Komatiitic and Iron-rich Tholeiitic Lavas of Munro Township, Northeast Ontario

Munro Township, in the Archean Abitibi greenstone belt of northeastOntario, contains volcanic and hypabyssal rocks of two magmaseries: (1) an Fe-rich tholeiitic series of basaltic to daciticlava flows (3–10 m thick), layered peridotite-pyroxenite-gabbroflows (120 m thick), and layered sills (700 m thick); (2) anultramafic-mafic komatiitic series, comprising discrete lavaflows of peridotitic to andesitic composition (1–17 mthick), layered peridotite-gabbro flows (120 m thick), and layeredsills (500 m thick). The komatiitie lavas form a successionabout 1000 m thick that is both underlain and overlain by thickersuccessions of tholeiitic volcanic rocks. Three types of komatiite are recognized: peridotitic, pyroxenitic,and basaltic komatiites. The most ultramafic are peridotiticcumulates rich in forsteritic olivine (Fo_89–94), at thebases of flows and sills. Many less mafic peridotitic komatiites(MgO: 20–30 per cent), which typically form the upperparts of flows and the marginal parts of small intrusions, exhibitspinifex textures indicative of their formation from ultrabasicliquids. Pyroxenitic komatiites (MgO: 12–20 per cent)also may contain olivine, but are dominated by clinopyroxene,usually in spinifex textures. Basaltic komatiites (MgO <12per cent) are composed mainly of clino-pyroxene and plagioclaseor devitrified glass, rarely in spinifex texture and more commonlyin equigranular textures. Peridotitic komatiite with roughly30 per cent MgO appears to represent a parental liquid fromwhich the more ultramafic komatiites formed by accumulationof olivine, and the less mafic types were derived by fractionationof olivine, joined and finally succeeded in later stages byclinopyroxene and plagioclase. Komatiites of Munro Township share many of the characteristicsof the komatiites from the Barberton Mountain Land, South Africa(Voljoen & Viljoen, 1969a and b), but lack the high CaO/Al₂O₃ratios that distinguish the Barberton rocks. The Munro komatiitesare identical in this respect to ultramafic volcanic rocks inAustralia, Canada, Rhodesia, and India. It is proposed thatthe definition of the term komatiite be broadened so that itincludes all members of this ultramafic-mafic rock series, notonly those from Barberton Mountain Land. The proposed criteriaare: (1) highly ultramafic compositions in noncumulate lavas;(2) unusual volcanic structures such as spinifex texture andpolyhedral jointing; (3) low Fe/Mg ratios at given Al₂O₃ valuesor high CaO/Al₂O₃ ratios; low TiO₂ at given SiO₂; and high MgO,NiO, and Cr₂O₃.     

Spinifex and swirling olivines in a komatiite lava lake,Munro Township,Canada

Spinifex veins, and veins filled with swirling, tabular olivine grains, intrude the upper part of a 120 m thick lava lake in Munro Township, Ontario, Canada. These veins are 20–200 cm wide, tens of metres long and are oriented roughly parallel to the upper surface of the lava lake. The spinifex texture differs from that found in simple layered komatiite flows: rather than increasing downwards, the size of skeletal olivine grains is greatest in the centres of the veins, or varies irregularly. Upper contacts meander and veinlets penetrate overlying rock; lower contacts are transitional. The other type of vein contains 50–70% of ∼5 mm long hopper olivine grains whose orientation varies widely to produce a swirling, contorted fabric. Both types of vein are believed to have formed when lava from the centre of the lake flowed into fractures in the solidified crust.The lava lake has a dunitic lower portion and an olivine porphyry upper portion. The composition of chill samples, the compositions of olivine phenocrysts, and variations in major element abundances throughout the unit are used to show that the lava originally was komatiite with about 22% MgO. The lava lake is overlain by mafic scorias and breccias with moderate vesicularity and welding structures suggesting eruption in shallow water.     

Forward calculation of zoned garnet growth with limited diffusion transport in the matrix

Summary A forward model is proposed to reproduce the formation of garnet under conditions of sluggish diffusion transport in the matrix. Starting from a matrix consisting of chlorite and quartz, the amount of garnet growth and the chemical composition was calculated at each P–T increment in the system MnO–FeO–MgO–Al₂O₃–SiO₂–H₂O. Sluggish diffusion transport was introduced considering the local equilibrium between garnet surface and the matrix within a given diffusion distance (equilibration volume). Varying the diffusion distance, calculations were performed along the prograde P–T path of the Sambagawa metamorphic belt, Japan. The final size of the garnet grains was largely proportional to the diffusion distance. In contrast to the model without diffusion limitations, a shorter diffusion distance resulted in a rise of the Mg/(Mg + Fe) ratio in garnet before Mn approached zero. These results indicate that the chemical composition trend in zoned garnet from the Sambagawa belt is consistent with growth under sluggish material transport. The calculated amount of garnet growth increases dramatically with temperature. The amount of newly grown natural garnet in the Sambagawa metamorphic rocks was plotted against temperatures, where chemical compositions of garnet were calibrated against temperatures with the Gibbs’ method. This trend was also consistent with the modelled garnet behaviour.     

The origin of garnet-cordierite-sillimanite bearing rocks from Chandos Township,Ontario

Garnet-cordierite-sillimanite bearing rocks from the contact aureole of the Precambrian Loon Lake pluton in Chandos Township, southeastern Ontario were analyzed for the major and rare-earth elements. In comparison with the associated Apsley biotite gneisses, they are rich in Al, Mg, and Fe, low in Si, Na, and K and their REE distribution patterns show a depletion of light REE with a negative Eu anomaly. These rocks are probably residuum left after partial melting of biotite gneiss. Leucogranite associated with the GCS rocks may represent the extracted anatectic material. It is suggested that some of the garnet cordierite-sillimanite gneisses which frequently occur in high-grade regionally metamorphosed areas of the Grenville Province of the Canadian Shield may also be of a similar residual origin as proposed by Lal and Moorhouse (1969).     

Intrusion and Crystallization of a Spinifex-Textured Komatiite Sill in Dundonald Township, Ontario

Although komatiite has been defined as an ultramafic volcanicrock characterized by spinifex texture, there is a growing recognitionthat similar textures can also form in high-level dykes andsills. Here, we report the results of a petrological and geochemicalinvestigation of a 5 m thick komatiite sill in Dundonald Township,Ontario, Canada. This unit forms part of a series of komatiitesand komatiitic basalts, some of which clearly intruded unconsolidatedsediments. The komatiite sill is differentiated into a spinifex-texturedupper part and an olivine cumulate lower part. Features characteristicof the upper sections of lava flows, such as volcanic brecciaand a thick glassy chilled margin, are absent and, instead,the upper margin of the sill is marked by a layer of relativelylarge (1–5 mm) solid, polyhedral olivine grains that gradesdownwards over a distance of only 2 cm into unusually large,centimetre-sized, skeletal hopper olivine grains. This is underlainby a 1 m thick zone of platy spinifex-textured olivine and coarse,complex, dendritic, spinifex-textured olivine. The texture ofthe olivine cumulate zone in the overlying unit is uniform rightdown to the contact and a lower chilled margin, present at thebase of all lava flows, is absent. The textures in the silland the overlying unit are interpreted to indicate that thesill intruded the olivine cumulate zone of the overlying unit.Thermal modelling suggests that soon after intrusion, a narrowinterval of the overlying cumulate partially melted and thatthe liquid in the upper part of the sill became undercooled.The range of olivine morphologies in the spinifex-textured partof the sill was controlled by nucleation and crystallizationof olivine in these variably undercooled liquids. KEY WORDS: komatiite; intrusion; spinifex texture; olivine     

Nature and origin of nodular textures in anorthositic cumulates from the East Bull Lake Intrusion,Ontario, Canada

Summary Nodular anorthosite, containing rounded aggregates (nodules) of plagioclase set in a gabbroic to pyroxenitic matrix, is developed in constrictions (< 500 m wide) within the central and eastern parts of the Early Proterozoic East Bull Lake Intrusion, central Ontario, Canada. The nodules typically occur as spherical to ovoidal aggregates ranging from 1 cm to 14 cm in diameter. Larger nodules are physical agglomerations of smaller nodules that formed in isolation. Plagioclase core compositions for the nodules display little variation in anorthite content (An_68–75), indicating a homogeneous and nonfractionating parent magma during the initial stages of nodule development.The restricted occurrence of the nodular anorthosite is consistent with a model in which the nodules formed in response to elevated degrees of undercooling within dike-like constrictions in the proto-chamber. Undercooling resulted in the rapid nucleation and growth of plagioclase upon pre-existing plagioclase phenocrysts (disequilibrium crystallization). The resultant primary nodules then aggregated together forming secondary nodules, possibly through an agglomeration process involving a Mg-Fe-rich residual liquid. Migration of some of this relatively dense liquid out of the nodular anorthosite unit during the final stages of consolidation resulted in compaction and promoted the formation of large tertiary nodules.

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Zusammensetzung und Entstehung nodularer Texturen in anorthositischen Kumulaten von der East Bull Lake Intrusion, Ontario, Canada

Zusammenfassung Nodulare Anorthosite, die gerundete Aggregate (nodules) von Plagioklas in einer gabbroischen pyroxenitischen Matrix enthalten, kommen in beschränkten Bereichen (weniger als 500 m breit) in zentralen und östlichen Teilen der frühproterozoischen East Bull Lake Intrusion im zentralen Ontario, Canada, vor. Die Nodules liegen vorwiegend als kugelförmige bis ovoidale Aggregate vor, deren Durchmesser von 1 bis 14 cm schwankt. Größere Nodules sind aus Gruppen von aneinander gelagerten kleineren Nodules entstanden. Die Zusammensetzung von Plagioklaskernen in den Nodules zeigen geringe Variationen im Anorthit-Gehalt (An_68–78), und dies weist auf ein homogenes und nicht fraktioniertes Ursprungsmagma während der ersten Stadien der Nodule-Entwicklung hin.Das beschränkte Vorkommen des nodularen Anorthosits läßt sich durch ein Modell erklären, in dem die Nodules als Reaktion auf Unterkühlung innerhalb gangartiger Bereiche in der Proto-Magmenkammer entstehen. Unterkühlung resultierte in rascher Nukleation und im Wachstum von Plagioklas auf präexistierenden Plagioklaskristallen (Ungleichgewichts-Kristallisation). Die resultierenden primären Nodules verbinden sich dann und bilden sekundäre Nodules, möglicherweise durch einen Prozeß, der eine Mg-Fe-reiche Restschmelze involviert. Migration von einem Teil dieser relativ dichten Schmelze aus der nodularen Anorthosit-Einheit während der Endstadien der Verfestigung führte zur Kompaktion und erleichterte die Bildung von großen tertiären Nodules.

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With 4 Figures     

Nepheline-bearing rocks from the Poohbah Lake complex,Ontario: Malignites and Malignites

Malignites from the Poohbah Lake complex of northwestern Ontario, Canada are melanocratic cumulates. Cumulus pyroxene and apatite are poikilitically enclosed in a groundmass of large plates of intercumulus orthoclase and nepheline. Nepheline-feldspar fingerprint-like intergrowths occur. Nephelines are commonly zeolitized and pyroxenes altered to aggregates of biotite and/or garnet by deuteric alteration. Pyroxenes are weakly zoned from Di₇₁ Hd¹⁸Ac₁₁ to Di₆₃Hd₂₂Ac₁₅, and are similar to the least evolved pyroxenes of other alkaline rocks. Nephelines all have compositions within the Morozewicz-Buerger convergence field and feldspars have a limited compositional range from Or⁸⁸ to Or⁹⁵. Perthites are absent.Inconsistancies in the usage of the terms malignite and juvite are discussed and it is considered that a non-genetic petrographic classification of nepheline syenites leads to the obscuration of a group of potassic nepheline syenites, characterized by the presence of nepheline plus orthoclase which are typically associated with saturated to over-saturated alkaline rocks, contain pseudo-leucite or nepheline-orthoclase intergrowths, are emplaced in mobile belts and are not associated with rocks of the ijolite-carbonatite suite.A genetic classification of nepheline syenites is suggested and it is proposed that; (1) mafic-rich nepheline syenites be referred to as mela-nepheline syenites (sensu lato) rather than as malignites; (2) the term malignite be used for magmatic potassic nepheline syenites characterised by the presence of nepheline plus a single potassium-rich feldspar (orthoclase or microcline) and devoid of exsolution perthite under subsolvus conditions; (3) the metasomatic malignites and juvites of ijolite-carbonatite complexes be referred to as varieties of fenites.     

Geochemistry of the shawmere anorthosite complex,kapuskasing structural zone,Ontario

The Archean Shawmere Anorthosite Complex, at the southern end of the Kapuskasing Structural Zone, consists dominantly of anorthosite (An_{65 –85}) with minor gabbroic and ultramafic units, which are completely enclosed and cut by tonalites. Both the anorthosites and the tonalites are themselves cut by narrow dikes of gabbroic anorthosite. All of the rocks have undergone high grade metamorphism and are recrystallized so that few igneous textures remain.The anorthosites, gabbros and ultramafic rocks of this complex are cumulates which contain calcic plagioclase (An_65–95) and have atomic Mg/(Mg + Fe²⁺) ratios (Mg#) greater than 0.6; less than 3 ppm Rb; 150–210 ppm Sr; and less than 60 ppm Ba. REE abundanees range from 0.2 to 10 times chondritic and exhibit both light-enriched and light-depleted REE patterns. The lower Mg^# for the samples having more enriched light REE indicates substantial fractions of ferromagnesian minerals crystallized in addition to plagioclase during fractional crystallization, suggesting that the parent magma was basaltic, and not anorthositic. The ranges in Sr, Ba and REE abundances required for the magmas are typical of those for tholeiitic basalts from Archean greenstone belts. Thus the Shawmere Anorthosite Complex may represent cumulates of a crustal-level magma chamber which could have been the immediate source of basic Archean volcanics.One gabbroic anorthositic dike sample has a steeply fractionalted REE pattern with heavy REE abundances less than chondrites and a large positive Eu anomaly. The proposed interpretations is that this rock formed by partial melting of mafic cumulates, perhaps those of the Shawmere Anorthosite Complex itself.     

The Silent Lake Pluton,Ontario: a nodular,sedimentary, intrusive complex

The Precambrian Silent Lake pluton, Cardiff township, Ontario, consists of granitic rocks containing, in many localities, abundant quartz-sillimanite nodules.The intrusive form of the Pluton and massive character of most rocks suggests a magmatic origin. Lack of (a) sharp contacts, (b) apophyses, (c) contact metamorphism, (d) evidence of systematic magma evolution in the haplogranite system, (e) country-rock xenoliths, suggest non-magmatic origin. Polyhedral textures indicate annealing after emplacement. Bulk chemical composition, including very low Ca, low Mg, low Sr, high Sr/Ca indicate sedimentary (arkosic) origin. The rocks appear to be Hermon formation leptites mobilized and emplaced by ductile flow, followed by annealing.Structural, mineralogical, chemical and oxygen isotopic evidence indicate the nodules to have formed by deformation and de-alkalization of quartz-feldspar veins within the mobilized leptites.     

Plagioclase replacement textures in partially eclogitised gabbros from the Sanddal mafic-ultramafic complex, Greenland Caledonides

The Sanddal mafic‐ultramafic complex (SMUK) is a cluster of variably eclogitised mafic and ultramafic bodies that comprise the westernmost known eclogite facies locality in the North‐East Greenland eclogite province (NEGEP). Although there are no true eclogites in the SMUK, we interpret three distinct textural types of plagioclase replacement to record sequential stages in adjustment of SMUK olivine gabbro‐norites to eclogite facies conditions. The earliest stage, in which plagioclase was replaced by omphacite/spinel symplectite before nucleation of garnet (Type 1A & 1B) has not previously been described. Documentation of this texture provides clear evidence that, at least in some cases, garnet nucleation is delayed relative to nucleation of omphacite and is a rate‐limiting step for eclogitisation. Type 1C domains were produced by scattered nucleation of garnet in the same sample. In Type 2 domains, plagioclase was replaced by a layered corona with an outer layer of garnet, an inner layer of omphacite and an interior of inclusion‐rich plagioclase. In Type 3 domains, the omphacite layer was overgrown by the garnet rim, and omphacite is preserved only as inclusions in garnet. In more coarse grained leucogabbros, recrystallization was more complete, plagioclase replacement textures were less localised, and could not be divided into distinct stages. Plagioclase replacement in SMUK samples was not isochemical, and required diffusion of at least Mg and Fe from replacement of mafic phases in the surroundings. Strong compositional gradients in garnet reflect disequilibrium and were controlled by the different diffusion rates of Mg/Fe and Ca, different local chemical environments, and progress of the plagioclase breakdown reaction. The presence of small amounts of hydrous minerals (amphibole, phlogopite and clinozoisite) in local equilibrium in plagioclase domains of most SMUK samples indicates that a small amount of H₂O was present during high pressure metamorphism.     

Geology of the mineralized zone of the Wanapitei complex,Grenville Front,Ontario

The Wanapitei Complex (6 km×2.5 km), lying 0.4 km southeast of the Grenville Front, consists of a northwestern zone of gabbro and folded injection breccia and a southeastern layer of intensely folded hornblendeplagioclase gneiss. Disseminated Ni-Cu sulphides are unevenly distributed in a zone between the injection breccia and the folded gneiss.Rocks of the mineralized zone occur in southeastern and northeastern areas. The former area consists of hornblende norite, the major host rock of the sulphides, and olivine norite. Steeply-dipping cross-bedded primary layers and chemical trends indicate the top faces southeast. In the latter area olivine norite, hornblende norite, and hornblende gabbro grade eastward into recrystallized rocks and breccia. The olivine norites are characterized by corona reaction rims. Reactions are: olivine+plagioclase bronzite+diopside-spinel; olivine+pyroxene bronzite; and pyroxene+plagioclase diopside-spinel. Molecular proportion ratio variation diagrams suggest that rocks evolved from a common parent magma that underwent fractionation dominated by olivine and plagioclase. Sulphide mineralization (pyrrhotite, chalcopyrite, pentlandite, pyrite) is interstitial to the silicates and appears to be of primary magmatic origin.Northeasterly-trending shear zones, felsic dikes, and matic dikes are metamorphosed to the same degree as the rocks they cut (amphibolite facies). The sequence of events for the mineralized zone are: intrusion deep in the crust; tilting; brecciation; shearing; felsic and mafic dike emplacement; metamorphism; and injection of granite pegmatite dikes.Deceased (8-16-1986)     

Mineralogy of the deadhorse creek volcaniclastic breccia complex,northwestern Ontario,Canada

The Proterozoic Deadhorse Creek volcaniclastic breccia complex was emplaced in Archean metasedimentary and metavolcanic rocks of the Schreiber-White River greenstone belt adjacent to the Proterozoic Coldwell alkaline complex. The western sub-complex of the Deadhorse Creek breccia consists of metasomatically-altered breccia, a U-Be-Zr-rich main mineralized zone and a Zr-Y-Th-rich carbonate vein. The main mineralized zone is enriched in beryllium, thorium, uranium, first and second row transition elements, and rare earth elements. The major minerals present include: albite; potassium feldspar; quartz; calcite; apatite; and phenakite. Accessory minerals include: aegirine-jervisite; aegirine-natalyite; allanite; barite; barylite; coffinite; Ca-Mn-silicate; magnetite; monazite-(Ce); niobian vanadian rutile; pyrite; thorite; thorogummite; thortveitite; uraninite; vanadian crichtonite; xenotime-(Y); zircon and hydrated zircon; and zircon-thorite-coffinite solid solutions. The carbonate vein consists of dolomite-ankerite and calcite with accessory zircon, xenotime, and monazite. Barite, baotite and Ba-rich feldspars, were formed during metasomatism of the earlier-formed and genetically-unrelated volcaniclastic breccia adjacent to the main mineralized zone. The complex mineral assemblage of the fault-controlled main mineralized zone is considered to have formed in three stages. An initial emplacement of a “granitic” melt/fluid was followed by introduction of CO₂-bearing Cr-Nb-V-Ti-enriched alkaline fluids. The latter reacted with minerals which had crystallized from the “granitic” melt/fluid to produce the exotic V-, Sc- and Nb-bearing mineral assemblage. Subsequently, a supergene suite of minerals, consisting principally of calcite, thorogummite, hollandite and tyuyamanite, formed during post-Pleistocene alteration was superimposed onto the pre-existing Proterozoic age mineral assemblage. The major mineralogy of the main mineralized zone is essentially ‘granitic” and the melts/fluids are considered to be derived from an A-type granite source. However, the Deadhorse Creek mineralization is older (1129±6 Ma) than the A-type quartz syenites of the adjacent Coldwell complex (1108±1 Ma) which are the nearest potential sources of such melts. Thus, the source of the “granitic” melt together with that of the Cr-Nb-V-Ti-bearing alkaline fluids remains enigmatic.     

Origin of Archean anorthosites: Evidence from the Bad Vermilion Lake anorthosite complex,Ontario

The Bad Vermilion Lake anorthosite complex (2,700 m.y.) is exposed over an area of about 100 km² near Rainy Lake, Ontario. As is typical of other Archean anorthosites, it is composed of coarse (1–30 cm across), equidimensional, euhedral to subhedral, calcic (An₈₀) plagioclase, in a finer grained mafic matrix. The amount of mafic matrix in individual samples ranges from none to about 70% by volume. The complex has been variably metamorphosed to greenschist facies. Zoisite, chlorite, and hornblende are abundant, but primary plagioclase is preserved in many places. The anorthosite complex is associated with gabbro and with mafic to felsic metavolcanic rocks, and is cut by tonalite plutons and by mafic dikes. Some gabbros contain local concentrations of Fe-Ti oxides and/or apatite, but no chromite. The mafic groundmass of the anorthositic rocks is similar in major and trace element chemistry, including rare earth elements, to the associated basaltic metavolcanics, suggesting that the anorthositic complex may have accumulated from a subvolcanic magma chamber which fed mafic lavas to the surface during its crystallization. Mafic flows and dikes chemically similar to the mafic metavolcanics contain plagioclase megacrysts akin to those of the anorthositic rocks, and thus may represent a link between the anorthosite complex and associated mafic lavas. Elongate pretectonic tonalite intrusions were comagmatic with the felsic metavolcanics, but not with the anorthosites or metabasalts. These silicic rocks may represent low-pressure partial melts of the mafic rocks. There is no direct or indirect evidence for significant volumes of ultramafic material at the present exposure level of the complex. An estimate of the bulk composition of all rocks presumed to be comagmatic with the anorthosites, including gabbros and mafic metavolcanics, is an aluminous basalt with about 20 wt.% Al₂O₃. This composition has REE abundances unlike those of typical Archean high-Al basalts and probably does not represent that of a primary or evolved melt. The possibility must be considered, therefore, that a substantial fraction of material comagmatic with the anorthosites has been separated from the complex, either by magmatic or tectonic processes.     

Tourmaline nodules from Capo Bianco aplite (Elba Island,Italy): an example of diffusion limited aggregation growth in a magmatic system

The morphology of tourmaline nodules occurring in the Capo Bianco aplite (Elba Island, Italy) is studied. Outcrop features indicate that tourmaline nodules are the product of magmatic crystallization, as they are aligned along flow fields developed within the magmatic hosting mass. Mesoscopic observations indicate that nodule morphologies are very variable, from rounded to dendritic. Morphometric analyses show that tourmaline nodules are fractals and that fractal dimension quantifies their degree of irregularity. Numerical simulations of nodule growth are performed by using a Diffusion-Limited Aggregation process. The presence in natural samples of nodules with different morphologies is explained by considering a chaotic magmatic system characterized by a complex interplay between growth rate in different dynamical regions, latent heat of crystallization, and local convection dynamics. It is suggested that higher growth rates correspond to growth of tourmaline nodules in dynamical regions where the transfer of nutrients is very efficient. In such conditions, the latent heat released by the growing nodule is high, inducing strong local convection dynamics, destabilizing the nodule interface, and promoting the formation of dendritic morphologies. On the contrary, the growth of nodules in dynamical regions characterized by weak transfer of nutrients is inhibited leading to weak local convection dynamics and, consequently, to the formation of rounded morphologies.     

Diffusion-controlled olivine corona textures in granitic rocks from Lofoten, Norway: calculation of Onsager diffusion coefficients, thermodynamic modelling and petrological implications

Reactions occurring during cooling of charnockitic intrusives on the Lofoten Islands produce characteristic diffusion-controlled textures around fayalite and Fe–Ti oxides. Thermobarometry indicates the corona textures formed at 780–840 °C and pressures of 4–10 kbar, whereas the magmatic assemblage of the charnockite (clinopyroxene–olivine–quartz) crystallized at about 850–870 °C and 4 kbar. The succession olivine|orthopyroxene+magnetite|orthopyroxene+garnet and olivine|orthopyroxene+magnetite|amphibole developed where olivine reacted with adjacent plagioclase or K-feldspar, but the modes and the thicknesses of the corona textures vary according to the feldspar type, indicating that the primary magmatic ternary feldspar was already exsolved into albitic plagioclase and alkali feldspar when the corona formation began. Simultaneously, in other parts of the rock, primary magmatic clinopyroxene reacted to amphibole and Fe–Ti oxides reacted to orthopyroxene+garnet coronas or to amphibole. Textures demonstrate significant Al diffusion in the rocks under granulite facies conditions and they suggest that no pervasive fluid influx occurred and that amphibole formation was dependant on a local source of H₂O probably related to water-release during the last stages of magmatism. Calculation of the net reaction by accounting for all observed reactions at different sites in the rock indicates that the system can be regarded as balanced on a hand-specimen scale with respect to all elements except for Na and H₂O. The larger variety of textures developed in rocks of granitic bulk composition provide more constraints than textures from gabbroic compositions, and permitted calculation of a set of relative diffusion coefficients which also reproduce textures in the gabbroic and anorthositic rocks from the Lofoten Islands. The following set of relative diffusion coefficients (L_i/L_Fe) reproduces the observed textures in the Lofoten rocks: Si=0.82, Mg=0.59, Mn=0.05, Na=0.38, K=0.39, Al=0.05 and Ca=0.07.     

Polymetamorphism during the Grenvillian Orogeny in SE Ontario: Results from trace element mapping,in situ geochronology,and diffusion geospeedometry

The Flinton Group is a metasedimentary succession of the Grenville Province in SE Ontario, potentially allowing insight into the tectono-thermal evolution of continental crust during the Mesoproterozoic. At its Green Bay locality, Flinton Group metapelites of the staurolite zone contain abundant, post-kinematic garnet porphyroblasts. Whereas the larger garnet crystals are typically impinged, smaller crystals are isolated from each other, occasionally exhibiting elongated shapes with apparently trigonal morphology. Central sections of the garnet population of a representative sample reveal that garnet is composed of different compositional and microstructural domains. In the largest crystals of the population, garnet contains rectangular to rhombic domains, marked by sharp increases in the concentrations of Nb, V, Ti, and Cr. These domains are associated with irregularly shaped patches, characterized by spatially heterogenous enrichments of Ca and LREE, and depletions in the contents of P, Y, MREE, and HREE, accompanied by increased densities of comparatively coarse-grained quartz inclusions hosting apatite. Microstructural relationships indicate that these domains correspond to portions of garnet that pseudomorphed biotite, with the enrichments of Nb, V, Ti, and Cr outlining the original biotite shapes. The compositional patterns formed by Ca, P, Y, and REE indicate that apatite participated in the grain-fluid interactions that operated during the metasomatic replacement of biotite by garnet. The statistical analyses of the garnet number and size distributions confirm that garnet initially nucleated on biotite, controlled by the kinetics of attachment and detachment processes at the garnet/biotite interface, resulting in the typical impingement habit. In situ Lu–Hf garnet geochronology applied to garnet that did not pseudomorph biotite, and hence is enriched in HREE, points to a first metamorphic event at c. 1080 $\pm$ 31 Ma. Subsequent pseudomorphism of staurolite by white mica in a Al₂O₃- and FeO-mobile system resulted in the concomitant crystallization of a new garnet generation, forming overgrowths on the first garnet generation and nuclei in the fine-grained matrix. Garnet that nucleated during this event grew to isolated and elongated crystals with apparently trigonal morphology, aligned in a direction c. perpendicular to the rock matrix foliation. The open-system behaviour during this event limits the use of whole-rock-based geochronological and thermobarometrical applications. However, previously published in situ U–Pb ages of monazite included in the rims of the garnet crystals and in the rock matrix indicate that this event took place at c. 976 $\pm$ 4 Ma, likely associated with a period of increased hydrothermal activity late in the metamorphic history of the Grenvillian Orogeny. Diffusion geospeedometry calculations indicate that garnet growth during this hydrothermal event lasted for less than 6 Myr.     

Characteristics of Corona Textures in the Huangtuling Granulite in the Dabie Comples,China and Their Implications for Tectonic Settings

Widely developed in the Dabie complex are various disequilibrium textures which provide direct evidence for the evolution of metamorphism and late-stage uplifting history.The typical mineral assemblasge in the Opx-Gt-Pl-Bi gneiss in Huangtuling,Luotian County,Hubei Province,Is Opx(I) Gt Pl(I) Bi(I) Q.The corona composed of cordierite and orthopyroxene(Ⅱ）growing around garnets in the granulite makes it clear that there occurres the following metamorphic reaction:Gt Q→Cd Opx(Ⅱ）.It is estimated that the gran ulite-forming temperature(T)and pressure(P)are 857-998℃ and 1.18-1.23Gpa,respectively,and the corona was formed under the following conditions:T=829-911℃ and P=0.52-0.59GPa.The above results indicate that There occurred a rapid and nearly adiabatic uplifting event and a decompressional metamorphism in the Dabie complex after the formation of granulite.As compared with the granulites worldwidely distributed in 90 locations(Harley 1989),the Huangtuling granulite should belong to the high-pressure type,which represents the composition of the crust at a depth of more than 40 kilometers.     

A secondary clinopyroxene-chlorite-spinel assemblage in clinopyroxenite of the Mann complex, Abitibi Belt, Ontario: An unusual hydrothermal alteration suite

Summary Small dispersed patches (< 1 cm) of chlorite + secondary clinopyroxene ± spinel occur within a unit of clinopyroxenite in a mafic/ultramafic complex located in Mann Township, approximately 47 km NE of Timmins, Ontario. Prior to hydrothermal alteration that resulted in formation of the chlorite patches, clinopyroxenite was a homogeneous, medium-grained clinopyroxene adcumulate. The abundances of major and trace lithophile elements and the compositions of magmatic clinopyroxene (augite, with average Mg number of 84.3, = 1.1, n = 80) are uniform across the section sampled. The most altered portions of clinopyroxenite consist of a total of 30 to 40% chlorite plus secondary diopsidic pyroxene with traces of spinel. Chlorite patches are, to some extent connected by very thin veins. Multiple generations of chlorite are inferred from cross-cutting relationships and variations in chlorite chemistry. Adjacent to chlorite patches, the magmatic clinopyroxene is occasionally converted to diopside. The secondary clinopyroxene is typically zoned from diopside to salite, and is characterized by very low minor element concentrations and a positive MnO-FeO correlation. Spinel in chlorite patches is iron-rich chromite. Magmatic chromite and iron-rich chromite are commonly zoned with outer rims of either ferrochromite or Cr magnetite or both. Occasionally trace amounts of copper-rich sulphides accompanied by platinum-group minerals occur only with the chlorite-clinopyroxene-spinel alteration assemblage. A proposed paragenetic sequence for the secondary minerals is based on reaction of magmatic clinopyroxene with a hydrothermal fluid during subsolidus cooling of the intrusion. The assemblage secondary clinopyroxene + chlorite ± iron-rich chromite suggests a fluid with maximum temperature of approximately 500°C.

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Eine sekundäre Klinopyroxen-Chlorit-Spinell Paragenese in Klinopyroxeniten, Mann Complex, Abitibi Belt, Ontario: Eine ungewöhnliche hydrothermale Alterationsabfolge

Zusammenfassung Kleine, dispers verteile Nester (< 1 cm) von Chlorit + sekundärem Klinopyroxen ±Spinell kommen in Klinopyroxeniten eines mafisch/ultramafischen Komplexes in Mann Township, ca. 47 km NE von Timmins, Ontario, vor. Vor der Alteration, die in der Bildung der Chloritnester resultierte, handelte is sich bei diesen Gesteinen um homogene mittelkörnige Klinopyroxen-Adkumulate. Die Verteilung der Haupt- und lithophilen Spurenelemente und die Zusammensetzung der magmatischen Klinopyroxene (Augit, mg 84.3, = l. l, n = 80) sind entlang des beprobten Profils einheitlich. Die am stärksten alterierten Klinopyroxenite bestehen aus 30-40% Chlorit plus sekundärem Diopsid und Spuren von Spinell. Die Chloritnester stehen über sehr dünne Gängchen miteinander in Verbindung. Auf Grund der Texturen und des Mineralchemismus sind mehrere Chloritgenerationen zu unterschieden. In der Nähe der Chloritnester wandelt sich magmatischer Klinopyroxen fallweise in Diopsid um. Der sekundär gebildete Klinopyroxen ist zonar gebaut (Diopsid zu Salit), weist sehr geringe Gehalte an Nebenelementen auf und ist durch eine positive Korrelation von FeO mit MnO charakterisiert. Der Spinell in den Chloritnestern ist Fe-reicher Chromit. Magmatischer und Fe-reicher Chromit sind normalerweise zoniert mit Rändern von Ferrochromit oder/und Cr-Magnetit. Spuren von Cu-reichen Sulfiden, die von PGM begleitet werden, kommen vereinzelt mit der Chlorit-Klinopyroxen-Spinell Alterationsgesellschaft vor. Die vorgeschlagene paragenetische Abfolge wird durch Subsolidus-Reaktion magmatischer Klinopyroxene mit hydrothermalen Fluiden im Zuge der Abkühlung der Intrusion erklärt. Die Vergesellschaftung von sekundärem Klinopyroxen + Chlorit ± Fe-reicher Chromit läßt auf ein Fluid mit ca. 500°C Maximaltemperatur schließen.

     

Pyroxene in enclaves and syenites of the Red Hill complex,New Hampshire: an ion and electron microprobe study

The Red Hill complex, New Hampshire, contains both silica-undersaturated and silica-saturated to oversaturated syenites. Ion microprobe analyses of pyroxene from the Nepheline Sodalite Syenite (NSS) and its enclaves reveal that the REE abundances increase in a systematic manner from low values in the enclave pyroxenes to higher values in the NSS host rock pyroxenes. This variation is interpreted to have resulted from differentiation and coupled with previously published mineral and bulk-rock compositions, suggests that the enclaves represent samples of NSS parental liquids that intruded into residual, syenitic liquids in a zoned magma chamber. Pyroxene analyses of the Garland Peak Syenite (GPS) and its enclaves indicate that the enclaves are of several populations: some may be related to the GPS, others are not. The GPS itself is heterogeneous and pyroxene trace element zoning is difficult to explain by fractionation processes. The silica-oversaturated Outer Coarse Syenite (OCS) contains pyroxenes with trace element characteristics that are distinct from the NSS. The low V concentrations suggest that the OCS exprienced magnetite fractionation prior to pyroxene growth. It is proposed that high f O₂in the OCS magma caused Fe-Ti oxide crystallization, which in turn, influenced magmatic silica activity. The chondrite normalized REE patterns of OCS pyroxenes are also suggestive of titanite crystallization, another indication of high f O₂. In contrast to the influence of magnetite crystallization, the high and rimwardly increasing Y and Yb concentrations in both the NSS and the OCS pyroxenes suggest that amphibole fractionation was not the major influence on silica activity. Therefore, it is unlikely that the OCS was derived by amphibole fractionation from a NSS precursor magma.     

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