[SMP41-P06] Fast and Slow Schists: Constraints from phengite geochronology
Keywords:Pacific-type HP-UHP metamorphic belts, Exhumation rate, strain rate, phengite geochronology, Fast and Slow Schists
Multi-stage exhumation models have been proposed for HP-UHP metamorphic sequences. The models are that the exhumation rates were high from the deepest level to the lower crust and the rates decrease to the shallow crustal levels. In the Sanbagawa HP schist belt in central Shikoku, the eclogite facies metamorphic rocks exhumed faster than the lower grade rocks. The early stage of exhumation from the eclogite facies to the overprinting amphibolite facies gives 9 mm/y. The rates are much lower in the later stage of exhumation. Lago di Cignana HP-UHP units in the western Alps have very short duration of deformation, in particular, less than 5 Myr in Lago di Cignana UHP unit, suggesting the exhumation rate is higher than 18 mm/y in the early stage of exhumation from the deepest level (ca. 120 km) to the lower crust (ca. 30 km), being two times higher than that of the Sanbagawa HP schist belt. Lago di Cignana HP-UHP units and Sanbagawa HP schist belt both are Pacific-type HP-UHP metamorphic belts consisting of the metamorphosed oceanic lithology that usually record only a single metamorphic cycle though the former has been considered to be part of collisional orogeny. Lago di Cignana HP-UHP units having the higher exhumation rate are likely due to the subsequent continental collision event because Sanbagawa HP schist belt with the lower exhumation rate did not experience the subsequent continental collision event. Thus, the former with the high exhumation (strain) rate makes “Fast schist” sequence that the several unit boundaries are distinct fault and the later with the low exhumation (strain) rate, “Slow schist” sequence having the large scale recumbent fold.