The Weinsberg granite, a coarse-grained biotite granite with abundant K-feldspar megacrystals, is the volu­metrically dominant and most characteristic granite type of the late-Variscan Moldanubian Batholith in the Moldanubian zone of the Bohemian Massif. In the western batholith area, a local orthopyroxene-bearing variant (charnockite) of the Weinsberg granite has been identified and given the name of the Sarleinsbach quartz-monzodiorite in previous studies. Whole rock analysis of the charnockite and the relatively mafic Weinsberg granite in the immediate neighborhood show no significant geochemical differences with respect to either the major or trace elements. The mineralogy and petrology of the charnockite and surrounding granite are the same except for the presence of orthopyroxene ± clinopyroxene in the charnockite. In addition, the charnockite is characterized by the presence of dark grey, glassy orthoclase megacrysts with only some partial conversion to microcline, whereas in the granite the K-feldspar megacrysts consist of white microcline. The Fe–Mg silicates in the charnockite (orthopyroxene, clinopyroxene, amphibole, and biotite) are relatively Fe-rich (X_Fe = 0.6–0.7) whereas the plagioclase is more albitic (X_Ab = 0.6) than anorthitic. Fluid inclusions from the granite and associated charnockite are investigated and the results compared. The basic conclusion is that the magma responsible for the granite was dominated by an H₂O-rich fluid with a CaCl₂ component. The magma responsible for the charnockite was dominated by a CO₂-rich fluid with a minor NaCl component, which lowered the H₂O activity sufficiently below 1 such that orthopyroxene ± clinopyroxene was the stable Fe–Mg silicate phase during crystallization as opposed to the biotite in the granite. Taking into account that CO₂-rich and H₂O-rich fluids are immiscible in the presence of NaCl and CaCl₂ over the P–T range of the overall crust, the implication is that in granitoid melts, if CO₂ is present, there will be regions dominated by CO₂ and regions dominated by H₂O. The extent of either region will be determined by the overall CO₂/H₂O ratio in the melt. In the CO₂-dominated regions, the H₂O activity could be sufficiently lowered such that orthopyroxene is the stable Fe–Mg silicate phase during crystallization, though this will also be dependent on the Fe/Mg ratio of these phases and the overall whole rock chemistry of the melt. In addition to incipient solid state charnockitization, commonly seen in the Archean terranes of southern India and elsewhere, this suggests that a certain subset of granites and granitoids worldwide should have patches and/or limited areas of charnockite if the amount of CO₂ present in the original magma goes above a certain fraction.