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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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Europe
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Western Europe
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United Kingdom
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Great Britain
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England
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Derbyshire England (1)
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igneous rocks
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extrusive rocks (1)
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igneous rocks
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plutonic rocks
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granites
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charnockite (1)
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minerals
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carbonates
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calcite (1)
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ikaite (1)
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hydrates (1)
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Primary terms
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crystal growth (1)
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Europe
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Western Europe
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United Kingdom
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Great Britain
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England
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Derbyshire England (1)
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ground water (1)
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igneous rocks
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plutonic rocks
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granites
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charnockite (1)
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magmas (1)
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tunnels (1)
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Unusual morphologies and the occurrence of pseudomorphs after ikaite (CaCO 3 ·6H 2 O) in fast growing, hyperalkaline speleothems
Igneous charnockites are characterised by distinctively high abundances of K 2 O, TiO 2 , P 2 O 5 and LIL elements and low CaO at a given SiO 2 level compared to metamorphic charnockites, and I-, S- and A-type granites. They form a distinctive type of intrusive igneous rocks, the Charnockite Magma Type (CMT or C-type), which generally lack hornblende and consist of pyroxene, alkali feldspar, plagioclase, quartz, biotite, apatite, ilmenite and titanomagnetite. Although this mineral assemblage superficially resembles that of metamorphic charnockites, magmatic charnockites are characterised by inverted pigeonite, exceptionally calcic alkali feldspar, potassic plagioclase, and coexisting opaque oxides, all with crystallisation temperatures of 950–1050°C. Apatite is a ubiquitous phase which, together with the very high concentrations of Zr and TiO 2 , over a wide silica range, is consistent with the derivation of the Charnockite Magma Type by very high temperature partial melting and fractionation. The credibility of intrusive charnockites as a magmatic type has historically foundered because of their apparent restriction to granulite belts and the absence of any reported extrusive equivalents. We report examples of volcanic rocks, of various ages, with the same distinctive major and trace element compositions, mineral assemblages and high temperatures of crystallisation as intrusive charnockites. The Charnockite Magma Type is considered to be derived by melting of a hornblende-free or poor, LILE-enriched fertile granulite source which had not been geochemically depleted by a previous partial melting event but which was dehydrated in an earlier metamorphism. Whereas H 2 O-saturated melting produces migmatites or “failed” granites, and vapour-absent melting of an amphibolite can produce I-type granites, according to this model the vapour-absent melting of a hornblende-free or hornblende-poor granulite at even higher temperatures produces charnockites.