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Syenite

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Syenite

Syenite
leucocratic variety of nepheline syenite from Sweden (särnaite)

Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (<5%). Some syenites contain larger proportions of mafic components and smaller amounts of felsic material than most granites; those are classed as being of intermediate composition.

Composition of syenites

The feldspar component of syenite is predominantly alkaline in character (usually orthoclase). Plagioclase feldspars may be present in small proportions, less than 10%. Such feldspars often are interleaved as perthetic components of the rock.

When ferromagnesian minerals are present in syenite at all, they usually occur in the form of hornblende, amphibole and clinopyroxene. Biotite is rare, because in a syenite magma the formation of feldspar consumes nearly all the aluminium.

Most syenites are either peralkaline with high proportions of alkali elements relative to aluminum, or peraluminous with a higher concentration of aluminum relative to alkali and earth-alkali elements (predominantly K, Na, Ca).

Formation of syenites

Syenites are products of alkaline igneous activity, generally formed in thick continental crustal areas, or in Cordilleran subduction zones. To produce a syenite, it is necessary to melt a granitic or igneous protolith to a fairly low degree of partial 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 produce plagioclase, and hence a granite, adamellite or tonalite.

At very low degrees of partial melting a silica undersaturated melt is produced, forming a nepheline syenite, where orthoclase is replaced by a feldspathoid such as leucite, nepheline or analcime.

Conversely in certain conditions, large volumes of anorthite crystals may precipitate from thoroughly molten magma in a cumulate 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.

Occurrence of syenites

Syenite is not a common rock. Regions where it occurs in significant quantities include the following.

Etymology

The term syenite was originally applied to hornblende granite like that of Syene (now Aswan) in Egypt, from which the name is derived.

Episyenite

Episyenite (or epi-syenite) is a term used in petrology to describe to the result of alteration of material rich in SiO2, to SiO2-depleted rock.

A process that results in SiO2 depletion often is termed episyenitization. The term refers only to the macroscopic effect of relative SiO2 depletion in a rock; it does not imply anything about the nature of the physical processes leading to the SiO2 depletion in any particular case, because many different processes in various metamorphic environments may lead to episyenitization. For example:

  • chemical components in a stagnant melt may diffuse under the influence of chemical potential gradients that cause their segregation from low- SiO2 components when the melt begins to solidify
  • a SiO2-undersaturated fluid may dissolve quartz from rock and remove it by advection, thus leaving the parent rock depleted of silica.
  • a marginally molten rock mass may retain its unmolten silica-rich components, while the molten, silica-depleted fluid cools to form a syenite.
  • on beginning to cool, a fully molten silica-rich melt might precipitate its silica-containing components, leaving the silica-depleted melt to form a syenite afterwards.

References

  • E. Wm. Heinrich. Microscopic Petrography, McGraw-Hill, 1956
  1. ^ Lieber, Oscar Montgomery (1856). Report on the Survey of South Carolina.  

See also

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