The Chilled Marginal Gabbro and Other Contact Rocks of the Skaergaard Intrusion

Rocks in the outer selvage of the Skaergaard intrusion havea range of textures and compositions, and among these are materialsrepresenting quenched Skaergaard magma. Pristine chilled marginalgabbro (CMG), however, is not ubiquitous at the intrusive contact,because many of the "contact" rocks have been hydrothermallyor metasomatically altered, contaminated with gneiss or olivinexenocrysts, while others contain accumulated minerals. Materialrepresenting quenched magma appears to be restricted to contactrocks that are texturally and mineralogically similar to diabase,and free of accumulated minerals. Where it exists, the CMG isfound within one to three meters of the exposed intrusive contactexcept at the roof of the intrusion where its thickness is greater.CMG was distinguished from the diverse group of contact rocksby petrographic and geochemical screening of over 80 specimens.Samples of CMG from the eastern and western margins and fromthe roof of the intrusion have relatively uniform compositionsimilar to that of ferrobasalt, and are noticeably richer iniron (mg-number=0?51-0?54), TiO₂ K₂O, and P₂O₅ than other unmodifiedcontact rocks. CMG's also have trace element compositions distinctfrom most other rocks in the outer Marginal Border Series (MBS).They have incompatible element contents up to 3–6 timesgreater than in LZa-type cumulates, negligible Eu anomalies,and Ni and Cr contents and Ni/Cr ratios that are among the lowestof rocks in the outer MBS. The results of melting experiments corroborate selection ofthis material as CMG. The composition of glasses obtained frompartial melting experiments of LZa-type cumulates are essentiallyidentical to those of the CMG. The 1-atm. liquidus phase relationsfor one of the CMG samples (KT-39) is largely consistent withthe sequence and composition of cumulus minerals observed withdistance inward through the MBS and upward through the LayeredSeries. Solidification of magma at the outer margin of the intrusionis interpreted to have involved locally efficient quench crystallizationfollowed by initial primocryst growth in an undercooled transitionzone a short distance inward that finally extended into regionsof near equilibrium crystallization. The similarity in composition between samples of chilled marginalgabbro from the exposed roof and sides of the intrusion, andthose of reconstituted trapped liquid from early cumulates inthe outer MBS suggests that a single magma, similar in compositionto ferrobasalt, was parental to the Skaergaard intrusion. Thisinterpretation corroborates geophysical evidence of a significantlysmaller mass for the intrusion than that estimated by Wager,and provides a basis for revision of models of its chemicalevolution. Samples chosen by Wager as chilled marginal gabbrobelong spatially, texturally, and compositionally to the groupof LZa-type cumulates in the MBS, and should no longer be regardedas chilled marginal gabbro.     

Petrology of the Marginal Border Series of the Skaergaard Intrusion

The Marginal Border Series (MBS) of the Skaergaard intrusionconsists of rocks formed by in situ crystallization againstthe walls of the intrusion. Most of these rocks are productsof fractional crystallization, though samples believed to representchilled liquid occur locally at the intrusive contact. The MBScomprises only 5% of the exposed volume of the intrusion, butwithin its thickness, the order of crystallization and the compositionsof fractionated rocks and minerals vary systematically withdistance inward from the intrusive contact in largely the samemanner as rocks and minerals upward through the Layered Series(LS). Earliest differentiates are cumulates of olivine and plagioclase.The most basic compositions of cumulus plagioclase (An₇₂) andolivine (Fo₈₄) in these rocks indicate that the amount of fractionationpreceding formation of the exposed LS was substantially lessthat previously believed. Field and compositional data indicatethat picritic blocks are xenoliths rather than cumulates ofthe Skaergaard magma. Xenoliths of gneiss in all stages of reactionare locally abundant; however, there is no evidence that uppercrustal material contaminated the magma from which the MBS cumulatesformed. Compositions of cumulus minerals in the MBS differ fromthose in comparable LS rocks. Cumulates in the lower marginscontain more calcic plagioclase, more magnesian augite in allbut the late differentiates, and more iron-rich olivine. Thecompositions of cumulus olivine and to a lesser degree thoseof other mafic silicates, were modified to more iron-rich compositionsby re-equilibration with relatively large amounts of interstitialliquid. The lower MBS and LS crystallized from the same magma, but fractionationoccurred at different rates on the walls and floor of the intrusion.The upper margin may have crystallized from a magma of modifiedcomposition and fractionated at rates different from that inthe lower margin and Upper Border Series (UBS). Crystals onthe floor and roof of the intrusion accumulated faster or moreefficiently than on the walls. At any given stage of fractionation,crystals also accumulated against all sides of the magma chamberat about the same rate. Either the rates of cooling, crystallization,and crystal retention affected accumulation rates locally asfunctions of rock type and geometry of the walls, or these rateswere largely independent of wall rock owing to buffering ofconductive heat loss possibly to an envelope of hydrothermalfluid circulating around the crystallizing magma. The appearanceor disappearance of cumulus minerals in the lower MBS occursat higher structural levels than in the LS and at lower structurallevels than in the UBS. These relationships together with cumulusmineral compositions indicate that magma at the margins wasalways somewhat less fractionated than that at the floor androof of the chamber. It is proposed that these relationshipsreflect the combined effects of liquid and crystal fractionationof the magma within largely independent convection systems inthe lower and upper parts of the chamber.     

Pyroxenes of the Dufek Intrusion, Antarctica

The Dufek intrusion is a stratiform mafic body, 24,000 to 34,000km² in area and 8 to 9 km thick, in the Pensacola Mountainsof Antarctica. Textures, structures, magmatic stratigraphy,and chemical variation indicate that layered gabbros and relatedrocks of this body developed by accumulation of crystals thatsettled on the floor of a magma chamber. The major cumulus phasesin the exposed part of the intrusion are plagioclase, pyroxene,and iron-titanium oxides. The base of the Dufek intrusion is not exposed, and both Ca-richand Ca-poor pyroxene coexist as cumulus phases in the lowerexposed rocks. The Ca-rich pyroxenes belong to an augite-ferroaugiteseries (Ca_36.4Mg_48.7Fe_14.9-Ca_30.0Mg_23.5Fe_46.5) that extendsup through the 300 m thick capping granophyre. The Ca-poor pyroxenesbelong to a bronzite-inverted pigeonite series (Ca_3.5Mg_69.1Fe_27.4-Ca_11.4Mg_34.0Fe_54.6)that extends only to about 200 m below the granophyre layer.In addition to the cumulus pyroxenes some rocks contain post-cumulusgreen calcic augite and ferrohypersthene. The compositional change of the cumulus pyroxenes with stratigraphicheight is one of general iron enrichment. Superimposed on thistrend are (1) a 1 km thick section in the lower part of thebody that shows slight to no iron enrichment and (2) a markedreversal in the Fe/(Fe+Mg) ratio about 1 km below the top ofthe body. The variations from the general trend are associatedwith cyclic units and are best explained by convective overturnof the magma. In general, the pyroxene compositional trends are similar tothose of the Skaergaard and Bushveld intrusions. One significantdifference in the Dufek intrusion is the limited iron enrichmentof its Ca-rich pyroxenes, that may relate to a slower decreaseof P_O2 during crystallization of the Dufek magma.     

Phase Equilibrium Constraints on Intensive Crystallization Parameters of the Ilimaussaq Complex, South Greenland

The 1·13 Ga Ilímaussaq intrusive complex, SouthGreenland, is composed of various types of alkali granite andsilica-undersaturated alkaline to agpaitic nepheline syenitesrelated to three subsequently intruded magma batches. Mineralchemistry indicates continuous fractionation trends within eachrock type, but with distinct differences among them. The last,peralkaline magma batch is the most fractionated in terms ofX_Fe^{mafic mineral}, feldspar composition and mineral assemblage.This indicates that an evolving magma chamber at depth discontinuouslyreleased more highly fractionated alkaline melts. Fluid inclusionsin some sodalites record a pressure drop from 3·5 to1 kbar indicating that crystallization started during magmaascent and continued in the high-level magma chamber. On thebasis of phase equilibria and preliminary fluid inclusion data,crystallization temperature drops from >1000°C (augitesyenite liquidus) to <500°C (lujavrite solidus) and silicaactivity decreases from     

Mafic Mineralogy of Ferroaugite Syenite from the Coldwell Alkaline Complex, Ontario, Canada

A 1500 m thick sheet-like body of ferroaugite syenite is divisiblemineralogically into an upper and lower series of syenites.The lower syenites are characterized by well developed igneouslayering defined by mafic cumulus minerals. The syenites aresaturated to oversaturated and contain as cumulus phases alkalifeldspar, olivine (Fa_83–93), ferroaugite (Di₅₀Hd₄₅Ac₅–Di₅Hd₉₀Ac₅)and ilmeno-magnetite. Amphiboles which crystallized from theintercumulus liquid range in composition from ferrohastingsitichornblende to ferroedenitic hornblende to ferroactinolitic edenite.The upper series are coarse grained cumulates with poorly definedlayering and abundant patch pegmatites. Cumulus phases are alkalifeldspar, olivine (Fa₉₃), and acmitic-hedenbergite (Di₅Hd₅₀Ac₅–Ac₅₀Hd₅₀).Intercumulus liquids are peralkaline and crystallized to aenigmatiteand amphiboles which range in composition from ferrorichteritickatophorite to ferrorichterite, Patch pegmatites are peralkalinerocks composed of ferrorichterite, ferroactinolite, alkali feldspar,aenigmatite, quartz and zircon. Extreme differentiation of ferroaugitesyenite magma generates residua which are ironrich, oversaturatedand peralkaline. Initial and final temperatures of crystallizationare estimated from mineral stability data to be 800–900°C to 500–550 °C respectively. Thermodynamic dataand mineral compositions indicate that during crystallizationthe oxygen fugacity of the magma decreased from approximately10^–15 to 10^–23–10^–24 bars. Ferroaugitesyenite pyroxene compositional trends are similar to those ofundersaturated peralkaline syenites (llimaussaq) and demonstratethat acmite enrichment trends are independent of silica activityand take place under decreasing oxygen fugacities.     

Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway). Part I: Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent

Whole-rock major element compositions are investigated in 99 cumulates from the Proterozoic Bjerkreim–Sokndal layered intrusion (Rogaland Anorthosite Province, SW Norway), which results from the crystallization of a jotunite (Fe–Ti–P-rich hypersthene monzodiorite) parental magma. The scattering of cumulate compositions covers three types of cumulates: (1) ilmenite–leuconorite with plagioclase, ilmenite and Ca-poor pyroxene as cumulus minerals, (2) magnetite–leuconorite with the same minerals plus magnetite, and (3) gabbronorite made up of plagioclase, Ca-poor and Ca-rich pyroxenes, ilmenite, Ti-magnetite and apatite. Each type of cumulate displays a linear trend in variation diagrams. One pole of the linear trends is represented by plagioclase, and the other by a mixture of the mafic minerals in constant proportion. The mafic minerals were not sorted during cumulate formation though they display large density differences. This suggests that crystal settling did not operate during cumulate formation, and that in situ crystallization with variable nucleation rate for plagioclase was the dominant formation mechanism. The trapped liquid fraction of the cumulate plays a negligible role for the cumulate major element composition. Each linear trend is a locus for the cotectic composition of the cumulates. This property permits reconstruction by graphical mass balance calculation of the first two stages of the liquid line of descent, starting from a primitive jotunite, the Tjörn parental magma. Another type of cumulate, called jotunite cumulate and defined by the mineral association from the Transition Zone of the intrusion, has to be subtracted to simulate the most evolved part of the liquid line of descent. The proposed model demonstrates that average cumulate compositions represent cotectic compositions when the number of samples is large (> 40). The model, however, does not account for the K₂O evolution, suggesting that the system was open to contamination by roof melts. The liquid line of descent corresponding to the Bjerkreim–Sokndal cumulates differs slightly from that obtained for jotunitic dykes in that the most Ti-, P- and Fe-rich melts (evolved jotunite) are lacking. The constant composition of the mafic poles during intervals where cryptic layering is conspicuous is explained by a compositional balance between the Fe–Ti oxide minerals, which decrease in Fe content in favour of Ti, and the pyroxenes which increase in Fe.     

Neoproterozoic tholeiitic arc plutonism: petrology of gabbroic intrusions in the El-Aradiya area,Eastern Desert,Egypt

Gabbroic intrusions of the El-Aradiya area are a part of the Neoproterozoic basement cropping out in the central Eastern Desert of Egypt. They are composed mainly of gabbroic cumulates (diopside-plagioclase cumulate and plagioclase-augite cumulate) and fine-grained noncumulate gabbro. Mineral chemistry data indicate that the plagioclase core compositions of the gabbroic cumulates range between An₉₀ and An₆₀, whereas fine-grained noncumulate gabbro plagioclase core compositions are An₆₁₋₅₆ and rim compositions are An₅₄₋₄₂. The clinopyroxenes are diopside and augite in the gabbroic cumulate, and augite in the fine-grained noncumulate gabbro. Chemical re-equilibration between pyroxenes of gabbroic cumulates vary from 1150-900°C and for fine-grained noncumulate gabbro range from 1200-1100°C. The amphiboles are calcic, varying from tschermakite and tschermakitic hornblende, and Mg-hornblende in the gabbroic cumulate and only Mg-hornblende in the fine-grained noncumulate gabbro. They indicate an island-arc tholeiitic setting for gabbroic intrusions of the El-Aradiya area. Major and trace element data suggest arc tholeiite characters, a comagmatic suite and subduction-related magma with enrichment of LILE and depletion in HFSE relative to MORB. The estimated parent magma is similar to tholeiitic Aleutian arc primary magma. The gabbroic intrusions are analogous to intrusions emplaced in an immature island-arc setting in which the oceanic crust was thin.     

Stratigraphy and Petrology of the Mulcahy Lake Layered Gabbro: An Archean Intrusion in the Wabigoon Subprovince, Ontario

The Mulcahy Lake gabbro is an Archean layered intrusion of tholeiiticbulk composition located in the Wabigoon subprovince. The intrusionis 6 km thick at the thickest and is exposed over an area of63 km². It intrudes basaltic to siliceous volcanics of the CrowLake-Savant Lake greenstone belt and is intruded by the Atikwabatholith. Zircon U-Pb data indicate crystallization at 27322+1·0/–0·9m·y. Principal phases are plagioclase, orthopyroxene, augite (andpigeonite in iron-rich rocks), olivine, hornblende and magnetite.Olivine is confined to several horizons. Apatite and then zirconare prominent accessory phases at advanced stages of fractionation.Plagioclase, pyroxenes and olivine are cumulate phases. Hornblendeis invariably an intercumulus phase. Magnetite is ubiquitousthroughout the intrusion, generally as a cumulate phase, andforms centimeter thick layers in fractionated rocks. Fractionationfollowed a tholeiitic trend with iron enrichment in the liquid. The intrusion is divided into lower, mixed, middle, upper andmarginal zones. The lower and middle zones are 2·0 and2·5 km thick respectively. The upper zone is approximately1 km thick, and the marginal zone is measured in hundreds ofmeters. A 200 m thick mixed zone is interposed between the lowerand middle zones. The base of the lower zone consists of ultramaficunits containing olivine of Fo₈₂. The top of the zone has olivineof Fo₂₈. Fractionation of the lower zone, from the floor up,was interrupted by the introduction of pristine liquid whichmixed with more dense and cooler residual liquid in the chamberto form the mixed zone. Further introduction of several minorpulses of liquid constructed the lower part of the middle zone.The upper part of the middle zone was constructed from a majorpulse of liquid plus several minor pulses each of which is representedby reversals in cryptic layering. The upper zone consists ofultramafic to iron-rich gabbro cumulates formed by cooling throughthe roof plus horizons formed by influx of pristine liquid.Marginal zone rocks represent cooling through the walls of theintrusion. Rhythmic layering is well developed in lower and middle zonecumulates. Petrofabric data show that orthopyroxene has a lineationin the plane of layering and parallel to structures suggestiveof flow. Plagioclase laths also have a preferred orientationin many cumulates and in unlayered gabbros as well. Flow, possiblylaminar, of liquid-crystal material is suggested and may belinked to the ultimate development of layering. Pressure during the course of crystallization probably was greaterthan 2 and less than 5 kb. Temperatures estimated from pyroxenesvaried from approximately 1200 to 1000 °C.f_o², is not wellconstrained but was sufficient to allow the formation of thinlocal magnetite cumulates late in the crystallization. The primarymelt was hydrous as indicated by the presence of hornblende.It is very unlikely that the melt was saturated with water duringcrystallization of the cumulate phases.     

Pyroxenes of the Bushveld Intrusion, South Africa

New analyses are presented, for major, minor, and trace elements,of eleven Ca-rich pyroxenes, four bronzites, and two invertedpigeonites from the Bushveld layered basic intrusion. The twenty-threeanalyses now available are believed to represent the entireBushveld fractionation sequence. The Ca-rich pyroxene trendis from Ca_45.4Mg_49.6Fe_5.0 to Ca_42.7Mg_0.6Fe_56.8, the ferrohedenbergitesshowing no evidence of inversion from ferriferous ß-wollastonites.The Ca-poor pyroxene trend is from bronzite (Ca_2.8Mg_85.0Fe_12.2)through pigeonites to ferropigeonites (approximately Ca₃Mg₂₇Fe₅₅).All the pigeonitic pyroxenes have inverted to orthopyroxene. The compositional trends are remarkably similar to those ofthe Skaergaard pyroxene series, but the Bushveld sequence isthe most complete known for a single fractionated intrusion.The compositional and other variations of the pyroxenes, consideredtogether with those of the coexisting olivines and feldspars,leave little doubt that the Bushveld rocks originated by crystalaccumulation from a slowly cooled and fractionated intrusionof tholeiitic basalt magma. The slight but significant differences between the Bushveldand Skaergaard pyroxene trend characteristics can be explainedin terms of a displacement, in one intrusion as compared withthe other, of the liquidus and solidus surfaces relative tothe solvus and inversion surfaces in the system Wo—En—Fs.This may be due to minor differences in the initial magma compositionsof the two intrusions. Differences in the Mg/Fe ratios of Bushveldand Skaergaard coexisting pyroxene pairs are believed to bedue, at least in part, to the greater depth of the Bushveldmagma chamber. The Bushveld trends are briefly discussed in the light of recentexperimental studies on compositions within the Di-Hed-En-Fspyroxene quadrilate     

Petrological, Geochemical and Isotopic Constraints on the Origin of the Harzburg Intrusion, Germany

We present mineralogical, petrological and geochemical datato constrain the origin of the Harzburg mafic–ultramaficintrusion. The intrusion is composed mainly of mafic rocks rangingfrom gabbronorite to quartz diorite. Ultramafic rocks are veryrare in surface outcrops. Dunite is observed only in deepersections of the Flora I drill core. Microgranitic (fine-grainedquartz-feldspathic) veins found in the mafic and ultramaficrocks result from contamination of the ultramafic magmas bycrustal melts. In ultramafic and mafic compositions cumulatetextures are widespread and filter pressing phenomena are obvious.The order of crystallization is olivine pargasite, phlogopite,spinel plagioclase, orthopyroxene plagioclase, clinopyroxene.Hydrous minerals such as phlogopite and pargasite are essentialconstituents of the ultramafic cumulates. The most primitiveolivine composition is Fo_89·5 with 0·4 wt % NiO,which indicates that the olivine may have been in equilibriumwith primitive mantle melts. Coexisting melt compositions estimatedfrom this olivine have mg-number = 71. The chemical varietyof the rocks constituting the intrusion and the mg-number ofthe most primitive melt allow an estimation of the approximatecomposition of the mantle-derived primary magma. The geochemicalcharacteristics of the estimated magma are similar to thoseof an island-arc tholeiite, characterized by low TiO₂ and alkalisand high Al₂O₃. Geochemical and Pb, Sr and Nd isotope data demonstratethat even the most primitive rocks have assimilated crustalmaterial. The decoupling of Sr from Nd in some samples demonstratesthe influence of a fluid that transported radiogenic Sr. Leadof crustal origin from two isotopically distinct reservoirsdominates the Pb of all samples. The ultramafic rocks and thecumulates best reflect the initial isotopic and geochemicalsignature of the parent magma. Magma that crystallized in theupper part of the chamber was more strongly affected by assimilatedmaterial. Petrographic, geochemical and isotope evidence demonstratesthat during a late stage of crystallization, hybrid rocks formedthrough the mechanical mixing of early cumulates and melts withstrong crustal contamination from the upper levels of the magmachamber. KEY WORDS: Harzburg mafic–ultramafic intrusion; Sr–Nd–Pb isotopes; magma evolution; crustal contamination     

Zoned Pyroxenes from Shoshonite Lavas of Lesbos, Greece: Inferences concerning Shoshonite Petrogenesis

Samples from several stratigraphic levels in the Miocene volcanicsuccession on Lesbos, Greece, contain complexly zoned clinopyroxeneand feldspar phenocrysts. Electron microprobe analyses havebeen made of zoned clinopyroxenes and feldspars, and their inclusions;and phenocryst mineralogy is compared with host-rock chemicalcomposition. The commonest pyroxene zoning sequence is coresof greenish salite or augite mantled by diopside which, in turn,may be rimmed by augite, with each type of pyroxene in opticaland chemical discontinuity with the next. None of the pyroxeneshas high pressure characteristics. The Ts content of phenocrystcores is correlated with the K-content of the host rock; andthe most diopsidic overgrowths also occur in the most K-richrocks. Augite rims occur only in Al-rich rocks. Most of the features of the pyroxenes can be accounted for bya hypothetical fractional crystallization scheme. However, twoobservations suggest that mixing of small magma batches mayalso be important. The earliest crystallized diopsides thatrim salite are rich in Cr₂O₃, suggesting precipitation froman unfractionated magma. Anorthoclase (implying advanced crystallizationof feldspars) occurs as cores to feldspar phenocrysts and asinclusions in clinopyroxene cores in several samples.     

Petrogenesis of the Bondla layered mafic-ultramafic complex,Usgaon, Goa

The Bondla mafic-ultramafic complex is a layered intrusion that consists predominantly of peridotites and gabbronorites. A chromitite-pyroxenite-troctolite horizon serves as a marker to subdivide the intrusion into two zones. The Lower Zone displays gravity stratified layers of chromite that alternate with those of olivine, which up-section are followed by olivine+pyroxene-chromite cumulates. The Upper Zone comprises gabbroic rocks that exhibit uniform layering. On the basis of modal and cryptic variation exhibited by the minerals this zone can be subdivided in to several lithohorizons starting from the troctolites at the base to gabbronorites and leucogabbros at the top. The junction between the two zones is marked by the distinct reversal in cryptic variation exhibited by the chromites and pyroxenes. The peridotite chromites contain higher Al₂O₃ and lower Cr₂O₃ than those from the chromitite above. Similarly clinopyroxenes from pyroxenite and troctolites are more magnesian that those from the peridotites stratigraphically below them. The complex in general is characterized by a gabbroic mineral assemblage in which both Ca-rich and Capoor pyroxenes coexist and displays a Fe-enrichment trend providing evidence of evolution from a contaminated tholeiitic magma. The rocks are characterized by low-TiO₂; Ni, Cr and V, show negative correlation with Zr whereas the large ion lithophile elements (LILE) are positively correlated and the Nb/La ratio varies from 0.4–0.6. These characteristics are consistent with a low-TiO₂ sub-alkaline tholeiitic magma that may have been modified by fractional crystallization and successive injections of more primitive melts in the magma chamber. The complex evolved in a periodically replenished magma chamber that consisted of two separate but interconnected sub-chambers.     

Composition of rock-forming minerals in the Kivakka layered massif,northern Karelia,and systematic variations in the chemistries of minerals in the rhythmic layering subzone

Data obtained on the compositions of rock-forming minerals from the Kivakka layered intrusion indicate that the #Fe of olivine increases up the vertical section of the intrusion, and the #Mg of the pyroxenes and the anorthite contents of the plagioclase simultaneously decrease, which reflects the fractionation of the parental melt. Emphasis is placed onto the compositional variability of minerals in the layered unit. A method is developed for the reliable calculation of the composition of pyroxenes before their exoslution (separately for ortho-and clinopyroxene). The composition of minerals in the zone of intercalating melano-, meso-, and leucocratic norites remains practically unchanged, which is generally consistent with the results of mathematical simulations: compositional variations of the bronzite (#Mg) and plagioclase (An content) during the crystallization of the bronzite-plagioclase cotectic were no greater than 5 mol %. The variations in the concentrations of trace elements in the cumulus and intercumulus clinopyroxene occurred to be the most informative. It was determined that the Cr concentration in the cumulus augite is statistically significantly lower than that in the intercumulus augite, which is at variance with fractionation laws and led us to suggest that the rocks with cumulus pyroxenes were formed by a melt with a higher degree of fractionation than those of the intercumulus melts of the adjacent layers.     

Petrology of the Upper Border Series of the Skaergaard Intrusion

The Upper Border Series of the Skaergaard intrusion consistsof a 960 m thick sequence of rocks that crystallized againstthe roof of the magma chamber. The texture and composition ofthe unit vary systematically from top to bottom as a resultof changes that occurred in the magma during the solidificationof the intrusion. The order of crystallization of primocrystminerals in the Upper Border Series was: olivine; + plagioclase;+ apatite; + ilmenite; + magnetite; + Ca-rich pyroxene;—olivine;+ olivine; + ferrobustamite. The major silicate phases varyfrom high-temperature compositions to low-temperature compositionswith increasing distance from the upper contact. Post-crystallizationre-equilibration has affected the compositions of the oxideminerals and to a lesser extent the compositions of olivineand Ca-rich pyroxene. The Upper Border Series differentiationsequence differs from the Layered Series sequence, in that:(1) apatite appears much earlier; (2) magnetite precipitatedbefore Ca-rich pyroxene rather than after it; (3) orthopyroxeneis much less common; (4) the plagioclase is systematically poorerin K₂O; and (5) the rocks are systematically richer in K₂O andSiO₂. The upper part of the Skaergaard magma appears to havebeen enriched in H₂O, K₂O, SiO₂, and P₂O₅ relative to the partthat was parental to the Layered Series.     

Ca-rich Ilvaite-Epidote-Hydrogarnet Endoskarns: a Record of Late-Magmatic Fluid Influx into the Persodic Ilimaussaq Complex, South Greenland

Endoskarn assemblages involving the Ca-silicates ilvaite, epidoteand Ca-rich garnet occur along fracture zones in the persodicIlímaussaq intrusion, South Greenland. The 1·16Ga intrusion solidified at a depth of about 3–4 km, belowa cover of sandstones and pillow-basalts of the Eriksfjord Formation.In contrast to typical skarn assemblages, the Ilímaussaqendoskarns contain albite as a main phase and they did not formin metacarbonate rocks, as these are completely lacking in thevicinity of Ilímaussaq. Instead, they record late- topost-magmatic interaction of possibly external Ca-rich fluidswith the alkaline to agpaitic magmatic rocks. Accordingly, endoskarntextures clearly reflect the magmatic textures of the precursorrocks. Phase relations in two endoskarn varieties with epidote+ albite + andradite-rich garnet ± ilvaite ± retrogradeprehnite suggest their formation at about 500°C at highoxygen fugacities slightly above the hematite–magnetiteoxygen buffer [FMQ (fayalite–magnetite–quartz) +5 to FMQ + 7] with later small modifications as a result offluid influx or cooling of the original fluid at about 300–350°C(formation of prehnite) and at about 200–250°C (oxygenisotopic re-equilibration of the albite). One model for theformation of the observed assemblages is the decomposition ofCa-bearing minerals, such as primary eudialyte, clinopyroxeneor ternary feldspar, and redistribution of the Ca by a metasomatizinglate-magmatic fluid. Stable isotope (O, H) investigations, however,favour a model in which seawater was the metasomatizing fluid,which entered the Eriksfjord basalts above the intrusion, reactedwith them (spilitization) and brought about 10^–3 mol/lCa along fractures into the metasomatized rocks. Fluid–rockinteraction in the Eriksfjord basalts is documented by abundantchlorite–epidote–quartz assemblages; high fluid/rockratios allowed the fluid to retain its seawater oxygen isotopecomposition. KEY WORDS: agpaite; endoskarn; Ilímaussaq; ilvaite; metasomatism; seawater     

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.     

Fe isotope systematics of coexisting amphibole and pyroxene in the alkaline igneous rock suite of the Ilímaussaq Complex,South Greenland

The Ilímaussaq intrusion, South Greenland, provides an exceptional test case for investigating the changes of stable Fe isotope fractionation of solidus phases with changes in the Fe³⁺/∑Fe ratio of an evolving melt. The intrusion comprises a sequence of four melt batches that were fed from the same parental alkali basaltic magma. Differentiation produced cumulate rocks that range from augite syenite (phase I) over peralkaline granite (phase II) to agpaitic syenites (phases IIIa and IIIb). Fe³⁺/∑Fe ratios in amphiboles increase substantially from phase I to phase II and III rocks and mark a major change in the parental magma composition from augite syenites to peralkaline granites and agpaitic syenites. Before this transition, olivine, clinopyroxene, and amphibole in augite syenite, the most primitive rock type in the Ilímaussaq Complex, have a uniform Fe isotope composition that is identical to that of the bulk of igneous crustal rocks and approximated by the average isotopic composition of basalts (δ^56/54Fe_IRMM-014 = 0.072 ± 0.046‰). After the transition, amphiboles in the peralkaline granites and agpaitic syenites yield significantly heavier Fe isotope compositions with δ^56/54Fe_IRMM-014 values ranging from 0.123 to 0.237‰. Contamination of the Ilímaussaq magma by ongoing crustal assimilation as cause for this increase can be excluded on the grounds of Nd isotope data. Large-scale metasomatic overprint with an external fluid can also be dismissed based on amphibole O and Li isotope systematics. Rather, the increase towards heavy Fe isotope compositions most likely reflects the change in chemical compositions of amphiboles (calcic in augite syenite to sodic in the agpaitic syenites) and their Fe³⁺/ΣFe ratios that mirror changes in the chemical composition of the melt and its oxygen fugacity. A sensitive adjustment of equilibrium Fe isotope fractionation factors to amphibole ferric/ferrous ratios is also supported by beta-factors calculated from Mössbauer spetroscopy data. Comparison of the measured isotope fractionation between clinopyroxene and amphibole with that predicted from Mössbauer data reveal Fe isotope systematics close to equilibrium in augite syenites but Fe isotopic disequilibrium between these two phases in phase IIIa agpaitic syenites. These results are in agreement with O and Li isotope systematics. While amphiboles in all Ilímaussaq lithologies crystallized at temperatures between 650 and 850 °C, textural evidence reveals later clinopyroxene crystallization at temperatures as low as 300–400 °C. Therefore, isotopic equilibrium at crystallization conditions between these two phases can not be expected, but importantly, subsolidus reequilibration can also be dismissed.     

Crystallochemistry and origin of pyroxenes in komatiites

We present a detailed mineralogical and major- and trace-element study of pyroxenes in two Archean komatiitic flows in Alexo, Canada. The pyroxenes in spinifex-textured lavas commonly are zoned with cores of magnesian pigeonite and rims of augite. Concentrations of incompatible trace elements are low in pigeonite and jump to higher values in the augite mantles, a variation that can be modelled using accepted partition coefficients and assuming crystallization from komatiitic liquids. Crystallization sequences are very different in different parts of both flows. In the flow top, the sequence is olivine followed by augite: deeper in the spinifex sequence, pigeonite crystallizes after olivine, followed by augite; in lower cumulates, orthopyroxene or augite accompany olivine. In spinifex lavas, pigeonite crystallizes sooner than would be predicted on the basis of equilibrium phase relations. We propose that contrasting crystallization sequences depend on the position in the flow and on the conditions of crystal growth. In the flowtop, rapid cooling causes quench crystallization. Deeper in the spinifex layer, constrained growth in a thermal gradient, perhaps augmented by Soret differentiation, accounts for the early crystallization of pigeonite. The cumulus minerals represent a near-equilibrium assemblage. Augites in Al-undepleted Archean komatiites in various localities in Canada and Zimbabwe have high moderate to high Wo contents but their Mg# (Mg/(Mg + Fe) are lower than in augites in komatiites from Barberton, South Africa. We attribute the combination of high Wo and high Mg# in Barberton rocks to the unusually high CaO/Al₂O₃ of these Al-depleted komatiites.     

The origin and mode of emplacement of lujavrites in the Ilímaussaq alkaline complex, South Greenland

Lujavrites are rare meso- to melanocratic agpaitic nepheline syenites that are characterized by elevated contents of elements such as Li, Be, Zr, REE, Nb, Th and U. They are the most evolved members of the three large composite agpaitic complexes – Lovozero, Kola Peninsula, Russia; Pilansberg, South Africa; and Ilímaussaq, South Greenland – and are inferred to stem from the same deep fractionating magma sources that fed the earlier members of the complexes. The composition of the melts that evolved into lujavrites is, however, not well known. The agpaitic part of the Ilímaussaq complex is divided into a roof series, a floor series of cumulates and an intermediate series of lujavrites sandwiched between the two. In the traditional view, the lujavrites formed from residual melts left between the downward crystallizing roof series and the floor cumulates. New field observations and geochemical data suggest that the floor cumulates and the main mass of lujavrites constituted a separate intrusive phase which was emplaced into the already consolidated roof series rocks largely by piecemeal stoping. Studies of the contact facies of the floor cumulates indicate that the initial magma of the floor cumulate–lujavrite sequence was peralkaline nepheline syenitic with enhanced contents of Zr, Hf, HREE, Y, Nb, Ta, F, Ba and Sr. Subsequent crystallization in a closed system resulted in the formation of the floor cumulates and lujavrites. Chemical analyses of dykes within and outside the complex represent stages in the magmatic evolution of the agpaitic rocks.     

东天山黄山南镁铁-超镁铁岩体的岩浆演化过程:矿物学、Sr-Nd同位素特征的指示

黄山南镁铁-超镁铁质岩体位于东天山造山带觉罗塔格构造带内,属于土墩-黄山-图拉尔根镁铁-超镁铁质岩带。本文在前人研究的基础上,从橄榄石、辉石矿物学组成和全岩Sr、Nd同位素等方面对黄山南岩体进行了分析研究,并与黄山、黄山东、香山等典型含矿岩体作了对比,旨在进一步查明黄山南岩体的岩浆源区和母岩浆性质及其岩浆演化过程。Sr-Nd同位素特征表明黄山南岩体来自一个弱亏损的地幔源区,相较黄山、黄山东、香山等岩体的源区具有更加富集的特征。黄山南岩体中的橄榄石属于贵橄榄石,斜方辉石主要为古铜辉石,少数为紫苏辉石,单斜辉石主要为顽透辉石、普通辉石和少数的透辉石。单斜辉石和橄榄石的成分特征表明形成黄山南岩体的母岩浆为演化程度较低的拉斑玄武质岩浆,且母岩浆成分具有高Mg、高Ni的特点。计算得到黄山南岩体母岩浆的Fe O~T=13.20%、MgO=16.96%、Ni=377.2×10~(-6),且母岩浆在结晶分异过程中没有经历充分的硫化物熔离作用,这也是造成母岩浆中Ni含量较高以及岩体含矿性较差的主要因素。