[S-MP46_28PM2] Deformed rocks, Metamorphic rocks and Tectonics

[SMP46-P02_PG] 3D imaging of the Mn-caldera shaped zoning of the garnet found from the Sanbagawa metamorphic belt and its origin.

3-min talk in an oral session

*Kenta YOSHIDA¹, Takao HIRAJIMA¹ (1.Graduate School of Science, Kyoto University)

Keywords:garnet, Sanbagawa metamorphic belt, compositional zoning, disequilibrium crystal growth

Garnets with a complex compositional zoning were found from the northern proximal area of the Western Iratsu body of the Sanbagawa metamorphic belt of the Besshi district, southwest Japan. The studied garnet shows incipient Mn-reverse (increasing) zoning part (defined as core) and subsequent Mn-bell shape (decreasing) zoning part (defined as mantle), which is almost identical to the “Mn-caldera shaped zoning” described by Banno et al. (2004) in the Asemigawa region of the central Shikoku. In order to describe the chemical characteristic sterically, X-ray chemical mapping were performed by each 0.2-0.3 mm depth step, for one very-coarse-grained garnet with ca. 11 mm in diameter. The result clearly shows that the core/mantle boundary has the highest Mn content with euhedral shape, and that the chemical composition continuously changes through the grain. Internal schistosity defined by sigmoidal inclusion arrays cross-cuts the core/mantle boundary. This fact also suggests the continuous growth of garnet from the central part to the outer part. In the same sample, garnets with Mn-bell shape type zoning are also observed, which are relatively fine-grained up to 5 mm. Raman barometry and thermodynamic modeling suggest the climax P-T conditions of the studied sample did not reach the eclogite facies, which are consistent with the conditions of the oligoclase-biotite zone of the Sanbagawa metamorphic belt (610 ℃ and 1.0 GPa, Enami, 1994).Contrary to the simple Mn-bell shape type zoning which grown up with progressive regional metamorphism, “Mn-caldera shaped zoning” could be generated from the crystal nucleation under oversaturated environment (Matsumoto and Kitamura, 2004). Such oversaturation is expected in a rapid increase of temperature. Recently, Aoya et al. (2013) proposed the eclogite nappe covering the large part of the Besshi district. However, the exact boundary between the eclogite nappe and lower grade surrounding rocks is still under the debate. The conjunction of the eclogite nappe and the lower-grade surrounding rocks are thought to have taken place near the peak metamorphic stage of the surrounding rocks (500-600 ℃ and ca. 1 GPa, Aoya et al., 2013). Mn-caldera shaped zoning garnet found in the Besshi district (this study; Xu et al., 1994) are both found from the northern proximal of the hypothesized eclogite nappe. Those Mn-caldera shaped zonings are possibly originated from the conjunction of the eclogite nappe and surrounding crystalline schist, and corresponding rapid heating. Such features of garnet can help to determine the boundary of the eclogite nappe in the Besshi district.