Dating low-temperature hydrothermal fluid activity with in situ Rb/Sr geochronology of partially chloritized biotite

Chloritization of biotite by fluids is common in low-temperature tectonic regimes. Using three metapelites from the Tatric Superunit in the Western Carpathians (northern Slovakia), we demonstrated that in situ Rb/Sr geochronology of partially chloritized biotite can be used to date chloritization. The Superunit records high-temperature Variscan tectonics overprinted by low-temperature Alpine tectonics. Petrographic and chemical characteristics demonstrate that muscovite did not react with fluids and have uniform Rb/Sr geochronology, broadly reflecting Variscan metamorphism. In contrast, biotite grains are partially chloritized and yield dispersed 87Rb/86Sr and 87Sr/86Sr values, plotting as wedge-shaped arrays in a Nicolaysen isochron diagram. The arrays are consistent with re-equilibration of the Variscan Rb/Sr records in the remaining biotite. The youngest isochrons extracted for the three metapelites overlap at ca. 55−50 Ma, coeval with Alpine tectonism. Therefore, biotite Rb/Sr systematics can be reset by low-temperature hydrothermal fluid circulation, which has important implications for dating shallow-crustal tectonics and hydrothermal ore deposits that are otherwise difficult to date.