Possible uranium sources for the largest uranium district associated with volcanism: the Streltsovka caldera (Transbaikalia, Russia)
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Keywords:
Caldera
Peralkaline rock
Phenocryst
Uranium ore
Magma chamber
Phenocryst
Peralkaline rock
Silicic
Xenolith
Magma chamber
Igneous differentiation
Fractional crystallization (geology)
Caldera
Topaz
Magmatic water
Alkali feldspar
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Abstract Gallium abundances, determined by laser ablation-inductively coupled plasma-mass spectrometry, are presented for phenocrysts and glassy matrices from a metaluminous trachyte and five peralkaline rhyolites from the Greater Olkaria Volcanic Complex, Kenya Rift Valley. Abundances in the glasses range from 28.9 to 33.3 ppm, comparable with peralkaline rhyolites elsewhere. Phenocryst Ga abundances (in ppm) are: sanidine 31.5–45.3; fayalite 0.02–0.22; hedenbergite 3.3–6.3; amphibole 12; biotite 72; ilmenite 0.56–0.72; titanomagnetite 32; chevkinite-(Ce) 364. The mafic phases and chevkinite-(Ce) are enriched in Ga relative to Al, whereas Ga/Al ratios in sanidine are smaller than in coexisting glass. Apparent partition coefficients range from <0.01 in fayalite to 12 in chevkinite-(Ce). Coefficients for hedenbergite, ilmenite and titanomagnetite decrease as melts become peralkaline. The sharp increase in Ga/Al in the more fractionated members of alkaline magmatic suites probably results from alkali feldspar-dominated fractionation. Case studies are presented to show that the Ga/Al ratio may be a sensitive indicator of such petrogenetic processes as magma mixing, interaction of melts with F-rich volatile phases, mineral accumulation and volatile-induced crustal anatexis.
Peralkaline rock
Phenocryst
Trachyte
Sanidine
Amphibole
Ilmenite
Felsic
Igneous differentiation
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Peralkaline rock
Cape
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Mayor Island, New Zealand is a small isolated occurrence of a peralkaline rhyolite volcano 60km away from the calc-alkaline Taupo Volcanic Zone. During the volcano's 130,000 year sub-aerial history the erupted magma increases in peralkalinity, with the Agpaitic Index (AI) rising from 1 in the very oldest lavas to ~ 1.25 in the most recent eruptions, straddling the compositional divide between comendite and pantellerite. In addition to this an ~40% enrichment of REE's and certain trace elements (e.g. Zr,Y,Nb) is observed. Phenocryst phases present include alkali feldspar, quartz, aenigmatite, Na-hedenbergite, aegerine and fayalitic olivine. With the slight exception of the feldspar which has a slight bimodality in Na/K ratio the major element compositions of these phases remains stable throughout the evolution of the magma. Analysis of melt inclusions, matrix glass and phenocryst phases has allowed an estimation of (i) volatile element partitioning between melt and vapour and, (ii) non-volatile trace element partitioning between phenocrysts and melt. These data allow an assessment of the role that the phenocrysts have played in the evolving magma composition through fractional crystaUisation and they give insight into the effect of degassing on magma chemistry. Trace elements were measured using the secondary ion mass spectrometer (ion microprobe) analysis and the volatile components of both inclusion and matrix glass utilised FTIR (H20), electron microprobe (C1) and ion microprobe (H,F) techniques.
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Trace element
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This chapter contains sections titled: Introduction Geologic Setting Evolution of the Volcanic Center Discussion
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Center (category theory)
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Peralkaline rock
Phenocryst
Felsic
Molybdenite
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Bimodal associations of basalt and rhyolite of Upper Ordovician age which were erupted in a submarine environment occur within the Caledonian orogenic belt of South Britain at Parys Mountain (Anglesey), in Snowdonia (North Wales) and at Avoca (SE Ireland). The volcanic rocks have experienced hydrothermal alteration and low-grade metamorphism, and therefore immobile elements (e.g. Ti, Zr, Nb, Y) have been used to identify the original geochemical characteristics. The basalts have characters transitional between volcanic ‘arc’ and ‘within plate’ types consistent with eruption on an extensional part of an active continental margin. Two groups of rhyolites have been identified. A low-Zr group (Zr<500ppm), represented at all three locations, is interpreted as originally of high-K, subalkaline type. A high-Zr group (Zr>500ppm), represented at Snowdonia and Avoca, is interpreted as originally being peralkaline in composition; their high Zr/Nb ratios (>10) are typical of peralkaline rhyolites erupted above subduction zones. The bimodal nature of the associations and the peralkaline character of some rhyolites indicates magma production in a complex tectonic setting, transitional between an active continental margin/island arc and an extensional environment. Associated sulphide mineralization is volcanogenic and probably syn-sedimentary. High-level, rhyolitic magma chambers are thought to have driven convection of the hydrothermal fluids from which the sulphides precipitated.
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Phenocryst
Trachyte
Peralkaline rock
Magma chamber
Caldera
Radiogenic nuclide
Igneous differentiation
Melt inclusions
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Phenocryst
Peralkaline rock
Pyroxene
Sanidine
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