Hyperpotassic granulites from the Blanský les Massif (Moldanubian Zone, Bohemian Massif) revisited

Abstract

In the Blanský les granulite Massif (Moldanubian Unit, South Bohemia), built mainly by felsic calc-alkaline HP-HT garnet ± kyanite granulite, occur rare small bodies of hyperpotassic granulite with garnet (Plešovice type) or pyroxene (Lhotka type). The Plešovice type is dominated by slightly perthitic K-feldspar and almandine-pyrope rich garnet, the latter variously retrogressed to biotite ± plagioclase. The other conspicuous euhedral crystals are apatite and zircon. In the rock occur also Zr-Nb-rich rutile and much rarer primary monazite; originally LREE-rich apatites decompose into small Th-poor, MREE-rich monazite grains. The Lhotka type granulites contain pyroxene, often altered to actinolite, instead of garnet. The predominant perthitic K-feldspar encloses a small amount of unmixed celsian. Typical are large euhedral crystals of apatite with small unmixed monazite grains; primary monazite is rare. Noteworthy is the occurrence of secondary Ti phases with high REE, Y and Zr, formed at the expense of the primary pyroxene. The studied granulite types are highly potassic (K2O > 7 %, up to c. 14 %, K2O/Na2O = 3.1-9.2 wt. %), silica-poor (SiO2 < 65 %), with low contents of most major- and minor-element oxides, apart from K and P. Characteristic are high concentrations of Cs, Rb, Ba and U at variable enrichments in Zr and Hf. Whole-rock contents some HFSE (Ti, Nb and Ta) are extremely low. The REE patterns show marked negative Eu anomalies and variable LREE enrichments increasing with rising silica due to a conspicuous drop in HREE. The Sr-Nd isotopic ratios document the derivation from mature crustal sources (εNd337 - -5.5; 87Sr/86Sr337 = 0.7272 and 0.7279 for the Plešovice and 87Sr/86Sr337 = 0.7109 for the Lhotka types). Apatite saturation temperatures are high (∼1070 °C), close to the previously estimated conditions of (re)crystallization for both calc-alkaline and hyperpotassic granulites. The zircon saturation temperatures tend to be more variable, some exceeding 1000 °C but many unrealistically low, reflecting effects of disequilibrium melting, heterogeneous distribution of accumulated crystals in the magma and/or sampling bias. The hyperpotassic granulites are interpreted as Viséan igneous rocks. The parental magma could have originated by low degrees of HP-HT, non-eutectic partial melting. The low-scale melt was most likely expelled close to the HP-HT metamorphic peak or at early stages of decompression from common Moldanubian calc-alkaline granulites. This genetic link is documented by the presumed P-T conditions, similar age and complementary geochemical and Sr-Nd isotopic signatures. Subsequently the magma could have developed by garnet, apatite and zircon dominated fractional crystallization, with or without some crystal accumulation.