The problem of dating high-pressure metamorphism:: a U-Pb isotope and geochemical study on eclogites and related rocks of the Marianske Lazne Complex, Czech Republic

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 Lazne Complex in the western Bohemian Massif. Concordant U-Pb zircon analyses from the mafic to intermediate samples disclose the Marianske Lazne 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 Lazne Complex was interpreted as a dismembered Cambro-Ordovician ophiolite complex affected by Variscan subduction. Metamorphic zircon in mafic rocks of the Marianske Lazne 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.