Ultrahigh-Temperature Anatexis of Greywackic Granulites Generates Peraluminous Charnockites in the Jining Complex, North China Craton

Abstract

Abstract Charnockites form an important component of the lower continental crust. Quantitative investigation on the properties of magmas that can stabilize orthopyroxene at solidi is crucial to understanding the petrogenesis of igneous charnockites. This study has performed detailed petrologic analyses and thermodynamic constraints on the Paleoproterozoic high-maficity (FeOT + MgO = 5–14 wt%) and peraluminous charnockites from the Jining Complex in the North China Craton. These charnockites occur as intrusions in granulite facies terranes and contain the mineral assemblages including prevalent perthitic/antiperthitic feldspars and garnet with minor biotite (usually less than ~5 vol%) beside orthopyroxene. The Jining charnockitic magmas were ascertained to have ultrahigh temperatures up to 1050–1100°C, poor H2O contents around 0.14–0.42 wt% and limited aluminum saturation indexes (ASIs) of 1.0–1.3. The stabilization of orthopyroxene at the solidus is attributed to low magmatic H2O contents and ASIs, which have maxima of 1.2 wt% and 1.5, respectively, and are positively correlated to bulk-rock maficity. Such charnockitic magmas could not release H2O-rich fluids near solidi, as the H2O is buffered by the orthopyroxene–biotite pair. Moreover, combined geochemical discrimination and progressive melting modelling reveal that the Jining charnockites were generated by partial melting of a greywackic granulite source, with about 15%–40% entrainment of solid phases in mushy magmas. The melting occurs at temperatures as high as 1050–1100°C, obviously beyond biotite stabilities, and involves quartz, feldspars and garnet as melting reactants, which differs from the previous proposition that peraluminous charnockites are related to biotite dehydration melting. The resultant magmas are substantially enriched in maficity and depleted in H2O due to both the melt compositions per se and the high entraining capability. Such peraluminous charnockite plutons massively emplaced in granulite facies terranes indicate post-orogenic ultrahigh-temperature anatexis of metasedimentary rocks in conditions close to the crustal dry solidus.