Hyperpotassic granulites from Blanský les (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 (K 2 O > 7 %, up to c. 14 %, K 2 O/Na 2 O = 3.1-9.2 wt. %), silica-poor (SiO 2 < 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 Sr/ 86 Sr 337 = 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.

Heat sources and trigger mechanisms of exhumation of HP granulites in Variscan orogenic root

“…This is particularly true for U and Th hosted by accessories such as zircon or monazite, which would resist fluid loss in course of the progressive heating. On the other hand, they should dissolve readily even at low degrees of melting, as the partial melt was likely to be rather corrosive, being hot and rich in alkalis with fluorine (Finger & Cooke, 2004; Janoušek et al. , 2007).…”

“…As in box plots, the median, 25th and 75th percentiles are marked with horizontal lines across the box. Compositions of the hyperpotassic Plešovice granulites (data from Janoušek et al. , 2007) (b) and Lower Austrian glimmerite veins (Becker et al.…”

“…A good candidate for complementary small‐volume, HP–HT (>900 °C) melt that managed to separate from the calc‐alkaline granulites are the rare hyperpotassic granulites from Plešovice, Blanský les Massif (Vrána, 1989; Janoušek et al. , 2007).…”

Geochemistry and petrology of high-pressure kyanite-garnet-albite-K-feldspar felsic gneisses and granulites from the Kutná Hora Complex, Bohemian Massif

“…Geochemical evidence shows that the most widespread felsic kyanite-garnet granulites of leucogranite-granite composition were dominantly derived from Lower Palaeozoic calc-alkali magmatic rocks (Janoušek et al 2004). Nonetheless, other compositional types of granulites, such as highly differentiated rocks, are also of interest since they carry information, which is not revealed by studying the most widespread calc-alkali granulite types (Vrána 1989;Janoušek et al 2006aJanoušek et al , 2007Kotková 2007; this study).…”

High (ultrahigh) pressure metamorphic terrane rocks as the source of the detrital garnets from the Middle Jurassic sands and sandstones of the Cracow Region (Cracow-Wieluń Upland, Poland)

“…Representative analyses of the garnets from various rocks (kimberlites, garnet peridotites, UHP eclogites, eclogites, HP granulites, LP granulites, retrograde eclogites), were taken from the following papers: garnets from HP granulites in the Góry Sowie Mts. (O'Brien et al 1997); garnets from peridotites, eclogites and granulites from the Bohemian Massif (Messiga and Bettini 1990;Nakamura et al 2004;Seifert and Vrána 2005;Vrána et al 2005;Medaris et al 2006a,b;Janoušek et al 2006Janoušek et al , 2007Racek et al 2008); garnets from HP and UHP eclogites and garnet peridotites from the Norway Western Gneiss Region (Krogh Ravna and Terry 2004); garnets from kimberlites (Schulze 1997); garnets from eclogites with inclusions of diamond (Schulze 1997); garnets from HP granulites, from UHP eclogites with inclusions of coesite and from garnet peridotites from the Saxonian Erzgebirge and Granulitgebirge (Massonne and Bautsch 2004). The resulting triangular diagram was divided to three sectors -A, B, C, with the latter subdivided into the sectors C1 and C2 (see Text-figs 6, 7, white fields): (A) garnets from high pressure (HP) and ultrahigh pressure (UHP) conditions; (B) garnets from HP eclogite and HP granulite facies conditions; and (C) garnets from amphibolite facies conditions: Subsector C1 represents a transitional subgroup metamorphosed under transitional P-T conditions between the granulite and amphibolite facies conditions and Subsector C2 represents a subgroup of amphibolite facies conditions.…”