Alkali fedspar syenite

Syenite, from Latin "Lapis Syenitis" (lapis = stone), from Syene (an ancient city of southern Egypt), is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts (<5%). The feldspar component of syenite is predominantly alkaline in character (usually orthoclase). Plagioclase feldspars may be present in small quantities, less than 10%. Syenites are usually either peralkaline with high proportions of alkali elements relative to aluminum, or peraluminous with a higher concentration of aluminum relative to alkali elements (K, Na, Ca).

Syenites usually occur as relatively small independent intrusions or more commons as satellite bodies, related to larger intrusions with different overall compositions. In many areas Syenites are comagmatic with Granitic intrusions. As these Syenites tend to form marginal igneous facies to much larger granitic bodies, the former are often interpreted as having evolved from the latter. This poses problems, because if a Syenite is to evolve from a granite, significant amounts of SiO₂ have to removed; and significant amounts of MgO, total Fe, MnO and TiO₂ and also CaO and Na₂O have to be added.

Such changes in chemical composition may be locally accomplished by the assimilation of mafic/or carbonate rocks, and the escape of volatiles containing dissolved silica. Many Syenite, however, are interpreted as being product of the fractional crystallization of basaltic magma. Chapman and Williams (1935) demonstrated that the removal of 53% of Plagioclase, 10% of Pyroxene, 10% of Olivine and 4.5% of Ilmenite form the parental basaltic magma would produce a monzonitic magma; and the removal of 17% of plagioclase, 16% of pyroxene and 2% of Ilmenite from this parental magma would produce a Syenitic magma. This fractional crystallization process require the removal of high portion of plagioclase, and this may help explain the close association of Syenites and Anorhosites.

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Alkali fedspar syenite: Alkali fedspar (red) e pyroxene (dark). From Sand atlas

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Hornblende and titanite in a alkali fedspar syenite. PPL image. 2x (Field of view = 7mm)	Hornblende and titanite in a alkali fedspar syenite. XPL image. 2x (Field of view = 7mm)	Hornblende and titanite in a alkali fedspar syenite. PPL image. 2x (Field of view = 7mm)
Hornblende and titanite in a alkali fedspar syenite. XPL image. 2x (Field of view = 7mm)	orthoclase in a alkali fedspar syenite. XPL image. 2x (Field of view = 7mm)	Hornblende in a alkali fedspar syenite. PPL image. 2x (Field of view = 7mm)
Hornblende and orthoclase in a alkali fedspar syenite. XPL image. 2x (Field of view = 7mm)	orthoclase in a alkali fedspar syenite. XPL image. 10x (Field of view = 2mm)	Hornblende in a alkali fedspar syenite. PPL image. 2x (Field of view = 7mm)
Hornblende and orthoclase in a alkali fedspar syenite. XPL image. 2x (Field of view = 7mm)	Hornblende and orthoclase in a alkali fedspar syenite. XPL image. 2x (Field of view = 7mm)	Hornblende in a Alkali Fedspar Syenite. PPL image. 2x (Field of view = 7mm)
Hornblende and orthoclase in a alkali fedspar syenite. XPL image. 2x (Field of view = 7mm)