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Ferropicritic magmatism has produced a variety of volcanic and subvolcanic formations in the upper part of the early Proterozoic Pechenga Group, including sills, tuffs, pillowed and massive lavas, and thick, differentiated lava flows that possess spinifex-textured upper parts and lower cumulate parts enriched in olivine. Various olivine and pyroxene spinifex types can be distinguished. The occurrence...
The 2.7 Ga-old volcanic sequence at Kambalda comprises a lower formation of massive and pillowed tholeiitic basalts (Lunnon basalt); a middle formation (Kambalda komatiite) of channel- and sheet-flow facies komatiitic peridotites with thin, intercalated sediments (Silver Lake member) and thin, massive, aphyric and differentiated spinifex-textured komatiites (Tripod Hill member); upper formations of...
The introduction of high temperature, high magnesium komatiitic and picritic magmas into the Earth's upper crust has given rise either directly or indirectly to many of the world's major ore deposits. There is a direct association between komatiitic magmatism and Kambalda-style Ni-Cu-PGE deposits. The link between magma type and mineralization is not as clear cut in the case of the Noril'sk Cu-Ni-PGE...
New major, trace element and Sr and Nd isotope data are presented for selected lavas from the three uppermost formations in the Siberian Trap, and on over 60 samples of the associated intrusive rocks. The lavas from a 1400 m section are remarkably homogeneous and, apart from four samples of basaltic andesites, SiO 2 =48.4-49.6%, MgO=8.1-6.3%, and Mg * =54-58, TiO 2 =1.05-1...
A tholeiitic picrite erupted from the summit region of Kilauea volcano, Hawaii in 1959. Work by Wright, Helz and Schwindinger and Anderson shows that the 1959 magma was a mixture of new, hot, MgO-rich magma that rose through and mixed with cooler stored magma causing crystallization of abundant olivine phenocrysts. CO 2 dissolved in melt inclusions in olivine phenocrysts from the eruption...
The Perseverance (formerly known as Agnew) and the neighbouring Rocky's Reward nickel deposits are associated with metamorphosed komatiite flows erupted onto a substrate of felsic crystal tuffs. The deposits are overlain by the Perseverance Ultramafic Complex, which consists of a central dunite lens, 700 m thick by 3 km wide, flanked by olivine orthocumulates and spinifex textured komatiites. A...
The Archaean supracrustal sequence at Forrestania contains at least five komatiitic belts which can be traced along strike for distances in excess of 30 km. Lithologies range from olivine cumulates, which crystallised at the interface between substrate and flowing lava, to spinifex-textured rocks, which underwent rapid crystallisation within ponded lava lobes. Characteristic associations of different...
The Tertiary volcanism of the North Atlantic region provides one of the most extensive and best described magmatic records of interaction between a starting plume and continental lithosphere, and the role of plumes in continental break-up. Many authors consider the entire volcanic province, from Baffin Island in the north-west to Scotland in the south-east, to be the product of a single proto-Icelandic...
Meimechites are highly magnesian alkaline lavas from the Meimecha-Kotuj region of northern Siberia. They contain abundant large phenocrysts of olivine and smaller grains of chromite in a matrix of smaller olivine grains, titanian clinopyroxene, ilmenite, altered glass, and in most but not all cases, biotite. A small proportion of the larger olivine grains have a pronounced parallel parting or show...
Komatiites in the western part of the Norseman-Wiluna greenstone belt in the Yilgarn Block of Western Australia display a wide variety of volcanic facies, ranging from very thin differentiated Munro Township-type flow units through to very thick olivine-rich cumulate flow units containing high proportions of adcumulate dunite.Cumulate flow units have been mapped in detail in the Agnew-Wiluna segment...
The 3.4 Ga Nondweni greenstone belt is situated near the southern margin of the Kaapvaal Craton and consists of predominantly pillowed and spinifex-textured volcanic rocks. Primary volcanic structures and compositional characteristics of this succession are unique in currently known Archaean geology. None of the lavas exceeds 23% MgO (anhydrous) and pyroxene rather than olivine dominates the crystallisation...
The success of recent dynamic models of adiabatic partial melting of the Earth's mantle has lead to an increasing acceptance that much of the compositional spectrum of terrestrial basalts can be explained in terms of variations in the physical parameters of melting of a common anhydrous-lherzolite mantle source. A comparison of normalized compositions of terrestrial picritic lavas with the results...
Precise U-Pb ages have been obtained from zircon and baddeleyite in plutonic rocks from the Siljan-Skrim area in the Vestfold Graben Segment in the south-western part of the Permian Oslo Rift. In addition, common lead data from feldspars in the same rocks are presented. The measured common Pb compositions of the feldspars are used to correct for initial common Pb in the zircons. The common Pb data...
Field and petrographic evidence together with major element geochemistry suggest that mixing and mingling of magmas of contrasting compositions were important petrogenetic processes in the Fazenda Nova/Serra da Japeganga plutonic complex of Northeast Brazil. The complex was emplaced at pressures of 300-500 MPa in amphibolite facies metamorphic rocks of Neoproterozoic age and consists of three main...
Ultramafic rocks occur within polydeformed high grade metasedimentary rocks throughout the Vammala Nickel Belt (Fennoscandian Shield) and have been subdivided into cumulate-textured intrusive bodies and high-Mg (picritic) eruptive units.The crystallisation sequence olivine(+ cotectic chromite)-orthopyroxene-clinopyroxene-amphibole, the decreased stability field of olivine and fractionation of relatively...
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