Age and thermal history of the Maksyutov metamorphic complex: 40Ar/39Ar evidence

The ⁴⁰Ar/³⁹Ar method with stepwise heating was used to date phengite and glaucophane in the contact zone of garnet glaucophanite an omphacite-garnet rock (eclogite) from the lower unit of the Maksyutov metamorphic complex. The correlation of the measured age and the sizes of the phengite flakes indicates that the behavior of radiogenic Ar in them was controlled by the mechanisms of volumetric diffusion. Taking into account the fact that all of the rocks have the same thermal history, the dates most close to the age of metamorphism are those of the largest phengite flakes from garnet glaucophanite: 392 Ma. The age values obtained on phengite from an omphacite-garnet rock sampled at the maximum distance from the contact are equal to 378 Ma and correspond to the time when the rocks cooled to temperatures below 350°C. The results of numerical simulations indicate that the metamorphic age is no younger than 400 Ma, and the linear cooling rate can be estimated at 3.40 ?0.75/+1.24°C/m.y. The maximum values of the phengite ages are consistent with the dates of glaucophane from three rock samples: 389–411 Ma.