Petrography and Petrogenesis of Charnockite

Charnockite

  • Charnockites sensu lato (charnockite-enderbite series) are lower crustal felsic rocks typically characterised by the presence of anhydrous minerals including orthopyroxene and garnet.
  • They either represent dry (H2O-poor) felsic magmas that are emplaced in the lower crust or granitic intrusions that have been dehydrated during a subsequent granulite facies metamorphic event.
  • Post magmatic high-temperature recrystallisation may result in widespread metamorphic granulite microstructures, superimposed or replacing the magmatic microstructures.
  • They are either granitic rocks metamorphosed to the granulite facies (metamorphic charnockites) or rocks whose pyroxene crystallized directly from magma.
  • Charnockitic rocks (or series) usually show spatial association of rocks (charnockite– charnoenderbite– enderbite) differing in modal abundance of the dominant feldspar species.
  • For both magmatic charnockites and dehydrated granites, subsequent fluid-mineral interaction at intergrain boundaries during retrogradation are documented by microstructures including K-feldspar microveins and myrmekites.
  • So, the definition is “Orthopyroxene (or fayalite)-bearing granitic rock that is clearly of igneous origin or that is present as an orthogneiss within a granulite terrane.”

  • Charnockites have a common compositional range and can be broadly divided into intermediate charnockites and felsic charnockites, with most reported occurrences showing variation from intermediate to felsic compositions.
  • Intermediate charnockites are dominantly calc-alkalic (in terms of modified alkali lime index) and ferroan to magnesian (in terms of Fe-number), while felsic charnockites are dominantly alkali-calcic and ferroan.
  • The relatively iron-enriched nature of felsic charnockites imparts to them a predominantly tholeiitic affinity, in contrast to the predominantly calcalkaline affinity of the intermediate charnockites.

Metamorphic vs. igneous charnockites

  • Earlier thought that charnockites had to be granitic rocks dehydrated during metamorphism.
  • Alternatively, charnockites can also represent dry granitic melts in which orthopyroxene has crystallised from the magma.
  • Mostly, emplacement of charnockitic magmas occurs in a lower crustal (granulite facies) environment in which the charnockitic magmas and the metamorphic host rock (in particular ultrahigh-temperature granulites) are in (near) thermal equilibrium.
  • This process prohibits widespread contact metamorphism except in rare cases in which charnockites are emplaced in amphibolite-facies rocks causing high-temperature, low-pressure contact metamorphism.
  • The general absence of a temperature difference between intruding charnockitic magmas and its surroundings in the lower crusts is the prime cause of the ambiguities between a magmatic and metamorphic origin of charnockites.
  • Recrystallisation at granulite facies conditions may result in the (partial) overprinting of igneous microstructures if charnockites are emplaced in the lower crust.

Microstructures in igneous charnockites

  • Microstructures in igneous charnockites include the following: (i) magmatic mineral textures that have been preserved during subsequent recrystallisation, (ii) melt remnants, which are proof of the existence of a melt phase and thus magmatic origin of a rock and (iii) post-magmatic high-temperature metasomatic features, typically occurring along mineral intergrain boundaries.

Igneous mineral textures

  • Igneous microstructures in charnockites include a porphyritic texture, characterised by euhedral to subhedral pyroxene and feldspar.
  • Feldspar may show Carlsbad twinning and compositional zoning in the case of plagioclase.
  • Charnockitic augen gneiss in which the augen represent K-feldspar phenocrysts are an example of charnockites showing such a porphyritic texture.
  • Other igneous microstructures include myrmekitic intergrowths of biotite-quartz ± plagioclase. This texture can be evidence for the presence of a melt phase.
  • The accessory minerals apatite and zircon are sometimes assumed to indicate a magmatic origin, especially if they are euhedral or contain melt inclusions. However, they may occur in magmatic charnockites as well as in granites, which means that they can be of little use to distinguish between magmatic charnockites and dehydrated granites.

Melt remnants

  • Melt remnants are a clear indication of the igneous origin of charnockites and can be in the form of melt (or melt derived) inclusions or typical melt-related microstructures.

Melt inclusions: Melt inclusions in lower crustal rocks are not likely to be preserved due to chemical re-equilibration and recrystallisation during (slow) cooling unless cooling has been interrupted.

Melt-related microstructures: Traces of former melts may also be obtained from melt remnants in pore spaces at the intersection of three or more mineral grains.

  • These remnants occur as a single mineral phase. A major difference with melt inclusions is that, in this case, the composition of the melt patch is monomineralic and not comparable to a melt composition. i.e., the mono-mineralic phases represent the final melt pockets after the other mineral phases have already been crystallised, which is referred to as sequential crystallisation.
  • Other possible evidence for melt remnants includes the occurrence of isolated, rounded zircons in antiperthite patches within plagioclase phenocrysts in igneous charnockites.

Post-magmatic high-temperature metasomatism:

  • Metasomatic features give evidence of percolation of an interstitial low-H2O activity fluid phase in the form of high salinity brines.
  • Traces of brines are indeed found in many lower crustal rocks in the form of fluid inclusions, together with pure CO2 fluid inclusions.
  • High-temperature metasomatic features, i.e. myrmekites and K-feldspar microveins, are quite common in charnockites and identical to those described in granulite facies metamorphic rocks.
  • These metasomatic features are found in both igneous and metamorphic charnockites and as such are not distinctive of either one of them.
  • Myrmekites (vermicular intergrowths of quartz with Ca-rich plagioclase along K-feldspar grain boundaries) typically occur along the margins of large K-feldspar phenocrysts in the charnockitic augen gneisses.
  • Another typical metasomatic feature is the presence of K-feldspar veining along the grain boundaries of minerals such as quartz, biotite, and plagioclase.
  • For igneous charnockites, the metasomatic features are restricted to the charnockite body. This indicates that the low-H2O activity fluids were contained in the magma and not derived from the immediate surroundings.

Metamorphic microstructures in charnockites: Metamorphic recrystallisation, either in charnockites emplaced at granulite conditions or in dehydrated granitic rocks, results in a typical granoblastic granulite texture.

  • Dynamic recrystallisation may also result in the development of elongated quartz crystals or ribbons, sometimes of extraordinary size. Recrystallisation may result in the (partial) disappearance of solid, fluid and/or melt inclusions.
  • Some of the solid inclusions such as zircons may be found as isolated crystals in the groundmass implying that fluid induced dissolution and reprecipitation during solid-state recrystallisation occurred.
  • This fluid is most likely an alkali-bearing aqueous solution. The occurrence of supposed minerals of metamorphic origin, in particular garnet and also other Al-rich mineral phases such as cordierite, is sometimes used as an indication for a metamorphic origin of the charnockite.
  • A metamorphic origin for charnockite can only be inferred if textural evidence is found for the crystallisation order of orthopyroxene relative to biotite/amphibole. The formation of orthopyroxene is according to the reactions Bt + Qtz → Opx + Kfs + H2O and Hbl + Qtz → Opx + Cpx + Pl + Kfs + H2O

Controls on formation of Charnockites:

  • The occurrence of pyroxene in granitic rocks is primarily controlled by composition of coexisting fluid phase.
  • If the fluid phase has a low water activity, Opx forms in a granitoid melt.
  • Three additional factors favour the crystallization and formation of Opx.
    • High Fe/Fe+Mg ratio in the melt.
    • Presence of mafic melts as a source of CO2 (thus lower water activity).
    • Deep crustal levels favour crystallization of pyroxene bearing granites.
  • Indian Occurrences: Pallavaram massif, Nilgiri Hill, Cardamom hill.