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Concept
Syenite
Summary
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%). It is considered a granitoid. 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.
The extrusive equivalent of syenite is trachyte.
The feldspar component of syenite is predominantly alkaline in character (usually orthoclase). Plagioclase feldspars may be present in small proportions, between 10% and 35% of the feldspar content. Such feldspars often are interleaved as perthitic components of the rock.
When ferromagnesian minerals are present in syenite at all, they usually occur in the form of amphibole (typically hornblende) and clinopyroxene. Biotite is rare, because in a syenite magma the formation of feldspar consumes nearly all the aluminium. However less Al-rich phyllosilicates may be included, such as annite.
Other common accessory minerals are apatite, titanite, zircon and other opaques.
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 (predominantly K and Na) and earth-alkali (predominantly Ca) elements.
Syenites are products of alkaline igneous activity, generally formed in thick continental crustal areas, or in Cordilleran subduction zones. The formation of syenites can be theorized to be from the melt of 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.
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