P-T-t modelling, fluid circulation, and 39Ar-40Ar and Rb-Sr mica ages in the Aar Massif shear zones (Swiss Alps)

Abstract

Two different 1D forward numerical models of P-T-t paths for the Grimsel granodiorite are evaluated, in order to estimate its retrograde path independently of isotopic ages. The tectonostratigraphic evolution of the External Crystalline Massifs, metamorphic conditions and fluid inclusion data are the essential constraints used to construct these models. Modelled ages are compared with measured ages on micas from the Aar massif, which formed the basis for the controversial “closure temperature” hypothesis. We applied “classical” 39Ar-40Ar and Rb-Sr techniques to two deformation transects in shear zones of the Aar granite and the Grimsel granodiorite. Biotites and white micas crystallised in these shear zones during Miocene deformation under greenschist-facies conditions. Isotopic ages do not reflect the cooling trajectory calculated by the P-T-t models and cannot be considered as “cooling ages”. Instead, K-Ar ages of micas are best viewed as dating local crystallization during ductile greenschist-facies deformation around 21–17 Ma. Whole-rock-mica Rb-Sr ages of ca. 12 Ma demonstrate a chemically open behaviour due to late fluid circulation. The age results indicate that the Rb-Sr record in these rocks, which were deformed under hydrated conditions, provides chronohygrometric information, i.e. on the timing of documented fluid circulation events.