• Ocean and Polar Research
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• v.25 no.1
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• pp.119-128
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• 2003

Although phoscorites and carbonatites form only a minor proportion of the earth's crustal rocks, these unusual rocks and their intimate relation are of both academic and economic importance. Rare metal (Nb, Zr, Ta) and REEs mineralizations are in close relation with the differentiation of these phoscorite-carbonatite complexes (PCCs). Recent integrated petrological and geochemical data on PCCs in the Kola Alkaline Province, Arctic, indicate that phoscorites and associated carbonatites are differentiated from common 'carbonated silicate patental magma'. Various hypotheses for the genesis of phoscorite-carbonatite complexes have been proposed during the last half-century. A simple magmatic fractionation scheme can not explain the chemical and mineralogical characteristics of phoscorite and conjugate carbonatite. Instead, the hypotheses involving liquid immiscibility and coeval accumulation processes are favored to explain the mineralogical and geochemical characteristics of phoscorite and carbonatite association.

• Proceedings of the Mineralogical Society of Korea Conference
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• 2004.05a
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• pp.32-34
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• 2004

• Proceedings of the Petrological Society of Korea Conference
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• 2004.05a
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• pp.32-34
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• 2004

• Proceedings of the Mineralogical Society of Korea Conference
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• 2003.05a
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• pp.67-67
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• 2003

The chemical compositions and textural relationships of the Nb-Zr oxide minerals including pyrochlore [ideally (Ca,Na)$_2$Nb$_2$O$\sub$6/(OH,F), with up to 24% UO$_2$ and 16% Ta$_2$O$\sub$5/] and baddeleyite [ideally ZrO$_2$, with up to 6% Nb$_2$O$\sub$5/] in the Sokli carbonatite complex, Kola Peninsula, Arctic are described. These two minerals in carbonatites are the major hosts for the HFSEs such as U, Th, Ta, Nb, Zr and Hf and thus are interest both economically and petrologically. The Sokli carbonatite complex (360-370 Ma) in Northern Finland, which forms a part of the Paleozoic Kola Alkaline Province (KAP), is mainly composed of multi-stages of carbonatite and phoscorite associations (P1-C1 P2-C2, P3-C3, D4 and D5) surrounded by altered ultramafic rocks (olivinite and pyroxenite) and cut by numerous small dikes of ultramafic lamprophyre. The Sokli complex contains the highest concentration in niobium and probably in tantalum, which are economically very important to modern steel technology, among the ultramafic-alkaline complexes of the KAP. Pyrochlore and baddeleyite mostly concentrate in the phoscorites. Pyrochlores in the Sokli complex are generally rounded octahedra and cubes in shape, red brown to grey yellow in color, and 0.2 to 5 mm in size. They are found in all calcite carbonatites, phoscorites and dolomite carbonatites, except P1-C1 rocks. These pyrochlores display remarkable zonations which depend on host rock compositions, and have significant compositional variations with evolution of the Sokli complex. The common variation scheme is that (1) early pyrochlore is highly enriched in U and Ta; (2) these elements decrease abruptly in the intermediate stage, while Th and Ce increase, and (3) late stage pyrochlore is low in U, Ta, Th, and Ce, and correspondingly high in Nb. Baddeleyites in the Sokli complex occur in the early P1-C1 and P2-C2 rocks and rarely in P3. They crystallized earlier than pyrochlores, and occasionally show post-magmatic corrosion and replacement. The FeO and TiO$_2$ contents of baddeleyites are much lower than those of the other terrestrial and lunar baddeleyites, whereas Nb$_2$O$\sub$5/ and Ta$_2$O$\sub$5/ contents are the highest among the reported compositions. Ta/Nb and Zr/Nb ratios of pyrochlores and baddeleyites decrease towards later stage facies, which is in accordance with the whole rock compositions. The variation of Ta/Nb and Zr/Nb ratios of pyrochlores and baddeleyites is considered to be a good indicator to trace an evolution of the carbonatite complexes.

• The Journal of the Petrological Society of Korea
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• v.12 no.4
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• pp.170-183
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• 2003

The Sokli complex in the northeastern Baltic Shield (Finland) forms a part of the extensive Devonian Kola Alkaline Province. The complex contains ultramafic lamprophyres occurring as dikes of millimetric to metric thickness. The Sokli ultramafic lamprophyres have petrographical and geochemical affinities with aillikite. High concentrations of Cr and Ni with low Al$_2$O$_3$ content of the Sokli aillikites indicate a strongly depleted harzburgitic source. However, compared to the kimberlites, the lower Cr and Ni contents and mg-number with weaker HREE depletion of the Sokli aillilkites imply a smaller proportion of garnet in the source and thus suggest a shallower melting depth of the source. In order to account for high concentrations of all incompatible elements and LREEs, with high volatile content (especially CO$_2$), an additional enriched material is thought to have been incorporated into the Sokli aillikite source. An anomalous enrichment of K in the Sokli aillikites, compared to nearby ultrapotassic rocks and world-wide ultramafic lamprophyres, indicate a presence of K-rich phase (probably phlogopite) in the source mantle.