Contrasting mafic to felsic HP-HT granulites of the Blanský les Massif (Moldanubian Zone of southern Bohemia): complexity of mineral assemblages and metamorphic reactions

Abstract

*Corr esponding author A detailed petrological and geochemical study of the Blanský les Granulite Massif (BLGM; Moldanubian Zone of the Bohemian Massif) provides evidence for processes and mineral assemblages previously unrecognized, or not fully appreciated, in the South Bohemian HP–HT Granulite Complex. It also underlines the “patchwork”-like nature of the BLGM, caused by distinct protolith compositions as well as the differences in occurrence, or preservation, of mineral assemblages, reaction textures, and/or variable changes in chemical compositions. Such a complexity reflects super position of several processes operating from the eclogite-facies metamorphic peak to the final emplacement, as well as variable resistance of individual rock types with distinct metamorphic fabrics. Even on a thin section scale, domains preserving evidence for early phases of eclogite-facies metamorphism, including original textural relations and largely intact HP–HT mineral assemblages, occur in close proximity to domains dominated by phases produced during later decompression. Consistent high-pressure estimates, 2.2–2.3 GPa (GASP), are newly obtained for S 1 /S 2 fabrics in the prevailing (type 1) felsic granulites, and correlate well with the recent finds of rare omphacite relics in the mafic granulites. It indicates that much of the HP history of the felsic and mafic rock types was shared and took place under eclogite-facies conditions. Three important metamorphic processes leading to chemical changes in felsic granulites are described. (1) The small bodies of Kfs-dominated hyperpotassic granulites with minor Grt (or, rarely, Di) are fairly common and provide direct evidence for operation of non-eutectic HP melting. These occurrences are accompanied by compositionally complementary residual (type 2) felsic granulites, rich in Qtz and Ky and partly depleted in alkalis with Zr. Owing to their Ca-poor composition, type 2 granulites were not involved in some of the decompression reactions, such as formation of plagioclase rims around kyanite and garnet breakdown, and thus their mineral assemblages remained effectively “frozen” during decompression. They further imply that the partial melting and genesis of the hyperpotassic melts took place under eclogite-facies conditions. (2) Another case of syn-metamorphic modification is represented by a local limited base-cation loss during superimposed amphibolite-facies de-alkalization, accompanying fluid flux along S 3 shear domains (formation of feldspar-free Qtz–Sill bands). (3) On a microscale, the retrogression reactions, often incomplete, were clearly associated with transfer of ions. The felsic Moldanubian HP–HT granulites were assumed by several authors to have undergone only limited modification during peak metamorphism, most notably loss of Cs, Th and U. Clearly, some of them may have suffered more significant changes, such as perceptible loss of alkalis or even relatively immobile elements (e.g., zirconium), compared to the likely primary protolith(s) composition.