Emil Jelínek

Thermobarometry, diffusion modelling and cooling rates of crustal garnet peridotites: Two examples from the Moldanubian zone of the Bohemian Massif

Abstract Different origins for crustal garnet peridotites are illustrated by two Variscan bodies in adjacent, tectonically juxtaposed terranes in the Moldanubicum of the Bohemian Massif (eastern Moravia). The Nove Dvory garnet peridotite contains lenses of eclogite and is allochthonous and allofacial with respect to surrounding Gfohl gneiss. The Mohelno spinel-garnet peridotite is devoid of eclogite and has concordant contacts with surrounding felsic granulite. The bulk of the Mohelno body is spinel peridotite, and garnet occurs only along its margins, where it has formed at the expense of spinel. Thermobarometry of mineral cores from garnet-bearing assemblages yields values for Nove Dvory from 1195°C, 54,6 kbar to 970°C, 41.8 kbar and for Mohelno from 1265°C, 27.9 kbar to 1070°C, 23.4 kbar. The compositional zoning profiles in garnets at both localities are matched closely by diffusion modelling in which bilinear cooling rates are utilized. At Nove Dvory initial cooling rates of 50–200°C/Ma are followed by slower rates of 10–50°C/Ma. In contrast, at Mohelno an extremely rapid initial cooling rate of 1°C/year was followed by one of 500°C/Ma. An idealized tectonic model is proposed for the emplacement of the Mohelno peridotite to account for its cooling history. The predicted garnet zoning profiles that result from conductive cooling agree closely with the observed zoning profiles. Both peridotites were derived from the upper mantle. The Nove Dvory body was emplaced from a deep fault zone into the crust as garnet peridotite, and the Mohelno body was emplaced as a hot slab of spinel peridotite, in which garnet formed only along contacts as the body cooled into the garnet stability field.

Mantle-Derived, UHP Garnet Pyroxenite and Eclogite in the Moldanubian Gföhl Nappe, Bohemian Massif: A Geochemical Review, New P-T Determinations, and Tectonic Interpretation

The Gföhl nappe, the uppermost structural unit in the Moldanubian Zone of the Variscan Bohemian Massif, contains a distinctive association of HP crustal granulite (900-1000°C, 15-18 kbar) and UHP mantle garnet peridotite (875-1150°C, 33-60 kbar). Ultrahigh-pressure (UHP) garnet peridotite is host to layers and lenses of garnet pyroxenite and eclogite, which formed by high-pressure crystal accumulation of garnet and pyroxene (± trapped melt) from transient melts in subcontinental lithosphere. The source of such melts was subducted, hydrothermally altered oceanic crust. New analyses of garnet websterite, orthopyroxene eclogite, and kyanite eclogite yield temperatures of 840-950°C and pressures of 34-43 kbar, comparable to those of enclosing peridotite, although kyanite eclogite at one locality (Ührov) yields significantly different values of 1030-1200°C and 17-22 kbar. Most petrological and geochemical features of the Gföhl crustal and mantle association can be explained in terms of Devonian (Emsian to Famennian) convergence and subduction of Moldanubia beneath Tepla-Barrandia, culminating in Early Carboniferous (Tournaisian) continental collision. However, this tectonic scenario fails to account for a pressure gap of ∼20 kbar between HP granulite and UHP peridotite-pyroxenite-eclogite, which remains problematic.

Geochronology and geochemistry of eclogites from the Mariánské Lázně Complex, Czech Republic: Implications for Variscan orogenesis

The Mariánské Lázně complex (MLC) is located in the Bohemian Massif along the north-western margin of the Teplá-Barrandian microplate and consists of metagabbro, amphibolite and eclogite, with subordinate amounts of serpentinite, felsic gneiss and calcsilicate rocks. The MLC is interpreted as a metaophiolite complex that marks the suture zone between the Saxothuringian rocks to the north-west and the Teplá-Barrandian microplate to the south-east. Sm-Nd geochronology of garnet-omphacite pairs from two eclogite samples yields ages of 377±7, and 367±4 Ma. Samples of eclogite and amphibolite do not define a whole rock Sm-Nd isochron, even though there is a large range in Sm/Nd ratio, implying that the suite of samples may not be cogenetic. Eclogites do not have correlated ɛNd values and initial 87Sr/86Sr ratios. Five of the eight eclogite samples have high ɛNd values (+10.2 to +7.1) consistent with derivation from a MORB-like source, but variable 87Sr/86Sr ratios (0.7033 to 0.7059) which probably reflect hydrothermal seawater alteration. Three other eclogite samples have lower ɛNd values (+ 5.4 to −0.8) and widely variable 87Sr/86Sr ratios (0.7033 to 0.7096). Such low ɛNd values are inconsistent with derivation from a MORB, source and may reflect a subduction or oceanic island basalt component in their source. The MLC is an important petrotectonic element in the Bohemian Massif, providing evidence for Cambro-Ordovician formation of oceanic crust and interaction with seawater, Late Devonian (Frasnian-Famennian) high- and medium-pressure metamorphism related to closure of a Saxothuringian ocean basin, Early Carboniferous (Viséan) thrusting of the Teplá terrane over Saxothuringian rocks and Late Viséan extension.

Geochemistry of subsurface Precambrian plutonic rocks from the Brunovistulian complex in the Bohemian massif, Czechoslovakia

Abstract The Precambrian Brno pluton consists of ultramafic rocks, gabbronorites and gabbros, diorites, quartz diorites, leucotonalites, leucogranodiorites, granites and leucogranites that have the chemical characteristics of a volcanic arc assemblage. The granitoid components are subalkaline, meta-aluminous (I-type) with a calcium to sodium differentiation trend. The magmatic evolution of the Brno pluton started by generation of tholeiitic magma from the mantle, followed by possible contamination of ascending magma with immature crustal material and differentiation of tonalitic melt. The pluton is interpreted as representing the products of fractional crystallization of a melt of tonalitic composition. It is a late tectonic intrusion in the Brunovistulian complex which makes up the easternmost exposed part of the Bohemian massif and constitutes a juvenile segment of crust. Welding of the volcanic arc on to Fennosarmatia took place during the Cadomian orogeny, which is mainly Precambrian in age (630–550 Ma), although its latter stages span the Precambrian-Cambrian boundary (one age of arc magmatism is 584 ± 5 Ma).

Gold content of ectomycorrhizal and saprobic macrofungi from non-auriferous and unpolluted areas

Ectomycorrhizal and saprobic macrofungi growing in the wild were collected from non-auriferous and unpolluted areas and analyzed for gold. Gold was determined using long-term instrumental neutron activation analysis (INAA). In total, 154 samples, including 67 species of ectomycorrhizal fungi and 22 species of terrestrial saprobes, were examined. Gold contents of the both groups were mostly less than 20 ng g(-1) of D.W. The highest concentrations (expressed in D.W.) were found in the ectomycorrhizal species Amanita strobiliformis (136 ng g(-1)), Russula claroflava (148 ng g(-1)), Cantharellus lutescens (156 and 210 ng g(-1)), and Boletus edulis (235 ng g(-1)). Among the saprobic fungi, the highest values were found in Langermannia gigantea (160 ng g(-1)) and Morchella esculenta (189 ng g(-1)). Species of Agaricus commonly had relatively high gold values, 10s of ng g(-1). The gold content of macrofungal fruit bodies was considerably higher than that of vascular plants, and parallels concentrations found in plants growing in auriferous areas.

Zinc partitioning between glass and silicate phases in historical and modern lead–zinc metallurgical slags from the Příbram district, Czech Republic

Metallurgical slags of different ages resulting from Pb-metallurgy in the Přibram district (Czech Republic) have been studied. The chemical analysis (EPMA) of melilite, clinopyroxene, olivine and glassy matrix showed the following ZnO concentrations (in wt. %): 3.20–11.93 (melilite), 1.56 (clinopyroxene), 1.29–7.82 (olivine), 1.58–6.58 (glass). The Zn partition coefficient D = Cs / Cl between crystallized phases and coexisting glass was calculated. The values obtained are: 1.96–2.16 (melilite), 0.41 (clinopyroxene) and 0.79–1.19 (olivine). The distribution of zinc between the crystalline phases and glass depends on the phase assemblage, which reflects the blast furnace charge and temperature, as well as the cooling conditions of slags.

Tertiary alkaline Roztoky Intrusive Complex, České středohoří Mts., Czech Republic: petrogenetic characteristics

The České středohoří Mts. is the dominant volcanic center of the Ohře (Eger) rift zone. It hosts the Roztoky Intrusive Complex (RIC), which is made up of a caldera vent and intrusions of 33–28-Ma-old hypabyssal bodies of essexite–monzodiorite–sodalite syenite series accompanied by a radially oriented 30–25-Ma-old dike swarm comprising about 1,000 dikes. The hypabyssal rocks are mildly alkaline mostly foid-bearing types of mafic to intermediate compositions. The dike swarm consists of chemically mildly alkaline and rare strongly alkaline rocks (tinguaites). The geochemical signatures of the mildly alkaline hypabyssal and associated dike rocks of the RIC are consistent with HIMU mantle sources and contributions from lithospheric mantle. The compositional variations of essexite and monzodiorite can be best explained by fractional crystallization of parent magma without significant contributions of crustal material. On the other hand, the composition of monzosyenite, leuco-monzodiorite and sodalite syenite reflects fractional crystallization coupled with variable degrees of crustal assimilation. It is suggested that the parent magmas in the Ohře rift were produced by an adiabatic decompression melting of ambient upper mantle in response to lithospheric extension associated with the Alpine Orogeny.

Application of Environmental Radionuclides in Radiochronology

Since the discovery of radioactivity by Henri Becquerel in 1896, naturally occurring radionuclides (Chapt. 2) have greatly contributed to our ability to measure the ages of terrestrial and extra-terrestrial rocks and minerals, groundwater reservoirs, and biological samples, such as bones or made-made objects. Radiochronology utilizes the radioactive decay of terrestrial nuclides to their progeny atoms, as well as the decay of nuclides produced by cosmic radiation, to derive the ages of natural and man-made objects and the time and duration of processes in nature. Since the publication of the first radiometric age determination by Ernest Rutherford in 1904, radiochronology has grown into a separate discipline of science that is integrated into isotope geology, cosmochemistry, hydrogeology, archaeology and the environmental sciences.