Карбонатит тас құрамы және геологиялық ерекшеліктері

Igneous rock with more than 50% carbonate minerals

Carbonatite (/k//ɑr/'/b//ɒ//n//ə/,/t//aɪ//t/) is a type of intrusive or extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. 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, being composed largely of soluble carbonates, are easily weathered and are therefore unlikely to be preserved in the geologic record. Carbonatite eruptions as lava may therefore not be as uncommon as thought, but they have been poorly preserved throughout the Earth's history. Carbonatite liquid compositions are significantly more alkaline than what is preserved in the fossil carbonatite rock record as composition of the melt inclusions shows. Only one carbonatite volcano is known to have erupted in historical time, the active Ol Doinyo Lengai volcano in Tanzania. It erupts with the coolest lava in the world, at The lava is natrocarbonatite dominated by nyerereite and gregoryite.

Primary mineralogy is highly variable, but may include natrolite, sodalite, apatite, magnetite, barite, fluorite, ancylite group minerals, and other rare minerals not found in more common igneous rocks. Recognition of carbonatites may be difficult, especially as their mineralogy and texture may not differ much from marble except the presence of igneous minerals. They may also be sources of mica or vermiculite. Carbonatites are classed as calcitic sovite (coarse textured) and alvikite (finer textured) varieties or facies. The two are also distinguished by minor and trace element composition. The terms rauhaugite and beforsite refer to dolomite and ankerite rich occurrences respectively. The alkali carbonatites are termed lengaite. Examples with 50–70% carbonate minerals are termed silico carbonatites.

Carbonatites are unusual igneous rocks composed predominantly of carbonate minerals. Most carbonatites tend to include some silicate mineral fraction; by definition an igneous rock containing >50% carbonate minerals is classified as a carbonatite. Silicate minerals associated with such compositions are pyroxene, olivine, and silica undersaturated minerals such as nepheline and other feldspathoids. Geochemically, carbonatites are dominated by incompatible elements (Ba, Cs, Rb) and depletions in compatible elements (Hf, Zr, Ti). This together with their silica undersaturated composition supports inferences that carbonatites are formed by low degrees of partial melting. A specific type of hydrothermal alteration termed fenitization is typically associated with carbonatite intrusions. This alteration assemblage produces a unique rock mineralogy termed a fenite after its type locality, the Fen Complex in Norway. The alteration consists of metasomatic halos consisting of sodium rich silicates arfvedsonite, barkevikite and glaucophane along with phosphates, hematite and other iron and titanium oxides.

Carbonatites may contain economic or anomalous concentrations of rare earth elements, phosphorus, niobium–tantalum, uranium, thorium, copper, iron, titanium, vanadium, barium, fluorine, zirconium, and other rare or incompatible elements. Apatite, barite and vermiculite are among the industrially important minerals associated with some carbonatites. The largest REE carbonatite deposits are Bayan Obo, Mountain Pass, Maoniuping, and Mount Weld. Vein deposits of thorium, fluorite, or rare earth elements may be associated with carbonatites and may be hosted internal to or within the metasomatized aureole of a carbonatite. As an example, the Palabora complex of South Africa has produced significant copper (as chalcopyrite, bornite and chalcocite), apatite, vermiculate along with lesser magnetite, linnaeite (cobalt), baddeleyite (zirconium–hafnium), and by product gold, silver, nickel and platinum.