Moldanubian Zone Research Articles

Magnetotelluric study of the eastern margin of the Bohemian Massif: relations between the Cadomian, Variscan, and Alpine orogeny

Eastern margin of the Bohemian Massif is a geologically remarkable area, where three different orogenic cycles are meeting—the oldest, Cadomian, building a basement for younger cycles; a younger, Variscan; and the youngest, Alpine, covering the older units. In the past, this area was investigated by gravimetry and seismic methods. Recently, we have supplemented broadband magnetotelluric measurements within the period range of 0.001–1000 s, carried out on 29 stations distributed along a 140-km-long west-east regional profile. The profile direction was based on local geology and then confirmed by a dimensionality and directionality analysis. Data showed moderate effects of cultural noise in the signals and could be successfully processed by robust methods. We carried out a 2D inversion of the data using the REBOCC approach. The inversion results confirm the known near-surface geology and reveal deeper structures. On the west and in the central part of the profile, units of the Bohemian massif (the Moldanubian Zone, the Brunovistulicum) are interpreted. In the east, the Western Carpathians units are encountered. Short-period data agree well with the known near-surface geology of all inner smaller units and bring new knowledge in particular on their thickness. In the shallow structure, several conductive anomalies have been identified which are hypothesized to be related to graphitized layers in the Moldanubian Zone. From long-period data, a new image of the Moldanubian/Brunovistulicum contact and about the structure of the Brunovistulicum, especially the Brunovistulian Massifs, was obtained.

Milarite-group minerals from the NYF pegmatite Velká skála, Písek district, Czech Republic: sole carriers of Be from the magmatic to hydrothermal stage

Four distinct paragenetic, morphological and compositional types of milarite-group minerals were distinguished in the NYF pegmatite Velka skala, Pisek district, Czech Republic (Moldanubian Zone). The simply zoned (Kfs,Plg,Ab,Qz > Bt > Ap > Tur) pegmatite dike, ∼60 cm thick, is enclosed in durbachite (K,Mg-rich melasyenite). The milarite-group minerals include: yellowish, subhedral, prismatic grains of homogeneous milarite I (milarite s.s.); prismatic subparallel aggregates of white to grayish milarite II (zoned milarite s.s. to Sc-rich milarite, up to ∼20 mol% of the oftedalite component) from small pockets; colorless hexagonal prismatic crystals of milarite III (milarite s.s.) lining open fractures and microscopic fillings in recrystallized K-feldspar locally associated with REE-bearing minerals (allanite, aeschynite, titanite); very rare milarite IV (milarite s.s. to agakhanovite) as prismatic crystals and thin overgrowths with complex oscillatory zoning, ≤200 D:\Palash\HWNLM\07_06m in size, on crystals of milarite II and milarite III. Their mineral assemblages, textures and compositions suggest primary (magmatic) for milarite I and hydrothermal origin for milarite II, III, and IV. The simplified compositional evolution in these milarites is: Al + Ca → Be + Sc → Be + Y, HREE →Be+ LREE . Milarite-group minerals are the sole carriers of Be in the pegmatite, no relics or pseudomorphs after other Be-rich minerals were found; thus milarite is now established as an additional primary Be-rich mineral in granitic pegmatites. Mineral assemblages, chemical composition of minerals and close relation to the host durbachite suggest a NYF affinity of the Velka skala pegmatite as well as a close relation to euxenite-type pegmatites of the Třebic Pluton. Milarite-group minerals from Velka Skala exhibit very similar compositional evolution to those found in the Heftetjern pegmatite, Tordal, Norway.

Felsic granulite with layers of eclogite facies rocks in the Bohemian Massif; did they share a common metamorphic history?

High pressure granulite and granulite gneiss from the Rychleby Mountains in the East Sudetes form an approximately 7km long and 0.8km wide body, which is enclosed by amphibolite facies orthogneiss with a steep foliation. Well preserved felsic granulite is located in the central part of the body, where several small bodies of mafic granulite are also present. In comparison to other high pressure granulites in the Bohemian Massif, which show strong mineral and textural re-equilibration under granulite facies conditions, the mafic granulite samples preserve eclogite facies minerals (garnet, omphacite, kyanite, rutile and phengite) and their field and textural relations indicate that both mafic and felsic granulites shared common metamorphic history during prograde eclogite facies and subsequent granulite facies events. Garnet from both granulite varieties shows prograde compositional zoning and contains inclusions of phengite. Yttrium and REEs in garnet show typical bell-shaped distributions with no annular peaks near the grain rims. Investigation of major and trace elements zoning, including REEs distribution in garnet, was combined with thermodynamic modelling to constrain the early eclogite facies metamorphism and to estimate pressure-temperature conditions of the subsequent granulite facies overprint. The first (U)HP metamorphism occurred along a low geothermal gradient in a subduction-related environment from its initial stage at 0.8GPa/460°C and reached pressures up to 2.5GPa at 550°C. The subsequent granulite facies overprint (1.6–1.8GPa/800–880°C) affected the rocks only partially; by replacement of omphacite into diopside+plagioclase symplectite and by compositional modification of garnet rims. The mineral textures and the preservation of the eclogite facies prograde compositional zoning in garnet cores confirm that the granulite facies overprint was either too short or too faint to cause recrystallisation and homogenisation of the eclogite facies mineral assemblage. The results of this study are compared with other granulite massifs in the Moldanubian Zone. In addition, a possible scenario for the Variscan eclogite and subsequent granulite facies metamorphism in the Bohemian Massif is discussed.

Zinc in diopside in marbles of southern part of the Moldanubian Zone (Czech Republic)

Zinc in diopside in marbles of southern part of the Moldanubian Zone (Czech Republic)

Significance of Zr-in-Rutile Thermometry for Deducing the Decompression P–T Path of a Garnet–Clinopyroxene Granulite in the Moldanubian Zone of the Bohemian Massif

Significance of Zr-in-Rutile Thermometry for Deducing the Decompression P–T Path of a Garnet–Clinopyroxene Granulite in the Moldanubian Zone of the Bohemian Massif

Origin of the pegmatite veins within the skarn body at Vevčice near Znojmo (Gföhl Unit, Moldanubian Zone)

Origin of the pegmatite veins within the skarn body at Vevčice near Znojmo (Gföhl Unit, Moldanubian Zone)

Tourmaline, an indicator of external Mg-contamination of granitic pegmatites from host serpentinite; examples from the Moldanubian Zone, Czech Republic

Dominant primary solidus and minor subsolidus tourmalines from a variety of granitic pegmatites enclosed in serpentinites of the Moldanubian Zone, Czech Republic were examined, mainly by electron probe micro-analyser, to reveal the degree of external Mg(Ca)-contamination from their host rocks. The rocks include: (i) homogeneous to slightly heterogeneous nests of plagioclase-tourmaline rocks (group A) of anatectic or metasomatic origin, (ii) subhomogeneous to simply zoned barren pegmatite dikes (group B), and (iii) Li-bearing zoned pegmatite dikes of rare-element class (group C). The plagioclase-tourmaline rocks (group A) show spatial relation to pegmatites of the group B. Mostly black primary tourmalines (dravite, oxy-dravite, uvite, schorl, oxy-schorl, fluor-schorl) show extensive Mg- and Ca-contamination (group A), moderate Mg- and locally minor Ca-contamination (group B plus the locality Věžna I of the group C) and weak Mg-contamination of the tourmaline solely from outermost pegmatite units (group C); tourmalines from internal units of the pegmatites are typically Mg-free. The substitution mechanisms include MgR2+ −1 (R2+ = Fe2+ > Mn2+) in all groups, NaR2+ (□Al)−1 and R2+ (OH) (AlO)−1 in Ca-poor tourmalines and CaO (NaOH)−1 combined with the substitution CaR2+ (NaAl)−1 in Ca-enriched tourmalines (group A). Both Mg- and Ca-contamination events were very likely contemporaneous. The extent of contamination is higher in small and texturally simple plagioclase-tourmaline rocks (group A). Larger and more highly evolved Li-bearing pegmatites (group C) with zoned internal structure show a high degree of undercooling; consequently, rapid crystallization of outer zones with biotite and/or tourmaline depleted melt in almost all Mg and isolated the pegmatite body from further external contamination during solidus crystallisation. The granitic pegmatites (group B and group C) were open to the host serpentinite during early solidus crystallization immediately after emplacement of melt and then in early and/or late subsolidus crystallization (hydrothermal stage); in plagioclase-tourmaline rocks (group A) the system was likely continuously open to host serpentinite. This study affirms tourmaline as a very useful indicator of external contamination and elevated contents of Mg in tourmaline or in other minerals are the most reliable sign of external contamination in granitic rocks.

Uraninite, Coffinite and Brannerite from Shear-Zone Hosted Uranium Deposits of the Bohemian Massif (Central European Variscan Belt)

New mineralogical data are presented for shear-zone hosted uranium mineralisation from selected uranium deposits that occur in the Bohemian Massif. The uranium mineralisation is in high-grade metamorphic rocks of the Moldanubian Zone and/or in granitic rocks of the Moldanubian batholith and Bor pluton as complex uraninite–coffinite and uraninite–coffinite–brannerite assemblages. For analysed coffinites and brannerites, anomalous enrichment of Y (up to 3.4 wt % Y2O3) and Zr (up to 13.8 wt % ZrO2) is significant. The microprobe data indicate that coffinites from the Rožná and Okrouhlá Radouň uranium deposits contain variable PbO (0–4.3 wt %), FeO (0–2.5 wt %), Al2O3 (0–3.5 wt %), P2O5 (0–1.8 wt %), and CaO (0.7–3.5 wt %). Brannerite is present in unaltered and altered grains with variable concentrations of U4+ (0–0.5 apfu), U6+ (0.06–0.49 apfu), Ti (0.90–2.63 apfu), Ca (0.09–0.41 apfu), and low concentrations of Al (0–0.19 apfu), Th (0–0.04 apfu), Y (0–0.08 apfu), Zr (0–0.13 apfu) and REE (0–0.14 apfu).

Mineralogical and geochemical (stable C and O isotopes) variability of marbles from the Moldanubian Zone (Bohemian Massif, Czech Republic): implications for provenance studies

Metacarbonates of the Moldanubian Zone (Bohemian Massif, Czech Republic) were studied to obtain qualitative and quantitative mineralogical-petrographic as well as stable isotopic data for the purpose of stone provenance studies, potentially applicable in material research studies of cultural heritage artefacts. Twenty-six samples from twelve different historical quarries, as well as two samples from historical artefacts, were analysed by both mineralogical-petrographic and geochemical methods including: polarizing microscopy, cathodoluminescence, scanning electron microscopy with microanalysis, petrographic image analysis, powder X-ray diffraction, and isotope ratio mass spectrometry. The petrographic characteristics allowed for the discrimination of groups of (1) calcitic marbles, (2) dolomitic marbles, and (3) carbonate–silicate rocks. These groups exhibit characteristic features such as (1) the presence/abundance of major rock-forming minerals, (2) grain geometric characteristics (specifically, mean carbonate grain size and index of grain size homogeneity), and (3) the presence of specific accessory phases. The content of non-carbonate minerals, some rock fabric parameters, as well as the carbon and oxygen isotope data exhibited significant variability, even within a single quarry in the case of some impure marbles and carbonate–silicate rocks. Although the carbon and oxygen isotopic ranges displayed overlaps among the quarries studied, the isotopic signatures throughout the Moldanubian Zone allowed for discrimination of a group of white calcitic marbles with high carbon and oxygen depletion, as well as white dolomite–calcitic marble with higher carbon isotope values when compared with other marble resources of the Bohemian Massif. A combination of the isotopic signature with detailed mineralogical-petrographic characteristics seems to provide sufficient information for discrimination of the Moldanubian marbles from one another. The provenance of the Vrchotovy Janovice artefact is very probably from the Rabi quarry, among the Moldanubian marbles. The provenance of the artefact from the Prague Klementinum was not definitively assigned; however, the Nehodiv quarry was considered its probable source locality.

Ultrapotassic dykes in the Moldanubian Zone and their significance for understanding of the post-collisional mantle dynamics during Variscan orogeny in the Bohemian Massif

We report mineral textures, geochemistry and age relations of two ultrapotassic dykes from a dyke swarm in the Western part of the Moldanubian Zone at contact to the Teplá-Barrandian Block. The dykes have orientation almost perpendicular to the NNE–SSW trending Central Bohemian plutonic complex and cross cut metamorphic foliation in basement gneisses and migmatites. Based on mineral compositions and geochemistry, the dykes show close relations to Mg-K syenite plutons in the Moldanubian Zone. The two dykes are vaugnerite and syenite in compositions and contain talc pseudomorphs after olivine within a fine-grained matrix that consists of K-feldspar, phlogopite with small amounts of clinopyroxene and accessory quartz, apatite, titanite and sulphides of Fe, Cu, Ni. The syenite porphyry dyke cross cuts the vaugnerite. It contains quartz phenocrysts and comparing to vaugnerite has lower modal content of talc pseudomorphs.The vaugnerite and syenite porphyry have high K2O (6 to 7wt.%) and mg-number (0.66–0.74), but low CaO and Na2O contents. The vaugnerite is markedly rich in P2O5 (>2wt.%) and comparing to syenite porphyry has higher amount of mantle-incompatible elements (e.g. Rb, Cs, Ba, Pb, Th, U), V and Cr. ID-TIMS analyses on titanite in vaugnerite and on zircon in syenite porphyry yielded 338.59±0.68 and 337.87±0.21Ma, respectively. Mineral and bulk rock chemistry of the dykes indicates that the source magma was formed by a low degree of partial melting of a highly anomalous domain in the upper mantle which subsequently fractionated and was contaminated with crustal material during its ascent. Formation of ultrapotassic magma is explained by transcurrent shear zones in the mantle lithosphere that occurred due to block rotation and post-collisional mantle dynamics initiated by slab break-off and asthenosphere upwelling into the Moldanubian accretionary complex during the Variscan Orogeny.

Cordierite from a high-temperature low-pressure shear zone of the south-western Bohemian Massif (Moldanubian terrain, Austria)

A medium-scale shear zone exposed in the gneiss rocks of the South-western Bohemian Massif (Moldanubian Zone) contains cordierite, whose Na p.f.u. is subject to a significant increase from the centre to the edge of the deformation area, whilst other elements only show negligible variations. Coexisting mineral phases of cordierite include garnet, biotite, and sillimanite. According to the results obtained from the garnet-cordierite Fe2+/Mg2+-exchange thermometer a decrease of peak temperature from 639°C in the central mylonite to 593°C in the marginal mylonite can be observed, which indicates significant shear heating. Lithological pressures were estimated by considering the position of cordierite-forming reactions in the P-T field and the stability of coexisting sillimanite. They are subject to a reduction from 0.35GPa in the highest deformed mylonite to 0.31GPa at the margin of the shear zone. According to the results of comprehensive petrographic and mineralogical studies the investigated shear zone underwent a Variscan HT-LP metamorphic event implying the formation of cordierite and an Alpine MT-LP event entailing the rotation and decomposition of the cordierite phase.

Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic

The Bor pluton belongs to the Variscan granitoid plutons of the Moldanubian zone of the central European Variscides. Along with similar granitoid plutons of the western part of the Bohemian Massif (Leuchtenberg, Babylon, Fichtelgebirge), it is close in its composition to low-F biotite granites of the Saxothuringian zone. This paper investigates the hydrothermal alteration of the Bor pluton in relation to uranium mineralisation. Uranium mineralisation of the Bor pluton is associated with shear zones occurring on the western margin of the pluton on its boundary with high-grade metasediments of the Moldanubian zone (Zadní Chodov) and with metasomatic mineralisation evolved in hydrothermally altered biotite granites of the Bor pluton (Vítkov II, Lhota). Uranium mineralisation in altered granites is accompanied by their intense hematitisation, albitisation, chloritisation and carbonatisation. Hydrothermal alterations of granites were accompanied by the enrichment in U, Na, P, Ti, Mg, Ca and depletion in Si and K. The altered high-grade metasediments occurring in shear-zones on the western boundary of the Bor pluton are enriched in U, Ca, Mg and P and carbonaceous matter. In the Bor pluton area with typical coffinite–uraninite association the presence of brannerite is significant for uranium mineralisation. The observed coffinite is enriched in Y (up to 4.3wt.% Y2O3) and Zr (up to 2.0wt.% ZrO2). Unaltered brannerite displays low Ca (0.9–4.5wt.% CaO), Al (0.0–0.5wt.% Al2O3), Fe (0.7–2.2wt.% FeO), Pb (0.0–1.4wt.% PbO) and Th (0.0–2.8wt.% ThO2) concentrations. The highly altered brannerite is depleted in U, enriched in Ti, Si and Al. The temperature of the ore stage was estimated using chlorite thermometry and ranged from 122°C to 258°C.

Carbonatitic and granitic melts produced under conditions of primary immiscibility during anatexis in the lower crust

Carbonatites are peculiar magmatic rocks with mantle-related genesis, commonly interpreted as the products of melting of CO2-bearing peridotites, or resulting from the chemical evolution of mantle-derived magmas, either through extreme differentiation or secondary immiscibility. Here we report the first finding of anatectic carbonatites of crustal origin, preserved as calcite-rich polycrystalline inclusions in garnet from low-to-medium pressure migmatites of the Oberpfalz area, SW Bohemian Massif (Central Europe). These inclusions originally trapped a melt of calciocarbonatitic composition with a characteristic enrichment in Ba, Sr and LREE. This interpretation is supported by the results of a detailed microstructural and microchemical investigation, as well as re-melting experiments using a piston cylinder apparatus. Carbonatitic inclusions coexist in the same cluster with crystallized silicate melt inclusions (nanogranites) and COH fluid inclusions, suggesting conditions of primary immiscibility between two melts and a fluid during anatexis. The production of both carbonatitic and granitic melts during the same anatectic event requires a suitable heterogeneous protolith. This may be represented by a sedimentary sequence containing marble lenses of limited extension, similar to the one still visible in the adjacent central Moldanubian Zone. The presence of CO2-rich fluid inclusions suggests furthermore that high CO2 activity during anatexis may be required to stabilize a carbonate-rich melt in a silica-dominated system. This natural occurrence displays a remarkable similarity with experiments on carbonate–silicate melt immiscibility, where CO2 saturation is a condition commonly imposed.In conclusion, this study shows how the investigation of partial melting through melt inclusion studies may unveil unexpected processes whose evidence, while preserved in stiff minerals such as garnet, is completely obliterated in the rest of the rock due to metamorphic re-equilibration. Our results thus provide invaluable new insights into the processes which shape the geochemical evolution of our planet, such as the redistribution of carbon and strategic metals during orogenesis.

Coincidence of gabbro and granulite formation and their implication for Variscan HT metamorphism in the Moldanubian Zone (Bohemian Massif), example from the Kutná Hora Complex

Leucocratic metagabbro and amphibolite from a mafic–ultramafic body within migmatite and granulite in the Kutná Hora Complex were investigated. The mafic–ultramafic rocks show amphibolite facies metamorphism, but in the central part of the body some metagabbro preserves cumulus and intercumulus plagioclase, clinopyroxene and spinel. Spinel forms inclusions in both clinopyroxene and plagioclase and shows various degree of embayment structure, that was probably a result of reaction with melt during magmatic crystallization. In the metagabbro, garnet forms coronae around clinopyroxene at the contacts with plagioclase. Amphibolite contains garnet with prograde zoning and plagioclase. Phase relations of igneous and metamorphic minerals indicate that magmatic crystallization and subsequent metamorphism occurred as a result of isobaric cooling at a depth of 30–35km. U–Pb dating on zircon from leucogabbro yielded a Variscan age (337.7±2Ma) that is similar or close to the age of granulite facies metamorphism (ca 340Ma) in the Moldanubian Zone. Based on the calculated PT conditions and age data, both the mafic–ultramafic body and surrounding granulite shared the same exhumation path from their middle–lower crustal position at the end of Variscan orogeny. The coincidence of mafic–ultramafic intrusives and granulite–amphibolite facies metamorphism is explained by lithospheric upwelling beneath the Moldanubian Zone that occurred due to slab break-off during the final stages of subduction of the Moldanubian plate beneath the Teplá Barrandian Block. The model also addresses questions about the preservation of minerals and/or their compositions from the early metamorphic history of the rocks subjected to ultradeep subduction and subsequent granulite facies metamorphism.

Petrological, Geochemical and Sr–Nd–O Isotopic Constraints on the Origin of Garnet and Spinel Pyroxenites from the Moldanubian Zone of the Bohemian Massif

Garnet pyroxenites, spinel pyroxenites, and eclogites in the Moldanubian Zone of the Bohemian Massif form layers and lenses in mantle-derived peridotites that are enclosed by migmatitic gneisses and granulites. We have analysed major and trace elements, and Sr, Nd and oxygen isotopes for a suite of pyroxenites, which vary in composition and origin, from nine localities (Becvary, Horni Bory, Drahonin, Nihov, Mohelno, Nove Dvory, Horni Kounice, Karlstetten and Meidling-im-Tal) in the Gfohl Unit and Kutna Hora Complex. Based on conventional geothermobarometry, most pyroxenites yield a restricted range of temperatures (similar to 875-975 degrees C) over a wide span of pressures (similar to 1.0-3.0 GPa). The pyroxenite suite exhibits large variations in elemental and isotopic compositions, reflecting its complex origin and evolution. Based on the rare earth element (REE) compositions of clinopyroxene (Cpx), three types of pyroxenite can be distinguished: Type A with a light REE (LREE)-depleted patterns, Type B with an LREE-enriched pattern and Type C with a convex-upward REE pattern. Such REE patterns reflect derivation of the melts from depleted (Type A) and enriched (Types B and C) mantle sources. Pyroxenites from eight localities originated as high-pressure crystal cumulates from transient basaltic melts migrating through the lithospheric mantle. In contrast, pyroxenites at the Becvary locality represent fragments of metamorphosed gabbroic cumulates from oceanic crust. For the pyroxenite suite as a whole, a positive correlation between Sr/Nd and Eu/Eu*, radiogenic Sr-87/Sr-86 and negative epsilon(Nd) values in clinopyroxene, and variable delta O-18 values in coexisting garnet argue for the presence of a crustal component in the parental pyroxenite melts. Variations in compatible elements (Ni, Sc, and Co) indicate that combined assimilation and fractional crystallization was important in the evolution of most of the pyroxenite parental melts, although fractional crystallization alone is recorded by the fragments of oceanic crust, perhaps reflecting their pre-subduction crystallization history.

Evolution history of pelitic gneiss and HP-granulites in the Moldanubian Zone of the southern Bohemian Massif.

Evolution history of pelitic gneiss and HP-granulites in the Moldanubian Zone of the southern Bohemian Massif.

High-pressure granulites of the Podolsko complex, Bohemian Massif: An example of crustal rocks that were subducted to mantle depths and survived a pervasive mid-crustal high-temperature overprint

The Podolsko complex, Bohemian Massif, is a mid-crustal migmatite–granite dome exposed along a tectonic boundary separating the upper crust from the deeply eroded interior of the Variscan orogen, referred to as the Moldanubian Zone. This study examines metamorphic history of mafic and felsic granulites that occur in this complex as minor lenses or layers hosted in pervasively anatectic rocks. The mafic granulite contains garnet with preserved high-Ca cores, which based on pseudosection modelling indicates pressure conditions near the coesite stability field at temperatures of ca. 550–600°C. The relicts of an earlier eclogite-facies stage have been overprinted by a later granulite-facies assemblage consisting of ternary feldspar, orthopyroxene, and spinel in the mafic granulite and sillimanite and spinel in the felsic granulite. Composition of younger garnet (in rims and as smaller grains) in both granulites suggests that a near isothermal decompression of these rocks was followed by heating that reached temperature of ca. 900°C at pressure of ca. 0.5GPa. It is thus concluded that the granulites underwent at least two temporally separate tectonometamorphic events: they were first subducted to mantle depths and exhumed rapidly at relatively low temperatures and then near isobarically heated at mid-crustal levels. The preservation of earlier eclogite-facies garnet in the mafic granulite indicates that the latter event was short-lived and was followed by near isobaric cooling. The geologically brief granulite-facies metamorphism was previously explained as a result of slab break-off and mantle upwelling after the main phase of microplate convergence in the Bohemian Massif. To put the Podolsko complex into a broader tectonic context, we synthesize the available petrologic and structural data from the correlative (U)HP assemblages of the Moldanubian Zone to suggest that they typically do not preserve structural record of the subduction stage, only rarely preserve an early exhumation phase to the middle crust, and most, if not all, have been extensively overprinted by late-stage processes at shallower crustal levels. This synthesis also reveals that kinematics and thus tectonic processes driving the final exhumation of the (U)HP rocks were diverse and may vary from one location to another. These local settings include exhumation in the footwall of major extensional detachments, due to folding and subsequent lateral extrusion, and along crustal-scale strike-slip shear zones. The extensive late-stage overprint masking the earlier (pre-340Ma) subduction history is greatly exemplified in the Podolsko complex where the granulites are only small relics in pervasively anatectic rocks melted at mid-crustal conditions and then rapidly exhumed along extensional detachments to upper-crustal levels.

Durbachites from the Eastern Moldanubicum (Bohemian Massif): erosional relics of large, flat tabular intrusions of ultrapotassic melts—geophysical and petrological record

The results of the airborne survey comprising gamma-ray spectrometry and proton magnetometry, ground gravity survey, and field geological observations (e.g., deep borehole profiles, contact aureole in dolomite marbles, distribution of granitic pegmatites within the Třebic pluton) suggest that the ultrapotassic Třebic and Jihlava plutons are flat intrusions. They intruded distinct deep levels of the crust, 2–4 kbar for Třebic pluton, and 5–7 kbar for Jihlava-pluton. Current thickness of the intrusions is generally less than 2 km, with two exceptions: (1) central part of the Jihlava pluton and (2) a small body near Věžnice, where the estimated depth of tube-shaped stocks of shoshonitic and ultrapotassic gabbros or monzogabbros is around 2.5 km. These stocks could represent feeding pipes of basic and alkaline and dry magmas protruding to the upper crust level. The NE part of the Třebic pluton is a bottom part of this body, whereas the NW corner and the southern promontory of the pluton could represent an upper (roof) part of the intrusion. Small isolated durbachite bodies located within the Moldanubian gneisses and migmatites of the Stražek Unit represent rootless remnants of a former large and flat durbachite body initially extending significantly more to N and NE and eroded since Lower Carboniferous. Discrepancy between the long-wave magnetic and gravity anomalies, and surface geological structure of the eastern part of the Moldanubian Zone indicates a crucial role of the thrust tectonics.

Evidence for partial melting of eclogite from the Moldanubian Zone of the Bohemian Massif, Czech Republic

Evidence for partial melting of eclogite from the Moldanubian Zone of the Bohemian Massif, Czech Republic

Two Paragenetic Types of Cookeite From the Dolní Bory-Hatě Pegmatites, Moldanubian Zone, Czech Republic: Proximal and Distal Alteration Products of Li-Bearing Sekaninaite

Cookeite occurs in pseudomorphs after sekaninaite associated with muscovite, quartz, tourmaline, albite, nontronite, and chlorite in pegmatite No. 1 (proximal cookeite type), and in pockets lined with albite and quartz in the Oldrich pegmatite (distal cookeite type), Dolni Bory-Hate, Bory pegmatite district, Moldanubian Zone, Czech Republic. The mineral assemblages of the Bory pegmatites with common Al,Fe-rich minerals (annite, schorl, sekaninaite, andalusite, diaspore, ilmenite, and almandine) suggest their primitive, peraluminous, LCT geochemical signature. Proximal cookeite forms aggregates of sheets, up to similar to 500 mu m in diameter, intergrown with prevailing muscovite and quartz. Beige to grayish, porous aggregates of distal cookeite, up to 15 cm in size, consist of thin sheets, up to similar to 70 mu m in diameter, locally associated with small grains of pyrite and pale brown siderite as a matrix. Electron microprobe data of cookeite from the No. 1 and Oldrich pegmatites yielded very similar results: Si (3.23-3.27, 3.21-3.32 apfu) and Al (4.59-4.65, 4.48-4.65 apfu), respectively, and low concentrations of Fe (Fe-tot <= 0.08 apfu), Mg (<= 0.07 apfu), and K (<= 0.15 apfu). Concentrations of Li, B, and Be obtained by LA-ICP-MS (and wet chemical analysis-Li, Oldrich) of cookeite from both pegmatite dikes yielded an average of 2.22 wt.% Li2O (0.78 apfu) and 2.38 wt.% Li2O (0.84 apfu), low contents of B = 370-780 ppm and 250-430 ppm, and very low Be <= 16 ppm and <= 1.2 ppm for pegmatite No. 1 and the Oldrich pegmatite, respectively. Strong hydrothermal alteration of common Li-bearing sekaninaite (0.11-0.55 wt.% Li2O) is evident and a potential source of Li for both the proximal and distal cookeites. Cookeite precipitated from hydrothermal fluids at T similar to 350-270 degrees C and P < similar to 2 kbar. These data are consistent with the cookeite formation in granitic pegmatites and the distal cookeite demonstrates the high mobility of Li in hydrothermal conditions. The crystallization of cookeite at the No. 1 and Oldrich pegmatites was followed by crystallization of an Fe-rich mineral-nontronite and siderite, respectively.