Rb-Sr multimineral isochron data for metamorphic veins allow to date separate increments of the mineral reaction history of polymetamorphic terranes. Granulite facies rocks of the Lindås nappe, Bergen Arcs, Norway, were subducted and exhumed during the Caledonian orogeny. The rocks show petrographic evidence for two distinct events of local fluid infiltration and vein formation, along fractures and shear zones. The first occurred at eclogite facies (15-21 kbar, 650-750 °C) and a later one at amphibolite facies conditions (8-10 kbar, 600°C). The presence of fluids enabled local metamorphic equilibration only near fluid pathways. In fluid-absent domains, preexisting assemblages were metastably preserved. This resulted in a heterogeneity of metamorphic signatures on m-to µm-scales. Well-preserved granulite facies rocks preserve their Proterozoic Rb-Sr mineral ages, as does the U-Pb system of zircon in most lithologies. Six Rb/Sr multimineral isochron ages for eclogite facies veins and their immediate wallrocks date the fluid-induced eclogitization at 429.9 ± 3.5 Ma (2σ, weighted average, MSWD = 0.39). An eclogite facies vein has yield metamorphic zircon with concordant U-Pb ages of 429 ± 3 Ma, identical to the U-Pb age of 427.4 ± 0.9 Ma for zircon xenocrysts in an amphibolite facies vein. Seven Rb/Sr mineral isochron ages date amphibolite-facies fluid infiltration at 414.2 ± 2.8 Ma (MSWD = 1.5), an age value testifying to residence of the rocks in the deep orogenic crust at temperatures >600°C for nearly 15 Ma. The new data show that Rb-Sr mineral isochron ages effectively date fluid-induced (re)crystallization events rather than stages of cooling. The direct link between isotopic ages and distinct petrographic equilibrium assemblages aids to constrain the evolution of rocks in the P-T-reaction-time space, which is essential for understanding exhumation histories and the internal dynamics of orogens in general.

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Recent findings for a young (31.5 ± 0.7 Ma) age of high‐pressure metamorphism at ∼90 km depths in the Eclogite Zone of the Tauern Window, Eastern Alps, prompt the question about the timing of the structural development of the Tauern Window and its relation to high‐pressure metamorphism. We show that all major structures in the Tauern Window, resulting from strong N‐S lithospheric shortening and simultaneous minor E‐W extension, began developing coevally with high‐pressure metamorphism in the Eclogite Zone. Large‐scale strike‐slip shear zones started to form at ∼32–30 Ma and facilitated the spatial accommodation of simultaneous shortening and extension. At least some of the strike‐slip and extensional shear zones operated into the Middle Miocene, either continuously or intermittently, with pronounced activity at ∼21–15 Ma. The considerable exhumation of the Eclogite Zone from ∼90 km depths into the middle crust, and the tectonic development of its framework occurred within only 1–2 Ma after eclogitization. This is evidenced by almost identical ages for eclogite facies metamorphism and for the development of the major structures that bound the Eclogite Zone under blueschist‐ and greenschist facies metamorphic conditions. We discuss a tectonic model in which considerable transpressional shortening and thickening took place in the present central‐southern part of the Tauern Window. We propose that the Tauern Window nucleated here and that most of the regional deformation at ∼32–30 Ma is today found at the periphery of the window and in the adjacent Austroalpine units. Afterward, transpression continued, the window grew to the E, W, and N, and deformation progressed to those parts of the window. Ductile deformation in the present‐day surface level ceased at ∼15 Ma.

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