Genesis of a synmagmatic charnockite associated with the Weinsberg granite, southern Bohemian Batholith, northern Austria

Abstract

: 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 neighbor-hood 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 ± clino - pyroxene 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 H 2 O-rich fluid with a CaCl 2 component. The magma responsible for the charnockite was dominated by a CO 2 -rich fluid with a minor NaCl component, which lowered the H 2 O 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 CO 2 -rich and H 2 O-rich fluids are immiscible in the presence of NaCl and CaCl 2 over the P–T range of the overall crust, the implication is that in granitoid melts, if CO 2 is present, there will be regions dominated by CO 2 and regions dominated by H 2 O. The extent of either region will be determined by the overall CO 2 /H 2 O ratio in the melt. In the CO 2 -dominated regions, the H 2 O 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 CO 2 present in the original magma goes above a certain fraction.