Dating the evolution of C–S microstructures: a combined 40Ar/39Ar step-heating and UV laserprobe analysis of the Alpine Roffna shear zone

A technique combining 40Ar/39Ar step-heating analyses, in situ UV laserprobe dating and electron microprobe chemical characterization has been developed to unravel the relationships between microstructures, metamorphism and K/Ar systematics. This technique has been applied to white micas of a discrete shear zone in the Roffna metarhyolite in the Suretta nappe (SE Switzerland). Neoformed phengites from the Roffna metarhyolite crystallised in shear zones during Tertiary Alpine deformation under blueschist- and greenschist-facies conditions. In situ 40Ar/39Ar UV laser dating of white micas defining the schistosity and C-planes yield systematically younger isotopic dates in the C-planes, which are also correlated with decreasing celadonite content (i.e., Si/Al ratio). These results suggest that the timing of the high-pressure deformation event is preserved in the schistosity; high-Si micas appear to have lost no Ar. Later deformation was localised preferentially in C-planes giving younger 40Ar/39Ar dates probably related to a later period of phengite growth. The age sequence is broadly consistent with the textural constraints and geological field relations.