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

Both structural and petrological data can be wed to constrain the P-T path of an eclogitic schist unit (the Seba basic schist) in the Sambagawa belt o f SW Japan. The relationships between these two sets of data are well defin ed by porphyroblastic and other microstructures. The derived P T path for t he Seba basic schist has an overall clockwise trajectory with the decompres sion, or exhumation-related, path taking place under a lower P/T gradient t han the burial, or subduction-related, path. The clockwise nature of the P T path is qualitatively, supported by chemical zoning of amphibole coexisti ng 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 t he second in the epidote-amphibolite facies. The existence of two temperatu re maxima gives a simple explanation for the observation that metamorphic z onal boundaries postdating the eclogite facies metamorphism cross-cut the d istribution of the main eclogite bodies in the Sambagawa bell. Estimates of metamorphic pressure using the jadeite content of clinopyroxene in the Seb a area demonstrate the existence of a tectonic discontinuity, between the e clogitic schist and surrounding non-eclogitic schist. Structural studies sh ow, that although these two units have experienced very different peak meta morphic conditions, they became juxtaposed during a single ductile deformat ion affecting both units. This deformation is related to exhumation of the eclogitic schist and subduction of the non-eclogitic schist, indicating tha t both were formed during the same subduction event. The presence of a majo r tectonic boundary between two units with a similar or gain as subducted a nd accreted material, but contrasting metamorphic histories, can be interpr eted in terms of nappe tectonics, and the existence of an 'eclogite nappe t he third nappe of the Sambagawa belt, is proposed.