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High-pressure (HP) complexes can compile a memory of their individual P-T-time-deformation paths through evolving orogens. Reading this memory requires both deciphering of the geochronological information, and correlating it with data on pressure, reaction history, deformation and, last not least, temperature. Isotopic signatures of minerals, like the Rb-Sr system of white mica, have long been considered as particularly sensitive to temperature. This often prompted direct translation of sets of mineral ages into cooling paths, with delineation of exhumation paths from inferred thermal histories. However, this approach is problematic because of the common observation of microscale spatial correlations between isotopic ages, mineral growth zonation, and recrystallization fronts. These correlations indicate that factors like availability of fluids, mineral recrystallization, and the modal rock composition similarly play crucial roles in determining the behaviour of isotopic system of mineral grains.
As an alternative approach, assemblages which demonstrably remained closed for intermineral isotope exchange can be used to generate Rb-Sr 'event ages' for specific metamorphic processes. It is shown that the Rb-Sr system of white mica in eclogite may persist unchanged even through very high temperatures (>650°C). Key requirements for such high thermal stability are a) absence of free aqueous fluids after assemblage crystallization, and b) modally controlled closed system behaviour, i.e., presence of white mica in an assemblage solely of phases like garnet and omphacite which do not allow for significant diffusional Sr isotope exchange even at the highest temperatures reached. A valid Rb-Sr multimineral isochron comprising all phases of an eclogitic assemblage can be interpreted as dating eclogitization, and linked to PT data to constrain eclogitization conditions. The same concept applies to assemblages formed later in a HP rocks' history. Veins, precipitated from fluids during exhumation, may be used to determine age and PT conditions of amphibolite- or greenschist facies overprints. Different event ages combine to an exhumation history, established beyond the pitfalls of closure temperature theory.
In the Proterozoic granulites of the Bergen Arcs, West Norway, repeated events of local fluid infiltration find their petrological expression in local eclogitization at 429 ± 3 Ma (based on U-Pb zircon and Rb-Sr multimineral data for eclogitization-related veins), followed by partial amphibolitization at 414 ± 3 Ma, and local greenschist facies deformation at ∼400 Ma. In meter-scale lenses of unreacted Proterozoic granulites, even the Rb-Sr system of biotite remained largely unaffected, despite of temperatures >600°C prevailing for nearly 15 Ma in Caledonian times. This example, together with similar examples from the Western Gneiss Region, illustrates the potential of petrology-guided Rb-Sr multimineral geochronology to constrain the history of HP terranes.
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