P-T-D path of eclogite from the Sambagawa belt deduced from combination of petrological and microstructural analyses

Abstract

Both structural and petrological data can be used to constrain the P-T path of an eclogitic schist unit (the Seba basic schist) in the Sambagawa belt of SW Japan. The relationships between these two sets of data are well defined by porphyroblastic and other microstructures. The derived P-T path for the Seba basic schist has an overall clockwise trajectory with the decompression, or exhumation-related, path taking place under a lower P/T gradient than the burial, or subduction-related, path. The clockwise nature of the P-T path is qualitatively supported by chemical zoning of amphibole coexisting with eclogitic minerals. The significant feature of the P-T path is the presence of two temperature maxima, the first in the eclogite facies and the second in the epidote-amphibolite facies. The existence of two temperature maxima gives a simple explanation for the observation that metamorphic zonal boundaries postdating the eclogite facies metamorphism cross-cut the distribution of the main eclogite bodies in the Sambagawa belt. Estimates of metamorphic pressure using the jadeite content of clinopyroxene in the Seba area demonstrate the existence of a tectonic discontinuity between the eclogitic schist and surrounding non-eclogitic schist. Structural studies show that although these two units have experienced very different peak metamorphic conditions, they became juxtaposed during a single ductile defornation affecting both units. This deformation is related to exlumation of the eclogitic schist and subduction of the non-eclogitic schist, indicating that both were formed during the same subduction event. The presence of a major tectonic boundary between two units with a similar origin as subducted and accreted material, but contrasting metamorphic histories, can be interpreted in terms of nappe tectonics, and the existence of an 'eclogite nappe', the third nappe of the Sambagawa belt, is proposed.