Structural and microstructural constraints on the mechanism of eclogite formation in the Sambagawa belt, SW Japan

Eclogites in the Seba basic schist of the Sambagawa belt show contrasting textures. In one type of eclogite, coarse-grained omphacite is randomly oriented and cross-cuts the schistosity suggesting post-tectonic growth. In contrast, in the second type, fine-grained omphacite is preferentially aligned parallel to the schistosity suggesting pre- to syn-tectonic growth. The former type of eclogite has been regarded as a product of solid-state contact metamorphism after the formation of the schistosity while the latter type has been related to the Sambagawa regional metamorphism. The idea that eclogite can form by contact metamorphism is unconventional and, if true, could have major implications for the interpretation of eclogitic rocks in general. In the Seba basic schist, it has generally been assumed that the dominant schistosity formed during a single phase of deformation. However, field and microstructural studies show: (i) there are two different schistosities, one formed before and the other during eclogite formation; and (ii) the differences in eclogite texture reflect the strength of deformation during the second stage. Therefore, the various textural types of eclogite in the Seba basic schist all formed during a single phase of regional metamorphism and there is no need to invoke contact metamorphism.