Microtectonic control of 40Ar/39Ar white mica age distributions in metamorphic rocks (Erzgebirge, N-Bohemian Massif): Constraints from combined step heating and multiple single grain total fusion experiments

Abstract

Felsic rocks of the Erzgebirge in the Central European Variscides experienced ultra-high pressure metamorphism at ∼340 Ma, followed by nearly isothermal decompression during channel like rapid exhumation within ∼10 Ma. Despite the general time frame of exhumation and emplacement within mid-crustal levels is known, available geochronological data do not provide a detailed timescale of individual stages of the entire process. Addressing this problem we combined white mica 40Ar/39Ar ages obtained from multi-grain step heating and multiple single grain total fusion experiments with mineral chemical, structural and tectono-metamorphic constraints. Gneisses of the channel center are characterized by E-W elongated white mica exclusively aligned in the continuous foliation. The rocks of the channel contact zone and in the hanging and footwall contain NW-SE stretched white mica parallel to the foliation, but also components of ductile to brittle-ductile shear bands truncating the main foliation. Well defined weighted plateau ages of all samples range between 340.0 ± 1.1 Ma and 327.9 ± 1.3 Ma, confirming previously published data, but represent presumably mixed ages due to multi-phase deformation. Single grain age distributions, in contrast, exhibit two statistically significant age peaks at 338.6 ± 0.2 Ma and at 332.8 ± 0.3 Ma. Rocks without shear bands exclusively contain white mica belonging to the older age fraction, whereas rocks with shear bands show a broad age scatter including both age fractions. We interpret these age distributions as being independent of closure temperature, but reflecting different dynamic recrystallization events. The older age fraction reflects the formation of the main foliation during west-directed emplacement of the ultra-high pressure rocks in mid-crustal levels at ∼339 Ma, whereas the shear band related neo- or recrystallization is responsible for the younger age fraction dating the final transport in the upper crust at ∼333 Ma.