[S-MP28] Deformed rocks, Metamorphic rocks and Tectonics

[SMP28-P12] Saline fluids migration recorded in albite-spots of quartz schists in Sambagawa metamorphic rocks, Tenryu district, Japan

*takumi matsumoto ¹, Miki Tasaka¹, Tatsuhiko Kawamoto¹ (1.Shizuoka University)

Keywords:Sambagawa metamorphic rocks, albite-spots, quartz schists, Saline fluids migration

Fluid-rock interaction, deformation and alteration in the presence of fluid, is an important process in subduction zones(Ernst et al., 1970; Ochiai, 1989; Inui et al., 2023). In the Sambagawa metamorphic belt, albite porphyroblasts, also called albite-spots, are known to result from abnormal grain growth of plagioclase. Previous petrological studies have suggested that these albite-spots can be formed by the transition of mineral assemblage with increasing temperature, since the albite-spots are observed only in the high-temperature metamorphic zones above the garnet zone (Inui et al., 2023). Yet, the formation process remains better understood, and only a few studies have been conducted to observe these processes. In this study, we collected and analyzed 10 samples of quartz schist from the Sambagawa metamorphic belt located on the boundary between the garnet and chlorite zones along the Atako River in the Sakuma and Tenryu areas, Shizuoka Prefecture, Japan, to investigate the formation process of the albite-spots. There are two types of albite textures in the quartz schists: (1) band of small albite crystals with about 0.05mm in size, and (2) isolated large albite porphyroblast with 1~2 mm in size. The bands of small albite crystals are observed in all the samples, but the albite porphyroblasts are found only along the upper side in the river in the garnet zone. Electron backscatter diffraction (EBSD) analysis of crystallographic orientation revealed that quartz crystals were deformed by dislocation creep due to the presence of sub-grain boundaries and a distinct crystallographic preferred orientation (CPO). In contrast, the albite crystals showed few subgrain boundaries and no clear CPO, indicating a lack of intracrystalline deformation. Chemical composition analysis of albite using electron probe micro analyzer (EPMA) investigated no compositional zoning with the albite content [XAb= Na/(Na+ Ca)]) between 1 to 0.98 in all the analyzed samples. In addition, micro-XRF analysis of thin sections indicates that the bands of albite crystals are distributed on the foliation and slightly oblique to the lineation of the host rock, whereas the albite porphyroblasts are isolated around the bands of albite crystals. The observed heterogeneous distribution of albite crystals cannot be explained by simple metamorphism associated with increasing temperature, as proposed by previous studies. Fluid inclusions were only observed within the large albite porphyroblasts. The Raman spectroscopy shows that they are aqueous fluids. Microthermometry measurements of the fluid inclusions showed that the homogenization temperature of the fluid inclusions was155±7℃ and the salinity was 4.4±3.6 wt. % NaCl equivalent. These results suggest that fluid derived from the subduction zone preferentially migrated parallel to the pre-existing deformed structure in the quartz shist along the foliation and sub-parallel to the lineation in the analyzed samples. The albite crystals precipitated from an albite supersaturated saline fluid, potentially which resulted in the abnormal grain growth consuming the smaller albite crystal components and the formation of albite-spots. Consequently, we proposed that the observed albite-spots provides a direct piece of evidence of external migration of saline fluids during the Sambagawa metamorphism.

Reference

Ochiai, T., (1989). Geological Structure around Shirakura Mountain in the Tenryu Area of the Sambagawa Metamorphic Belt. Graduation Thesis of the Department of Earth Sciences, Faculty of Science, Shizuoka University.

Ernst, W. G., Y. Seki, H. Onuki, M. C. Gilbert, (1970). Comparative study of low-grade metamorphism in the California coast ranges and the outer metamorphic belt of Japan, The Geological Society of America, Inc., 124.

Inui, M., Kojima, S., Yoshiya, N., (2023). Grain size reduction of albite porphyroblasts in pelitic schists of the Sanbagawa metamorphic belt, Kanto Mountains, Japan. Journal of Mineralogical and Petrological Sciences, 118-021.