The Paleoarchean (ca. 3.5-3.3 Ga) Onverwacht Suite (OS) of the Barberton Greenstone Belt consists of a 15-km thick imbricate tectonic stack of seven complexes consisting predominantly of volcanic rocks and intrusions. Tectonostratigraphically from base to top they are the Sandspruit, Theespruit, Komati, Hooggenoeg, Noisy, Kromberg and Mendon Complexes. The Hooggenoeg and Noisy Complexes in the middle of the OS are separated by a significant unconformity resulting from the uplift of the submarine lavas and deep erosion, demonstrating the onset of tectonic accretion prior to 3455 Ma. The basic lavas of the tectonostratigraphic lower (Theespruit, Sandspruit and Komati) and upper (Mendon) complexes are composed of komatiite, komatiitic basalt and high-MgO basalt, whereas those in the middle part (Hooggenoeg and Kromberg) are predominantly high- to low-MgO tholeiitic basalts. Felsic volcanic rocks and intrusions are important in two of the complexes (Theespruit and Noisy). The ultramafic to basaltic lavas show REE patterns that are almost flat and resemble those of modern MORB, whereas those of the felsic rocks are flat from Lu to Gd and moderately to strongly enriched in LREE, similar to modern arcs. Average epsilon(Nd) ((T)) values are close to depleted mantle growth curves. In MORB-normalised multi-element diagrams, the komatiitic to basaltic rocks exhibit flat patterns from Lu through La and consistent relative enrichment in the elements Pb, U, Th, Ba and Cs. Apart from the Komati Complex, the majority of the lavas show significant negative Nb and Ta anomalies. Enrichment in non-conservative incompatible elements (Cs, Ba, Th, LREE) relative to conservative elements (Ta, Nb, Zr, Hf, Ti, Y, HREE) shows that the komatiitic to basaltic magmas were generated from metasomatised mantle above subducting altered oceanic crust. The geochemistry of the felsic rocks indicates an origin by melting of subducted amphibolite and eclogite. The tectonostratigraphy and the geochemical characteristics of the lavas and intrusions are consistent with successive obduction and accretion of segments of oceanic crust formed in back-arc basins and volcanic arcs. (C) 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

The basis of this study comprises 540 geochemically analysed samples collected systematically from eighteen stratigraphic sections (2257 m in total length) through the submarine volcanic rocks of the tectonically separated Hooggenoeg, Kromberg and Mendon Complexes in the southwestern part of the Paleoarchean Barberton Greenstone Belt. The lavas are predominantly high- to low-Mg tholeiitic basalts but include minor komatiite and komatiitic basalt. They are non-deformed and preserve delicate igneous textures, but have been allochemically altered and regionally metamorphosed. Comparison of pillow cores and rims, samples from massive flows and interelement correlations demonstrate that Ti, Al, Cr, V, Nb, Ta, Zr, Hf, Y, Pb, Th and the REE were relatively immobile during alteration and hence preserve geochemical evidence bearing on the origin and tectonic setting of the lavas.Chondrite-normalized REE patterns are slightly LREE depleted in komatiite to slightly LREE enriched in basalts. MORB-normalized values of non-conservative elements (Cs, Ba, Pb, Th) are high relative to conservative elements (Ta, Nb, Zr, Hf, Y, HREE) in all of the volcanic rocks, particularly those of the Hooggenoeg Complex. Most of the samples exhibit enrichment of Cs and Ba, Pb anomalies and depletion in Nb and Ta, consistent with a subduction-related oceanic environment. With the exception of the lavas of the Hooggenoeg Complex, using primordial mantle (PM) values for normalisation generally subdues the enrichments of the non-conservative elements seen in MORB-normalised multi-element diagrams. However, negative Nb and Ta anomalies relative to La remain significant. High Ba/Th ratios indicate relatively shallow level enrichment of the magma source in large ion lithophile elements by aqueous fluids, whilst enhanced Th concentrations reflect deeper partial melting. Estimated subduction related contributions to Th vary in the ca. 2700 m thick section of the Hooggenoeg Complex and suggest changes in the depth to the subducting slab, which may relate to variation in the subduction angle in the course of ca. 10 million years. epsilon(Nd(T)) values suggest earlier melt extraction and possible incorporation of older crustal material, probably subducted elastic sediments.Our inferred model for the formation of the Upper Onverwacht Suite, based on the lithological and structural development of the lavas and their geochemistry invokes eruption in intra-oceanic back-arc basins and volcanic island arcs. Magmas were generated by variable degrees of partial melting at different depths and temperatures of metasomatised mantle above subducting and dehydrating oceanic lithosphere, and were subsequently modified by fractional crystallization and hybridization. In terms of MORB- and PM-normalised multi-element patterns, as well as Nd-isotope ratios, the volcanic rocks of the Onverwacht Suite are comparable with the west Pacific-Indonesian arc systems.

We dispute Duchesne and Charlier's (Duchesne, J.C., Charlier, B., 2005. Geochemistry of cumulates from the Bjerkreim–Sokndal Intrusion (S. Norway). Part I: Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent. Lithos 83, 229–254.) postulate that the major-element compositions of cumulates in the Bjerkreim–Sokndal Intrusion vary linearly between plagioclase and mafic “poles” and their inference that this supports an origin by in situ crystallisation. We use a larger set of major-element data for plagioclase–orthopyroxene–ilmenite cumulates to show that some linear trends in Harker diagrams simply reflect varying amounts of hemo-ilmenite relative to plagioclase and orthopyroxene, while others are probably spurious and induced by variations in modal plagioclase, the most abundant cumulus mineral. Ratios of oxides that enter almost exclusively into orthopyroxene and hemo-ilmenite are shown to be highly dispersed, reflecting differential sorting of the mafic minerals.

To constrain the amount and rate of crustal contamination that is possible in basaltic and jotunitic magma, and to gain an insight into the physical and thermal processes of assimilation in crustal magma chambers, we have modelled published Sr and Nd isotopic data from three layered intrusions. Well-exposed sequences of cumulates with no evidence of magma recharge provide direct records of concurrent assimilation and fractional crystallization (AFC). The key to the modelling is that F, the mass fraction of magma remaining in the chamber, can be estimated from the thicknesses of the studied cumulate sequences. This allows AFC model curves to be fitted to the isotopic data by varying r, the ratio of the rate of mass assimilated to the rate of mass crystallized. The results of modelling show that r is nearly constant in 800 to 2000 m thick sequences of cumulates displaying up-section decreases in anorthite content of plagioclase, increases in whole-rock Sr0 (initial 87Sr/86Sr) and decreases in whole-rock εNd0 (initial εNd). The r-values of the layered sequences range from ∼0.12 in the Fongen–Hyllingen Intrusion, over 0.20 in the Bjerkreim–Sokndal Intrusion, to 0.27 in the Hasvik Intrusion. The total amount of assimilation, the bulk crust/magma ratio, reaches values of 0.08, 0.19 and 0.28 at the level of the most contaminated samples after 60% to 80% crystallisation, whereas the instantaneous crust/magma ratio of the most contaminated magmas were respectively 0.14, 0.46, and 0.70, for the three intrusions.Innumerable country rock xenoliths occur in the three layered intrusions and played a crucial role in the assimilation process. The xenoliths spalled off the roofs of the magma chambers during magma emplacement and their initial temperature and composition relate to r in the intrusions. In the Hasvik Intrusion (r=0.27), the initial temperature of the country rocks was ∼450 °C and the xenoliths were fusible metasediments and therefore produced a high fraction of partial melt that could be assimilated. In the Bjerkreim–Sokndal Intrusion (r=0.20), the country rocks were initially at temperatures of 640–880 °C but included both refractory massif-type anorthosite and fusible gneisses. In the Fongen–Hyllingen Intrusion (r=0.12), the country rocks were cooler (∼300 °C) and the xenoliths include refractory metabasalt (dominant) and fusible metapelite. We argue that the refractory metabasalt and anorthosite xenoliths acted mainly as heat sinks, resulting in reduced r-values in Fongen–Hyllingen and Bjerkreim–Sokndal Intrusions.Heating of refractory and fusible xenoliths, and melting of fusible xenoliths absorbed sensible and latent heat of the magma. Energy-balanced modelling shows that up to 75% of the heat available was absorbed by xenoliths within the magma chambers, promoting higher rates of cooling and crystallisation than would have resulted from loss of heat to the envelope of country rocks alone. The high r-values reflect the amount of heat absorbed by heating and melting country rock within the magma chambers themselves, and their constancy reflects the ready availability of fusible xenoliths.

The stratigraphy of the well‐preserved Solund‐Stavfjord Ophiolite Complex in the West Norwegian Caledonides documents the volcanic evolution of a spreading center in a Late Ordovician back‐arc basin. Basaltic sheet flows, pillow lavas and volcanic breccias are the main components of the ∼470–800 m thick extrusive sequence, and are organized stratigraphically in a cyclic manner. Cyclic units vary in thickness from ∼5 m to 225 m and are typically composed of basal sheet flows or lava flows with large pillows that are succeeded by flows with progressively smaller pillows and volcanic breccias. Thick, independent breccia units also occur in the stratigraphy. In sheet‐flow dominated parts of the sequence the cyclic units are thicker (average 85 m) than in pillow‐dominated parts (average 20 m). Detailed logging of closely spaced profiles (∼1 km or less apart) shows that the proportions of sheet flows, pillow lavas and volcanic breccias varies laterally. Along an axial segment of less than 10 km, the volcanic products change from predominantly sheet flows, reflecting robust volcanic centers, to pillow lavas to volcanic breccias. Sheet‐flow dominated volcanic centers seem to be spaced at intervals of ∼25–30 km, and we tentatively interpret their regularity as an expression of volcanic segmentation at an intermediate‐ to fast spreading center. Observations of modern ocean crust suggest that sheet flows dominate at fast spreading ridges, while pillow lavas dominate at slow‐spreading ridges. Volcanic breccias are apparently rare in both of these environments. These features contrast with the stratigraphy of the Solund‐Stavfjord Ophiolite Complex, where the proportion of different volcanic products varies laterally and volcanic breccias are common. We emphasize the importance of detailed studies of the volcanic stratigraphy of ophiolites, as complements to those of in‐situ oceanic crust, in order to provide a more complete picture of volcanic evolution of oceanic crust in different spreading regimes.

The Bjerkreim–Sokndal Layered Intrusion contains an up to 3-m-thick layer of sulphide-bearing orthopyroxenite or melanorite that can be followed for about 30 km along the boundary between megacyclic units II and III. The layer of orthopyroxenite is developed within a sequence of ilmenite leuconorites and defines the base of zone IIIa. The leuconorites are generally succeeded by ilmenite–magnetite leucotroctolite, which defines the base of zone IIIb. The orthopyroxenite layer has an initial Sr-isotopic ratio similar to the leucotroctolites (ca. 0.7051) and lower than the enveloping ilmenite leuconorites of zones IIc and IIIa (ca. 0.7054). Variations of tetrahedrally coordinated Al in orthopyroxene and elevated Cr/TiO2 in the orthopyroxenite layer and the leucotroctolite imply that the orthopyroxenite is genetically more related to the leucotroctolite than the enveloping leuconorites. On the basis of field observations and analytical results, we conclude that the orthopyroxenite crystallised as a result of mixing between relatively primitive magma and differentiated resident magma, both of which were sulphide-saturated, during chamber replenishment. The injected magma formed a buoyant plume that spread out laterally at its level of neutral buoyancy within the compositionally zoned resident magma, some distance above the magma-chamber floor. Mixing in the plume resulted in a hybrid saturated in orthopyroxene, ilmenite and sulphide melt. Batches of relatively dense magma containing crystals of orthopyroxene and ilmenite and droplets of sulphide melt sank to the floor of the chamber from the hybrid magma layer and formed a layer of orthopyroxenite on the magma-chamber floor. Removal of orthopyroxene from the hybrid melt resulted in the sporadic crystallisation of plagioclase at the orthopyroxene–plagioclase cotectic. Plagioclase joined orthopyroxene and sulphide droplets in the dense melt batches which sank to the floor, resulting in the local development of melanorite instead of orthopyroxenite. Crystallisation of ilmenite leuconorite from the resident magma below the hybrid magma layer resumed after formation of the orthopyroxenite layer. This continued as the hybrid layer thickened and eventually came into contact with the chamber floor when leucotroctolites of zone IIIb started to crystallise on the deeper parts of the floor. Elsewhere, lower-temperature cumulates formed on elevated portions of the floor from magma higher in the stratified column. The sulphide-bearing orthopyroxenite layer is not associated with PGE mineralisation because both the replenishing and differentiated resident magmas had lost PGE through earlier sulphide saturation.

The stratigraphy and geochemistry of the uppermost 200–300 m of the metabasalt sequence of the Solund-Stavfjord Ophiolite Complex of western Norway has been investigated over a lateral distance of ~6 km. Volcanologically, this sequence was constructed in a cyclic manner. The lowest part of a volcanic cycle is characterised by sheet flows and/or large pillows followed by lavas in which pillows become progressively smaller upwards. In several of the cycles, pillow lavas are differentiated Fe–Ti basalts at the base, and successive flows become gradually more primitive stratigraphically upwards. The compositional differences between the bottom and top of a cycle can be substantial (e.g. TiO2 is 2.98–1.00 wt%, Zr 227–76 ppm, and Cr 125–520 ppm respectively). Nd isotopic and other data suggest that all the basaltic magmas were generated from a uniform source. Concomitant with the upward decrease in the Ti and Fe contents of the metabasalts through a volcanic cycle, estimated magma densities also decrease. We attribute the geochemical stratigraphy of the metabasalts to mainly reflect magma mixing in a frequently replenished magma chamber. Hybrids in the chamber repeatedly mixed with inflowing magmas which were more primitive and less dense. During the time intervals between cycles, the magma chamber was essentially closed and extensive fractional crystallisation took place. In some sequences, however, volcanic cycles and stratigraphical geochemical trends are poorly defined. We tentatively propose that in such cases the volcanics were erupted from more than one magma chamber.

New mapping shows that the 1160-m-thick Layered Series of the mafic to ultramafic Intrusion 4 of the Honningsvåg Intrusive Suite, North Norwegian Caledonides, comprises approximately 25% feldspathic peridotite, 55% melatroctolite (olivine-rich troctolite), 5% troctolite, <2% olivine melagabbro (olivine-rich olivine gabbro) and 13–15% olivine gabbro. The cumulate sequence can be divided into eight macrocyclic units, the base of each unit resulting from magma replenishment. Compositions of olivine and plagioclase obtained from ca. 1700 electron microprobe point analyses, display complex and limited variations (Fo87–74 and An80–67) relative to stratigraphic height.

contaminated layered intrusions known. Thousands of recrystallized Strontium and neodymium isotopic data for mafic cumulates, chilled tabular xenoliths of metasedimentary origin enclosed in the cumulates margins, and adjacent crustal rocks of the Hasvik Layered Intrusion, are thought to represent the remnants of the assimilated material. North Norwegian Caledonides, are reported together with new The xenoliths spalled off the roof during magma emplacement, and, mineralogical and whole-rock analytical data to constrain the extent together with the elevated temperatures (400–600°C) of the midand effect of the assimilation of crustal xenoliths in a basaltic crustal country rocks, led to a high degree of assimilation in the magma chamber. Initial Sr/Sr (700 Ma) of 0·7045 and eNd Hasvik magma chamber. (700 Ma) of+3·03 for the chilled margin, which has a tholeiitic composition akin to the chilled rocks of the Skaergaard intrusion, demonstrate that the parental magma was derived from a depleted mantle source. The basal cumulates (0–335 m) show an up-section decrease in Sr/Sr from 0·7045 to 0·7038 and a correlative

The Late-Proterozoic Bjerkreim-Sokndal Layered Intrusion (BKSK) consists of andesine anorthosite, leuconorite, troctolite, norite, gabbronorite, jotunite, mangerite, quartz mangerite and charnockite. The sequence of appearance of cumulus minerals and their compositions suggest a parent magma that was evolved, had plagioclase (±olivine) on the liquidus, was sufficiently TiO2-rich for hemo-ilmenite to crystallise early, and low in CaO and CaOAl2O3 compared to basalts as reflected by the sodic plagioclases and the delayed appearance of cumulus augite. Fine- to medium-grained jotunites found along the northern contact of the BKSK consist of plagioclase (An45–53), inverted pigeonite (Mg# = 55-50), sparse augite (Mg# = 69-59), Fe-Ti oxides, K-feldspar, quartz and apatite. They are basic to intermediate rocks with relatively high FeOtotal, high TiO2, low MgO/MgO + FeO, moderate Al2O3 and low CaO and normative diopside. The jotunites have compositions that are consistent with the parental magma for the lower part of the BKSK Layered Series, and are interpreted as being marginal chills. Similar, but slightly more differentiated, jotunite magmas were subsequently emplaced into the BKSK and the surrounding region as broad dykes and small plutons. Jotunite is a minor rock type in most massif-type anorthosite provinces but may have an important petrological significance.

Abstract Wedge-shaped layers of ultramafic and mafic cumulates in Intrusion II of the Caledonian Honningsvåg Intrusive Suite suggest crystallization on an inclined magma chamber floor from a compositionally-zoned and density-stratified magma. Cyclic unit 8 (140−100 m thick) consists of a distally-thinning olivine gabbro (denoted paoC) macrolayer overlain by a distally-thickening gabbronorite, pahC. New mineral data in four traverses across cyclic unit 8 show systematic compositional changes; the Mg# of the mafic phases decreases upwards through the unit and distally, both along the base and along the paoC/pahC interface. A crystallization model based on an effectively continuously-zoned magma chamber with numerous, relatively thin, double-diffusive magma layers is proposed. Differential migration of horizontal isopleths (e.g. Mg# and aSiO₂) in response to fractional crystallization and assimilation of country rock can explain the variations in the Mg# of the cumulates.

Abstract Field relations in the upper part of Intrusion II of the Caledonian Honningsvåg Intrusive Suite show that some peridotite sheets transgress, and include in situ rafts of, the adjacent gabbroic cumulates. Modal and textural analyses of three olivine melagabbro sheets show non-cotectic mineral proportions that are likely to result from crystal-melt reactions. Discordant, replacive fingers and pipes of feldspathic peridotite along interfaces between peridotite and overlying olivine melagabbro also suggest crystal-melt reactions. It is proposed that several picritic sills intruded porous gabbroic cumulates in the upper part of Intrusion II. Lateral infiltration of picritic magma led to crystal-melt reactions, mainly assimilation of plagioclase and precipitation of olivine, resulting in the formation of olivine melagabbro and peridotite sheets, and replacive fingers and pipes of feldspathic peridotite.

The Bjerkreim-Sokndal Layered Intrusion is a large (~230 km2), discordant, Late Proterozoic, post-orogenic pluton in the Egersund-Farsund Igneous Province. The intrusion was emplaced shortly after massif-type anorthosite plutons and is cut by jotunite dykes. It contains a >7000 m thick Layered Series consisting of rocks belonging to the anorthosite kindred: andesine anorthosite, leuconorite, troctolite, norite, gabbronorite, mangerite, and quartz mangerite. Cumulates in the Layered Series are organized in 6 megacyclic units (MCU 0 to IV), individually up to 1800 m thick, but varying considerably in thickness and development along strike. The highest-temperature cumulates are troctolites containing plagioclase of ~An54 and olivine of ~Fo77. Phase contacts in the macrocyclic units reflect crystallization of the silicate minerals in the order plagioclase (± olivine), orthopyroxene, Ca-rich pyroxene, pigeonite. Ilmenite crystallized early and apatite appeared as a cumulus mineral at about the same time as Ca-rich pyroxene. Cumulus magnetite followed orthopyroxene and preceded Ca-rich pyroxene in MCU III and IV, but crystallized after Ca-rich pyroxene in MCU IB. MCUs 0, IA and II do not contain cumulates with cumulus magnetite or Ca-rich pyroxene. Olivine (~Fo50) reappears in the uppermost part of the Layered Series where there is a rapid stratigraphic transition to mangerite and quartz mangerite. The basal parts of MCUs III and IV are characterized by thin sequences of plagioclase, plagioclase-orthopyroxene-ilmenite and orthopyroxene-ilmenite cumulates in which there are systematic upward decreases in initial Sr isotope ratios. They are overlain by troctolite (plagioclase-olivine cumulate) and are believed to have crystallized from hybrid magmas. The MCUs, the discordant geometry of phase contacts, the stratigraphic variations in initial 87Sr/86Sr ratio (0.7049-0.7085), and the abundance of xenoliths suggest crystallization of the cumulates at the base of a periodically-replenished, compositionally-zoned magma chamber that was continually assimilating country rocks. The parent, as indicated by medium-grained jotunite along country-rock contacts, appears to have been an evolved, Ti-rich magma similar to ferrobasalt, but poor in diopside components. Systematic stratigraphic variations in initial 87Sr/86Sr ratio at the base of MCU III and MCU IV suggest that influx of magma

Difficulties in removing the tracheal tube from the trachea are relatively uncommon. We report here a case of difficult extubation which was precipitated by pulling off the pilot balloon and valve assembly in order to deflate the cuff.