Partial melting studies at crustal pressures in SiO2-rich portions of the system KAlSiO4–Mg2SiO2–H2O–CO2 can be used to model the anatectic origin of charnockites. The univariant reaction
phlogopite + sanidine + quartz + vapor = enstatite + liquid

produces a SiO2-rich melt (granite analog) at 3 kbar; the vapor composition at the solidus is buffered to high H2O-contents by the coexistence of phlogopite with its breakdown products. At higher pressures, 8 and 15 kbar, the fluid phase is buffered to higher CO2-contents and the melt composition becomes enriched in K2O and MgO (charnockite analog). Melting relations are controlled by the expansion of the quartz liquidus field relative to the enstatite and sanidine fields with increasing pressure. Partial melts generated at the base of the crust in the presence of a CO2-rich fluid will be of an alkaline nature and will crystallize enstatite at lower pressures.

CO2-saturated melting of similar SiO2-rich bulk compositions (phlogopite-absent) by the reaction
enstatie + sanidine + quartz + CO2=liquid

occurs at temperatures in excess of 1000°C to about 15 kbar. Liquid compositions show analogous trends, however, with increasing pressure, to those observed in the 5-component system as a consequence of the expansion of the quartz liquidus surface relative to the enstatite surface.

This partial melting model for charnockite genesis satisfies the constraints of observed charnockite mineralogies, P and T estimations for charnockite assemblages (5–12 kbar and 750°–1000°C), and reports of high-temperature CO2-rich fluid inclusions that are believed to approximate solidus vapor compositions (Ormaasen, 1977; Konnerup-Madsen, 1979). Also, observations of anatexis preceding granulite facies metamorphism are reconciled: Influx of CO2 vapor (to a melt-crystal system) in sufficient amount to exhaust the vapor phase buffer will result in H2O extraction from the melt and crystallization of the melt, although temperatures may continue to rise.

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