K–Ar phengite geochronology of HP–UHP metamorphic rocks –An in–depth review–

Abstract

The reported discordant and anomalously old K–Ar (⁴⁰Ar/³⁹Ar) phengitic white mica ages from collisional orogenic belts are due to the fact that white micas in continental lithologies are not reset completely during high– to ultrahigh–pressure (HP–UHP) metamorphism because the closure temperature of white mica is much higher than the generally accepted value, approximately 600 °C. On the other hand, phengites in HP–UHP schists experience deformation–induced recrystallization during exhumation of the host lithology. The radiogenic argon is released from the deformed phengite, as documented by comparison of the in situ ⁴⁰Ar/³⁹Ar dating of phengite included in rigid garnet and of stretched phengite in the matrix. These non–resetting and argon–release phenomena give inconsistent phengitic white mica ages in metamorphosed continental lithologies. The heterogeneity in the deformational process due to differences in lithological compositions, local domains or even within single mica crystals results in inconsistent ages, as documented from the in situ ⁴⁰Ar/³⁹Ar dating of the deformed micas. The Sanbagawa HP schist belt and Lago di Cignana HP–UHP units both consist of metamorphosed oceanic lithologies that usually record only a single metamorphic cycle and have phengites without any inherited excess argon. The duration of deformation during exhumation spans from the peak metamorphism to the end of deformation in the crust, making it possible to estimate the exhumation rates of the metamorphic sequences. The low exhumation rates (<6 mm/y) of the Sanbagawa belt suggest a slow strain rate during rock deformation, resulting in a ‘slow schist’ sequence with a recumbent fold structure. The rapid exhumation rates (<26 mm/y) of Lago di Cignana suggest a high strain rate during rock deformations, resulting in a ‘fast schist’ sequence consisting of several units with fault–bounded contacts. The Lago di Cignana UHP unit, which underwent the highest exhumation rate, could indicate a subsequent continental collision event, whereas the Sanbagawa belt did not experience a subsequent continental collision event.