- 著者
- Yuzuki SHINJI Tatsuki TSUJIMORI
- 出版者
- Japan Association of Mineralogical Sciences
- 雑誌
- Journal of Mineralogical and Petrological Sciences (ISSN:13456296)
- 巻号頁・発行日
- vol.114, no.1, pp.26-32, 2019 (Released:2019-03-08)
- 参考文献数
- 40
- 被引用文献数
- 7 8
Retrograde pumpellyite was newly found in garnet blueschist that is Mg–rich equivalent of late Paleozoic retrograde eclogite of the Yunotani Valley in the Omi area, Hida–Gaien Belt. The pumpellyite with high Al/(Al + Mg + Fe) occurs in pressure shadows around garnets; it is associated with secondary glaucophane, epidote, chlorite, titanite, phengite, albite, and quartz, which all characterize a retrograde blueschist–facies mineral assemblage after peak eclogite–facies mineral assemblage. This feature is comparable with retrograde pumpellyite in late Paleozoic garnet blueschist (with relict eclogite–facies mineral assemblage) in the Osayama area of the Chugoku Mountains. Equilibrium phase calculation confirmed that the pumpellyite is stable at a low temperature and pressure portion of the lawsonite–blueschist–facies. T–bulk–composition (Mg) pseudosection suggests that pumpellyite appears preferentially in high Mg/(Mg + Fe) bulk composition. The limited occurrence of retrograde pumpellyite in the Yunotani garnet blueschist and retrograde eclogite would be explained by Mg–rich bulk compositions. Also, the limited occurrence in pressure shadows around garnets suggests that the fluid trapped in the pressure shadows might have enhanced growth (or precipitation) of pumpellyite. This finding provides a strong evidence that the deeply subducted (eclogite–facies) metabasaltic rocks both in the Hida–Gaien Belt and the Chugoku Mountains were subjected to a very similar blueschist–facies overprinting locally reached the pumpellyite stability field. The ‘Franciscan–type’ cooling path suggests a ‘steady–state’ underflow of the paleo–Pacific oceanic plate in late Paleozoic at a convergent margin of the South China Craton.
- 著者
- Yuzuki SHINJI Tatsuki TSUJIMORI Tatsuhiko KAWAMOTO
- 出版者
- Japan Association of Mineralogical Sciences
- 雑誌
- Journal of Mineralogical and Petrological Sciences (ISSN:13456296)
- 巻号頁・発行日
- vol.114, no.6, pp.302-307, 2019 (Released:2020-01-22)
- 参考文献数
- 31
- 被引用文献数
- 2
Late Paleozoic Yunotani eclogites in the Itoigawa–Omi area of the Hida–Gaien Belt were subjected to a blueschist–facies recrystallization and deformation after the peak eclogite–facies metamorphism. We studied fluid inclusions in quartz domains of different metamorphic stages in the retrograde eclogite. Coarse–grained quartz filling pressure shadow of porphyroblastic garnets contains transgranular fluid inclusions (Group–I) with high salinity [6.0–8.0 eq mass% NaCl, with a mean value 7.2 ± 0.6 eq mass% NaCl (n = 12)]. In contrast, transgranular fluid inclusions in later stage quartz–rich band (Group–II) in parallel to a penetrative foliation developed in matrix are characterized by lower salinity [1.1–5.8 eq mass% NaCl, with a mean value 3.4 ± 1.2 eq mass% NaCl (n = 12)]. The occurrences of fluid inclusion trails suggest that each group corresponds to the fluids entrapped prior to the blueschist–facies recrystallization and the fluids entrapped during the blueschist–facies stage, respectively. The Group–II fluid inclusions formed at blueschist–facies conditions support the idea that aqueous fluids in the subduction channel have a salinity similar to seawater (~ 3.5 eq mass% NaCl). Moreover, the Group–I fluid inclusions indicate a possibility that aqueous fluids at the eclogite–facies depth in subduction channels possess a salinity higher than seawater.