| Igneous rock | |
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| Composition | |
|---|---|
| Classification | Felsic |
| Plagioclase and K-Feldspars, depleted in quartz | |
| Texture | Phaneritic |
| Equivalents | Trachyte extrusively |
Syenite is a coarse-grainedintrusiveigneous rock with a general composition similar to that ofgranite, but deficient inquartz, which, if present at all, occurs in relatively small concentrations (< 5%). It is considered agranitoid. Some syenites contain larger proportions ofmafic components and smaller amounts offelsic material than most granites; those are classed as being ofintermediate composition.
Theextrusive equivalent of syenite istrachyte.[1]
Thefeldspar component of syenite is predominantlyalkaline in character (usuallyorthoclase).Plagioclase feldspars may be present in small proportions, between 10% and 35% of the feldspar content. Such feldspars often are interleaved asperthitic components of the rock.
Whenferromagnesian minerals are present in syenite at all, they usually occur in the form ofamphibole (typicallyhornblende) andclinopyroxene.Biotite is rare, because in a syenite magma the formation of feldspar consumes nearly all the aluminium. However less Al-richphyllosilicates may be included, such asannite.
Other common accessory minerals areapatite,titanite,zircon and other opaques.
Most syenites are eitherperalkaline with high proportions of alkali elements relative to aluminum, orperaluminous with a higher concentration of aluminum relative toalkali (predominantly K and Na) andearth-alkali (predominantly Ca) elements.
Syenites are products of alkaline igneous activity, generally formed in thickcontinental crustal areas, or in Cordilleransubduction zones. The formation of syenites can be theorized to be from the melt of granitic or igneousprotolith to a fairly low degree ofpartial melting. This is required because potassium is an incompatible element and tends to enter a melt first, whereas higher degrees of partial melting will liberate more calcium and sodium, which produceplagioclase, and hence agranite, adamellite ortonalite.
At very low degrees of partial melting asilica undersaturated melt is produced, forming anepheline syenite, where orthoclase is replaced by afeldspathoid such asleucite,nepheline oranalcime.
Conversely in certain conditions, large volumes ofanorthite crystals may precipitate from thoroughly molten magma in acumulate process as it cools. This leaves a drastically reduced concentration of silica in the remainder of the melt. The segregation of the silica from the melt leaves it in a state that may favour syenite formation.[citation needed]
Some syenites are also theorized to be the product of thefractional crystallization of basaltic magmas.[2]
Syenite is not a common rock. Regions where it occurs in significant quantities include the following.
The termsyenite was originally applied to hornblende granite like that ofSyene (now Aswan) inEgypt, from which the name is derived.
Episyenite (orepi-syenite) is a term used inpetrology to describe veins, pods, or lenses of rock originally rich insilicon dioxide (SiO2) from which quartz has been severely depleted.[9] This is often accompanied by strong enrichment in potassium andrare earth elements, leaving the altered rock a distinctive brick red color,[10] or by albitization (enrichment in sodium), leaving the altered rock a conspicuous white color.[11]
Episyenites are heterogenous in their properties, but all have experienced nearly complete disappearance of quartz at sub-solidus temperatures; that is, at temperatures below the melting point of the host rock. The formation of episyenites (episyenitization) typically takes place through leaching of quartz by mildly saline hydrothermal fluids, typically near a coolingintrusion. Because episyenitization usually takes place ingranitoid rock and usually involves alkalinemetasomatism (addition of alkali metal oxides to the rock) the result is a rock that has the mineral composition of an igneous syenite.[11]
In addition to rare earth elements,[10] episyenites may be important sources ofuranium[12] and other valuable metals.[11]
Notable occurrences of episyenite are found in theCentral Iberian Massif of Spain,[9] inCambrian toOrdovician beds of New Mexico and Colorado,[10] Scandinavia, Brazil, and Ukraine.[11]
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