Two plates — Many subduction zones: The Variscan orogeny reconsidered

An Andean type Palaeozoic convergence in the Bohemian Massif

U Pb, Hf and O isotope evidence for two episodes of fluid-assisted zircon growth in marble-hosted eclogites from the Dabie orogen

The Palaeozoic Variscan oceans revisited

Eclogites from the deepest structural levels beneath the Semail ophiolite, Oman, record the subduction and later exhumation of the Arabian continental margin. Published ages for this high pressure event reveal large discrepancies between the crystallisation ages of certain eclogite-facies minerals and apparent cooling ages of micas. We present precise U-Pb zircon (78.95 ± 0.13 Ma) and rutile (79.6 ± 1.1 Ma) ages for the eclogites, as well as new U-Pb zircon ages for trondhjemites from the Semail ophiolite (95.3 ± 0.2 Ma) and amphibolites from the metamorphic sole (94.48 ± 0.23 Ma). The new eclogite ages reinforce published U-Pb zircon and Rb-Sr mineral-whole rock isochron ages, yet are inconsistent with published interpretations of older 40Ar/39Ar phengite and Sm-Nd garnet dates. We show that the available U-Pb and Rb-Sr ages, which are in tight agreement, fit better with the available geological evidence, and suggest that peak metamorphism of the continental margin occurred during the later stages of ophiolite emplacement.

Dating the geologic history of Oman’s Semail ophiolite: insights from U-Pb geochronology

This study presents new geochemical (major and trace element, Nd–Sr isotope) and U–Pb zircon, monazite, titanite and rutile data for various rock types (eclogite, high-pressure granulite, amphibolite, orthogneiss, leucosome) of the high-grade metamorphic Marianske Lazně Complex in the western Bohemian Massif. Concordant U–Pb zircon analyses from the mafic to intermediate samples disclose the Marianske Lazně Complex as a mixture of c. 540 Ma oceanic rocks juxtaposed at depth with lower-crustal rocks of the structurally overlying Tepla–Barrandian Unit. This interpretation refutes earlier models in which the Marianske Lazně Complex was interpreted as a dismembered Cambro-Ordovician ophiolite complex affected by Variscan subduction. Metamorphic zircon in mafic rocks of the Marianske Lazně Complex, as well as monazite from orthogneiss, and titanite in leucosome yield ages around 380 Ma. On the basis of detailed petrography and cathodoluminescence imaging we conclude that the c. 380 Ma age reflects the timing of Variscan exhumation and associated decompression melting under upper amphibolite-facies to granulite-facies conditions. Thereby, the data derived from metamorphic zircon of eclogites and high-pressure granulite, unexpectedly, do not date the timing of eclogitization, which could have happened just before Variscan exhumation, or even shortly after Late Cadomian protolith formation.

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The Problem of Dating High-pressure Metamorphism: a U–Pb Isotope and Geochemical Study on Eclogites and Related Rocks of the Mariánské Lázně Complex, Czech Republic

Chromium isotope systematics in three mantle-derived domains of Central European Variscides: Relationship between δ53Cr values and progressive weathering of serpentinized ultramafic rocks

V. Štědrá

Papers

The Problem of Dating High-pressure Metamorphism: a U–Pb Isotope and Geochemical Study on Eclogites and Related Rocks of the Mariánské Lázně Complex, Czech Republic

Chromium isotope systematics in three mantle-derived domains of Central European Variscides: Relationship between δ53Cr values and progressive weathering of serpentinized ultramafic rocks