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

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 superposition 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.

Chronology, petrogenesis and heat sources for successive Carboniferous magmatic events in the Southern–Central Variscan Vosges Mts (NE France)

“…Becker et al (1999) and Janoušek & Holub (2007) suggested that the very common spatial and temporal link of the Moldanubian Mg-K magmatism with HP-HT granulites might indicate that the magmas are the result of partial melting of a lithospheric mantle contaminated by, and/or metasomatized with fluids or small-scale melts derived from, a mature crustal material. This can be explained in the context of an Andean-type subduction, passing into underplating of the Moldanubian Domain by mostly felsic metaigneous rocks of Saxothuringian affinity, their relamination at the base of the autochthonous Moldanubian crust, and the contamination of the interlayered lithospheric mantle (Janoušek et al 2004b;Janoušek & Holub 2007;Schulmann et al 2009bSchulmann et al , 2014.The degree of non-eutectic, HP-HT anatexis in the underplated refractory felsic metaigneous lithologies might not have been extensive, and the resulting felsic HP granulites are indeed mostly melt-undepleted (Janoušek et al 2004b;Vrána et al 2013). These 'dirty' mantle domains were thought to have melted by heat advected from the rising asthenospheric mantle, shortly after a slab breakoff (Janoušek & Holub 2007) or produced in situ by radioactive decay in the relaminated felsic crust .…”

Ca-rich garnets and associated symplectites in mafic peraluminous granulites from the Gföhl Nappe System, Austria

“…The P-T estimates in Table 1 are based on three approaches. The first approach used conventional continuousreaction thermobarometry involving integral ternary feldspar compositions from perthite and antiperthite, Na-bearing clinopyroxene and the Ca content in garnet (O'Brien, 2008;Vrána et al, 2013, and references therein). The calculated P-T conditions are about 1000 • C and pressure of 15-20 kbar, occasionally more.…”

Geochemical variability of granite dykes and small intrusions at the margin of the Granulite Complex in southern Bohemia

“…Granulites carrying HP/HT record of metamorphism at the base of a thickened continental crust (P = 2.1-2.3 GPa, T = 950-1050 °C), were formed from felsic quartz-feldspathic rocks corresponding largely to granites (Fiala et al 1987;Janoušek et al 2004a;Vrána et al 2013) at 341 to 339 Ma (Aftalion et al 1989;Wendt et al 1994;Kröner et al 2000;Svojtka et al 2002;Sláma et al 2007Sláma et al , 2008O'Brien 2008). The granulite complex was rapidly uplifted to the level of the mid-continental crust, with resulting metamorphic re-equilibration.…”