Rb-Sr multimineral isochron data for metamorphic mobilisates allow to date separate increments of the mineral reaction history of polymetamorphic terranes. Granulitic 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 mobilisate 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 record 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 mobilisate 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 mobilisate. Amphibolite facies fluid infiltration occurred at 414.2 ± 2.8 Ma (weighted average of seven Rb/Sr mineral isochron ages, MSWD = 1.5). 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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Glodny, J.; Kühn, A.; Austrheim, H. (2008): Geochronology of fluid-induced eclogite and amphibolite facies metamorphic reactions in a subduction-collision system, Bergen Arcs, Norway. Contributions to Mineralogy and Petrology, 156, 1, 27-48.