The two are also distinguished by minor and trace element composition. The alkali-carbonatites are termed lengaite. Natrocarbonatite is made up largely of two minerals, nyerereite named after Julius Nyerere , the first president of independent Tanzania and gregoryite named after John Walter Gregory , one of the first geologists to study the East African Rift and author of the book The Great Rift Valley.
These minerals are both carbonates in which sodium and potassium are present in significant quantities. Both are anhydrous , and when they come into contact with the moisture in the atmosphere, they begin to react extremely quickly. The black or dark brown lava and ash erupted begins to turn white within a few hours. Carbonatite is composed predominantly of carbonate minerals and extremely unusual in its major element composition as compared to silicate igneous rocks, obviously because it is composed primarily of Na 2 O and CaO plus CO 2.
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.
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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.
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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. Associated igneous rocks typically include ijolite , melteigite , teschenite , lamprophyres , phonolite , foyaite , shonkinite , silica undersaturated foid-bearing pyroxenite essexite , and nepheline syenite. Carbonatites are typically associated with undersaturated low silica igneous rocks that are either alkali Na 2 O and K 2 O , ferric iron Fe 2 O 3 and zirconium -rich agpaitic rocks or alkali-poor, FeO-CaO-MgO-rich and zirconium-poor miaskitic rocks.
The Mount Weld carbonatite is unassociated with a belt or suite of alkaline igneous rocks, although calc-alkaline magmas are known in the region. The genesis of this Archaean carbonatite remains contentious as it is the sole example of an Archaean carbonatite in Australia.
Carbonatite is known to form in association with concentrically zoned complexes of alkaline-igneous rocks, the typical example of this being Phalaborwa, South Africa. Carbonatites take the form of sills, lopoliths and rare dikes are reported in the Guyana Shield. The Mud Tank and Mount Weld carbonatites take the form of multi-stage cylindrical intrusive bodies with several distinct phases of carbonatite intrusion.
Smaller carbonatite sills and dikes are present in other Proterozoic mobile belts in Australia, typically as dikes and discontinuous pods. In , the discovery of a new carbonatite deposit was confirmed north-west of Prince George, British Columbia , in a region termed the "Rocky Mountain Rare Metal Belt".
Lengai Reference Materialx
Other older carbonatite volcanoes are located in the same region, including Mount Homa. 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. 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, vermiculte along with lesser magnetite, linnaeite cobalt , baddeleyite zirconium—hafnium , and by-product gold , silver , nickel and platinum. Volcanic rocks : Subvolcanic rocks : Plutonic rocks :. Picrite basalt Peridotite. Basalt Diabase Dolerite Gabbro. Andesite Microdiorite Diorite. Dacite Microgranodiorite Granodiorite. Rhyolite Microgranite Granite.
From Wikipedia, the free encyclopedia. Oxysalt minerals, Can. In: Shcherbak, N. Petrogenetic relationship between melilitite and lamproite in the Roman Region: The lavas of S. Venanzo and Cupaello, Contrib. Vulture volcanic complex Italy : Evidence for distinct parental magmas and for residual melts with melilite, Miner. A commentary, Peccerillo A.
E, Green D. Immiscible melt inclusions in nepheline phenocrysts from nephelinite lavas, Oldoinyo Lengai: heating experiments and compositional constraints.
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