Evidence of the lawsonite eclogite facies metamorphism from an epidote-glaucophane eclogite in the Kotsu area of the Sanbagawa belt, Japan

Abstract

The lawsonite (Lws) eclogite facies mineral assemblage [Lws + omphacite (Omp)] was discovered as an inclusion in garnet (Grt) within a unique epidote (Ep)-glaucophane (Gln) eclogite in the Kotsu area of the Sanbagawa belt in eastern Shikoku, Japan. One of the unique characteristics of this eclogite is the occurrence of Grt, displaying a distinct Mn-bell shaped zoning pattern {Xsps [= Mn/(Fe2+ + Mn + Mg + Ca)] > 0.4 at the core}. Lws inclusions are columnar shaped crystals of ̃ 20 μm in size, and are present from the Outer Core to Inner Rim {0.03 < 0.08}. Omp inclusions are anhedral, 10-100 μm in size, and are present from the Inner Rim to Outer Rim (0.06 < Xprp < 0.11). The mineral assemblage of the unique eclogite in the matrix (i.e., Grt, Omp, Ep, quartz, Gln/barroisite, and phengite) is identical to that of the common eclogite in the relevant area. However, the Mn bell-shaped zoning of Grt is weak in the common eclogite (Xsps < 0.1 throughout the grain). Combining a model petrogenetic grid within the NCKFMASH system and paragenesis of inclusion minerals in the zoned Grt allowed us to propose a new prograde P-T path for the Kotsu eclogite [i.e., (1) ̃ 400 °C and 0.8 GPa represented by an assemblage of Lws + albite + Gln + Ep + paragonite (Pg) in the Inner Core, (2) a subsequent P and T increase from ̃ 450 °C and 1.3 GPa to 550 °C and 1.8 GPa by an assemblage of Lws + Omp + Gln + Ep + Pg in the Inner Rim, and (3) 600 oC and 2.0 GPa by an assemblage of Grt + Omp + Gln + Ep + Pg in the Outer Rim/matrix]. The derived prograde P-T path is comparable to the thermal structure along the top of the present-day Philippine Sea slab beneath Shikoku Island, as calculated by Peacock (2009).