Petrography and Petrogenesis of Syenites

Syenites

  • Syenites are plutonic rocks composed essentially of alkali feldspars. They occupy the fields 6 and 7 in OAP diagram.
  • May contain a small amount of plagioclase (ranging up to 35 per cent) and quartz (less than 5 per cent) when Q+A+P=100
  • With little more oversaturation in silica. they grade to quartz syenites in which quartz ranges from 5 to 20 per cent. Fold bearing syenites contain feldspathoids but limited to less than ten per cent.
  • Syenites which are totally devoid of plagioclase, are termed alkali feldspar syenites and may be considered as plutonic equivalent of trachytes.
  • Syenites contain a variety of mafic minerals but they are normally hornblende and biotite.
  • Fe-rich olivine and pyroxene occur in subalkaline syenite e.g. ferrosyenite.
  • Peralkaline syenites (containing either quartz or feldspathoids or none) contain sodic pyroxene, e.g., aegirine or aegirine augite.
  • Syenites are medium to coarse grained hypidiomorphic granular rocks and are less commonly porphyritic.
  • The mafic minerals are usually subhedral, occasionally euhedral and alkali-feldspars are often tabular or equidimensional.
  • In porphyritic variety the tabular alkali feldspar grains may show preferred dimensional orientation to define directive (trachytoid) texture whereas the groundmass material comprises a mosaic of smaller subhedral/anhedral alkali feldspar and minor plagioclase (oligoclase).
  • Syenites can also be grouped as hypersolvus and subsolvus based on the feldspars.
  • Syenites that crystallize under low pressure and exhibit a single perthitic alkali feldspar are considered hypersolvus syenites. They subsequently produce perthite, mesoperthite and antiperthite.
  • Syenites that crystallize under high pressure conditions, exhibit 2 alkali feldspars are referred to as subsolvus syenites. They contain both microcline/orthoclase and plagioclase.
  • Based on colour index, syenites can be described as lusitanites (high colour index) or melanofelsic syenites (having mafic minerals within normal range but dark grey/greenish grey perthite feldspar).
  • Syenites normally do not form large independent intrusions or plutons but usually occur as small subvolcanic plugs or minor intrusions associated with larger bodies of different compositions.
  • Normal syenite (orthosyenite) an ideal silica saturated rock and can form in association with both silica oversaturated or undersaturated series.
  • Syenites therefore may occur as local variants, in granitic plutons as also in peralkaline foidsyenite intrusions.
  • Fenites are syenitic in composition but have different geological setting as they do not form igneous association.
  • Fenites are syenite looking rocks containing-alkali feldspar, aegirine- and sodic amphiboles-with or without quartz.
  • They are often formed by alkali metasomatism or fenitisation of granitic country rocks along contact zone of alkaline carbonatite intrusion.
  • Trachytes are volcanic and hypabyssal equivalents of syenites and like syenites they show a range in degree of saturation in respect to silica as also variation in Na/K ratio.
  • They grade in composition from oversaturated quartz trachytes to feldspathoidal trachytes or phonolites. Normal trachytes can be distinguished on TAS diagram.
  • 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.
  • 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 are interpreted as being product of the fractional crystallization of basaltic magma.
  • The removal of 17% of plagioclase, 16% of pyroxene and 2% of Ilmenite from this parental magma would produce a Syenitic magma.
  • Indian occurrence: Described in eastern ghats belts and Deccan volcanic province.