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     

Textures in Partially Solidified Crystalline Nodules: a Window into the Pore Structure of Slowly Cooled Mafic Intrusions

Abundant glass is present along grain boundaries in coarse-grained,glass-bearing, crystalline gabbroic and peridotitic nodulesentrained and erupted in lavas from Iceland, Santorini and MaunaLoa (Hawaii), even when the total porosity is less than a fewvolume per cent. The glass films vary from a few microns toa few tens of microns thick, and are associated with stringsof small lensoid grain boundary pockets formed by impingementduring crystal growth. Additional porosity occurs as extensiveliquid-filled pockets adjacent to included grains within oikocrystsand as large triangular pockets formed by impingement of planar-sidedgrains. Interstitial material within glass films, and the irregularityof film thickness along a single grain boundary, suggest thatthe present pore structure is representative of the pore structurebefore entrainment and eruption. Pore geometry is consistentwith a dominant control by crystal growth during solidification,with little or no evidence for control by minimization of internalenergies driven by textural equilibration. Similarities betweenliquid distribution in the crystalline nodules and that of late-stage,interstitial phases in fully solidified mafic cumulates fromthe Rum and Skaergaard intrusions demonstrate that the crystallinenodules provide information about the latest stages of solidificationin slowly cooled mafic plutons. The highly permeable networkof intersecting liquid films, lenses and pockets may promotein situ crystallization in the solidifying mush, explainingthe common presence of adcumulates in such intrusions. KEY WORDS: textures; liquid distribution; mafic cumulates; crystalline nodules     

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     

Metamorphic Evolution of Iron-rich Mafic Cumulates from the Otztal-Stubai Crystalline Complex, Eastern Alps, Austria

Olivine-rich rocks containing olivine + orthopyroxene + spinel+ Ca-amphibole ± clinopyroxene ± garnet are presentin the central Ötztal–Stubai crystalline basementassociated with eclogites of tholeiitic affinity. These rockscontain centimetre-sized garnet layers and lenses with garnet+ clinopyroxene ± corundum. Protoliths of the olivine-richrocks are thought to be olivine + orthopyroxene + spinel dominatedcumulates generated from an already differentiated Fe-rich () tholeiitic magma that was emplaced into shallowcontinental crust. Protoliths of the garnet-rich rocks are interpretedas layers enriched in plagioclase and spinel intercalated ina cumulate rock sequence that is devoid of, or poor in, plagioclase.U–Pb sensitive high-resolution ion microprobe dating ofzircons from a garnet layer indicates that emplacement of thecumulates took place no later than 517 ± 7 Myr ago. Aftertheir emplacement, the cumulates were subjected to progressivemetamorphism, reaching eclogite-facies conditions around 800°Cand >2 GPa during a Variscan metamorphic event between 350and 360 Ma. Progressive high-P metamorphism induced breakdownof spinel to form garnet in the olivine-rich rocks and of plagioclase+ spinel to form garnet + clinopyroxene ± corundum inthe garnet layers. Retrogressive metamorphism at T 650–680°Cled to the formation of Ca-amphibole, chlorite and talc in theolivine-rich rocks. In the garnet layers, högbomite formedfrom corundum + spinel along with Al-rich spinel, Ca-amphibole,chlorite, aspidolite–preiswerkite, magnetite, ilmeniteand apatite at the interface between olivine-rich rocks andgarnet layers at P < 0·8 GPa. Progressive desiccationof retrogade fluids through crystallization of hydrous phasesled to a local formation of saline brines in the garnet layers.The presence of these brines resulted in a late-stage formationof Fe- and K-rich Ca-amphibole and Sr-rich apatite, both characterizedby extremely high Cl contents of up to 3·5 and 6·5wt % Cl, respectively. KEY WORDS: cumulates; Variscan metamorphism; SHRIMP dating; högbomite; saline brines     

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     

A Textural Record of Solidification and Cooling in the Skaergaard Intrusion, East Greenland

The clinopyroxene–plagioclase–plagioclase dihedralangle, _cpp, in gabbroic cumulates records the time-integratedthermal history in the sub-solidus and provides a measure oftextural maturity. Variations in _cpp through the Layered Seriesof the Skaergaard intrusion, East Greenland, demonstrate thatthe onset of crystallization of clinopyroxene (within LZa),Fe–Ti oxides (at the base of LZc) and apatite (at thebase of UZb) as liquidus phases in the bulk magma is recordedby a stepwise increase in textural maturity, related to an increasein the contribution of latent heat to the total heat loss tothe surroundings and a reduction in the specific cooling rateat the crystallization front of the intrusion. The onset ofboth liquidus Fe–Ti oxide and apatite crystallizationis marked by a transient increase in textural maturity, probablylinked to overstepping before nucleation. Textural maturationat pyroxene–plagioclase–plagioclase triple junctionseffectively ceases in the uppermost parts of the Layered Seriesas a result of the entire pluton cooling below the closure temperaturefor dihedral angle change, which is 1075°C. Solidificationof the Layered Series of the Skaergaard intrusion occurred viathe upwards propagation of a mush zone only a few metres thick. KEY WORDS: magma; partial melting; asthenosphere; olivine; mantle     

Large-scale Mechanical Redistribution of Orthopyroxene and Plagioclase in the Basement Sill, Ferrar Dolerites, McMurdo Dry Valleys, Antarctica: Petrological, Mineral-chemical and Field Evidence for Channelized Movement of Crystals and Melt

The Jurassic Ferrar dolerite sills of the McMurdo Dry Valleys,Antarctica represent the plumbing system for flood basalt eruptionsassociated with the breakup of Gondwana. Among the Ferrar sills,the 350–450 m thick cumulate-textured Basement Sill isdifferentiated into a Lower Marginal Zone (LMZ) gabbronorite,a thick Lower Zone (LZ) orthopyroxene–plagioclase orthocumulatepyroxenite, a strongly layered mela- to leuco-gabbronorite MiddleZone (MZ), a thick Upper Zone (UZ) gabbronorite with ferrogabbroicpods, and an Upper Marginal Zone (UMZ) gabbronorite. Texturesand mineral compositions in the LZ pyroxenite and MZ–UZgabbronorites are nearly identical, the main distinction beingthe greater relative proportion of plagioclase in the MZ–UZgabbronorites, and of pigeonite in the UZ. Most orthopyroxenein the LZ, MZ and UZ occurs as sub-euhedral, normally zonedprimocrysts, commonly with rounded plagioclase inclusions. Plagioclaseis usually sub-euhedral and normally zoned, but can containsodic cores interpreted to be xenocrystic. Orthopyroxene andfeldspar compositions thoughout the sill are generally fairlyuniform, and resemble the compositions of micro-phenocrystsin the chilled margins. We infer that the sill was filled bya c. 1250°C slurry of orthopyroxene + plagioclase phenocrystsor primocrysts that subsequently unmixed in response to buoyancyforces. The LZ websterite contains numerous anorthosite to gabbronoriteschlieren, veins and pipes (< 2 m diameter), which we interpretas fossil segregation channels. Textures and mineral compositionsin these felsic channels are very similar both to UZ and MZgabbronorites, and to the groundmass separating accumulatedorthopyroxene primocrysts in the LZ and MZ. We infer that plagioclase-charged,hydrous pore melt from the pyroxenite may have segregated, pooledand ascended through these conduits to feed growth of the UZgabbronorite. Detailed mapping shows that the pipes are separatedby about 15 m on average. Calculations suggest that this numberdensity of conduits could have drained the LZ cumulates of theirinterstitial melt + plagioclase in about 8 days. Sequences (eachc. 5–10 m thick) of layered leuco-gabbronorite in theMZ could represent intra-cumulate sills that formed from plagioclase-richslurries ascending in segregation channels. Fe–Ti-richpyroxenitic veins and pods (some pegmatitic) and an unusualcoarse-grained plagioclase facies occur at the contacts betweenmassive leuco-gabbronorite layers in the MZ. Discordant ferro-pegmatitepods and dykes occur throughout the UZ. We interpret these Fe-richpegmatoidal rocks as evolved residual melts expelled from thecompacting gabbronoritic cumulates of the MZ and UZ. KEY WORDS: Ferrar; cumulates; differentiation; Antarctica; layering     

Origin of Fe-Ti Oxide Ores in Mafic Intrusions: Evidence from the Panzhihua Intrusion, SW China

Economic concentrations of Fe–Ti oxides occur as massive,conformable lenses or layers in the lower part of the Panzhihuaintrusion, Emeishan Large Igneous Province, SW China. Mineralchemistry, textures and QUILF equilibria indicate that oxidesin rocks of the intrusion were subjected to extensive subsolidusre-equilibration and exsolution. The primary oxide, reconstructedfrom compositions of titanomagnetite in the ores and associatedintergrowths, is an aluminous titanomagnetite (Usp₄₀) with 40wt % FeO, 34 wt % Fe₂O₃, 16·5 wt % TiO₂, 5·3 wt% Al₂O₃, 3·5 wt % MgO and 0·5 wt % MnO. This compositionis similar to the bulk composition of the oxide ore, as inferredfrom whole-rock data. This similarity strongly suggests thatthe ores formed from accumulation of titanomagnetite crystals,not from immiscible oxide melt as proposed in earlier studies.The occurrence of oxide ores in the lower parts of the Panzhihuaintrusion is best explained by settling and sorting of densetitanomagnetite in the ferrogabbroic parental magma. This magmamust have crystallized Fe–Ti oxides relatively early andabundantly, and is likely to have been enriched in Fe and Tibut poor in SiO₂. These features are consistent with fractionationof mantle-derived melts under relatively high pressures (10kbar), followed by emplacement of the residual magma at 5 kbar.This study provides definitive field and geochemical evidencethat Fe–Ti oxide ores can form by accumulation in ferrogabbro.We suggest that many other massive Fe–Ti oxide depositsmay have formed in a similar fashion and that high concentrationsof phosphorus or carbon, or periodic fluctuation of fO₂ in themagma, are of secondary importance in ore formation. KEY WORDS: ELIP; Fe–Ti oxide ore; layered intrusion; Panzhihua; QUILF     

Mafic Pegmatites Intruding Oceanic Plateau Gabbros and Ultramafic Cumulates from Bolivar, Colombia: Evidence for a 'Wet' Mantle Plume?

The fault-bounded Bolívar Ultramafic Complex (BUC) onthe eastern fringes of the Western Cordillera of Colombia wastectonically accreted onto the western coast of South Americain the late Cretaceous–early Tertiary, along with pillowbasalts of the Caribbean–Colombian Oceanic Plateau (CCOP).The complex consists of a lower sequence of ultramafic cumulates,successively overlain by layered and isotropic gabbroic rocks.The gabbros grade into, and are intruded by, mafic pegmatitesthat consist of large magnesiohornblende and plagioclase crystals.These pegmatites yield a weighted mean ⁴⁰Ar–³⁹Ar step-heatingage of 90·5 ± 0·9 Ma and thus coincidewith the timing of peak CCOP volcanism. The chemistry of theBUC is not consistent with a subduction-related origin. However,the similarity in Sr–Nd–Pb–Hf isotopes betweenthe CCOP and the BUC, in conjunction with their indistinguishableages, suggests that the BUC is an integral part of the plume-derivedCCOP. The parental magmas of the Bolívar complex wereprobably hydrous picrites that underwent 20–30% crystallization.The residual magmas from this fractionation contained     

Pyroxenites from the Southwest Indian Ridge, 9-16{degrees}E: Cumulates from Incremental Melt Fractions Produced at the Top of a Cold Melting Regime

The Southwest Indian Ridge (SWIR) at 9–16°E and 52–53°Sis characterized by ultra-slow, oblique spreading and containsone of the few documented occurrences of pyroxenite veins associatedwith abyssal peridotites. The origin of these uncommon lithologiesis still debated. We present a detailed study (including electronmicroprobe and laser ablation inductively coupled plasma massspectrometry) of spinel websterites collected during Cruise162, Leg 9, of the R.V. Knorr. Rare earth element patterns inclinopyroxenes (Cpx) lead us to discard a possible origin ofthe pyroxenites as residues from partial melting of garnet pyroxenites(i.e. relics of a layered mantle protolith). Their compositionand cumulate texture (when not obscured by mylonitization relatedto emplacement on the seafloor) are better interpreted in termsof fractional crystallization from a basaltic melt at relativelyhigh pressure. Evidence for a high pressure of crystallizationincludes the lack of plagioclase in the cumulate assemblageand the high Al₂O₃ contents of the pyroxenes: up to 5 wt % inorthopyroxene (Opx) and up to 7 wt % in Cpx. These values areamong the highest reported for pyroxenes in a mid-ocean ridgesetting. Sub-solidus breakdown of spinel to plagioclase (nowaltered) is observed in one sample, providing a rough estimateof the final equilibration pressure of these cumulates, around0· 6–0· 7 GPa (plagioclase–spineltransition for a bulk pyroxenite composition). The inferredpyroxenite parent melts were close to equilibrium with the associatedresidual peridotites; some samples have a slightly evolved compositionin terms of the Mg-number [Mg/(Mg + total Fe)]. These parentalmelts had major and trace element compositions consistent witha mid-ocean ridge basalt (MORB) affinity, although they werenot rigorously identical to MORB. Among other characteristics,these melts were relatively depleted in highly incompatibleelements. We propose that they correspond to the latest, shallowest,incremental melt fractions produced during fractional decompressionmelting of a normal MORB (N-MORB) mantle source. These meltsexperienced fractional crystallization as soon as they segregatedfrom the peridotite matrix, moved upward, and crossed the lithosphere–asthenosphereboundary (defined here as the base of the conductive lid). Asa consequence, these shallow melt fractions produced beneathmid-ocean ridges did not fully mix with melt fractions producedand extracted at greater depths. Our study provides concreteevidence for the actuality of pyroxene crystallization in meltchannels beneath mid-ocean ridges at relatively high pressures,a process frequently invoked to account for the ‘pyroxeneparadox’ in MORB petrogenesis. KEY WORDS: abyssal pyroxenites; cumulates; lithospheric mantle; melt migration; Southwest Indian Ridge     

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     

The Lower Zone-Critical Zone Transition of the Bushveld Complex: a Quantitative Textural Study

The Lower Zone–Critical Zone boundary of the BushveldComplex is an intrusion-wide, major stratigraphic transitionfrom ultramafic harzburgite and pyroxenite in the Lower Zoneto increasingly plagioclase-rich pyroxenites and norites inthe Critical Zone. Quantitative textural and compositional datafor 29 samples through this transition show the following: LowerZone orthopyroxene grains are larger, have higher aspect ratios,are better foliated and have a lower trapped liquid componentthan those of the Critical Zone. The larger grain size of theLower Zone results in crystal size distribution plots that arerotated to lower slopes and intercepts relative to those inthe Critical Zone. Although all rocks show differing amountsof foliation, mineral lineations are weak to absent. These dataare consistent with significant compaction-driven recrystallizationin the study section. Numerical modeling of concurrent compactionand crystallization provides a quantitative model of how theLower Zone–Critical Zone transition may have formed: plagioclaseis rare in the Lower Zone because compaction removes interstitialliquid before it reaches plagioclase saturation. However, asthe crystal pile grows, plagioclase saturation is reached inthe interstitial liquid before compaction is complete in moreevolved pyroxenites, producing more abundant but still modestamounts of plagioclase characteristic of the Lower CriticalZone. It is concluded that both the textures and the modal mineralogyare largely controlled by compaction and compaction-driven recrystallization;primary magmatic textures are not preserved. KEY WORDS: Bushveld Complex; compaction; crystal size distributions; crystal aging; igneous textures     

Crystal Size Distribution (CSD) and Textural Evolution of Accessory Apatite, Titanite and Allanite during Four Stages of Metamorphism: an Example from the Moine Supergroup, Scotland

Crystal size distributions (CSD) and shapes of accessory apatite,titanite and allanite are investigated in three texturally distinctgarnet zones (Z1–Z3) and in the matrix (Z4) of a garnet–epidote–biotitegneiss from the Moine Supergroup. Additionally, textural relationshipsand interactions between the accessory minerals and surroundingrock-forming minerals are considered, results of numerical CSDmodelling are presented, and geochronological and geologicalconsequences of the inferred CSD evolutions are discussed. Texturesand CSDs indicate that the accessory minerals were in, or near,a stage of nucleation and initial growth immediately prior togarnet Z1 overgrowth, and formed within less than 20 000 years,either by a size-independent or size-dependent growth mechanism.Subsequently, the CSDs were modified by different growth mechanisms,as supported by several parameters including CSDs, grain numbers,grain sizes, specific volumes and others. The apatite CSD evolutionfrom Z1 to Z4 is consistent with open-system LPE (Law of ProportionateEffects) growth accompanied and followed by supply controlledrandom ripening, whereas transformation of the original titaniteCSD is more consistent with Ostwald ripening, temporarily accompaniedby positive or negative McCabe growth. The allanite CSDs alsopoint to Ostwald ripening between Z3 and Z4. The textural observationsindicate that the growth evolution of the accessory phases wasinfluenced by mineral reactions with surrounding rock-formingminerals, as well as by deformation and matrix coarsening, ina manner similar to that found in more simple ceramic systems.The observed textures require a successive temperature risethroughout the tectono-metamorphic evolution of the investigatedrock, in agreement with existing P–T data. Fast nucleationand initial growth of the accessory minerals during Z1 was perhapsinitiated by contact metamorphism, whereas subsequent growthand annealing (Z2–Z4) result from regional metamorphicevents. KEY WORDS: apatite; titanite; allanite; CSD; Moine Supergroup; textures; kinetics     

The Shape and Volume of the Skaergaard Intrusion, Greenland: Implications for Mass Balance and Bulk Composition

Re-examination of the Skaergaard intrusion in the context ofits regional setting, combined with new data from explorationdrilling, has resulted in a revised structural model for theintrusion. It is modelled as an irregular box, c. 11 km fromnorth to south, up to 8 km from east to west, and 3·4–4km from the lower to the upper contact. The walls of the intrusionare inferred to follow pre-existing and penecontemporaneoussteep faults, and the floor and roof seem largely controlledby bedding planes in the host sediments and lavas, similar toregional sills. The suggested shape and volume are in agreementwith published gravimetric modelling. Crystallization alongall margins of the intrusion concentrated the evolving meltin the upper, central part of the intrusion, best visualizedas an ‘onion-skin’ structure inside the box. Thetotal volume is estimated to c. 280 ± 23 km³, of which13·7% are referred to the Upper Border Series (UBS),16·4% to the Marginal Border Series (MBS) and 69·9%to the Layered Series (LS). In the LS, the Lower Zone (LZ) isestimated to constitute 66·8%, the Middle Zone (MZ) 13·5%and the Upper Zone (UZ) 19·7%. The new volume relationshipsprovide a mass balance estimate of the major and trace elementbulk composition of the intrusion. The parental magma to theSkaergaard intrusion is similar to high-Ti East Greenland tholeiiticplateau basalts with Mg number c. 0.45. The intrusion representsthe solidification of contemporary plateau basalt magma trappedand crystallized under closed-system conditions in a crustalreservoir at the developing East Greenland continental margin. KEY WORDS: bulk composition; emplacement; mass proportions; Skaergaard intrusion; structure     

Ca-rich Garnet-Clinopyroxene Rocks at Hujialin in the Su-Lu Terrane (Eastern China): Deeply Subducted Arc Cumulates?

Layers of Ca-rich garnet–clinopyroxene rocks enclosedin a serpentinite body at Hujialin, in the Su–Lu terraneof eastern China, preserve igneous textures, relict spinel ingarnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite,Fe-rich spinel, and also amphibole in clinopyroxene. In termsof their major and trace element compositions, the studied samplesform a trend from arc cumulates towards Fe–Ti gabbros.Reconstructed augite compositions plot on the trend for clinopyroxenein arc cumulates. These data suggest that the rocks crystallizedfrom mantle-derived magmas differentiated to various extentsbeneath an arc. The Ca-rich garnet + diopside assemblage isinferred to have formed by compressing Ca-rich augite, whereasthe relatively Mg-rich cores of garnet porphyroblasts may haveformed at the expense of spinel. The protolith cumulates weresubducted from near the crust–mantle boundary (c. 1 GPa)deep into the upper mantle (4·8 ± 0·6 GPaand 750 ± 50°C). Negatively sloped P–T pathsfor the garnet–clinopyroxene rocks and the corollary ofcorner flow induced subduction of mantle wedge peridotite arenot supported by the available data. Cooling with, or without,decompression of the cumulates after the igneous stage probablyoccurred prior to deep subduction. KEY WORDS: arc cumulates; Ca-rich garnet; garnet–clinopyroxene rocks; Su–Lu terrane; UHP metamorphism     

Rates of Thermal and Chemical Evolution of Magmas in a Cooling Magma Chamber: a Chronological and Theoretical Study on Basaltic and Andesitic Lavas from Rishiri Volcano, Japan

Rates of magmatic processes in a cooling magma chamber wereinvestigated for alkali basalt and trachytic andesite lavaserupted sequentially from Rishiri Volcano, northern Japan, bydating of these lavas using ²³⁸U–²³⁰Th radioactive disequilibriumand ¹⁴C dating methods, in combination with theoretical analyses.We obtained the eruption age of the basaltic lavas to be 29·3± 0·6 ka by ¹⁴C dating of charcoals. The eruptionage of the andesitic lavas was estimated to be 20·2 ±3·1 ka, utilizing a whole-rock isochron formed by U–Thfractionation as a result of degassing after lava emplacement.Because these two lavas represent a series of magmas producedby assimilation and fractional crystallization in the same magmachamber, the difference of the ages (i.e. 9 kyr) is a timescaleof magmatic evolution. The thermal and chemical evolution ofthe Rishiri magma chamber was modeled using mass and energybalance constraints, as well as quantitative information obtainedfrom petrological and geochemical observations on the lavas.Using the timescale of 9 kyr, the thickness of the magma chamberis estimated to have been about 1·7 km. The model calculationsshow that, in the early stage of the evolution, the magma cooledat a relatively high rate (>0·1°C/year), and thecooling rate decreased with time. Convective heat flux fromthe main magma body exceeded 2 W/m² when the magma was basaltic,and the intensity diminished exponentially with magmatic evolution.Volume flux of crustal materials to the magma chamber and rateof convective melt exchange (compositional convection) betweenthe main magma and mush melt also decreased with time, from 0·1 m/year to 10^–3 m/year, and from 1 m/yearto 10^–2 m/year, respectively, as the magmas evolved frombasaltic to andesitic compositions. Although the mechanism ofthe cooling (i.e. thermal convection and/or compositional convection)of the main magma could not be constrained uniquely by the model,it is suggested that compositional convection was not effectivein cooling the main magma, and the magma chamber is consideredto have been cooled by thermal convection, in addition to heatconduction. KEY WORDS: convection; magma chamber; heat and mass transport; timescale; U-series disequilibria     

Late-Stage Mafic Injection and Thermal Rejuvenation of the Vinalhaven Granite, Coastal Maine

The Vinalhaven intrusive complex consists mainly of coarse-grainedgranite, inward-dipping gabbro–diorite sheets, and a fine-grainedgranite core. Small bodies of porphyry occur throughout thecoarse-grained granite. The largest porphyry body (roughly 0·5km by 2·5 km) occurs with coeval gabbro, hybrid rocks,and minor fine-grained granite in the Vinal Cove complex, whichformed during the waning stages of solidification of the coarse-grainedVinalhaven granite. Porphyry contacts with surrounding coarse-grainedgranite are irregular and gradational. Compositions of wholerocks and minerals in the porphyry and the coarse-grained graniteare nearly identical. Neighboring phenocrysts in the porphyryvary greatly in degree of corrosion and reaction, indicatingthat the porphyry was well stirred. Thermal rejuvenation ofa silicic crystal mush by a basaltic influx can explain thecomposition and texture of the porphyry. Comparable rejuvenationevents have been recognized in recent studies of erupted rocks.Weakly corroded biotite phenocrysts in the porphyry requirethat hydrous interstitial melt existed in the granite duringremelting. Field relations, along with thermal calculations,suggest that cooling and crystallization of coeval mafic magmacould have generated the porphyry by thermal rejuvenation ofgranite crystal-mush containing about 20% melt. Field relationsalso suggest that some of the porphyry matrix may representnew felsic magma that was emplaced during remelting. KEY WORDS: granite; magma chamber; mafic replenishment; rejuvenation     

Ultramafic Xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for Multiple Metasomatic Events in the Lithospheric Mantle beneath the Wyoming Craton

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al₂O₃contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al₂O₃ and TiO₂) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have ¹⁴³Nd/¹⁴⁴Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low ¹⁴³Nd/¹⁴⁴Nd values (0·5113)and extremely high ⁸⁷Sr/⁸⁶Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their Sr–Ndisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette     

Mafic and Felsic Magma Interaction in Granites: the Hercynian Karkonosze Pluton (Sudetes, Bohemian Massif)

The Hercynian, post-collisional Karkonosze pluton contains severallithologies: equigranular and porphyritic granites, hybrid quartzdiorites and granodiorites, microgranular magmatic enclaves,and composite and lamprophyre dykes. Field relationships, mineralogyand major- and trace-element geochemistry show that: (1) theequigranular granite is differentiated and evolved by smalldegrees of fractional crystallization and that it is free ofcontamination by mafic magma; (2) all other components are affectedby mixing. The end-members of the mixing process were a porphyriticgranite and a mafic lamprophyre. The degree of mixing variedwidely depending on both place and time. All of the processesinvolved are assessed quantitatively with the following conclusions.Most of the pluton was affected by mixing, implying that hugevolumes (>75 km³) of mafic magma were available. This maficmagma probably supplied the additional heat necessary to initiatecrustal melting; part of this heat could have also been releasedas latent heat of crystallization. Only a very small part ofthe Karkonosze granite escaped interaction with mafic magma,specifically the equigranular granite and a subordinate partof the porphyritic granite. Minerals from these facies are compositionallyhomogeneous and/or normally zoned, which, together with geochemicalmodelling, indicates that they evolved by small degrees of fractionalcrystallization (<20%). Accessory minerals played an importantrole during magmatic differentiation and, thus, the fractionalcrystallization history is better recorded by trace rather thanby major elements. The interactions between mafic and felsicmagmas reflect their viscosity contrast. With increasing viscositycontrast, the magmatic relationships change from homogeneous,hybrid quartz diorites–granodiorites, to rounded magmaticenclaves, to composite dykes and finally to dykes with chilledmargins. These relationships indicate that injection of maficmagma into the granite took place over the whole crystallizationhistory. Consequently, a long-lived mafic source coexisted togetherwith the granite magma. Mafic magmas were derived either directlyfrom the mantle or via one or more crustal storage reservoirs.Compatible element abundances (e.g. Ni) show that the maficmagmas that interacted with the granite were progressively poorerin Ni in the order hybrid quartz diorites—granodiorites—enclaves—compositedykes. This indicates that the felsic and mafic magmas evolvedindependently, which, in the case of the Karkonosze granite,favours a deep-seated magma chamber rather than a continuousflux from mantle. Two magma sources (mantle and crust) coexisted,and melted almost contemporaneously; the two reservoirs evolvedindependently by fractional crystallization. However, maficmagma was continuously being intruded into the crystallizinggranite, with more or less complete mixing. Several lines ofevidence (e.g. magmatic flux structures, incorporation of granitefeldspars into mafic magma, feldspar zoning with fluctuatingtrace element patterns reflecting rapid changes in magma composition)indicate that, during its emplacement and crystallization, thegranite body was affected by strong internal movements. Thesewould favour more complete and efficient mixing. The systematicspatial–temporal association of lamprophyres with crustalmagmas is interpreted as indicating that their mantle sourceis a fertile peridotite, possibly enriched (metasomatized) byearlier subduction processes. KEY WORDS: Bohemian Massif; fractional crystallization; geochemical modelling; hybridization; Karkonosze