Geochronology of Himalayan shear zones: unravelling the timing of thrusting from structurally complex fault rocks

Dating structurally complex fault rocks often results in internally inconsistent ages, as several mineral generations are intergrown at scales 10 μm and are almost always altered to various degrees. First, electron probe microanalysis is necessary to assess both inventory and spatial distribution of minerals and their retrogression or alteration phases.We then used 40Ar/39Ar step-heating combining two independent indicators that allow the discrimination of coexisting mica generations from each other: (1) mica stoichiometry, which is proxied by 39Ar concentration in combination with 37Ar/39Ar and 38Ar/39Ar (Ca/K and Cl/K) ratios; (2) furnace temperature at which the degassing peak accompanying dehydration and structural collapse is observed. As dehydration rates depend on average bond strength in the crystal structure, it is predicted and observed that the temperature of the differential Ar release peak is variable among different minerals.We observe that the Ca/Cl/K signatures of pure micas coincide with the Ar release peak. The Main Central Thrust zone in the Garhwal Himalaya records a protracted history. Foliation of the Vaikrita Thrust formed at 15–8 Ma, followed by static decompression at 7 Ma; foliation of the structurally lower Munsiari Thrust formed around 5 Ma. Our elaborate and time-consuming petrochronological procedure should become routine whenever analysing polydeformed metamorphic rocks.