The age of formation of a marble in the Moldanubian Varied Group, Bohemian massif, Czech Republic using Re-Os dating of molybdenite

摘要

A Re-Os geochronological investigation was undertaken on sparsely disseminated molybdenite occurring in a regionally metamorphosed carbonatite-like marble, which belongs to the crystalline sequence of the Moldanubian Varied Group. The mineralogical composition of the marble is complex and includes a Mo-Th-Nb-REE mineralization that is concentrated in silicate-rich horizons with a high component of alkaline volcanic-sedimentary detrital material. These mineralized horizons form a continuous layer that is present only in rocks underlying graphite bed. We suggest that a component of the mineralization may arise from me-tasomatic infiltration into more porous detrital horizons within the carbonatite-like marbles during regional metamorphism. The overlying sequence containing organic-rich units may have contributed Mo-Th to the mineralization, but we do not exclude the possibility that these components could have come from the hosting alkaline volcanic-sedimentary material, where Nb and REE may have been enriched. The confluence of both a porous medium and available ore components may have focused metal-enriched metamorphic fluids. We suggest that the Re-Os age of the molybdenite records regional amphibolite grade metamorphism at about 495 Ma in the southern Bohemian massif. Because the Re-Os chronometer in molybdenite is demonstrably robust through granulite facies metamorphism, it is not affected by subsequent thermal overprints associated with the Variscan orogeny, and preserves an important part of the Cadomian event in this region.