The study is focused on the composition of various types of Moldanubian dyke granites in the Bohemian Forest (Czech Republic).The studied area of about 200 km 2 is mainly in the northern environs of the Lipno dam lake on the Vltava River.This territory consists of metamorphic units such as Blanský les and Křišťanov granulite massifs associated with metasedimentary migmatite complexes of Monotonous and Varied units, intruded by Knížecí Stolec durbachite pluton and post-tectonic Variscan granitoids.The range of granite samples includes leucocratic rocks with muscovite, or with muscovite and biotite, and types with biotite as the single mica.Tourmaline-and garnet-bearing granites are less common.The set of 25 samples characterizes the composition of 20 dykes and small intrusions.A simple provisional division of granite samples into low-Ca (0.35-0.65 wt.% CaO) and medium-Ca (0.67-1.16 wt.% CaO) groups is used.Tourmaline granites (± Ms, Grt) contain schorl with 20-40 mol.% dravite.Garnets contain almandine and spessartine as the major components (c.30 mol. % Sps) but the sample from the Hrad hill exhibits an outer zone with up to 32 mol.% Grs.Apatite occurs in several generations, especially in low-Ca granites, which have a significant phosphorus substitution in feldspars: 1) primary fluorapatite, 2) minute anhedral apatite (containing P unmixed from albite) characterized by up to c. 10 mol.% of chlorapatite component in predominating fluorapatite, 3) very rare (hydrothermal) hydroxylapatite filling brittle fractures in tourmaline.Accessory cordierite, originally present in some leucogranites, is altered to pinite (muscovite + chlorite ± biotite aggregate).Several samples from the Smrčina area contained cordierite with low Be, which has been unmixed as a newly formed tiny beryl in pinite.The dataset exhibits geochemical heterogeneity.Low-Ca and medium-Ca granites are distinct in the Ba-Rb-Sr ternary, as well as in the of Zr/Hf vs. Y/Ho and SiO 2 vs. A/CNK plots.The low-Ca dyke granites show numerous chemical differences from the granites of the plutonic bodies (such as the Eisgarn or Deštná types of the Moldanubian Batholith).The medium-Ca granite dykes split into the Smrčina type and remaining types of muscovite-biotite granites.Several types of chondrite-normalized REE patterns can be distinguished in terms of the total REE contents, the degree of LREE over HREE enrichment and magnitude of the Eu anomaly; most of the patterns show clearly a tetrad effect.
The Hejný Pond, 500 × 400 m, southeast of the Zalužany village in central Bohemia, attracted attention as a possible impact structure.The pond has a notable bilateral symmetry with an axis oriented NW-SE, indicating a significant subhorizontal vector in the overall deformation pattern.A negative gravity anomaly elongated in the same direction (600 × 300 m) with the amplitude of -0.35 mGal was disclosed by detailed gravity survey.The incomplete ring of elevations suggests slightly uplifted basement segments with a diameter of 1 km.Search for glass and other possible indicators of shock metamorphism resulted in a find of glass fragments 22 to 37 mm across.The glass has composition comparable to the local Kozárovice hornblende-biotite granodiorite of the Central Bohemian Plutonic Complex.Unusual features include locally high contamination by iron (up to 10-13 wt.% FeOt) and abundant cristobalite.The absence of planar deformation features in quartz and feldspars together with the absence of platinum-group elements and Ni enrichment in local glass may be considered as obstacles in defining Zalužany as an impact structure.Although glass fragments are rare at the locality, and the considerable Quaternary erosion, the occurrence of rock inclusions in two of the fragments makes the Zalužany structure a promising object for future study.
Multidisciplinary research evaluates structural, metamorphic and petrochemical data of selected rock types in different units located in the northeastern part of the Bohemian Massif, Czech Republic: (1) the Strážek Unit in the northeastern part of the Moldanubian Zone, (2) the Svratka and Kutná Hora units correlated with the Orlice-Sněžník Unit in Western Sudetes as well as (3) the Polička, Hlinsko and Zábřeh units belonging to the Teplá-Barrandian Zone.Petrochemical data for metasediments of the Polička, Hlinsko and Zábřeh units are mutually comparable and confirm a lithological affinity to the upper-crustal Teplá-Barrandian Zone.The FeO t /MnO ratios in metasediments of the Strážek Unit and the Svratka Unit indicate differences in the origin of sedimentary protolith rocks.Relict pre-Variscan structures, including extensive migmatization, and high-grade mineral assemblages with peak metamorphic pressures of ~1.4 GPa in skarn bodies, are preserved in the Svratka Unit.The evidence for Palaeo-Variscan (390-355 Ma) HP and UHP events, recorded in the high-grade Kutná Hora and Orlice-Sněžník units, was observed neither in the NE part of the Moldanubian Zone (Strážek Unit) nor the Teplá-Barrandian Zone (Polička and Zábřeh units).The Variscan orogenic event imprinted in the Svratka, Polička and Zábřeh units was the right-lateral slip along WNW-ESE trending shear zones.This deformation was accompanied by metamorphism at ~580-650 °C and ~0.5-0.7 GPa (352-343 Ma) and intrusion of numerous small bodies of syn-deformation calc-alkaline granitoids in the Polička and Zábřeh units.The younger metamorphic fabrics in the northeastern part of the Moldanubian Zone reflect a fast exhumation of deep-seated high-grade complexes at ~340 Ma, which was constrained by post-tectonic emplacement of durbachites at ~339 Ma.Metamorphic development in felsic granulites of the Strážek Unit, metamorphosed c. 340 Ma ago at 850 °C and 1.8 GPa, was followed by decompression to T ≅ 790 °C and P = 1.3 GPa and finally T = 700 °C and P = ~0.4GPa.In contrast, Běstvina granulite in the Kutná Hora Unit, with the ~360 Ma high-grade metamorphism at 800-920 °C and 1.8-2.1 GPa, is free of such a HT-LP overprint.Thus the data indicate that the Svratka and Kutná Hora units, exhibiting numerous mutual differences, should not be considered as belonging to the Moldanubian Zone as they evolved as independent entities.The geochemical data on garnet-clinopyroxene skarns from the Moldanubian Zone, the Svratka and Kutná Hora units do not provide mutually distinguishing features.This is largely due to a very wide compositional variation in rocks interpreted as metamorphosed exhalite with detrital material admixture.Skarns from the Svratka Unit preserve clinopyroxenes with elevated jadeite component (0.5-24 mol.%) and prograde compositional zoning in garnet -features obliterated in samples from the Moldanubian Zone and the Kutná Hora Unit.
Detrital zircon U–Pb age data from the Moldanubian part of the Bohemian Massif obtained by laser ablation inductively coupled plasma mass spectrometry show variations between the lithotectonic units and point to their formation in spatially close but separated basins that were supplied by detritus from somewhat different source areas, possibly over different periods of time. The youngest detrital zircon component in metasediments of the Monotonous and Varied Units is Early Ordovician and Mid- to Late Devonian in age, respectively, suggesting their deposition in Palaeozoic times. Reliable interpretation of the Mid- to Late Devonian zircons recovered from metasediments of the Gföhl Unit is precluded by their high metamorphic grade and presence of anatectic melt. The Early Carboniferous zircons from the Gföhl Unit are interpreted as being of metamorphic origin. Comparison of detrital zircon age spectra from the Moldanubian, Teplá–Barrandian and Moravo-Silesian metasediments suggests that these were deposited in separate basins but the overall similarity of the Neoproterozoic and Palaeoproterozoic age maxima and sparse Neo- and Mesoarchaean ages suggest that the three crustal segments must have been spatially related prior to the Variscan orogeny. Future tectonic models of the Variscan assembly of the Bohemian Massif must account for nearly synchronous evolution of the Moldanubian, Teplá–Barrandian and Moravo-Silesian Palaeozoic sedimentary basins that shared a common crust and most of their sedimentary sources, as well as for a rapid burial of the Moldanubian sediments to mid- and lower crustal levels that was followed by their rapid exhumation to the upper crust in Mid-Devonian to Early Carboniferous times.
A multidisciplinary geoscience project on crustal structure of western Bohemia, Czech Republic, was implemented during the period [1991][1992][1993][1994].It included measurement of 200 km long deep seismic profile across several major units of the Variscan fold belt.Data and interpretations resulting from the project have now been published in English version as vol.47 (1997) of the Journal of Geological Sciences (series Geology) by the Czech Geological Survey, Prague.The results include information on crustal geology of the internal zones of Variscides in the eastern and NE surrounding of the 9101 m deep KTB scientific borehole in SE Germany.
Gem-quality garnet at the Sangu deposit occurs in plagioclase segregation veins confined to mafic granulite, as nodular crystals up to 10 cm in diameter. The colour is commonly red to brownish red in transmitted light, with a moderate brown hue. Faceted stones up to two-carats size have a pleasant strong red colour in incident light, but larger pieces show typically dark tone. Sangu garnet is pyrope-almandine with 46-47 mol% Prp, 39-40 mol% Alm, 9-10 mol% Grs, 2-3 mol% Adr, and 0.10 wt% TiO2. Indices of refraction are 1.7597-1.7632 and specific gravity ranges from 3.87 to 3.94. This garnet is characterized by the presence of abundant acicular rutile crystals. Mine production primarily consists of fractured pieces 1-3 cm in size, 0.5-2 grams in weight.
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