Syenite is a coarse-grained intrusive igneous rock of somewhat the same general composition as granite but with the quartz either absent or present in relatively small amounts (<5%). Some syenites contain larger degree of mafic components and are characterized as intermediate composition.
Syenites are formed from 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.
In certain conditions, large volumes of anorthite crystals may precipitate from a magmatic melt in a cumulate process, drastically reducing the silica content, thereby conducive to later syenite formation.
Syenite is not a common rock, some of the more important occurrences being in New England, Arkansas, Montana, New York (syenite gneisses), Switzerland, Germany, Norway, Plovdiv, Bulgaria, Malawi (Mulanje Mountain Forest Reserve) and Romania (Ditrău). The Malvern Hills, which are on the border between the English counties of Herefordshire and Worcestershire are also formed from syenite.
The process which results in SiO2 depletion can be termed episyenitization. This process is only referring to the macroscopic result of relative SiO2 depletion in a rock. The actual physical process leading to this SiO2 depletion may vary in a given metamorphic environment. Diffusion of chemical components in a stagnant fluid, related to differences in chemical potential or pressure as well as advection of a SiO2- undersaturated fluid may lead to the dissolution of quartz from the un-altered rock, thus depleting it of this component.
- E. Wm. Heinrich. Microscopic Petrography, McGraw-Hill, 1956
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