The Geology of the Great Dyke, Zimbabwe: Crystallization, Layering, and Cumulate Formation in the P1 Pyroxenite of Cyclic Unit 1 of the Darwendale Subchamber

The P1 layer of the Great Dyke is an 200 m thick pyroxenitesuccession in Cyclic Unit 1 and, as the topmost lithology ofthe Ultramafic Sequence, represents the transition from ultramaficto mafic rocks. Of critical importance to this part of the stratigraphyis the strong lateral environmental change from axis to marginas a result of the flared structure of the Great Dyke. Duringthe formation of the P1 layer the axial zone was underlain bya great thickness of hot ultramafic cumulates whereas the samelayer in the marginal zone progressively offiaps the lower ultramaficlayers and is in close proximity to the underlying wall/floorrocks. Heat loss through the floor was therefore much greaterin the marginal zone than in the axis. Major lateral variations are observed, with all lithologicalunits and layers thinning towards the margins of the subchambertogether with a progressive change in the form of the cumulates.Discordant relationships towards the margin between layer types(modal, cryptic, and form) are a feature of the P1 unit whichhas also been recognized in other parts of the Great Dyke (Prendergast,1991). Pyroxene compositions show significant variations withinan overall fractionation trend and decoupling occurs betweenmajor and minor element components of bronzite, suggesting strongcompositional heterogeneity of the magma. This type of crypticlayering has not previously been described and is informallycalled ‘cryptorhythmic’ layering. Pyroxene compositional variation is related to reaction andmodification by trapped intercumulus liquid, and few mineralspreserve liquidus compositions. A similar situation must existfor most layered intrusions. The strong dependence of pyroxenecompositions on incompatible element content in the whole-rockshows that the original liquidus compositions were modifiedby postcumu-lus overgrowth and reaction with the trapped intercumulusliquid. Well-constrained data arrays indicate that most cumulatesin the P1 layer behaved as a closed system with little or nomigration of intercumulus liquid. Liquidus compositions cantherefore be deduced and the residual porosity and degree ofpostcumulus formation were modelled using a computer program.Residual porosity is shown to be between 1 and 13% (by mass).Rocks in the marginal facies have a relatively large proportionof discrete postcumulus phases but instead of representing crystallizationof trapped liquid these are shown to be mainly heteradcumulusphases, i. e., interstitial minerals that have grown largelyby adcumulus processes in equilibrium with the main body ofmagma. The heteradcumulus component can be as high as 27%. Thesephases occur as oikocrysts which give rise to a well-developednodular pyroxenite (the ‘potato’ reef). The formationof the nodules caused local redistribution of primary sulphideliquid. The liquid layers which gave rise to cumulates in the marginalfacies are shown to be enriched in iron and incompatible elementscompared with the axial zone, indicating that the P1 pyroxenitelayer formed by crystallization of a magma which was eithercompositionally stratified or exhibited a strong lateral compositionalgradient.     

The Munni Munni Complex, Western Australia: Stratigraphy, Structure and Petrogenesis

The late Archaean Munni Munni Complex is a layered mafic-ultramaficintrusion emplaced into granitic rocks of the west Pilbara Block.It consists of a lower Ultramafic Zone with a maximum thicknessof 1850 m and an overlying Gabbroic Zone at least 3600 m thick.There are strong geometrical and stratigraphic similaritiesto the Great Dyke of Zimbabwe. The Ultramafic Zone comprises multiple macrorhythmic cyclesof olivine-clinopyroxene adcumulates and mesocumulates. Layeringdips towards the centre of the intrusion and trends laterallyinto a narrow and variably contaminated chilled margin. Higherlayers extend progressively further up the sloping floor ofthe intrusion. Cryptic layering is defined by rapid fluctuationsin Cr content of cumulus clinopyroxene, accompanied by relativelysmall variation in Fe/Mg ratio. The base of the Gabbroic Zone is marked by the first appearanceof cumulus plagioclase and the simultaneous appearance of pigeoniteas a persistent cumulus phase. Magnetite appears as a cumulusphase 400–600 m above this. Gabbroic Zone cumulates showa gradual linear upward increase in Fe/Mg and an absence ofcyclic layering, suggesting crystallization in a closed chamber. Chilled margin samples show evidence of in situ contamination,but indicate that the parent magma to the ultramafic portionof the intrusion was a high-Mg, low-Ti basalt with similaritiesto typical Archaean siliceous high-Mg basalts. Partial meltingof granitic wall rocks occurred along steep side walls but wasless extensive along the shallow-dipping floor. A pyroxenitedyke, the Cadgerina Dyke, intersects the floor of the intrusionat a level close to the top of the Ultramafic Zone, and appearsto have acted as a feeder conduit to the Gabbroic Zone and theuppermost layers of the Ultramafic Zone. The contact zone between the Ultramafic Zone and the GabbroicZone is a distinctive 30–50 m thick pyroxenite layer,the Porphyritic Websterite Layer, which also exlends laterallyup the side walls of the intrusion to form a 200 m thick marginalborder zone separating Gabbroic Zone cumulates from countryrock granites. A distinctive suite of bronzite-rich xenoliths,some containing Al-rich, Cr-poor spinel seams, occurs withinand just above the Porphyritic Websterite Layer in the centralpart of the intrusion. There is a steep gradient of decreasing Cr and increasing Fe/Mgin cumulus clinopyroxenes across the upper 100 m of the UltramaficZone. A sharp downward step in Cr occurs a few metres belowthe base of the Gabbroic Zone, immediately beneath a stronglyorthocumulate layer of augite cumulate containing disseminatedplatinum-group element (PGE)-rich sulphides. Lateral pyroxenecomposition trends within the Porphyritic Websterite Layer canbe accounted for by an increase in cumulus porosity as thislayer approaches the floor of the intrusion. Quantitative modelling of pyroxene composition trends indicatesthat Ultramafic Zone cumulates crystallized from relativelysmall volumes of magma, an order of magnitude less than thesize of the magma body inferred from trends in the GabbroicZone. This conclusion, together with the geometry of the PorphyriticWebsterite Layer, implies that the Porphyritic Websterite Layermarks a level at which the chamber expanded as a result of amajor new influx of magma. Pyroxene composition trends indicatethat this influx was of a distinetly different and more fractionatedcomposition than that parental to the Ultramafic Zone. Injection of fractionated tholeiitic magma into more primitivehigh-Mg basalt resident magma formed a turbulent fountain, whichentrained the resident magma and formed a cool, dense basalhybrid layer. Crystallization of the Porphyritic WebsteriteLayer occurred where the top of this hybrid layer impinged onthe sloping floor. Continuing injection of tholeiitic magmaexpanded the thickness of the hybrid layer, causing the PorphyriticWebsterite Layer to accrete progressively up the sloping floorand the walls. After the conclusion of the influx phase, thehybrid layer became homogenized to a final tholeiite-rich composition,which eventually crystallized to form the Gabbroic Zone. Thexenolithic rocks within and above the Porphyritic WebsteriteLayer were probably derived initially by crystallization ofa contaminated silica-enriched melt layer at the roof of theintrusion, followed by detachment and sinking or slumping tothe floor. Orthopyroxene phenocrysts within the PorphyriticWebsterite Layer may also have originated within this roof zone.     

Stratigraphy, geochemistry and platinum group element mineralisation of the central zone of the Selukwe Subchamber of the Great Dyke, Zimbabwe

The Selukwe Subchamber forms part of the South Chamber of the Great Dyke and is unique in its location and structure by its close association with the Selukwe greenstone belt. The contact with the greenstone belt (west side) is over 20 km in length and contrasts with the mainly granitic contact on the east side. The greenstone belt margin has caused a deflection of the magma chamber resulting in a narrowing of the subchamber in some parts. The narrowing of the chamber, combined with the contrasting marginal rocks, has resulted in several important petrological and geochemical features in the Selukwe Subchamber. These include abnormal compression of the stratigraphy from axis to margin by nearly 50%, geochemical differences in whole rock and mineral compositions, asymmetry in the forms and types of layers close to the east and west margins, and development of an extensive xenolith/autolith suite of rock fragments. The xenolith suite is derived from the greenstone belt, whereas the autolith suite resulted from fragmentation of the Border Group formed in contact with the wall-rocks early in the development of the magma chamber. The preservation of relics of the Border Group is the result of the more rapid cooling in the narrow chamber. In most areas of the Great Dyke, evidence of the Border Group has been largely eliminated. The style of the layering also contrasts with that in the larger North Chamber in that narrow layers of dominantly olivine pyroxenite characterise the sequence in the Selukwe Subchamber.The overall fractionation pattern of the Ultramafic Sequence in the central part of the chamber is represented by orthopyroxene compositions and, being similar to other parts of the Great Dyke, shows little change until the uppermost P1 Pyroxenite where marked fractionation is apparent. The platinum group element zone is associated with sharp compositional changes in orthopyroxene compared with the general trend of evolving pyroxene compositions in the P1 Pyroxenite layer.     

Cumulating processes at the crust-mantle transition zone inferred from Permian mafic-ultramafic xenoliths (Puy Beaunit,France)

Ultramafic and mafic xenoliths of magmatic origin, sampled in the Beaunit vent (northern French Massif Central), derive from the Permian (257 Ma) Beaunit layered complex (BLC) that was emplaced at the crust-mantle transition zone (∼1 GPa). These plutonic xenoliths are linked to a single fractional crystallisation process in four steps: peridotitic cumulates; websteritic cumulates; Al-rich mafic cumulates (plagioclase, pyroxenes, garnet, amphibole and spinel) and finally low-Al mafic cumulates. This sequence of cumulates can be related to the compositional evolution of hydrous Mg basaltic magma that evolved to high-Al basalt and finally to andesitic basalt. Sr and Nd isotopic compositions confirm the co-genetic character of the various magmatic xenoliths and argue for an enriched upper mantle source comparable to present mantle wedges above subduction zones. LILE, LREE and Pb enrichment are a common feature of all xenoliths and argue for an enriched sub-alkaline transitional parental magma. The existence of a Permian magma chamber at 30 km depth suggests that the low-velocity zone observed locally beneath the Moho probably does not represent an anomalous mantle but rather a sequence of mafic/ultramafic cumulates with densities close to those of mantle rocks.     

Parental magmas of the Nain Plutonic Suite anorthosites and mafic cumulates: a trace element modelling approach

Equilibrium melt trace element contents are calculated from Proterozoic Nain Plutonic Suite (NPS) mafic and anorthositic cumulates, and from plagioclase and orthopyroxene megacrysts. Assumed trapped melt fractions (TMF) <20% generally eliminate all minor phases in most mafic cumulate rocks, reducing them to mixtures of feldspar, pyroxene and olivine, which would represent the high-temperature cumulus assemblage. In anorthosites, TMF <15% generally reduce the mode to a feldspar-only assemblage. All model melts have trace element profiles enriched in highly incompatible elements relative to normal mid-ocean ridge basalt (NMORB); commonly with negative Nb and Th anomalies. Most mafic cumulates yield similar profiles with constant incompatible element ratios, and can be linked through fractional crystallization. High K-La subtypes probably represent crust-contaminated facies. Mafic cumulates are inferred to belong to a tholeiitic differentiation series, variably contaminated by upper and lower crustal components, and probably related to coeval tholeiitic basaltic dyke swarms and lavas in Labrador. Model melts from anorthosites and megacrysts have normalized trace element profiles with steeper slopes than those calculated from mafic cumulates, indicating that mafic cumulates and anorthosites did not crystallize from the same melts. Orthopyroxene megacrysts yield model melts that are more enriched than typical anorthositic model melts, precluding an origin from parental melts. Jotunites have lower K-Rb-Ba-Y-Yb and higher La-Ce than model residues from fractionation of anorthositic model melts, suggesting they are not cosanguineous with them, but provide reasonable fits to evolved mafic cumulate model melts. Incompatible element profiles of anorthositic model melts closely resemble those of crustal melts such as tonalites, with steep Y-Yb-Lu segments that suggest residual garnet in the source. Inversion models yield protoliths similar to depleted lower crustal granulite xenoliths with aluminous compositions, suggesting that the incompatible trace element budget of the anorthosites are derived from remobilization of the lower crust. The similarity of the highly incompatible trace elements and LILE between anorthositic and mafic cumulate model melts suggests that the basalts parental to the mafic cumulates locally assimilated considerable quantities of the same crust that yielded the anorthosites. The reaction between underplating basalt and aluminous lower crust would have forced crystallization of abundant plagioclase, and remobilization of these hybrid plagioclase-rich mushes then produced the anorthosite massifs.     

Re-Os and Sm-Nd Isotope and Trace Element Constraints on the Origin of the Chromite Deposit of the Ipueira-Medrado Sill, Bahia, Brazil

The chromite deposit of the Paleoproterozoic Ipueira–Medradosill is hosted in a single, thick (5–8 m), massive layer,which sets severe constraints for the origin of chromitites.It is divided from bottom to top into: (1) a Marginal Zone (5–20m); (2) an Ultramafic Zone (<250 m) consisting of duniteand harzburgite that host the chromitite layer, in which intercumulusamphibole is important and more abundant toward the top; (3)a Mafic Zone (<40 m). The parental magma was large ion lithophileelement and light rare earth element enriched and high fieldstrength element depleted. Sm–Nd isotopic compositionsare consistent with a 2 Ga age, but suggest a variable initialNd isotopic composition that correlates with the abundance ofamphibole. The more negative     

Highly refractory Archaean peridotite cumulates:Petrology and geochemistry of the Seqi Ultramafic Complex,SW Greenland

This paper investigates the petrogenesis of the Seqi Ultramafic Complex, which covers a total area of approximately 0.5 km~2. The ultramafic rocks are hosted by tonalitic orthogneiss of the ca. 3000 Ma Akia terrane with crosscutting granitoid sheets providing an absolute minimum age of 2978 ± 8 Ma for the Seqi Ultramafic Complex. The Seqi rocks represent a broad range of olivine-dominated plutonic rocks with varying modal amounts of chromite, orthopyroxene and amphibole, i.e. various types of dunite(s.s.),peridotite(s.l.), as well as chromitite. The Seqi Ultramafic Complex is characterised primarily by refractory dunite, with highly forsteritic olivine with core compositions having Mg# ranging from about 91 to 93. The overall high modal contents, as well as the specific compositions, of chromite rule out that these rocks represent a fragment of Earth's mantle. The occurrence of stratiform chromitite bands in peridotite, thin chromite layers in dunite and poikilitic orthopyroxene in peridotite instead supports the interpretation that the Seqi Ultramafic Complex represents the remnant of a fragmented layered complex or a magma conduit, which was subsequently broken up and entrained during the formation of the regional continental crust.Integrating all of the characteristics of the Seqi Ultramafic Complex points to formation of these highly refractory peridotites from an extremely magnesian(Mg# ~ 80), near-anhydrous magma, as olivinedominated cumulates with high modal contents of chromite. It is noted that the Seqi cumulates were derived from a mantle source by extreme degrees of partial melting(40%). This mantle source could potentially represent the precursor for the sub-continental lithospheric mantle(SCLM) in this region,which has previously been shown to be ultra-depleted. The Seqi Ultramafic Complex, as well as similar peridotite bodies in the Fiskefjord region, may thus constitute the earliest cumulates that formed during the large-scale melting event(s), which resulted in the ultra-depleted cratonic keel under the North Atlantic Craton. Hence, a better understanding of such Archaean ultramafic complexes may provide constraints on the geodynamic setting of Earth's first continents and the corresponding SCLM.     

Petrology of the G and H Chromitite Zones in the Mountain View Area of the Stillwater Complex, Montana

The Ultramafic Series of the Stillwater Complex in the MountainView area of the intrusion consists of 17 cyclic units thathave been numbered stratigraphically. A typical unit has olivinecumulates at the base, olivine–bronzite cumulates at intermediatelevels, and bronzite cumulates at the top. Most cyclic unitsalso have chromite-rich layers near their base, the thickestbeing the G and H chromitite zones in units 10 and 11. The Gand H zones are each separated from the top of the underlyingcyclic unit by 1–3 m of coarse-grained olivine cumulateand pegmatite; and they are both succeeded by thinner chromititezones, respectively called the hanging wall G (HWG) and thehanging wall H (HWH) zones, situated {small tilde}20 m and 5m above them. The G and H chromitite zones feature rhythmicsequences of thin layers that tend to progress upward from massivechromitite through chromite–olivine cumulate to olivine–chromitecumulate (the last with the minerals in approximately cotecticproportions of about 98:2). In cyclic units 10 and 11, variationsof Mg/Fe in the olivine and bronzite, and of Ni in the olivine,are small and show no clear stratigraphic fractionation trends.The abundance of Cr in the chromite in unit 10 does have a fractionationtrend, however, being generally highest at the bottom of theunit and lowest at the top, with a regression at the HWG zone.In general, Cr in chromite is highest at the base of a rhythmicunit and decreases upward, but it shows no overall decline throughsuccessive rhythmic units; Fe³ exhibits opposite variation,being lowest in the massive chromite, and highest in the disseminatedgrains. The G and H chromitite zones, in the Mountain View area, eachcontain enough chromite to form a single layer of massive chromitite{small tilde} 1 m thick. If their formation involved removalof only 30% of the Cr in the parental magmatic liquid (estimatedconcentration, 600 ppm), then this liquid could have amountedvolumetrically to an areally equivalent layer at least 2000m thick. Model calculations demonstrate that such a large volumeof liquid is consistent with the small variations of Mg/Fe inthe pyroxenes and olivines in the Stillwater cyclic units. We postulate that the G and H chromitite zones and cyclic unitsthat host them formed in response to the entry of new pulsesof primitive magmatic liquid into the Stillwater chamber. Fromexperimental observations, we infer that these pulses producedfountains in which the primitive liquid mixed with residualfractionated liquids, yielding hybrids that were compositionallywithin the chromite liquidus field (or volume) and that weresupercooled (supersaturated ) with respect to the oxide mineral.These effects may have been enhanced by low fO₂ (oxygen fugacity)in the primitive liquid and(or) by high fO₂ of the fractionatedliquid. The hybrid liquids probably collected at the bottomof the chamber in a zoned layer that then divided into double-diffusiveconvecting layers. In these circumstances, the lowest chromite-richlayer in a rhythmic sequence could have formed from the lowestdouble-diffusive liquid layer, and the next could then haveformed when this liquid mixed with the liquid layer above it—andso on up the sequence. We argue that the thick G and H chromititezones are situated toward the top of the Ultramafic Series becausethat level marks when the compositional contrasts between theinjected primitive liquid and the residual fractionated liquidsin the chamber were greatest.     

Investigations of the Stillwater Complex: Part V. Apatites as indicators of evolving fluid composition

Variations in the F, Cl and OH contents of apatite are not constrained by crystal-chemical factors (in contrast to micas and amphiboles), and thus changes in the abundance of these components provide an indicator of halogen fugacity variations and insights into the degassing history of igneous rocks. Microprobe analysis of intercumulus apatites from the Stillwater Complex reveal that Cl-rich apatites, typically containing <0.4 wt % F and >6.0 wt % Cl, occur throughout the lower 1/3 of the complex excluding the Basal series. A change from Cl-rich to more F-rich apatite occurs within olivine-bearing zone I (OB I) of the Banded series, the host zone of the platiniferous J-M Reef. Although apatite compositions are somewhat variable above the J-M Reef, more F-rich apatites predominante and typically contain >1.2 wt % F and <3.0 wt % Cl. The most F-rich apatites occur in the uppermost exposed cumulates. Pristine apatites from coeval sills and dikes from below the complex and from the Basal series are similarly F-rich. In all apatites, the Cl and F contents are lower in rocks affected by later metamorphic fluids. Rare earth element (REE) concentrations in chlorapatites show a marked peak in the olivine-rich rocks of the J-M Reef, and contain up to 2 wt % Ce₂O₃ + La₂O₃. The trend of first increasing, then decreasing Cl/F ratios with stratigraphic height is modeled by a vapor-driven zone refining process occurring within the cumulate pile causing Cl-enrichment in the interstitial melt accompanied by degassing at the top of the magma chamber causing overall loss of Cl from the magma as crystallization proceeded. The abrupt change from Cl-rich to more F-rich apatites within OB I is interpreted as the result of a breakdown of the Cl-rich zone refining front and mixing with Cl-poor supernatant melt. Any high temperature fluids that exsolved and circulated through the lower 1/3 of the complex must have been enriched in Cl and could have transported REE and trace metals.     

Compositional and Lithological Controls on the PGE-Bearing Sulphide Zones in the Selukwe Subchamber, Great Dyke: a Combined Equilibrium-Rayleigh Fractionation Model

The platinum group element (PGE)-bearing Main and Lower SulphideZones of the Selukwe Subchamber of the Great Dyke are made upof a series of subzones within which the ratios Pd:Pt are effectivelyconstant, whereas these ratios vary significantly between thesubzones. Fractionation of Pd with respect to Pt varies by afactor of 10 and cannot be modelled using a sulphide collectorphase and constant partition coefficients. The link with sulphideis indisputable and the control is likely to have been the degreeof oversaturation of PGE micro-nuggets in the magma. The apparentpartition coefficients for Pt and Pd between silicate and sulphideliquid are dependent on the degree of oversaturation and therebyexhibit spurious correlation with the PGE content of the sulphide.Modelling replicates the Pt and Pd distribution and ratios onlyby dramatically changing the effective partition coefficients.Pyroxene compositions (including TiO₂) are shown to be stronglydependent on the incompatible element content of the whole rock,and specific linear arrays relating these variables can be relatedto the PGE subzones. The overall control is Rayleigh fractionation,but constancy of the ratio Pd:Pt and the initial pyroxene composition(before re-equilibration with trapped liquid) within the subzonesis indicative of equilibrium crystallization. This layered structuremay have been derived from liquid layers in the magma chamber. KEY WORDS: platinum group elements; sulphide; Great Dyke; pyroxene compositions     

Sr Isotope and Trace Element Studies of the Great Dyke and Bushveld Mafic Phase and their Relation to Early Proterozoic Magma Genesis in Southern Africa

New Rb-Sr and trace element data are reported for the GreatDyke and Bushveld Mafic Phase layered intrusions. It is arguedthat geochemical characteristics, such as ⁸⁷Sr/⁸⁶Sr ratios andR.E.E. distribution patterns have been little modified by crustalcontamination. Rb-Sr data for whole-rocks of the Great Dyke yield an age of2514±16 m.y. and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70261±4.Mineral data are consistent with these results. The low errorson the results indicate no significant variation of ⁸⁷Sr/⁸⁶Srratios of successive magmatic influxes emplaced in differentmagma chambers. Earlier Great Dyke magmas were highly Mg-richand represent extensive partial melts of the source material.One such influx is shown to have a high Rb/Sr ratio (0.25) anda fractionated R.E.E. pattern (Ce_N/YB_N 12). These ratios areconsidered to approximate those of the source region. The Bushveld Mafic Phase has been dated accurately for the firsttime and has a Rb-Sr age of 2095±24 m.y. Initial ⁸⁷Sr/⁸⁶Srratios increase in a stepwise manner upwards in the intrusionfrom 0.70563±2 to 0.70769±6. Each increase isabrupt and occurs at a horizon also characterized by a suddenirregularity in cryptic variation. The Mafic Phase was emplacedas a succession of magmatic influxes each of which had higher⁸⁷Sr/⁸⁶Sr ratio than its predecessor. The first magma was both Mg-rich (MgO 21.5 per cent) and SiO₂-rich(50–55 per cent SiO₂) and was derived by extensive partialmelting of a shallow level upper mantle source. This sourcewas characterized by trace element abundance ratios (e.g. Rb/Sr 0.25; K/Rb 90; Ce_N/Yb_N 11), similar to those of kimberlitesand some potassic lavas and comparable with those deduced forthe Great Dyke source region. It is postulated that when the Rhodesian and Kaapvaal cratonsstabilized, underlying refractory mantle became fixed theretoto form a proto-lithosphere. Shortly afterwards, at about 2800m.y. ago, this proto-lithospheric mantle was enriched by passagethrough it of fluids with kimberlitic trace element chemistry.This sub-cratonic mantle thereafter evolved with a relativelyhigh Rb/Sr ratio. Magmas derived from it have anomalous chemicalcharacteristics with respect to those of ocean-floor basalts,reflecting major differences in the evolution of their respectivesource regions.     

Halogen Geochemistry of the Stillwater and Bushveld Complexes: Evidence for Transport of the Platinum-Group Elements by Cl-Rich Fluids

Compositional data on apatite, phlogopite, and amphibole indicatethat the high-temperature hydrothermal fluids which affectedthe lower portions of the Stillwater and Bushveld Complexeswere Cl-rich. Apatites from the platinum-group element (PGE)ore zones from both complexes are enriched in Cl relative toother cumulus and noncumulus apatites in these intrusions andto apatites from the Skaergaard and Kiglapait Intrusions andthe Great Dyke. Apatites from all five intrusions can be groupedinto three distinct compositional fields: (a) Cumulus apatitesare essentially fluorapatites with molar Cl/(Cl+OH+F) <0?03;(b) noncumulus apatites, with the exception of those from thePGE ore zones of the Stillwater and Bushveld Complexes, haveCl/(Cl+OH+F) <0?20; (c) Cl-rich apatites associated withPGE-rich zones have Cl/(Cl+OH+F) between 0?45 and 1?0. The REEcontent of noncumulus and Cl-rich apatites also show a positivecorrelation with Cl concentration. It is argued that becauseCl is less soluble in silicate melts than F and because meltswith extremely high Cl/F ratios are unknown, the Cl-rich apatitesequilibrated with Cl-rich hydrothermal fluids exsolved duringsolidification of the cumulate sequence. The Cl, F, and OH contents of phlogopites and amphiboles aremore variable. Compositional heterogeneity is due to crystal-chemicalcontrols on halogen contents, variation in the halogen contentof the original melt/fluid phase and subsolidus re-equilibrationduring cooling with both surrounding mineral phases and lowtemperature fluids. However, both the Stillwater and Bushveldphlogopites are enriched in Cl compared to those from the Skaergaardand Kiglapait Intrusions. The compositions of coexisting minerals from the platinum depositof Olivine-Bearing Subzone I of the Stillwater Complex are usedto compute a fluid composition. The fluid is rich in alkalisand iron as well as HCl, and the solution composition is consistentwith fluid compositions deduced for the PGE-bearing secondaryhortonolite pipes of the Bushveld Complex. The high (Pt+Pd)/Irratios of these deposits are also consistent with a hydrothermalorigin, as both Pt and Pd are more soluble in Cl-complexingfluids than Ir.     

Chromitites from the Luanga Complex,Carajás,Brazil: Stratigraphic distribution and clues to processes leading to post-magmatic alteration

The Neoarchean (ca. 2.75 Ga) Luanga Complex, located in the Carajás Mineral Province in Brazil, is a medium-size layered intrusion consisting, from base to top, of ultramafic cumulates (Ultramafic Zone), interlayered ultramafic and mafic cumulates (Transition Zone) and mafic cumulates (Mafic Zone). Chromitite layers in the Luanga Complex occur in the upper portion of interlayered harzburgite and orthopyroxenite of the Transition Zone and associated with the lowermost norites of the Mafic Zone. The stratigraphic interval that hosts chromitites (∼150 meters thick) consists of several cyclic units interpreted as the result of successive influxes of primitive parental magma. The compositions of chromite in chromitites from the Transition Zone (Lower Group Chromitites) have distinctively higher Cr# (100Cr/(Cr + Al + Fe³⁺)) compared with chromite in chromitites from the Mafic Zone (Upper Group Chromitites). Chromitites hosted by noritic rocks are preceded by a thin layer of harzburgite located 15–20 cm below each chromitite layer. Lower Cr# in chromitites hosted by noritic rocks are interpreted as the result of increased Al₂O₃ activity caused by new magma influxes. Electron microprobe analyses on line transverses through 35 chromite crystals indicate that they are rimmed and/or extensively zoned. The composition of chromite in chromitites changes abruptly in the outer rim, becoming enriched in Fe³⁺ and Fe²⁺ at the expense of Mg, Cr, Al, thus moving toward the magnetite apex on the spinel prism. This outer rim, characterized by higher reflectance, is probably related to the metamorphic replacement of the primary mineralogy of the Luanga Complex. Zoned chromite crystals indicate an extensive exchange between divalent (Mg, Fe²⁺) cations and minor to none exchange between trivalent cations (Cr³⁺, Al³⁺ and Fe³⁺). This Mg-Fe zoning is interpreted as the result of subsolidus exchange of Fe²⁺ and Mg between chromite and coexisting silicates during slow cooling of the intrusion. A remarkable feature of chromitites from Luanga Complex is the occurrence of abundant silicate inclusions within chromite crystals. These inclusions show an adjacent inner rim with higher Cr# and lower Mg# (100 Mg/(Mg + Fe²⁺)) and Al# (100Al/(Cr + Al + Fe³⁺)). This compositional shift is possibly due to crystallization from a progressively more fractionated liquid trapped in the chromite crystal. Significant modification of primary cumulus composition of chromite, as indicated in our study for the Luanga Complex, is likely to be common in non-massive chromitites and the rule for disseminated chromites in mafic intrusions.     

The Halogen Geochemistry of the Bushveld Complex, Republic of South Africa: Implications for Chalcophile Element Distribution in the Lower and Critical Zones

Halogen-bearing minerals, especially apatite, are minor butubiquitous phases throughout the Bushveld Complex. Interstitialapatite is near end-member chlorapatite below the Merensky reef(Lower and Critical Zones) and has increasingly fluorian compositionswith increasing structural height above the reef (Main and UpperZones). Cl/F variations in biotite are more limited owing tocrystal-chemical controls on halogen substitution, but are alsoconsistent with a decrease in the Cl/F ratio with structuralheight in the complex. A detailed section of the upper LowerZone to the Critical Zone is characterized by an upward decreasein sulfide mode from 0·01–0·1% to trace–0·001%.Cu tends to correlate with other incompatible elements in mostsamples, whereas the platinum-group elements (PGE) can behaveindependently, particularly in the Critical Zone. The decreasein the Cl/F ratio of apatite in the Main Zone is associatedwith a shift to more radiogenic Sr isotopic signature, implyingthat the unusually Cl-rich Lower and Critical Zones are notdue to assimilation of crustal rocks. Nor is the Main Zone moreCl rich where it onlaps the country rocks of the floor, suggestinglittle if any Cl was introduced by infiltrating country rockfluids. Instead, the results are consistent with other studiesthat suggest Bushveld volatile components are largely magmatic.This is also supported by apatite–biotite geothermometry,which gives typical equilibrium temperatures of 750°C. Theincreasingly fluorian apatite with height in the Upper Zonecan be explained by volatile saturation and exsolved a Cl-richvolatile phase. The high Cl/F ratio inferred for the Lower andCritical Zone magma(s) and the evidence for volatile saturationduring crystallization of the Upper Zone indicate the Lowerand Critical Zones magma(s) were unusually volatile rich andcould easily have separated a Cl-rich fluid phase during solidificationof the interstitial liquid. The stratigraphic distribution ofS, Cu and the PGE in the Critical Zone cannot readily be explainedeither by precipitation of sulfide as a cotectic phase or asa function of trapped liquid abundance. Evidence from potholesand the PGE-rich Driekop pipe of the Bushveld Complex implythat migrating Cl-rich fluids mobilized the base and preciousmetal sulfides. We suggest that the distribution of sulfideminerals and the chalcophile elements in the Lower and CriticalZones reflects a general process of vapor refining and chromatographicseparation of these elements during the evolution and migrationof a metalliferous, Cl-rich fluid phase. KEY WORDS: Bushveld Complex; chlorine; platinum-group elements; layered intrusions     

津巴布韦大岩墙北部红土型镍矿成矿条件及找矿标志

津巴布韦大岩墙北部红土型镍矿严格受橄榄岩控制,受地形地貌条件影响,位于橄榄岩出露的支谷山前缓坡地带的红土下部和靠山坡一侧.沟谷平坦和山上陡坡地段则无矿或有极薄氧化壳.该大岩墙上部橄榄岩、辉石岩、橄榄辉石岩带和铬铁矿层呈层状相互交替组成韵律性层序.在橄榄岩出露的地表植被稀疏,多为草本植物,而辉石岩、橄榄辉石岩带上呈现出茂密树丛.     

An early Palaeozoic supra-subduction lithosphere in the Variscides: new evidence from the Maures massif

Petrographic and geochemical studies of peridotites and melagabbros from the Maures massif (SE France) provide new constraints on the Early Palaeozoic evolution of the continental lithosphere in Western Europe. Peridotites occur as lenses along a unit rooted in the main Variscan suture zone. They are dominantly spinel peridotites and minor garnet–spinel peridotites. Spinel peridotites represent both residual mantle and ultramafic cumulates. Mantle-related dunites and harzburgites display high temperature textures, with olivine (Mg#_0.90), orthopyroxene (Mg#_0.90) and spinel (TiO₂ < 0.2%; Cr#_0.64–0.83) compositions typical of fore-arc upper mantle. Ultramafic cumulates are dunite adcumulates, harzburgite heteradcumulates and mesocumulates, melagabbro heteradcumulates and amphibole peridotites, with olivine (Mg#_0.85–0.89), orthopyroxene (Mg#_0.86–0.89) and Cr-spinel (TiO₂ = 0.5–3.3%; Cr#_0.7–0.98) compositions typical of ultramafic cumulates. Cr-spinel compositions of both spinel peridotite types suggest their genesis in a supra-subduction zone lithosphere. Core to rim zoning in spinel is related to the incomplete influence of regional metamorphism and serpentinisation. The covariation of major and minor elements with Al₂O₃ for cumulates is consistent with igneous processes involving crystal accumulation. Both mantle and cumulate dunites and harzburgites have U-shaped REE patterns and extremely low trace element contents, similar to peridotites from modern fore-arc peridotites (South Atlantic) and from ophiolites related to supra-subduction zones (Semail, Cyclops, Pindos, Troodos). Melagabbros also have U-shaped REE patterns similar to xenoliths from the Philippine island arc, but also similar to intrusive ultramafic cumulates from the Semail nappe of Oman related to a proto-subduction setting. A wehrlite has a REE pattern similar to that of amphibole peridotites reflecting metasomatism of clinopyroxene-bearing peridotites due to subduction-related fluids. The Maures spinel peridotites and melagabbros are therefore interpreted as the lowermost parts of a crustal sequence and minor residual mantle of lithosphere generated in a supra-subduction zone during Early Palaeozoic time. Garnet–spinel peridotites are chemically close to melagabbros, but have recorded high pressure metamorphism before their retrogression similar to spinel peridotites into amphibolites to greenschists facies metamorphism. They indicate burial to mantle depths of the margin of the supra-subduction lithosphere during the Early Palaeozoic continental subduction. Both peridotite types were exhumed during the Upper Palaeozoic continental collision. Comparable observations from other Variscan-related peridotites, in particular of the Speik complex of the Autroalpine basement, and a common age for the subduction stage allow extension of these regional conclusions to a broad area sharing the Cambrian suture zone, extending from the Ossa-Morena to the Bohemian massif.     

Magma Mixing and Intraplutonic Quenching in the Wingellina Hills Intrusion, Giles Complex, Central Australia

The Wingellina Hills intrusion is a small composite gabbroic/ultramaficintrusion and forms a tectonically dismembered segment of theUpper Proterozoic Giles complex in central Australia. Its 1600m of exposed magmatic stratigraphy formed in a continuouslyfractionating, periodically replenished magma chamber. Olivinegabbro and gabbronorite units alternate with lenticular strataboundintercalations of ultramafic (peridotite and pyroxenite) cumulates.A well-developed hybrid footwall zone of intermingled gabbroand pyroxenite underlies each ultramafic unit and demonstratesthe intrusive relationships of ultramafics into gabbroic cumulatemembers. The limited range of mg-number [100 ? Mg/(Mg+Fe)] of ferromagnesiansilicates indicates that the magmatic sequence covers a rathersmall spectrum in chemical fractionation and that the WingellinaHills intrusion represents the basal portion of a formerly largerlayered complex. The mg-number of olivine ranges from 89 to77, below which olivine is replaced by cumulus orthopyroxene.Clinopyroxene covers a wider mg-number range from 91 to 77 andis systematically enriched in MgO relative to coexisting orthopyroxeneand olivine. Anorthite content in plagioclase generally correlatespositively with mg-number changes of coexisting ferromagnesiansilicates. Interstitial plagioclase in clinopyroxenites containsexsolution lamellae of pure orthoclase. These antiperthitesare among the most calcic recorded, with plagioclase hosts betweenAn₆₀ and An₈₀. Bulk antiperthite compositions range around An₆₅–Ab₁₅–Or₂₀and straddle a high-temperature (Or₂₀) solvus in the plagioclasetriangle. The extent of former solid solution between calcicplagioclase and orthoclase indicates crystallization and coolingof the cumulates under moderate pressure and anhydrous conditions. Cryptic mg-number variations show that the intrusion experiencedweak iron enrichment with stratigraphic height. Normal fractionationis confined to the gabbroic members of the sequence, whereasultramafic intercalations are associated with sharp chemicalreversals toward more refractory mineral compositions. Reversalsof mg-number are considerably displaced into the underlyinggabbroic units by up to 50 m relative to the basis of ultramaficintercalations, which indicates extensive postcumulus infiltrationmetasomatism following the emplacement of fresh magma. The trivalentoxides in clinopyroxene have retained their pristine stratigraphicvariation patterns through later metasomatic events and stillcoincide with the cumulus layering. Macroscopic and cryptic layering in the Wingellina Hills intrusionare consistent with a continuously fractionating magma chamberwhose differentiation path was repeatedly reset by periodicinfluxes of primitive parent melt. Ultramafic and gabbroic cumulatemembers can be derived from a single olivine-saturated parentmelt by sequential separation of olivine, olivine-clinopyroxene,and finally olivine/orthopyroxene-clinopyroxene-plagioclase.A series of orthopyroxene-rich cumulates in the mixing zonesof the two melts crystallized from hybrids of the most primitiveand most evolved end-member compositions. Liquidus temperatures calculated for the resident and replenishingmelt components yield 1250 and 1350?C, respectively. As a resultof this temperature difference, fresh influxes of hot parentliquid crystallized rapidly under strongly undercooled conditionsas they ponded on, and quenched against,the chamber floor. Rapidcooling caused a temporary acceleration of the crystallizationfront and formation of impure cumulates with high trapped meltproportions, which resulted in a close coincidence of orthocumulateunits with stratigraphic levels of primitive melt addition.Grain sizes in orthocumulates vary with the cooling rate andpass through a maximum as the degree of undercooling increases.High cooling rates also influenced the composition of some cumulusphases. Clinopyroxenes from ultramafics in the mixing zonesare enriched in iron and aluminium (despite a more primitiveparent melt) and fall outside the fractionation path, especiallyif the batch of new hot magma was small compared with the poolof cooler resident liquid. Aluminous cumulus spinel is partof a metastable crystallization sequence and only crystallizedin the most magnesian ultramafics after episodes of intraplutonicquenching.     

S-rich apatite-hosted glass inclusions in xenoliths from La Palma: constraints on the volatile partitioning in evolved alkaline magmas

The composition of S-rich apatite, of volatile-rich glass inclusions in apatite, and of interstitial glasses in alkaline xenoliths from the 1949 basanite eruption in La Palma has been investigated to constrain the partitioning of volatiles between apatite and alkali-rich melts. The xenoliths are interpreted as cumulates from alkaline La Palma magmas. Apatite contains up to 0.89 wt% SO₃ (3560 ppm S), 0.31 wt% Cl, and 0.66 wt% Ce₂O₃. Sulfur is incorporated in apatite via several independent exchange reactions involving (P⁵⁺, Ca²⁺) vs. (S⁶⁺, Si⁴⁺, Na⁺, and Ce³⁺). The concentration of halogens in phonolitic to trachytic glasses ranges from 0.15 to 0.44 wt% for Cl and from <0.07 to 0.65 wt% for F. The sulfur concentration in the glasses ranges from 0.06 to 0.23 wt% SO₃ (sulfate-saturated systems). The chlorine partition coefficients (D_Cl^{apatite/glass}) range from 0.4 to 1.3 (average D_Cl^{apatite/glass} = 0.8), in good agreement with the results of experimental data in mafic and rhyolitic system with low Cl concentrations. With increasing F in glass inclusions D_F^{apatite/glass} decreases from 35 to 3. However, most of our data display a high partition coefficient (~30) close to D_F^{apatite/glass} determined experimentally in felsic rock. D_S^{apatite/glass} decreases from 9.1 to 2.9 with increasing SO₃ in glass inclusions. The combination of natural and experimental data reveals that the S partition coefficient tends toward a value of 2 for high S content in the glass (>0.2 wt% SO₃). D_S^{apatite/glass} is only slightly dependent on the melt composition and can be expressed as: SO_{3 apatite} (wt%) = 0.157 * ln SO_{3 glass} (wt%) + 0.9834. The phonolitic compositions of glass inclusions in amphibole and haüyne are very similar to evolved melts erupted on La Palma. The lower sulfur content and the higher Cl content in the phonolitic melt compared to basaltic magmas erupted in La Palma suggest that during magma evolution the crystallization of haüyne and pyrrhotite probably buffered the sulfur content of the melt, whereas the evolution of Cl concentration reflects an incompatible behavior. Trachytic compositions similar to those of the (water-rich) glass inclusions analyzed in apatite and clinopyroxene are not found as erupted products. These compositions are interpreted to be formed by the reaction between water-rich phonolitic melt and peridotite wall-rock.     

Evidence for a high Mg andesitic parental magma to the East and West satellite dykes of the Great Dyke,Zimbabwe: a comparison with the continental tholeiitic Mashonaland sills

One of the most significant mafic intrusive events in the Zimbabwe Craton was the emplacement of the Great Dyke layered ultramafic-mafic complex and its two parallel ‘satellite’ dykes at the end of the Archæan (∼2.6 Ga). The two satellite dykes, the East Dyke and the West (Umvimeela) Dyke, were far less affected by crystal accumulation and layering processes than the Great Dyke, and therefore may provide a clearer picture of parental magma compositions of the Great Dyke event. The geochemical character of this major episode of mafic magmatism is markedly different to that of more typical continental tholeiites, such as the ∼1.9 Ga Mashonaland sills, and reflects significant contrasts in petrogenetic influences. Despite its mid-continental setting, the parental magma of the satellite dykes had geochemical characteristics more often associated with magmas generated at destructive plate margins, suggesting that the nature of this magma was perhaps more similar to high Mg andesitic, rather than continental tholeiitic magmatism. Fine-grained samples with ∼14% MgO perhaps most closely approximate to the parental magma composition. Certain major and trace element concentrations (high MgO, SiO₂, LREE and LILE, and low Nb, Ta and Ti) and the lack of an Fe enrichment trend, suggest that the mafic magma had either suffered severe crustal contamination or had been derived from a metasomatised harzburgitic mantle source.     

Apatite Chlorine Concentration Measurements by LA‐ICP‐MS

Apatite incorporates variable and significant amounts of halogens (mainly F and Cl) in its crystal structure, which can be used to determine the initial F and Cl concentrations of magmas. The amount of chlorine in the apatite lattice also exerts an important compositional control on the degree of fission‐track annealing. Chlorine measurements in apatite have conventionally required electron probe microanalysis (EPMA). Laser ablation inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) is increasingly used in apatite fission‐track dating to determine U concentrations and also in simultaneous U‐Pb dating and trace element measurements of apatite. Apatite Cl measurements by ICP‐MS would remove the need for EPMA but the high (12.97 eV) first ionisation potential makes analysis challenging. Apatite Cl data were acquired using two analytical set‐ups: a Resonetics M‐50 193 nm ArF Excimer laser coupled to an Agilent 7700× quadrupole ICP‐MS (using a 26 μm spot with an 8 Hz repetition rate) and a Photon Machines Analyte Excite 193 nm ArF Excimer laser coupled to a Thermo Scientific iCAP Qc (using a 30 μm spot with a 4 Hz repetition rate). Chlorine concentrations were determined by LA‐ICP‐MS (1140 analyses in total) for nineteen apatite occurrences, and there is a comprehensive EPMA Cl and F data set for 13 of the apatite samples. The apatite sample suite includes different compositions representative of the range likely to be encountered in natural apatites, along with extreme variants including two end‐member chlorapatites. Between twenty‐six and thirty‐nine isotopes were determined in each apatite sample corresponding to a typical analytical protocol for integrated apatite fission track (U and Cl contents) and U‐Pb dating, along with REE and trace element measurements. ³⁵Cl backgrounds (present mainly in the argon gas) were ~ 45–65 kcps in the first set‐up and ~ 4 kcps in the second set‐up. ³⁵Cl background‐corrected signals ranged from ~ 0 cps in end‐member fluorapatite to up to ~ 90 kcps in end‐member chlorapatite. Use of a collision cell in both analytical set‐ups decreased the low mass sensitivity by approximately an order of magnitude without improving the ³⁵Cl signal‐to‐background ratio. A minor Ca isotope was used as the internal standard to correct for drift in instrument sensitivity and variations in ablation volume during sessions. The ³⁵Cl/⁴³Ca values for each apatite (10–20 analyses each) when plotted against the EPMA Cl concentrations yield excellently constrained calibration relationships, demonstrating the suitability of the analytical protocol and that routine apatite Cl measurements by ICP‐MS are achievable.