Ultrapotassic magmatism in the heyday of the Variscan Orogeny: the story of the Třebíč Pluton, the largest durbachitic body in the Bohemian Massif

Janoušek, Vojtěch; Hanžl, Pavel; Svojtka, Martin; Hora, John M.; Kochergina, Yulia V. Erban; Gadas, Petr; Holub, František V.; Gerdes, Axel; Verner, Kryštof; Hrdličková, Kristýna; Daly, J. Stephen; Buriánek, David

doi:10.1007/s00531-020-01872-2

Cited by 35 publications

(40 citation statements)

References 178 publications

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“…Both geochemical signature, as well as the isotopic Sr ( 87 Sr/ 86 Sr 337 = 0.709-0.7125) and Nd ( 337 Nd = -6.3) data suggest mixing of a durbachitic magma with (leuco)granitic melt derived from crustal rocks undergoing anatexis during rapid decompression (for details, see, e.g., Holub 1997;Janoušek et al 2003;Breiter 2008;Janoušek et al 2019). Radiometric dating of the durbachite (U/Pb zir-con) yielded ages of 342-335 Ma (Janoušek et al 2020 and references therein).…”

Section: Třebíč Plutonmentioning

confidence: 99%

Beryllium minerals as monitors of geochemical evolution from magmatic to hydrothermal stage; examples from NYF pegmatites of the Třebíč Pluton, Czech Republic

Zachař¹,

Novák²,

Škoda³

2020

Jour. Geosci.

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Mineral assemblages of primary and secondary Be-minerals were examined in intraplutonic euxenite-type NYF pegmatites of the Třebíč Pluton, Moldanubian Zone occurring between Třebíč and Vladislav south of the Třebíč fault. Primary magmatic Be-minerals crystallized mainly in massive pegmatite (paragenetic type I) including common beryl I, helvite-danalite I, and a rare phenakite I. Rare primary hydrothermal beryl II and phenakite II occur in miarolitic pockets (paragenetic type II). Secondary hydrothermal Be-minerals replaced primary precursors or filled fractures and secondary cavities, or they are associated with ,,adularia" and quartz (paragenetic type III). They include minerals of bohseite-bavenite series, less abundant beryl III, bazzite III, helvite-danalite III, milarite-agakhanovite-(Y) III, phenakite III, and datolite-hingganite-(Y) III. Chemical composition of the individual minerals is characterized by elevated contents of Na, Cs, Mg, Fe, Sc in beryl I and II; Na, Ca, Mg, Fe, Al in bazzite III; REE in milarite-agakhanovite-(Y) III; variations in Fe/Mn in helvite-danalite and high variation of Al in bohseite-bavenite series. Replacement reactions of primary Be-minerals are commonly complex and the sequence of crystallization of secondary Be-minerals is not defined; minerals of bohseite-bavenite series are mostly the latest. Beryl usually occurs in pegmatites with rare tourmaline, whereas helvite-danalite bearing pegmatites are tourmaline-rich. Abundant tourmaline in pegmatites with helvite-danalite and its scarcity in beryl-bearing pegmatites indicate that early tourmaline crystallization affected activity of Al in the parental medium and thus may have controlled formation of primary Be-minerals (beryl-higher Al, helvite-danalite-lower Al) which crystallized later. Secondary Be-minerals with dominant minerals of bohseite-bavenite series and milarite suggest high activity of Ca in fluids. Variations in chemical composition (Al contents) of bohseite-bavenite series were controlled by the chemical composition of the precursor. High variability of primary magmatic Be-minerals within a single pegmatite district is exceptional and it is constrained by variable activities of Si and mainly Al, divalent cations-Ca, Mn, Fe, Zn and Mg, trivalent cations-REEs, Sc, and B, S, and f O2 in the individual pegmatites.

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Section: Třebíč Plutonmentioning

confidence: 99%

Beryllium minerals as monitors of geochemical evolution from magmatic to hydrothermal stage; examples from NYF pegmatites of the Třebíč Pluton, Czech Republic

Zachař¹,

Novák²,

Škoda³

2020

Jour. Geosci.

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“…We, however, feel that the dating of this intrusion is not as generally problematic as previously mentioned when appropriate techniques for isotope analysis and data processing are applied. We argue that: (1) inappropriate consideration of decay-damage related Pb-loss in non-chemically abraded zircon, coupled with inheritance of old Pb leads to the previously observed age variations and (2) inaccurate calculation and reporting of laser-ablation, inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb ages and uncertainties in the recent paper of Janoušek et al (2020) have contributed to an inaccurate and problematic geochronological data set. In this comment, we explain our concerns on these published data, re-interpret the dates, and present new, high-precision chemical-abrasion, isotope-dilution, thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb dates that unequivocally constrain the age of zircon crystallization in the Třebíč melagranite melt.…”

mentioning

confidence: 99%

“…The commented paper by Janoušek et al (2020) presented an excellent and detailed assessment of the petrology, geochemistry and the mode of emplacement of the durbachitic Třebíč intrusion in the Bohemian Massif. The term "durbachite" denotes members of an intrusive series that comprises ultrapotassic, Mg-rich melagranites and ultrapotassic two-pyroxene melasyenites, which were emplaced into the exhumed high-grade core of the orogen during the late-orogenic stages (Žák et al 2014).…”

mentioning

confidence: 99%

“…The term "durbachite" denotes members of an intrusive series that comprises ultrapotassic, Mg-rich melagranites and ultrapotassic two-pyroxene melasyenites, which were emplaced into the exhumed high-grade core of the orogen during the late-orogenic stages (Žák et al 2014). Among those is the Třebíč pluton in the Moldanubian domain of the Bohemian Massif, a tabular intrusion that has been emplaced at shallow crustal levels (corresponding to 2-4 kbar, Novák 2006, Leichman et al 2017), and whose flat-lying fabrics, which are transitional between magmatic and solid state, are concordant with the regional foliation in the host rocks (Janoušek et al 2020). Therefore, its solidification dates the subvertical shortening of the Moldanubian domain, which is related to the underthrusting of the Brunia microcontinent (Verner et al 2006;Žák et al 2014).…”

mentioning

confidence: 99%

“…The dating of durbachite lithologies by zircon U-Pb has been described as problematic by many authors (e.g., Schaltegger et al 1996;Klötzli and Parrish 1996), and this is re-stated by Janoušek et al (2020) for the dating of the Třebíč pluton. We, however, feel that the dating of this intrusion is not as generally problematic as previously mentioned when appropriate techniques for isotope analysis and data processing are applied.…”

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confidence: 99%

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Comment on “Ultrapotassic magmatism in the heyday of the Variscan Orogeny: the story of the Třebíč Pluton, the largest durbachitic body in the Bohemian Massif” by Janoušek et al.

Schaltegger

Gaynor

Widmann

et al. 2021

Int J Earth Sci (Geol Rundsch)

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This comment addresses the incorrect treatment and presentation of data from laser ablation ICP-MS U–Pb age determinations of two samples of the Třebíč Pluton. This results in inaccurate ages and error assessment, invalidating the age interpretations of the authors. To corroborate our arguments, new high-precision chemical abrasion ID-TIMS data are presented that unequivocally define the emplacement age of the Třebíč pluton.

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Ultrapotassic plutons as a source of uranium of vein-type U-deposits (Moldanubian Zone, Bohemian Massif): insights from SIMS uraninite U–Pb dating and trace element geochemistry

Kubeš,

Leichmann,

Wertich

et al. 2024

Miner Deposita

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The Bohemian Massif hosts significant hydrothermal U-deposits associated with shear zones in the high-grade metamorphic basement. But there is a lack of evidence of a genetic link between mineralization and U-fertile igneous rocks. This contribution provides constraints on the major U source of the vein-type U-deposits, the timing of ore formation and the metallogenetic model. The anomalous trace element signatures of the low-temperature hydrothermal deposits (high Zr, Y, Nb, Ti, ∑REE) and their close spatial relation with ultrapotassic rocks of the durbachite series point to a HFSE and REE enriched source rock. The durbachites have high U content (13.4–21.5 ppm) mainly stored in magmatic uraninite and other refractory minerals (e.g., thorite, zircon, allanite) that became metamict over a time interval sufficient to release U from their crystal structure, as suggested by the time gap between emplacement of the durbachites (EMP uraninite U–Pb age ~ 338 Ma) and hydrothermal activity (SIMS uranium ore U–Pb age ~ 270 Ma). Airborne radiometric data show highly variable Th/U ratios (1.5–6.0), likely reflecting a combination between (1) crystallization of magmatic uraninite, (2) hydrothermal alteration, and (3) leaching and mobilization of U along NW–SE-trending fault zones, manifested by elevated Th/U values in the radiometric map. The presence of rare magmatic uraninite in durbachites suggests almost complete uraninite dissolution; EMP imaging coupled with LA-ICP-MS analyses of refractory accessory phases revealed extensive mobilization of U together with HFSE and REE, providing direct evidence for metal leaching via fluid-driven alteration of radiation-damaged U-rich minerals. The large-scale HFSE and REE mobilization, demonstrated by the unusual trace element signatures of the U-deposits, was likely caused by low-temperature (270–300 °C), highly alkaline aqueous solutions containing F-, P-, and K-dominated complexing ligands. The first SIMS U–Pb age of 270.8 ± 7.5 Ma obtained so far for U-mineralization from the Bohemian Massif revealed a main Permian U mineralizing event, related to crustal extension, exhumation of the crystalline basement, and basin formation, as recorded by U–Pb apatite dates (280–290 Ma) and AFT thermal history models of the durbachites. The Permo-Carboniferous sedimentary cover probably represented a source of oxidized basinal brines infiltrating the basement-hosted durbachite plutons and triggering massive metal leaching. The interaction between basin-derived brines and durbachites resulted in significant modification of the chemical composition of the hydrothermal system (K and F release during biotite chloritization, P liberation through monazite alteration), leading to the formation of ore-bearing fluids responsible for the metallogenesis of the basement-hosted unconformity-related U-deposits in shear zones in the Bohemian Massif.

show abstract

Ultrapotassic magmatism in the heyday of the Variscan Orogeny: the story of the Třebíč Pluton, the largest durbachitic body in the Bohemian Massif

Cited by 35 publications

References 178 publications

Beryllium minerals as monitors of geochemical evolution from magmatic to hydrothermal stage; examples from NYF pegmatites of the Třebíč Pluton, Czech Republic

Beryllium minerals as monitors of geochemical evolution from magmatic to hydrothermal stage; examples from NYF pegmatites of the Třebíč Pluton, Czech Republic

Comment on “Ultrapotassic magmatism in the heyday of the Variscan Orogeny: the story of the Třebíč Pluton, the largest durbachitic body in the Bohemian Massif” by Janoušek et al.

Ultrapotassic plutons as a source of uranium of vein-type U-deposits (Moldanubian Zone, Bohemian Massif): insights from SIMS uraninite U–Pb dating and trace element geochemistry

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