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.

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