[S-MP27] 鉱物の物理化学

[SMP27-01] Two different zoned garnets in the Khungui eclogite induced by replacement of Ti-bearing minerals during P-T evolution<quillbot-extension-portal></quillbot-extension-portal>

*Manzshir Bayarbold¹、Atsushi Okamoto¹、Masaoki Uno¹、OTGONBAYAR DANDAR¹、Noriyoshi Tsuchiya¹ (1.Graduate School of Environmental Studies, Tohoku University)

キーワード：Garnet, Ti-bearing minerals, HP metamorphism

Garnet is one of the most important minerals that record the dehydration process within the subduction zone and chemical composition of garnet is usually used to constrain the P-T conditions. Previous studies have revealed that different zoned garnets in metamorphic rocks could be formed due to mechanical mixing rocks in the mélange zone or local bulk composition (Lanari and Engi 2017; Enami et al. 2019). However, the mechanism of controlling different zoned garnets in metamorphic rocks is not fully understood within the subduction zone. In this study, we report the novel texture of garnet aggregate from the Khungui eclogite in the Zavkhan terrane, western Mongolia. We reveal that the variation of garnet texture and compositional zoning is closely related to the occurrence of the distribution of Ti-bearing minerals (rutile, ilmenite, and titanite). Based on the mineral assemblage, three metamorphic stages can be classified: prograde, eclogite (2.1–2.2 GPa, 445–610 °C), and decompression (0.5–1 GPa, 540–615 °C). The eclogite stage is presented by garnet + omphacite + barrosite + epidote + phengite + rutile (Bayarbold et al. 2022). Based on the garnet microstructure and main element zone, the Khungui eclogite comprises two types of garnet: garnet aggregate–Grt1 and euhedral garnet–Grt2. The major element (Fe, Ca, Mg, and Mn) compositional zoning of the Grt1 shows asymmetric zoning, whereas the Grt2 shows symmetric zoning. The EBSD analysis reveals that Grt1 contains numerous subdomains separated by high-angle orientations, and the grain boundary of the subdomains of Grt1 is not controlled by the major element zoning. The Grt1 shows the close spatial relationship with Ti-bearing minerals in the matrix, which is revealed through an analysis of thin section (Microscopy), element map (EPMA), and core sample (μX-ray CT). In addition, each garnet grains of Grt1 and Grt2 show trace element zoning, such as Ti. Moreover, asymmetric elemental zoning behavior of the Grt1 is often changed depending on whether Grt1 is contacted with Ti-bearing minerals or not. In contrast to previous models, these observations suggest that Grt1 composition is modified due to replacement of Ti-bearing minerals especially ilmenite to titanite at the exhumation stage in subduction zone.

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