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Carbonatites are intrusive or extrusive igneous rocks defined by mineralogic composition consisting of greater than 50 percent carbonate minerals.〔Bell, Keith (editor) (1989) ''Carbonatites: Genesis and Evolution'', London, Unwin Hyman〕 Carbonatites may be confused with marble, and may require geochemical verification. Carbonatites usually occur as small plugs within zoned alkalic intrusive complexes, or as dikes, sills, breccias, and veins. They are, almost exclusively, associated with continental rift-related tectonic settings. It seems that there has been a steady increase in the carbonatitic igneous activity through the earth's history, from the Archean Eon to the present. Nearly all carbonatite occurrences are intrusives or subvolcanic intrusives. This is because carbonatite lava flows are unstable and react quickly in the atmosphere. Carbonatite lavas may not be as uncommon as thought, but have been poorly preserved throughout earth's history. Only one carbonatite volcano is known to have erupted in historical time, the active Ol Doinyo Lengai volcano in Tanzania. It erupts with the lowest temperature lava in the world, at 500-600 °C. The lava is natrocarbonatite dominated by nyerereite and gregoryite. ==Genesis== Carbonatites are ''rare'', ''peculiar'' igneous rocks formed by unusual processes and from unusual source rocks. Three models of their formation exist: # direct generation by very low degree partial melts in the mantle and melt differentiation # liquid immiscibility between a carbonate melt and a silicate melt # peculiar, extreme crystal fractionation Evidence for each process exists, but the key is that these are unusual phenomena. Historically, carbonatites were thought to form by melting of limestone or marble by intrusion of magma but geochemical and mineralogical data discount this. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「carbonatite」の詳細全文を読む スポンサード リンク
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