The crystallisation history of the Igdlerfigssalik nepheline syenite intrusion,Greenland

During slow cooling of plutonic igneous rocks the initial high temperature minerals crystallised from the magma continue to re-equilibrate with each other to varying degrees with falling temperature. Thermodynamic studies of mineral equilibria are used to calculate crystallisation temperatures for the cumulus assemblage ol-cpx-mt-ne-fsp in the Igdlerfigssalik syenites and to calculate composition parameters for the original magmas. Cumulus crystallisation occurred in the range 900–980°C. Nepheline and alkali feldspar continued to equilibrate in some rocks down to 650°C, while macroscopic exsolution in alkali feldspar and titanomagnetite continued to temperatures below 600°C. Oxygen activities during the crystallisation of the cumulus minerals were below magnetite-wustite.     

The Kalka layered intrusion,central Australia

The Kalka Intrusion, central Australia has a 5000 m-thick layered sequence comprising Pyroxenite, Norite and Anorthosite Zones; an Olivine Gabbro Zone is enclosed by, and has a facies relationship with, the Norite Zone. Contamination is evidenced by high initial ⁸⁷Sr/⁸⁶Sr ratios ( 0.708) in the lower four-fifths of the intrusion, and resulted in pyroxenite rather than peridotite as a basal crystal accumulate. After an early phase of erratic buildup in contamination due to assimilation of ragged granulite wall-rock, armouring of the walls and mixing produced an homogeneous Norite Zone (threshold) magma body crystallising opx-cpx-plag. Within the succeeding 3500 m of section plagioclase An values have a general decline up sequence (An_74-60-feldspar threshold) with superimposed short term digressions to more calcic compositions. Initial ⁸⁷Sr/⁸⁶Sr ratios also fall very gradually (0.7081-0.7078 — isotopic threshold) with transient fluctuations to distinctly lower values. Maxima in plagioclase An contents and ⁸⁷Sr/ ⁸⁶Sr minima may be correlated with the spasmodic appearance of olivine and pronounced lithological variation. Such features are explained by the ponding of fresh bodies of uncontaminated magma on the floor of the chamber; these formed a hybrid with threshold magma that temporarily dominated crystallisation thereby perturbing plagioclase and isotopic compositions; eventually, mixing of the hybrid into the overlying threshold magma returned crystallisation to its initial state. The facies-controlled Olivine Gabbro Zone is the physical expression of ponded basal hybrid magmas. The onset of the Anorthosite Zone is marked by a pronounced decline in ⁸⁷Sr/⁸⁶Sr ratios to around 0.705 registering a major influx of new magma. In this instance the new magma dominated the system and a change to plagioclase as the dominant liquidus phase indicates a drastic change in input magma composition. The evolution of Kalka was determined by an interplay of crystal fractionation, fresh magma pulses, and magma mixing, with the latter effects producing both increases and decreases in plagioclase An contents; the complete process can only be revealed by combined mineral composition and isotopic data.     

Interaction between a mafic melt and host rocks during formation of the Kivakka layered intrusion,North Karelia

The study of interaction between mantle melts and crustal rocks is of great importance for deciphering the evolution of the Earth’s crust and for better understanding the composition of mantle sources, in particular, the degree of their compositional heterogeneity. This work presents the results of Rb-Sr and Sm-Nd isotopic studies of 37 samples taken from the Kivakka layered intrusion, host rocks, and rocks at the contact. The studies were aimed at verifying the hypothesis of possible crustal contamination of mafic melt during magma chamber crystallization. It was found that the section of the Kivakka layered massif is characterized by initial Sr and Nd isotopic heterogeneity, with negative correlation between initial Nd isotopic ratio and its content. The rocks of the massif have low ɛ_Nd(T) values.     

Geochemical structure of the Kivakka layered olivinite-norite-gabbronorite intrusion,North Karelia: Distribution of chalcophile elements

The distribution of elements and their correlations were analyzed throughout the section of the Kivakka intrusion to formulate the geochemical tendencies in the behavior of elements of different groups during formation of layered basic-ultrabasic complexes. (1) It was shown that the distribution of trace elements in the layered series is controlled by their ability or inability to enter isomorphically the cumulus minerals. The distribution of trace elements that occur as isomorphic admixtures in cumulus minerals (Cr, Mn, Zn, Co, Ni, and Ti), regardless of their geochemical type, is correlated with the crystallization order and distribution of minerals in the silicate matrix of the rocks. Elements that practically are not involved into silicates (S, Cu, Ag) show no any systematic variations; i.e., they are not controlled by fractionation of major rock-forming mineral. Their behavior is driven by the appearance of individual phase—sulfide melt, whose localization, formation time, and scales of fractionation are determined by saturation of parental melt in sulfide sulfur and general degree of its fractionation. (2) The comparison of cross-section variations of elements that isomorphically substitute for major elements in the Fe-Mg silicates, but differ in chalcophile affinity (in order of increase of chalcophile affinity: Mn → Zn → Co → Ni) reflects the contribution of unmixing and fractionation of sulfide melt during intrusion solidification. This is quantitatively defined primarily by partitioning coefficients of elements between cumulus silicates and magmatic melt, on the one hand, and between magmatic and sulfide melts, on the other hand. (3) The absence of simple correlation between local sulfide-rich horizons and silicate matrix of the rocks and signs of independent fractionation of sulfide melt prevent any attempts to predict the localization and scales of local sulfide mineralization within the layered series on the basis of petrochemical and related criteria. Only tendencies in the distribution of ore elements and sulfur across the section can be used for these purposes.     

Remediation and monitoring of a highway across an active landslide at Trevor, North Wales

In March 1989, a moderately steep slope forming sidelong ground below the B5434 Trevor to Froncysyllte Road failed after a period of exceptionally heavy rainfall and severed the highway. A long history of ground movements together with river erosion at the toe and a change in the groundwater regime within the slope created the preconditions for landslide. Site investigations characterised the ground conditions and identified the active slip surface within the compound landslide terrain. The permanent remedial measures adopted proved to be of considerable practical interest involving a bored pile anchored wall with foundations socketed into a deep sandstone foundation stratum. The anchored wall was designed to safeguard the highway and it was anticipated that deloading the slope would reduce the capacity for further movement. The wall achieved the design objective but progressive downslope movements resumed about one year after construction. They continue spasmodically to the present time with rates approaching 9 mm per year both laterally and vertically and although they affect the neighbouring areas, the highway corridor remains intact. The response of load cells attached to the ground anchors reveal two unusual patterns of increasing loads. At the eastern end of the wall, moderate load gains clearly relate to landsliding with reactivation along the existing slip surface and consequent removal of support to the wall as the ground progressively fails in a downslope direction. However, anomalous increases in loads recorded at the western end of the wall have proved more difficult to explain. They may be due either to a build up of water pressure behind the downhill portion of the wall or to localised weak ground conditions or swelling of degraded mudstones behind the wall. Monitoring of the load cells will continue in order to ensure the ongoing integrity of the highway.     

The differentiation of the Skaergaard intrusion

Conclusions We find no support for the claim that the Skaergaard magma followed the trend of common tholeiitic volcanic magmas, such as those of Iceland and the Scottish Tertiary. The end product of differentiation was not a large mass of rhyolite but an iron-rich, silica-poor liquid not unlike that deduced by Wager in 1960.The proposal that a large mass of rhyolitic liquid occupied the upper levels of the intrusion finds no support in the field. The thick series of ferrogabbos, which became richer in iron and poorer in silica until they reached a field of immiscibility cannot be reconciled with crystallization of a large mass of felsic magma. Mass-balance calculations that indicate otherwise are invalid, because they fail to take into account large volumes of rocks that differ in composition from those assumed in the calculations.While ignoring the existence of major units of the intrusion, Hunter and Sparks propose that lavas in Scotland and Iceland are more relevant to the liquid compositions than rocks that are intimately associated with the intrusion. Their argument that the Skaergaard Intrusion followed a trend of silica enrichment that is universal to tholeiitic magmas is based on an incomplete knowledge of the rocks and faulty calculations of mass-balance relations.We agree that much remains to be learned about the Skaergaard Intrusion and the basic mechanisms of magmatic differentiation. In this case, however, we are ready to hang our case on well-established field relations and a mass of laboratory data for what must be the most intensely studied body of rock on Earth.     

A late Devensian glaciolacustrine fan-delta at Rhosesmor,Clwyd, North Wales

Glacigenic sediments exposed at Rhosesmor, Clwyd, North Wales, were deposited in a small proglacial lake ponded by the stagnating margin of the Late Devensian Irish Sea icesheet. Three major fades assemblages are identified and show a prograding sequence of topset, foreset, and bottomset deposits associated with two Gilbert-style deltas stacked one above the other. The lower delta is associated with a lake level at c. 180 m OD. A subsequent ice-marginal readvance raised the lake to c. 192 m OD, drowned the lower delta and caused the growth of a second delta by an iceward shift of facies. It is estimated that the life of the lake was of the order of 90 years during which average sediment in-fill amounted to some 4±10⁴ m³ per annum.     

Diabasic intrusion and lavas,segregation veins,and magma differentiation at Kahoolawe volcano,Hawaii

A mafic sill-like intrusion, ~5?×?30 m, exposed along the eastern shoreline of Kahoolawe Island, Hawaii, represents tholeiitic magma emplaced as diabase among caldera-filling lavas. It differentiated from ~7.8 wt.% MgO to yield low-MgO (2.9 wt.%) vesicular segregation veins. We examined the intrusion for whole-rock and mineral compositions for comparison to Kahoolawe caldera-fill lavas (some also diabasic), to the Uwekahuna laccolith (Kilauea), and to gabbros, diabases, and segregations and oozes of other tholeiitic shield volcanoes (e.g., Mauna Loa and Kilauea lava lakes). We also evaluate this extreme differentiation in terms of MELTS modeling, using parameters appropriate for Hawaiian crystallization environments. Kahoolawe intrusion diabase samples have major and trace element abundances and plagioclase, pyroxene, and olivine compositions in agreement with those in gabbros and diabases of other volcanoes. However, the intrusion samples are at the low-MgO end of the large MgO range formed by the collective comparative samples, as many of those have between 8 and 20 wt.% MgO. The intrusion’s segregation vein has SiO₂ 53.4 wt.%, TiO₂ 3.2 wt.%, FeO 13.5 wt.%, Zr 350 ppm, and La 16 ppm. It plots in compositional fields formed by other Hawaiian segregations and oozes that have MgO <5 wt.%—fields that show large variances, such as factor of ~2 differences for incompatible element abundances accompanying SiO₂ from ~49 to 59 wt.%. Our MELTS modeling assesses the Kahoolawe intrusion as differentiating from ~8 wt.% MgO parent magma beginning along oxygen buffers equivalent to FMQ and FMQ-2, having magmatic H₂O of 0.15 and 0.7 wt.% (plus traces of CO₂ and S), and under 100 and 500 bars pressure. Within these parameters, MELTS calculates that <3 wt.% MgO occurs at ~1,086 to 1,060 °C after ~48 to 63 % crystallization, whereby the lesser crystallization percentages and lower temperatures equate to higher magmatic H₂O, leading to high SiO₂, ~56–58 wt.%. To contrast, greater crystallization is calculated for lower H₂O, for which it achieves less SiO₂, <55 wt.%. While MELTS reliably predicts SiO₂ approaching 58 wt.% for differentiation beyond <4 wt.% MgO, and shows that Kahoolawe intrusion’s segregations and those of Kilauea and Mauna Loa are all reasonably accommodated by the modeled parameters and SiO₂ differentiation curves, MELTS fails where it predicts that Fe enrichment is more robust under FMQ than FMQ-2 buffers. That failure not withstanding, MELTS differentiation from liquidus temperatures ~1,205–1,185 °C (depending on the various parameters) gradually increases fO₂ (up to ~0.4 log units, as normalized to FMQ) until magnetite crystallizes at ~1,090–1,085 °C, which reduces absolute fO₂ ~1 to 1.5 log units. The modeled Kahoolawe intrusion, then, exemplifies how tholeiitic magma differentiation can produce extreme SiO₂ and incompatible element compositions, and how Hawaiian segregations from shallow intrusions and lava lakes can be generally modeled under compositional and physical parameters appropriate for Hawaiian tholeiitic magmatism.     

The Paasivaara PGE reef in the Penikat layered intrusion,northern Finland

Summary Three major PGE-bearing mineralized zones have been found in the layered series of the early Proterozoic Penikat layered intrusion. These are designated as the Sompujärvi (SJ), Ala-Penikka (AP) and Paasivaara (PV) Reefs according to the site of their initial discovery.The uppermost of these, the PV Reef, has the highest Pt/Pd ratio. It is located in the transition zone between the fourth and the fifth megacyclic units. The main host rock is the uppermost anorthosite, disseminated sulphides and associated PGM being concentrated in the interstices of this plagioclase orthocumulate. The Reef has also been encountered in other parts of the transition zone, however, and sometimes even in the lowermost parts of the fifth megacyclic unit. The dominant sulphide paragenesis is chalcopyrite-pyrrhotite-pentlandite, whereas the PGM identified are represented by sperrylite (PtAs₂), kotulskite (PdTe), merenskyite (PdTe₂), isomertieite (Pd₁₁Sb₂As₂), stibiopalladinite (Pd₅Sb₂), cooperite (PtS) and braggite ((Pt, Pd, Ni)S).It is suggested that the PV Reef was formed in the mixing process when the fifth magma pulse intruded into the magma chamber. Mixing of the new magma with the older residual magma in the chamber accounted for the sulphide precipitation. Mixing and convection were probably turbulent at first and the sulphides were thus able to "scavenge" PGE from a large amount of silicate melt. The metal ratios in the mineralization point to a close genetic relationship with the fifth magma pulse.

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Das Paasivaara PGE Reef in der Penikat-Intrusion, Nord-Finnland

Zusammenfassung In den geschichteten Serien der frühproterozoischen Intrusion von Penikat kommen drei grössere PGE-führende Zonen vor. Diese werden als die Sompujärvi (SJ), Ala-Penikka (AP) und Paasivaara (PV) Reefs bezeichnet, entsprechend den Lokalitäten der Entdeckung.Das am höchsten gelegene PV Reef hat die höchsten Pt/Pd Verhältnisse. Es liegt in der Übergangszone zwischen der vierten und der fünften megazyklischen Einheit. Das wichtigste Wirtsgestein ist der oberste Anorthosit, wo disseminierte Sulfide und assoziierte PGM in den Zwischenräumen dieses Plagioklas-Orthokumulates vorkommen. Das Reef wurde auch in anderen Teilen der Überganszone beobachtet und manchmal sogar in den untersten Partien der fünften megazyklischen Einheit. Die dominierende Sulfidparagenese ist Kupferkies-Magnetkies-Pentlandit; PGM sind Sperrylith (PtAs₂), Kotulskit (PdTe), Merenskyit (PdTe₂), Isomertieit (Pd₁₁Sb₂As₂), Stibiopalladinit (Pd₅Sb₂), Cooperite (PtS) und Braggit ((Pt, Pd, Ni)S).Es wird angeregt, dass das PV Reef während der Mischungsvorgänge bei der Intrusion des fünften Magma Pulses in die Magmenkammer entstanden ist. Mischung des neuen Magmas mit dem alten Residual-Magma in der Kammer war für die Ausfällung der Sulfide verantwortlich. Mischung und Konvektion dürften anfangs turbulent gewesen sein, und so konnten die Sulfide die PGE aus einem beträchtlichen Anteil der Silikatschmelze entfernen. Die Metallverhältnisse dieser Vererzung lassen eine enge genetische Verbindung mit dem fünften Magmapuls erkennen.

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With 8 Figures     

新疆北山地区罗东镁铁质-超镁铁质层状岩体岩石成因

罗东镁铁质-超镁铁质岩体位于塔里木板块东北部的新疆北山地区,岩体平面形态为眼球状,出露面积约2.1 km2.由纯橄岩、单辉橄榄岩、斜长二辉橄榄岩、橄榄二辉岩、方辉辉石岩、橄长岩、橄榄辉长岩、辉长岩、苏长辉长岩和淡色辉长岩组成,堆晶结构和堆晶韵律发育,属于层状岩体.岩浆演化过程中主要分离结晶/堆晶相是橄榄石和单斜辉石,此...     

Geo-engineering problems at Llwyneinion hazardous waste site near Rhosllanerchrugog, North Wales

At Llwyneinion, a clay pit excavated in Carboniferous Bettisfield Formation was utilised for hazardous waste disposal prior to 1972. Approximately 94,000 t of acidic asphaltic sludges and pitches was poured directly into the pit together with 7500 t of spent bentonite containing absorbed oil. Numerous chemical drums were also dumped in the sludge. The unlined acidic asphaltic sludge and pitch lagoon occupies 1.3 ha, ranges up to 10 m deep and is underlain by fissured strata and extensive mineworkings including three shafts. The heterogeneous asphalt waste has developed a semi-solid asphalt layer at the base but mostly remains in a viscous and mobile condition. A surface layer of rainwater less than 0.4 m deep covers the waste. Surprisingly, detailed hydrogeological studies of the area surrounding the acidic asphaltic sludge and pitch lagoon failed to disclose any significant contamination of aquifers, however regular monitoring of water conditions is performed as a safeguarding measure. Various remediation strategies have been assessed and rejected. They fall into two main categories; those involving on site treatment of the acidic asphaltic sludges and pitches and encapsulation and those aimed at removal of the material to facilities elsewhere in the country. Investigations and deliberations continue in order to determine the optimum way forward.     

The nature and origin of the chrome-spinel of the Rhum layered intrusion

The textural features and mineralogy of chrome-spinel bearing rocks from the layered ultrabasic intrusion of Rhum indicate that in situ reaction of chromite with olivine, plagioclase and interstitial liquid has taken place. The compositions of the chrome-spinels vary throughout parts of the intrusion and also in a systematic way across a thin picotite rich seam at a feldspar cumulate — olivine cumulate junction. The origin of the chrome-spinel is discussed in relation to other occurrences of chromite in layered ultrabasic rocks.     

The origin of laminated and massive anorthosite,Sept Iles layered intrusion,Québec,Canada

Well-laminated plagioclase-rich rocks (‘anorthosite’) occur as layers in mineralogically similar but massive rocks in the Sept Iles intrusion. Several phases of crystallization of poikilitic clinopyroxene have preserved the intermediate stages in the production of these rocks. Plagioclase nucleated and initially grew in random orientations in the stationary part of the boundary layer. There, in the absence of other events, crystallization produced the massive anorthosite. The laminated anorthosite may have been produced by episodic increases in the velocity of the magma adjacent to the boundarylayer induced by magmatic density-currents. Such currents would have caused the zone of simple shear of the boundary-layer to migrate outwards into the previously stationary part. This simple shear would then have rotated the existing plagioclase crystals, as well as crystals that nucleated at that time, towards the shear-plane. Some rocks show evidence of several periods of enhanced flow-velocities. The production of the lamination may have weakened the crystal-mush and enabled slumping to occur, producing folds and contorted layers. Finally, compaction liberated large quantities of intercumulus magma of which some was trapped as granophyric segregations and some escaped by overturing and churning the crystal-pile. Earlier-formed granophyric segregations and laminated layers were disrupted during this process to produce a range of more-complex rocks including igneous breccias.     

Remediation and monitoring of a burning coal refuse bank affecting the Southsea Looproad at Brymbo, North Wales

Southsea Looproad at Brymbo suffers large and varied settlements where the highway crosses a smouldering coal refuse bank. A sudden surge in combustion activity during 1991 resulted in considerable subsidence within the carriageway and posed recurring and persistent difficulties for road safety and highway maintenance. Initial investigations involved subsurface temperature measurements to delimit the hot spots and provide a basis for the design of remedial works. The aim of remediation was to extinguish the migrating fire in the outer loop area and to control the burning throughout the remaining areas. The principal strand of the remediation strategy involved excavating the burning coal refuse, cooling, mixing with pulverized fuel ash and compacting. Supplementary containment measures included limestone-filled cut-off trenches, cementitious grout curtains and impervious clay blankets. The works achieved the design objectives, however, small pockets of smouldering material remained within the inner loop. Subsequent monitoring of the site disclosed two markedly different patterns of temperature response on each side of the road. On the outer rim of the looproad temperature trends were steadily downwards in accordance with expectations and indeed reached ambient temperature within about one year. However, on the inside of the looproad seasonal surges in temperature have been recorded during winter months every year since the completion of the remedial works in 1993. The explanation for this unusual pattern of combustion activity is thought to be related to climatic effects and particularly the combined influence of rainfall, atmospheric pressure, air temperature and wind strength and direction. The relative contribution of these factors remains uncertain. Temperature monitoring continues in order to identify the eventual end of combustion activity and determine the appropriate time to complete reinstatement of the carriageway.     

Slow,dense replenishments of a basic magma chamber: the layered series of the Newark Island layered intrusion,Nain, Labrador

The Newark Island layered intrusion is a composite layered intrusion within the Nain anorthosite complex, Labrador. The intrusion comprises a lower layered series (LS) dominated by troctolites, olivine gabbros and oxide-rich cumulates and an upper hybrid series (HS) characterized by a wide range of mafic, granitic and hybrid cumulates and discontinuous layers of chilled mafic rocks (Wiebe 1988). The HS crystallized from a series of replenishments of both silicic and basic magmas. The LS crystallized from periodically replenished basic magmas. The LS has a lower zone that consists mainly of olivine-plagioclase cumulates and contains minor cryptic reversals in mineral compositions that resulted from replenishments of relatively primitive magma. An upper zone is dominated by olivine-plagioclaseaugite-ilmenite cumulates. Cumulus titanomagnetite and pyrrhotite occur within some oxide-rich cumulates, and the stratigraphically highest layers contain cumulus apatite. At intermediate levels in the sequence, cumulus inverted pigeonite occurs in place of olivine. Several prominent regressions in the stratigraphy of the upper zone are marked by fine-grained troctolitic layers with much higher Mg no. [100 MgO/(MgO+FeO)] and anorthite than underlying cumulates. These layers coarsen upward and grade back to oxide-bearing olivine gabbros within thicknesses ranging from 10 cm to 15 m. Dikes that cut the LS have major- and trace-element compositions that strongly suggest that they are feeders for the replenishments. In the lower zone when olivine and plagioclase were the only cumulus phases, replenishments were less dense than the resident magma and rose as plumes and mixed with it. Precipitation of cumulus oxides in the upper zone lowered the density of resident magma so that subsequent replenishments were more dense than resident magma. Replenishments that occurred after oxides began to precipitate had small injection velocities. These post-oxide injections flowed along the interface between resident magma and the cumulate pile and precipitated flow-banded, fine-grained troctolites.     

Palynology of organic beds below Devensian glacigenic sediments at Pen-y-bryn,Gwynedd, North Wales

Pollen diagrams from organic facies overlain by glacigenic sediments at Pen-y-bryn, North Wales (53°7′N, 4°16′W), suggest that it is the first locality in the British Isles to provide evidence for several Devensian interstadials prior to Devensian stadial glaciation(s). The evidence is not, however, unequivocal. At least two main episodes of organic sedimentation are indicated, separated by an interval of uncertain duration. One eposide records a Pinus-Picea-Betula forest. A second, possibly later, event, with tree pollen less than 10% of total land pollen (TLP), suggests a cool, largely unforested environment. A third episode, with tree pollen up to 25% of TLP, may be related to the latter or be of intermediate age, whereas a fourth and later episode of organic accumulation may be of reworked material. Radiocarbon dating of organic deposits and of a wood macrofossil enclosed in basal till is inconclusive, as is preliminary uranium-series dating. The principal biogenic episodes may correlate with Oxygen Isotope Substages 5c and 5a, but as the pollen records reflect facies floras, the correlation remains tentative; it is also possible that other temperate periods are recorded. Further resolution of the chronostratigraphy is required to help clarify the problematic correlation of interstadial and stadial events in northwest Europe and to correlate more accurately the biogenic and glacigenic facies at Pen-y-bryn with their Pleistocene equivalents elsewhere.     

Petrology of the Treknattan intrusion in the Fongen-Hyllingen complex, Trondheim region, Norway: a late intrusion into an evolved layered complex

The basic-ultrabasic Treknattan intrusion is an important example of a late intrusion in a solidified, evolved, layered complex and sheds light on possible mechanisms by which such associations may develop. The Treknattan intrusion, emplaced into the basic Fongen-Hyllingen intrusion shortly after the latter had solidified, consists mainly of massive or weakly layered peridotite (olivine ± Cr-spinel cumulate) and troctolite (plagioclase + olivine ± Cr-spinel cumulate). The mineral compositional range partially overlaps the most primitive end of the much larger variation-interval in the Fongen-Hyllingen intrusion. The margin of the Treknattan intrusion is sometimes outlined by massive feldspathic websterite which appears to have formed by reaction between magma and melts of gabbroic country rock. The parental magma appears to have been a relatively water-rich picritic basalt with a possible genetic relationship to the magma parental to the enveloping Fongen-Hyllingen intrusion, both displaying tholeiitic relationship between olivine and Ca-poor pyroxene, and having crystallized from relatively water-rich magmas with an early crystallization order of olivine ± Cr-spinel-plagioclase-Ca-rich pyroxene. The recognition of the Treknattan intrusion as a separate body suggests that the bulk composition of the Fongen-Hyllingen intrusion is dioritic rather than gabbroic as previously thought.     

The petrology of the layered gabbro intrusion, eastern gabbro, Coldwell alkaline complex, Northwestern Ontario, Canada: evidence for multiple phases of intrusion in a ring dyke

The Coldwell alkaline complex is a large (> 350 km²) gabbro and syenite intrusion on the north shore of Lake Superior. It was emplaced at 1108 Ma during early magmatic activity associated with the formation of the Mid-Continent Rift of North America. The eastern gabbro forms a partial ring dyke on the outer margin of the complex and consists of at least three discrete intrusions. The largest of these is the layered gabbro that comprises a 300 m thick fine- to medium-grained basal unit overlain by up to 1100 m of variably massive to layered gabbroic cumulates which vary from olivine gabbro to anorthosite. Several xenoliths of Archaean metamorphic rocks that range in size from 10's to 100's of meters are present in the central part of the intrusion. Within discrete horizons in the layered gabbro are many centimeter- to meter-scale, gabbroic xenoliths. The main cumulus minerals, in order of crystallization, are plagioclase, olivine and clinopyroxene ± Fe-Ti oxides. Biotite and Fe-Ti-oxide are the dominant intercumulus phases. Orthopyroxene occurs not as a cumulus phase but as peritectic overgrowths on cumulus olivine. A detailed petrographic and mineral chemical study of samples from two stratigraphically controlled traverses through the layered gabbro indicates that the stratigraphy cannot be correlated along the 33 km strike of the ring dyke. Mineral compositions show both normal and reversed fractionation trends. These patterns are interpreted to record at least three separate intrusions of magma into restricted dilatant zones within the ring dyke possibly associated with ongoing caldera collapse. Calculations of parental melt composition using mineral — melt equilibria show that even the most primitive gabbros crystallized from an evolved magma with mg# of 0.42-0.49. The presence of orthopyroxene overgrowths on cumulus olivine suggests rising silica activity in the melt during crystallization and implies a subalkaline parentage for the layered gabbro.     

Characteristics of alkali magma differentiation at the Cape Verde Islands

Parental magmas of the Cape Verde Islands are high-Mg foidites, picrobasalts, and basanites. The rocks can be classified into the following two differentiated series: (1) a high-alkali series, which includes picrites, high-Mg foidites, low-Mg-foidites, and phonolites; and (2) a series of moderate alkalinity, which includes picrobasalts, basanites, tephrites, tephrophonolites, phonotephrites, phonolites, and trachytes.The differentiation of both series is associated with a decrease in the concentrations of Mg, Fe, Ti, and Ca and an increase in the contents of Al, Si, K, and Na. Rare lithophile elements (REE, Zr, Nb, Th, Rb, and U) progressively enrich younger derivatives. The Sr and Ba concentrations pass through a maximum, as is typical of alkaline series, which are characterized by broad crystallization fields of plagioclase and melilite, minerals that have high distribution coefficients of these elements. The analysis of the composition of volcanic rocks from the Cape Verde Islands within the scope of the system olivine-diopside-nepheline shows that the evolution of the parental magmas was controlled by crystallization differentiation in shallow-depth intermediate chambers. This conclusion finds further support in data on inclusions in minerals and a simulated crystallization model in a magmatic chamber.