphase assemblage: for systems in which only olivine and plagioclaseare liquidus (i.e. cumulus) phases in the main magma body abovethe crystal mush, the texture is significantly less mature thanthat in systems in which clinopyroxene is an additional liquidusphase. The difference in textural maturity reflects differencesin the cooling and solidification rate, and demonstrates directlythat the liquidus phase assemblage plays a role in determiningthe thermal history of plutons. KEY WORDS: cumulates; dihedral angles; Rum; Skaergaard; textures     

An Origin for Harrisitic and Granular Olivine in the Rum Layered Suite, NW Scotland: a Crystal Size Distribution Study

Dendritic crystal morphologies occur in a number of igneousrocks and are thought to originate from the rapid growth ofcrystals, yet many examples of dendritic morphologies are foundin plutonic igneous rocks where cooling rates should be low.Results from crystal size distribution (CSD) measurements onharrisitic olivines from Rum, Scotland, combined with estimatedolivine growth rates, suggest that the characteristic skeletalhopper and branching olivines of harrisitic cumulates that areup to centimetres long, may have exceptionally short crystalgrowth times (several hours to several hundreds of days). This,together with very low calculated nucleation densities for harrisiticolivine, supports the interpretation of harrisite being a disequilibriumtexture, developed in response to supersaturation of the magmain olivine. We propose that this supersaturation arose throughundercooling of thin picrite sheets emplaced along the Rum magmachamber floor, beneath cooler resident magma. It is envisagedthat the picrite sheets were largely free of suspended olivinecrystals. Coupled with the olivine-enriched composition of themelt and the increasing cooling rate, this allowed homogeneousnucleation of olivine to set in at deeper undercooling and greaterolivine supersaturation than if there had been plentiful suspendedolivines to act as heterogeneous nuclei. The enhanced supersaturationcaused rapid growth of olivine once nucleation began, with skeletaland dendritic shapes. It is suggested that the observed, interlayeredsequences of harrisite and cumulus peridotite found throughoutthe Rum Layered Suite are a result of multiple episodes of harrisitecrystallization resulting from picrite emplacement that alternatedwith periods of crystal growth and accumulation in the mainbody of magma at lesser degrees of undercooling. KEY WORDS: crystal size distribution; harrisite; crystal growth rates; Rum Layered Suite