Miloš René

The Saxo-Danubian Granite Belt: magmatic response to post-collisional delamination of mantle lithosphere below the southwestern sector of the Bohemian Massif (Variscan orogen)

The Saxo-Danubian Granite Belt: magmatic response to post-collisional delamination of mantle lithosphere below the southwestern sector of the Bohemian Massif (Variscan orogen) On the basis of the synchronicity of geochronological data and the similarity of granite types, it is proposed that the mid-Carboniferous Fichtelgebirge/Erzgebirge Batholith in the Saxothuringian Zone of the central European Variscan Fold Belt and the South Bohemian Batholith in the Moldanubian Zone (including the intervening Oberpfalz and Bavarian Forest granite areas) belong to one coherent and cogenetic, ca. 400 km long plutonic megastructure. Unlike older (syn-collisional) plutonic structures in the Bohemian Massif, this Saxo-Danubian Granite Belt (nov. nom.) has developed discordant to the Devonian/Early Carboniferous collision-related tectonic architecture of the Bohemian Massif. It is argued that the Saxo-Danubian Granite Belt formed in response to a post-collisional detachment of lithospheric mantle below the south-western sector of the Bohemian Massif.

The Blaník Gneiss in the southern Bohemian Massif (Czech Republic): a rare rock composition among the early palaeozoic granites of Variscan Central Europe

Metamorphosed and deformed tourmaline-bearing leucogranites with a Cambro-Ordovician formation age are widespread in the Monotonous Group of the Variscan southern Bohemian Massif, Czech Republic. The rocks, known locally as Blaník gneiss, are strongly peraluminous and classify as phosphorus-rich low-T, S-type granite. The magma formed from a metapelitic source, most likely through muscovite dehydration melting. With respect to its low-T origin and the abundance of tourmaline, the Blaník gneiss is exotic within the spectrum of Early Palaeozoic granites of the Variscan fold belt of Central Europe. Coeval granitic gneisses in the neighbouring Gföhl unit of the Bohemian Massif can be classified as higher T S-type granites and were probably generated through biotite dehydration melting. The geochemical differences between the Early Palaeozoic granitic magmatism in the Gföhl unit and the Monotonous Group support models claiming that these two geological units belonged to independent peri-Gondwana terranes before the Variscan collision. It is suggested here, that the Gföhl unit and the Monotonous Group represent zones of higher and lower heat flow within the Early Palaeozoic northern Gondwana margin, respectively. The geochemical data presented in this study could be helpful for terrane correlations and palaeogeographic reconstructions.

Source compositions and melting temperatures of the main granitic suites from the Moldanubian Batholith

The granitoids of the Moldanubian Batholith (Moldanubian Zone of the Bohemian Massif) are represented by three main suites: Ito I/S-type Weinsberg biotite granites–granodiorites, S-type Eisgarn two-mica granites and I/S-type Freistadt biotite granites–granodiorites. As shown by major-element data and zircon with monazite saturation thermometry, partial melting of metapelites likely produced most of the Eisgarn suite (Mrákotín/Číměř varieties) at c. 775–825 oC. Parental melts of the Weinsberg and Freistadt suites could have been generated by partial melting of a metagreywackes–metabasalts mixture at c. 740–940 oC. This study confirms that the CaO/Na2O ratio could be used for source assignment of S-type granite melts, whereas the CaO/(FeO + MgO + TiO2) ratio is more suitable for I/S-type granites. The observed systematic positive shift of Al2O3/ TiO2 temperatures compared with zircon and monazite saturation temperatures in all granitoids of the Moldanubian Batholith (up to 118 oC) may be explained by accumulation of ilmenite in the S-type Eisgarn suite. In the Iand I/S-granite types of the Weinsberg and Freistadt suites the Al2O3/TiO2 ratio is controlled by the accumulation of ilmenite and/or titanite. Consequently, the Al2O3/TiO2 ratio may be used as a thermometer only with caution.

DOKLAD AKTIVITY NÍZKOTEPLOTNÍCH Ca-Na-Cl SOLANEK VE FELDSPATITIZOVANÝCH GRANITECH ŽIVCOVÉHO LOŽISKA KRÁSNO (MASIV KRUDUM)

The quartz-fluorite NNW–SSE trending vein mineralization hosted by alkali-feldspar syenites and leucocratic topaz-albite granites of the Vysoký Kamen stock (Krudum granite body) formed from low-salinity (0–5 wt. % NaCl eq.) and low-temperature (Th =90–132 °C) aqueous fluids. Both quartz and fluorite host secondary fluid inclusions which contain high-salinity (16–26 wt. % NaCl eq.) low-temperature (Th = 89–138 °C) Ca-Na-Cl brines. Two compositional subtypes of brines can be identified (Na-rich hosted by FI in fluorite and Ca-rich hosted by FI in quartz). The microthermometric parameters of brine-bearing fluid inclusions are fully comparable to the high-salinity fluids that participated during formation of many types of vein mineralizations during the post-Variscan period in the Bohemian Massif. The brines probably migrated along the adjacent NNW–SSE trending Vysoký Kamen fault.