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.     

Differentiation of Komatiite Flows

Although layered komatiite flows with spinifex-textured upperparts and olivine-enriched lower parts are commonly thoughtto provide firm evidence of magmatic differentiation by olivinesettling, there are reasons to suspect that this may not betrue. Komatiite flows almost certainly convect vigorously asthey cool, with convective velocities that greatly exceed thesettling velocities of olivine grains. Other explanations forthe differentiation of komatiite flows into layers with differentolivine contents need to be investigated. One such explanation is provided by a detailed study of a komatiiteflow from Alexo, Ontario, Canada. This flow is about 16 m thickand has a spinifex-textured upper layer, and a lower layer,called the B-layer, which is composed of closely-packed, equantolivine phenocrysts. The composition of the initial liquid inthe flow is given by an upper chill sample, which has 28 percent MgO. In underlying spinifex lavas, MgO contents range from20 to 35 per cent MgO, and the B-layer has about 42 per centMgO. Olivines are most Fo-rich in the chill margin (Fo_94.1)and in the B-layer (Fo_93.8). In spinifex lavas, olivines aregenerally less magnesian, ranging from a high of Fo_93.8 in unusuallyMgO-rich chevron spinifex lavas, down to Fo₈₉ in the lower platespinifex lavas. It is believed that solidification of the upper part of theflow started only after it had become ponded, and that crystallizationproceeded with growth of spinifex olivines downwards from theroof of the flow. The manner in which the composition of silicateliquid within the flow changed during growth of the spinifexlayer can be calculated using the compositions of olivines inspinifex lavas. These calculations show that all the spinifexlavas are more magnesian than the liquids from which they formed:i.e. they all contain a component of excess olivine. Furthercalculations indicate, however, that liquid compositions inthe lower part of the flow changed more rapidly than can beexplained by accumulation of olivine in the spinifex lavas.This additional olivine must have crystallized in the lowerpart of the flow, and remained there as phenocrysts suspendedin the convecting liquid. As the spinifex-textured upper partcontinued to grow and thicken, olivine continued to crystallizewithin the flow, and the growing phenocrysts became more andmore concentrated in an ever-decreasing volume of liquid. Whenthe concentration exceeded about 50 vol. per cent, the viscosityof the lava became high enough to inhibit convection. The flowthen solidified completely, maintaining its olivine distributionwith a spinifextextured upper layer and an olivine-enrichedlower layer.     

The role of fluids in the metamorphism of komatiites,Agnew nickel deposit,western Australia

The Agnew nickel sulfide deposit is spatially associated with a lenticular body of ultramafic rocks which shows a concentric zonation in metamorphic mineralogy. Olivine + tremolite + chlorite + cummingtonite ±enstatite assemblages occur at the margin of the ultramafic lens, giving way to olivine + anthophyllite, olivine + talc and olivine + antigorite assemblages successively inwards. These rocks are interpreted as having crystallized from komatiitic lavas, and exhibit a spectrum of compositions from those of original flow tops to pure olivine adcumulates. The relative modal abundances of metamorphic olivine, tremolite and chlorite reflect original proportions of cumulus olivine and komatiite liquid in the protolith. Peak metamorphic conditions are estimated at 550° C, based on garnet-biotite thermometry, at a maximum pressure of 3 kb. This temperature falls within the narrow range over which metamorphic olivine may co-exist with enstatite, anthophyllite, talc or antigorite depending upon the fugacity of water in the metamorphic fluid. The observed mineralogical zonation is therefore attributed to infiltration by CO₂-rich fluids, generated by decarbonation of talc-carbonate rocks formed during pre-metamorphic marginal alteration of the ultramafic lens. Metamorphic fluids were essentially binary mixtures of water and CO₂, with minor H₂S having a maximum partial pressure less than 1 percent of total pressure. Enstatite-bearing assemblages formed in the presence of CO₂-rich fluids at fluid: rock volume ratios close to one, while anthophyllite, talc and antigorite bearing assemblages formed in the presence of progressively more water-rich fluids at progressively lower fluid-rock ratios.     

Low-Pressure Experimental Constraints on the Evolution of Komatiites

Melting experiments were performed on a komatiitic basalt with17 wt% MgO from Munro Township, Ontario, at I-atm pressure andan oxygen fugacity controlled approximately to the fayalite-magnetite-quartzbuffer. The experiments showed that olivine appears at 1344±5°C,spinel at 1334±6°C plagioclase at 1185±5°C,augite at 1176±5°C and pigeonite at 1154±6°C.Compositionally, olivine varies from Fo₉₀ to Fo₇₄ and displaysan average K^Fe/Mg_D (ol/liq) of 0•32. The spinels are chromitesand chromian spinels with Mg/(Mg + Fe²⁺) ratios between 0•66and 0•;32, which show a marked correlation with meltingtemperature. The pyroxenes show an average K^Fe/Mg_D (px/liq)of 0•26, identical for augite and pigeonite. Plagiodaseranges compositionally between An₈₂ and An₇₂ Plotted in thepseudo-quaternary basalt phase diagram, the liquid line of descentis similar to that observed for quartz tholeiitic magmas. Therefore,the low-pressure, late-stage evolution products of komatiiteand basaltic komatiite parental magmas will chemically and mineralogicallybe ferrobasaltic quartz tholeiites. High-temperature and high-pressuremodeling suggests that the main observed compositional variationof Munro komatiites can be explained by low-pressure crystalfractionation and accumulation of olivine into komatiite liquidswith below 21•5–23•5 wt% MgO and eruptive temperaturesbelow 1435–1465°C for oxygen fugacities between thefayalite-magnetite quartz (FMQ) and iron-wiistite (IW) buffers.The maximum magnesium content of liquid komatiites, assumingequilibrium Fo₉₄ olivine, is 27–29 wt% MgO and eruptivetemperatures are between 1515 and 1540°C. KEY WORDS: komatiites; experimental petrology; Munro Township; Ontario     

The Jinchuan Ultramafic Intrusion: Cumulate of a High-Mg Basaltic Magma

The Jinchuan intrusion, situated in Gansu province, China, isan ultramafic dyke-like body emplaced in the Longshoushan upliftedterrain on the southwest margin of the Sino-Korea platform.The intrusion is 6 km long, 350 m wide and hosts a major Ni-Cusulfide deposit. It comprises three subchambers: the west, west-central,and east. The two western subchambers are narrow and deep, andboth are laterally zoned from dunite in the core through Iherzoliteto olivine pyroxenite toward the margins. The eastern subchamberis shallow and wide, and it shows vertical stratification gradingfrom dunite at the base upward into Iherzolite and plagioclaseIherzolite, then back to Iherzolite at the top. Sixty rock samples from the Jinchuan intrusion have been analyzedfor major and trace elements, and 54 samples were also analyzedfor the REE. All samples contain > 24 wt.% MgO, with themajority having > 35% when recalculated to 100% anhydrous.Negative linear correlations are observed between MgO and mostother constituents (except for a few such as Na₂O, K₂O, Sr,and Rb, which may have been affected by alteration), and itappears that the rocks were essentially formed as mixtures ofcumulus olivine and primary magma. Electron microprobe analyses show olivine compositions fromFo₇₉ to Fo₈₆, with most between Fo₈₃ and Fo₈₅. The MgO/(MgO+ FeO) value of the primary magma is calculated to have been 0.64, and its MgO content is estimated to have been 12 wt.%.Thus, the Jinchuan igneous body is probably the ultramafic cumulateportion of an intrusion of a high-magnesium basaltic magma relatedto continental rifting. We suggest that the two western subchambers of the Jinchuanintrusion represent the main conduit to the original magma chamberand that their zoning was formed by flow differentiation. Theeastern subchamber probably represents a higher level of themagma chamber, where crystallization was marked by convectionand periodic replenishment. After consolidation, the Jinchuanintrusion was tilted to the east so the deeper parts of thewestern subchambers are now exposed to the same erosion levelas the shallower part of the eastern subchamber.     

Komatiite hosted Hunters Road nickel deposit,central Zimbabwe: physical volcanology and sulfide genesis

The nickel sulfide bearing Main Flow at Hunters Road is a thick komatiite unit situated at the base of a well developed lava channel complex overlying a prominent banded iron formation that caps a thick sequence of felsic lavas and volcaniclastic rocks intruded by a probably comagmatic feeder sill. The 300–350 m thick inner flank comprises a 200–250 m thick central olivine meso to adcumulate, relatively narrow lower and upper ortho to mesocumulates and a 2–10 m thick olivine spinifex zone at the top. Approximately 700 m towards the 90 m thick outer flank, the spinifex zone is up to 30 m thick, the central meso to adcumulate lenses out between the upper and lower mesocumulates, and the lower orthocumulate is more rich in clinopyroxene. In places, the flanks are directly overlain by rubbly hyaloclastite. The less well preserved, 500 m thick central axis coincides with a floor rock embayment of demonstrably primary origin, which is 200 m deep and at least 800 m wide. Nickel sulfide mineralisation is disseminated, texturally cocumulus to olivine and confined to the meso to adcumulate, the highest nickel grades being located towards the top. Large floor rock xenoliths occur in the mineralised meso to adcumulate of the inner flank within reaction envelopes of barren, pyroxene bearing, olivine ortho to mesocumulate. Formation of the embayment and xenoliths (clear evidence of large scale thermomechanical floor rock erosion) and of the olivine sulfide meso to adcumulate (the sulfur probably derived by the assimilation of sulfidic wall rocks adjacent to the feeder sill) is attributed to prolonged focused flow of hot turbulent lava close to the vent. The Main Flow is interpreted as the product of a thick channelised sheet flow comprising: (i) a turbulent lava river, open along the central axis and partly tubed over along the inner flanks; and (ii) a tubed over levée facies along the outer flanks accommodating overflow from the central axis by inflationary growth under laminar flow conditions.     

Contrasting komatiite belts,associated Ni–Cu–(PGE) deposit styles and assimilation histories

The Agnew–Wiluna greenstone belt in the Yilgarn Craton of Western Australia is the most nickel-sulfide-endowed komatiite belt in the world. The Agnew–Wiluna greenstone belt contains two mineralised units/horizons that display very different volcanological and geochemical features. The Mt Keith unit comprises >500 m-thick spinifex-free adcumulate-textured lenses, which are flanked by laterally extensive orthocumulate-textured units. Spinifex texture is absent from this unit. Disseminated nickel sulfides, interstitial to former olivine crystals, are concentrated in the lensoidal areas. Massive sulfides are locally present along the base or margins of the lenses or channels. The Cliffs unit is locally >150 m thick and comprises a sequence of differentiated spinifex-textured flow units. The basal unit is the thickest, and contains basal massive nickel-sulfide mineralisation. The Mt Keith and Cliffs units display important common features: (i) MgO contents of 25–30% in inferred parental magmas; and (ii) Al/Ti ratios of ～20 (Munro-type). However, the Mt Keith unit is highly crustally contaminated (e.g. LREE-enriched, high HFSEs), whereas the Cliffs unit does not display evidence of significant crustal assimilation. We argue that the distinct trace-element concentrations and profiles of the two komatiite units reflect their different emplacement style and country rocks: the Mt Keith unit is interpreted to have been emplaced as an intrusive sill into dacitic volcanic units whereas the Cliffs unit was extruded as lava flow onto tholeiitic basalts in a subaqueous environment. The mode of emplacement and nature of country rock is the single biggest factor in controlling mineralisation styles in komatiites. On the other hand, evidence of crustal contamination does not necessarily provide information of the prospectivity of komatiites to host Ni–Cu–(PGE) mineralisation, despite being a good proxy for the style of komatiite emplacement and the nature of country rocks.     

Komatiites and nickel sulfide ores of the Black Swan area,Yilgarn Craton,Western Australia. 3: Komatiite geochemistry,and implications for ore forming processes

The Black Swan komatiite sequence is a package of dominantly olivine-rich cumulates with lesser volumes of spinifex textured rocks, interpreted as a section through an extensive komatiite lava flow field. The sequence hosts a number of nickel sulfide orebodies, including the Silver Swan massive shoot and the Cygnet and Black Swan disseminated orebodies. A large body of whole rock analyses on komatiitic rocks from the Black Swan area has been filtered for metasomatic effects. With the exception of mobile elements such as Ca and alkalis, most samples retain residual igneous geochemistry, and can be modelled predominantly by fractionation and accumulation of olivine. Whole rock MgO–FeO relationships imply a relatively restricted range of olivine compositions, more primitive than the olivine which would have been in equilibrium with the transporting komatiite lavas, and together with textural data indicate that much of the cumulus olivine in the sequence was transported. Flow top compositions show evidence for chromite saturation, but the cumulates are deficient in accumulated chromite. Chromite compositions are typical of those found in compound flow-facies komatiites, and are distinct from those in komatiitic dunite bodies. Incompatible trace element abundances show three superimposed influences: control by the relative proportion of olivine to liquid; a signature of crustal contamination and an overprint of metasomatic introduction of LREE, Zr and Th. This overprint is most evident in cumulates, and relatively insignificant in the spinifex rocks. Platinum and palladium behaved as incompatible elements and are negatively correlated with MgO. They show no evidence for wholesale depletion due to sulfide extraction, which was evidently restricted to specific lava tubes or pathways. The lack of correspondence between PGE depletion and contamination by siliceous material implies that contamination alone is insufficient to generate S-saturation and ore formation in the absence of sulfide in the assimilant. Contamination signatures in spinifex-textured rocks may be a guide to Ni-sulfide mineralisation, but are not entirely reliable in the absence of other evidence. The widespread vesicularity of the sequence may be attributable to assimilated water rather than to primary mantle-derived volatiles, and cannot be taken as evidence for primary volatile-rich magmas. The characteristic signature of the Black Swan Succession is the presence of highly localised disseminated sulfide within a sequence showing more widespread evidence for crustal contamination and interaction with its immediate substrate. This has important implications for the applicability of trace element geochemistry in exploration for komatiite-hosted nickel deposits.Electronic Supplementary Material Supplementary material is available in the online version of this article at Editorial handling: Peter Lightfoot     

Potassic Mafic Lavas of the Bearpaw Mountains, Montana: Mineralogy, Chemistry, and Origin

The volcanic rocks of the Bearpaw Mountains are part of theMontana high-potassium province, emplaced through Archaean rocksof the Wyoming Craton between 54 and 50 Ma ago. Extrusive rocks,dominated by minettes and latites, have a volume of 825 km³.The minettes range in composition from 20 to 6% MgO. The moremagnesian varieties contain the phenocryst assemblage forsterite+ Cr-spinel + diopside phlogopite. More evolved rocks areolivine-free, with an assemblage of either salite + phlogopite+ pseudoleucite or salite + phlogopite + analcime. The analcimeis thought to be secondary after leucite, produced by loss ofpotassium from the minettes. Mineral chemistry and textures,especially of clinopyroxenes, indicate that mixing between minettemagmas of varying degrees of evolution was commonplace. Compositionalvariation was further extended by accumulation of olivine +spinel + clinopyroxene phenocrysts, and by the preservationof mantle xenocrysts in the minettes. The primary minette magmasare inferred to have had 12–14% MgO and to have been generatedat 30 kb from an olivine + diopside + phlogopite-bearing source.The primary magmas evolved dominantly by fractionation of olivine+ diopside. The minettes have high contents of large ion lithophile elements(LILE) and light rare earth elements (LREE), with K₂O up to6.18%, Ba 5491 ppm, Sr 2291 ppm, and Ce 99 ppm. (⁸⁷Sr/⁸⁶Sr)⁰ranges from 0.707 to 0.710 and Nd varies from –10 to–16. These data, plus high LILE/HFSE (high field strengthelements) values, are interpreted to show that the minettescontain at least three different mantle components. The lithospherewas initially depleted in Archaean times, but was metasomaticallyenriched in the Proterozoic and in the late Cretaceous and earlyTertiary. The latites have many chemical features in common with the minettes,such as potassic character and high LILE/HFSE values. They formedby fractional crystallization of minette magma in combinationwith assimilation of crustal rocks; this process enriched themagmas in SiO₂ and raised Na₂O/K₂O and ⁸⁷Sr/⁸⁶Sr values. Chemicaldata for phenocrysts and bulk rocks in minettes suggest mixingbetween minette and latite magmas.     

Abyssal peridotites >3,800 Ma from southern West Greenland: field relationships, petrography, geochronology, whole-rock and mineral chemistry of dunite and harzburgite inclusions in the Itsaq Gneiss Complex

Within the northern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on the southern side of the Isua supracrustal belt, enclaves up to ~500 m long of variably altered ultramafic rocks contain some relics of unaltered dunite-harzburgite. These are associated with mafic supracrustal and plutonic rocks and siliceous metasediments. SHRIMP U/Pb zircon geochronology on non-igneous zircons in altered ultramafic rocks and on igneous zircons from components of the surrounding orthogneisses intruding them, indicate an absolute minimum age for the ultramafic rocks of ca. 3,650 Ma, but with an age of ca. 3,800 Ma most likely. The diverse ultramafic and mafic rocks with rarer metasediment were all first tectonically intercalated and then became enclosed in much more voluminous tonalitic rocks dated at ca. 3,800 Ma. This is interpreted to have occurred during the development of a 3,810-3,790-Ma composite magmatic arc early in the evolution of the Itsaq Gneiss Complex. This northern part of the Itsaq Gneiss Complex is the most favourable for the geochemical study of early Archaean protoliths because it experienced peak metamorphism only within the amphibolite facies with little or no in-situ melt segregation, and contains some areas that have undergone little deformation since ca. 3,800 Ma. Most of the ultramafic enclaves are thoroughly altered, and largely comprise secondary, hydrous phases. However, the centres of some enclaves have escaped alteration and comprise dunite and harzburgite with >95% olivine (Fo_89-91) + orthopyroxene (En₈₉) + Al-spinel (Cr_8-20) assemblages. The dunites and harzburgites are massive or irregularly layered and are olivine-veined on 5-10-m to 10-cm scales. Their whole rock major and rare earth element, and olivine and spinel compositions differ significantly from xenoliths representing the Archaean cratonic lithospheric mantle, but are typical of some modern abyssal peridotites. The harzburgites and dunites show both LREE depleted and enriched patterns; however, none show the massive REE depletion associated with the modelled removal of a komatiite. They are interpreted as being the products of small degrees of melt extraction, with some showing evidence of refertilisation. These Greenland dunites and harzburgites described here are currently the best characterised 'sample' of the early Archaean upper mantle.     

Mineralogy and Petrogenesis of the Deccan Trap Lava Flows around Mahabaleshwar, India

Electron microprobe analyses of minerals of thirteen DeccanTrap lava flows at Mahabaleshwar have been carried out in thepresent study. All of these flows have tholeiitic bulk compositionsand all, except one (represented by MB-81-17 of Mahoney et al.,1982) contain olivine, plagioclase, two pyroxenes, and Fe-Tioxide minerals. Olivine and plagioclase appear as distinct phenocrystsin all but one flow, and Ca-rich pyroxene joins as a phenocrystphase in the younger flows. Pigeonite and Fe-Ti oxide minerals(titanomagnetite and ilmenite) occur in the groundmass. Olivineoccurs as both groundmass and phenocryst phase in MB-81-17 (whichis the only flow without low-Ca pyroxene phase); in all otherflows olivine appears only as phenocryst phase. In all but one(MB-81-17) flow olivine is completely altered. MB-81-17 olivinegrains are only partly altered, and in this rock the cores ofphenocrysts are rounded and have a composition of Fo₇₇ whereastheir euhedral rims have a composition around Fo₆₇. The groundmassolivine grains in MB-81-17 are Fo_41–32. Substantial Fe-enrichmentand zoning trends are shown by the pyroxenes in individual rocks.The cores of Ca-rich pyroxene phenocrysts of some of the flowshave as much as 4 wt. per cent A1₂O₃ and may have crystallizedat higher (crustal) pressures. Pigeonite thermometry (Ishii,1975) suggests an average of 1050?C for crystallization of thegroundmass pigeonite (eruption temperature?). Fe-Ti oxide mineralsare mostly altered in the older flows. In the younger flows,coexisting unaltered titanomagnetite and ilmenite yield maximumtemperature estimates for the crystallization of these phaseof about 1025?C and an oxygen fugacity of 10^–11.5 atm.The T-fo₂ path followed by these flows seems to have been consistentlysomewhat lower than that defined by the 1 atm. fayalite-magnetitequartz curve. All of the lavas examined have experienced extensivefractional crystallization of olivine and some clinopyroxeneat relatively higher pressures. These lavas were saturated orclose to being saturated with olivine+plagioclase+clinopyroxeneduring eruption. Plagioclase accumulation, although it appearsto have occurred, has not been significant. It is suggestedthat MB-81-17 magma was contaminated by a calcite-rich rock(limestone?) whereas the lower Group 1 magmas may have beenselectively contaminated by quartz-bearing contaminant. Alternately,parental magma of MB-81-1 (with the highest Mg-number and 8= -16) may have been produced in the upper mantle into whichminor masses of old crust was well mixed. Magma mixing, crystalfractionation, and contamination processes of Mahabaleshwarbasalts and possible genetic relationships with other DeccanTrap lavas are discussed.     

The Ruiga intrusion: A typical example of a shallow-facies paleoproterozoic peridotite-gabbro-komatiite-basaltic association of the Vetreny Belt,Southeastern Fennoscandia

The Ruiga differentiated mafic-ultramafic intrusion in the northwestern part of the Vetreny Belt paleorift was described for the first time based on geological, petrological, geochronological, and geochemical data. The massif (20 km² in exposed area) is a typical example of shallow-facies peridotite-gabbro-komatiite-basalt associations and consists of three zones up to 810 m in total thickness (from bottom to top): melanogab-bronorite, peridotite, and gabbro. In spite of pervasive greenschist metamorphism, the rocks contain locally preserved primary minerals: olivine (Fo _75–86), bronzite, augite of variable composition, labradorite, and Cr-spinels. A mineral Sm-Nd isochron on olivine melanogabbronorite from the Ruiga Massif defines an age of 2.39 ± 0.05 Ga, while komatiitic basalts of the Vetreny Belt Formation were dated at 2.40–2.41 Ga (Puchtel et al., 1997). The rocks of the Ruiga intrusion and lava flows of Mt. Golets have similar major, rare-earth, and trace element composition, which suggests their derivation from a single deep-seated source. Their parent magma was presumably a high-Mg komatiitic basalt. In transitional crustal chambers, its composition was modified by olivine-controlled fractionation and crustal contamination, with the most contaminated first portions of the ejected melt. In terms of geology and geochemistry, the considered magmatic rocks of the Vetreny Belt are comparable with the Raglan Ni-PGE komatiite gabbro-peridotite complex in Canada (Naldrett, 2003).     

Petrology of the Ejected Plutonic Blocks of the Soufriere Volcano, St. Vincent, West Indies

The ejected blocks of the Soufrière volcano consist ofthe mineral phases anorthite (An₉₆-An₈₉; average An₉₃), olivine(Fo₇₉-Fo₆₇; most frequent interval Fo_74-71), salite con taining5–6 per cent Al₂, O₃, hastingsitic amphibole, and magnetitecontaining 6 per cent A1₂O₃, 4 per cent MgO, 7 per cent TiO₂.The minerals occur in various proportions and textures. Theyare virtually unzoned and represent material which has beenejected at, and quenched from, a high temperature. The interstitialscoria present among the mineral grains in the blocks has thecomposition of a saturated sub-alkaline aluminous basalt, andis believed to represent the liquid phase with which the mineralswere in equilibrium at depth. The high-temperature mineralogy,the textures and structures of the rocks support the view thatthe blocks represent crystal cumulates which have crystallizedunder high water-vapor pressures from a fractionating basaltmagma at a depth approximating 6 km. The bulk composition ofthe blocks is such that the resultant liquid fractions are enrichedin silica. Fractional crystallization may be an important factorin the evolution of some calc-alkaline suites.     

Physical volcanology and geochemistry of Palaeoarchaean komatiite lava flows from the western Dharwar craton,southern India: implications for Archaean mantle evolution and crustal growth

ABSTRACT

Palaeoarchaean (3.38–3.35 Ga) komatiites from the Jayachamaraja Pura (J.C. Pura) and Banasandra greenstone belts of the western Dharwar craton, southern India were erupted as submarine lava flows. These high-temperature (1450–1550°C), low-viscosity lavas produced thick, massive, polygonal jointed sheet flows with sporadic flow top breccias. Thick olivine cumulate zones within differentiated komatiites suggest channel/conduit facies. Compound, undifferentiated flow fields developed marginal-lobate thin flows with several spinifex-textured lobes. Individual lobes experienced two distinct vesiculation episodes and grew by inflation. Occasionally komatiite flows form pillows and quench fragmented hyaloclastites. J.C. Pura komatiite lavas represent massive coherent facies with minor channel facies, whilst the Bansandra komatiites correspond to compound flow fields interspersed with pillow facies. The komatiites are metamorphosed to greenschist facies and consist of serpentine-talc ± carbonate, actinolite–tremolite with remnants of primary olivine, chromite, and pyroxene. The majority of the studied samples are komatiites (22.46–42.41 wt.% MgO) whilst a few are komatiitic basalts (12.94–16.18 wt.% MgO) extending into basaltic (7.71 – 10.80 wt.% MgO) composition. The studied komatiites are Al-depleted Barberton type whilst komatiite basalts belong to the Al-undepleted Munro type. Trace element data suggest variable fractionation of garnet, olivine, pyroxene, and chromite. Incompatible element ratios (Nb/Th, Nb/U, Zr/Y Nb/Y) show that the komatiites were derived from heterogeneous sources ranging from depleted to primitive mantle. CaO/Al₂O₃ and (Gd/Yb)_N ratios show that the Al-depleted komatiite magmas were generated at great depth (350–400 km) by 40–50% partial melting of deep mantle with or without garnet (majorite?) in residue whilst komatiite basalts and basalts were generated at shallow depth in an ascending plume. The widespread Palaeoarchaean deep depleted mantle-derived komatiite volcanism and sub-contemporaneous TTG accretion implies a major earlier episode of mantle differentiation and crustal growth during ca. 3.6–3.8 Ga.     

Dunite--Harzburgite--Chromitite Complexes as Refractory Residue in the Sangun--Yamaguchi Zone, Western Japan

Dunite, harzburgite and chromitite of alpine-type ultramaficcomplexes emplaced in the Paleozoic sediments in the Sangun—Yamaguchizone, western Japan, are massive and almost lacking in layeredstructure. Constituent minerals are more or less deformed andequilibrated at a relatively low temperature, about 700 °Cor lower. Chromian spinels in ultramafic rocks from dunite—harzburgite—chromititecomplexes in the Sangun—Yamaguchi zone are characterizedby the uniformity of the Cr/Cr + Al ratio, around 0.5, regardlessof locality and rock type, which is in contrast to the widevariation of the ratio of chromian spinel from the ordinaryalpine-type dunite—harzburgite complex. Mg/Mg + Fe" ratioof chromian spinel, on the other hand, is variable in parallelto the volume per cent of chromian spinel in ultramafic rocks.Olivine in ultramafic rocks is uniform in chemical composition,from Fo₈₈ to Fo₉₂, except for that in chromitite, of which itis Fo₉₅ to Fo₉₇. Primary chemical compositions (especially the Mg/Mg + Fe" ratio)of the constituent minerals have been modified to some extentby element redistribution at low temperature (700 °C orlower), the degree of modification depending on the volume ratioof the minerals. For example, the Mg/Mg + Fe" ratio of chromianspinel in peridotitic rocks has been lowered substantially,and inversely, that of olivine in chromitite has been raised.Primary Mg/Mg + Fe" ratios of olivine and chromian spinel canbe estimated, assuming a certain value of high temperature,e.g. 1200 °C, and the partition coefficient between olivineand chromian spinel at that temperature. As a result, the Mg/Mg+ Fe" ratio of olivine (0.88 to 0.92), and that of chromianspinel (0.78 to 0.80) were uniform, irrespective of rock typeand locality, at the high-temperature stage. Dunite—harzburgite—chromitite complexes in the Sangun—Yamaguchizone were accumulated as refractory residue after the relativelylarge-scaled partial fusion of some primordial peridotites,which resulted in the chemical uniformity of the residual minerals.They may have initially constituted the lowest part of an ophioliticsuite as ultramafic tectonite and been emplaced as dismemberedportions after the disruption of the parent body.     

Komatiite Flows from the Reliance Formation, Belingwe Belt, Zimbabwe: I. Petrography and Mineralogy

The 2.7-Ga Reliance Formation of the Ngezi Group, Belingwe GreenstoneBelt, Zimbabwe, contains extremely fresh komatiite lavas. Detailedfield mapping and a 200-m deep drill-hole, with excellent corerecovery, demonstrated the existence of a suite of lava flows.Each major flow is 10 m thick and characteristically exhibitschilled top and bottom margins, a spinifex zone dominated byrandom spinifex, a B1 zone, and a thick cumulate zone that typicallycomposes two-thirds of the flow thickness. Preservation of olivineand pyroxene mineralogy is superb by Archaean standards, tothe extent that even the tips of skeletal crystals survive.The matrix, although devitrified, is well preserved. Detailedstudy of two flows shows that skeletal grains from the spinifexzone have maximum Fo contents of 91.4. The Fo contents of microphenocrystsfrom the cumulate zone range from Fo_91.2 to Fo_91.6, but rarelarge phenocrysts ( 5 vol.%) have maximum Fo contents of 93.6.The Fo contents of the cumulate olivines do not vary with stratigraphicheight, implying that the cumulate zone formed rapidly, by accumulationof transported crystals. The cumulate zones contain 42–57%modal olivine and display reverse size grading of the olivinemicrophenocrysts. This grain-size variation is believed to resultfrom adcumulus growth within a cumulate pile formed by the formedby the gravitational settling of clusters of olivine crystals.Textural relationships indicate that the final part of the flowto start to crystallize was the lowermost part of the spinifexzone. Reprint requests.     

Petrology of Ultramafic Xenoliths from Loihi Seamount, Hawaii

Ultramafic xenoliths were recovered in four alkalic lava flowsfrom Loihi Seamount at depths between 2200 and 1400m. No xenolithbearing flows were sampled near the summit despite a concentrateddredge program. The flows, three of alkalic basalt and one ofbasanite, contain common olivine megacrysts and small xenolithsof dunite, rarer harzburgite, and a single wehrlite. Olivinemegacrysts as large as 8 mm are Fo_{84–88 6} and containmagnesiochromite inclusions with 1?1–3?5 wt.% TiO₂ Dunitecontains Fo_{83 5–88?5} olivine, magnesiochromite with l?5–6?9wt.% TiO₂ (avg. 3?2 wt.%), and extremely rare chrome-rich diopside.The wehrlite contains euhedral Fo_{85 9} olivine and magnesiochromitewith 1?9–4?7 wt.% TiO₂ poikilitically enclosed in chrome-richdiopside (Wo_{45 4}En_{48 0}Fs_6?6).Most of the olivine megacrysts,dunite, and the wehrlite are cumulates of Loihi alkalic lavasthat accumulated in a magma storage zone located at least 16kmbelow sea level. The rarity of dunite related to tholeiiticmagmas supports the interpretation that the alkalic lavas atLoihi generally predate the tholeiitic lavas. The harzburgitexenoliths have cataclastic textures and contain Fo_{89 5–926} olivine, enstatite (Wo_{2 0–2?7}En_{90?0–88 7}Fe_8?0–8?6),Cr-rich endiopside (Wo_{43 4–44 5}En_{52 0–50 0}Fs_{4 6–45}), and translucent red-brown magnesiochromite. The harzburgitexenoliths, which have 2-pyroxene temperatures of 1066 ? 35?C,originated in the uppermost mantle in a region of high strainrate, probably near the boundary between the mantle and theoverlying ocean crust. The presence of upper mantle xenolithsindicates that the magma storage zone is located below the baseof the ocean crust within the uppermost mantle.     

Pyroclastic rocks: another manifestation of ultramafic volcanism on Gorgona Island,Colombia

Tertiary ultramafic volcanism on Gorgona Island, Colombia, is manifested not only by komatiite flows, but also by a more voluminous sequence of tuff breccias, which is cut by comagmatic picrite dikes. The ultramafic pyroclastic rocks are chaotic to stratified mixtures of angular to subrounded glassy picritic blocks and a fine grained volcaniclastic matrix that consists primarily of plastically-deformed, glassy globules. The entire deposit is interpreted to have formed by an explosive submarine eruption of phenocryst-laden picritic magma. MgO contents of tuff breccias and picrite dikes range from 21 to 27 wt%. Relative to nearby komatiite flows, these rocks are MgO-rich, and FeO-, TiO₂- and Ni-poor. HREE concentrations are very low (N<0.2), indicating derivation from a more highly depleted mantle source region. Nd isotopic data suggest a cogenetic relationship between the picritic magma that formed the tuff breccia and associated dikes and that which produced the komatiite flows. Nevertheless Pb isotopic data as well as whole rock geochemistry preclude such a connection, either due to olivine fractionation/accumulation or to different degrees of partial melting. These ultramafic rock types crystallized from magmas which most likely were extracted from distinct mantle source regions.     

Petrology of the Partridge River Intrusion, Duluth Complex, Minnesota: 1. Relationships between Mineral Compositions, Density and Trapped Liquid Abundance

A 525-m-long drill core (DDH-221) through the Partridge Riverintrusion has been divided into four zones on the basis of changesin mineral abundances, compositions and grain size. The igneousrocks in the core consist of cumulate gabbro, troctolite andolivine gabbronorite, in which the original cumulate frameworkof plagioclase and olivine contained varying amounts of trappedintercumulus (pore) liquid. The compositions of the unzoned olivine (Fo_31–71) havebeen modified by reaction with Fe-rich in situ intercumulusliquid, but the plagioclase cores (An_59–73) have not.The compositions of postcumulus Ca-rich pyroxene, restrictedto En_36–44, and the more variable Ca-poor pyroxene (En_45–74),follow a downward Fe-enrichment trend similar to the Fe-enrichmentin the olivine. The cumulus olivine expected to be in equilibriumwith plausible parental magmas to these rocks was not preservedin the drill core, nor is the chilled margin to the intrusionsufficiently primitive to account for all the olivine. Revisedmass balance estimates of the primary magmatic compositionsof olivine are Fo_67–85. The new limiting value for theprimary olivine is similar to the Fo_83–85 olivine expectedto crystallize from the chilled margin to the nearby PigeonPoint olivine diabase sill under equilibrium conditions. Thechanges in the mineral compositions in core DDH-221 do not adequatelydescribe the behavior of parental melts on an equilibrium coolingpath, implying that the cumulus plagioclase and olivine crystallizedelsewhere, and were mixed with varying amounts of intercumulusliquid before introduction to the present crustal site of thePartridge River intrusion. Rock density increases with depth from 2?76 to 3?21, with amean of 2?98 g/cm³. Estimated trapped liquid densities rangefrom 2?56 to 2?92 g/cm³ at high temperatures. This is interpretedto mean that the intercumulus liquid could not have been expelledupward by compaction of the cumulate pile. The dense intercumulusliquid increased downward in abundance to form a series of rocksthat range continuously from variously packed framework cumulatesto chilled non-cumulate rocks in the basal zone. In situ crystallizationis concluded to be the dominant mode of solidification of thePartridge River intrusion, in which infiltration metasomatismis precluded by the high liquid density.     

Platinum group element and nickel sulphide ore tenors of the Mount Keith nickel deposit, Yilgarn Craton, Australia

A set of platinum group element (PGE) analyses of about 120 samples from a 250-m continuous drill core through the Mount Keith komatiite-hosted nickel orebody, combined with Ni, Cu, Co, S, and major elements, reveals a complex trend of covariance between the original cumulus components of a thick sequence of nearly pure olivine–sulphide liquid adcumulates. The intersection is divided into informal chemostratigraphic zones, defined primarily by combinations of fine-scale cyclicity in original olivine composition, defined by Mg#, and sulphide composition, defined by Pt/S and Ni/S. Contents of Ni and PGE in 100% sulphides (tenors) were determined from linear regressions of the Ni–S and PGE–S covariance for each zone. Inferred olivine compositions range from about Fo₉₂ to Fo_94.6 and show a broad decrease from bottom to top of the sequence complicated by numerous reversals, revealing crystallisation in an open conduit system. Ni and PGE tenors of Mount Keith sulphide ores have typical values similar to the type I deposits of the Kambalda Dome. Mobility of S, at least on the scale of 2-m sample composites, is evidently relatively minor. Tenors for the various zones range 12–22% Ni, 370–1540?ppb Pt, 970–3670?ppb Pd, 100–460?ppb Ir, 170–460?ppb Rh, and 710–1260?ppb Ru. Pt, Pd, and Rh tenors are very strongly correlated, but the iridium group of platinum group elements (IPGEs; Ir and Ru) less so. Tenor variations are predominantly controlled by variations in magma/sulphide ratio R (100–350), with a minor component of variance from equilibrium crystallisation trends in the parent magma. PGE depletion in the silicate melt due to sulphide liquid extraction is limited by entrainment of sulphide liquid droplets and continuous equilibration with the transporting silicate magma. Ratios of the PGEs to one another are similar to those in the host komatiite magma, with the exception of Pt, which is systematically depleted in ores, relative to Rh and Pd and relative to host magma, by a consistent factor of about 2 to 2.5. This anomalous Pt depletion relative to PGE element ratios in unmineralized komatiitic rocks matches that observed in bulk compositions of many komatiite-hosted orebodies. The highly consistent nature of this depletion, and particularly the very strong correlation between Pt, Pd, and Rh in the Mount Keith deposit, argue that this depletion is a primary magmatic signal and not an artefact of alteration. Differential diffusion rates between Pt and the other PGEs, giving rise to a low effective partition coefficient for Pt into sulphide liquid, is advanced as a possible but not definitive explanation.