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.
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.
- In North America syenite occurs in Arkansas and Montana. Regions in New England have sizable amounts, and in New York syenite gneisses occur.
- In Europe syenite may be found in parts of Switzerland, Germany, Norway, in Plovdiv, Bulgaria and in Ditrău, Romania. In the British Isles the Malvern Hills, which are on the border between the English counties of Herefordshire and Worcestershire, are largely syenite.
- In Africa there are syenite formations in Aswan, Egypt, and in Malawi in the Mulanje Mountain Forest Reserve.
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.
- E. Wm. Heinrich. Microscopic Petrography, McGraw-Hill, 1956
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