Quartz-phlogopite-liquid equilibria and origins of charnockites

Published experimental data for quartz + K-feldspar + phlogopite + enstatite + liquid + vapor suggest that the liquid in the low-pressure invariant assemblage has less than 3 wt% H₂O, and that a thermal divide exists on the liquidus below 5 kbar. With addition of CO₂ to the system, the thermal divide persists to at least 15 kbar. The small but not negligible solubility of CO₂ in the silicate liquids apparently results in (1) a temperature range of 50100 degrees over which quartz + K-feldspar + phlogopite + enstatite can coexist with liquid in the absence of vapor and (2) the possibility that influx of CO₂ into a solid-phase assemblage such as quartz + K-feldspar + phlogopite can initiate partial melting. Subsequent segregation of restite from liquid could lead to the formation of chamockite with a CO₂-rich vapor and biotite granite with an H₂O-rich vapor.