10:45 〜 11:00
[SMP27-01] Garnet-forming partial melting in the presence of C-O-H fluid prevailed in the migmatite zone of the low-P/T type metamorphic belt (Aoyama area, Ryoke belt, Japan)
キーワード：部分溶融、地殻流体、領家変成帯
Subsolidus rocks in the middle and lower crust have very low porosity, and the amount of pore fluid is very small [1]. Therefore, fluid–absent dehydration melting has long been considered as a dominant style of partial melting in high-temperature metamorphism of the crust [2]. Dehydration melting is an incongruent melting which forms peritectic mineral(s) and melt in the absence of a free fluid phase. Anatectic melt can be trapped as melt inclusions in peritectic mineral(s), and this texture is the direct evidence of partial melting in high-grade metamorphic rocks [e.g., 3]. Recently, however, melt inclusions and C-O-H fluid inclusions are found in the same domain of a peritectic garnet from a graphite-bearing pelitic migmatite [e.g., 4]. This texture indicates that C-O-H fluid was present during the garnet-forming partial melting. Whether such fluid-present incongruent melting is prevailing in the active middle and lower crust can be investigated by observing high-temperature metamorphic rocks in exposed crustal sections.
The Ryoke metamorphic belt is a Cretaceous low-P/T type metamorphic belt distributed for ~ 800 km in SW Japan [e.g., 5]. Partial melting in the migmatite zones of the Ryoke metamorphic belt has been explained by dehydration melting reactions consuming biotite [6, 7, 8]. However, [9] reported C-O-H fluid inclusions enclosed in peritectic garnet from a diatexite sample (B81) from the metatexite-dominant part of the Grt-Crd zone of the Aoyama area (Ryoke metamorphic belt, SW Japan). The outer rim of the garnet encloses abundant ilmenite and rare euhedral plagioclase inclusions (textural indicator of partial melting, e.g., [10]) in addition to the C-O-H fluid inclusions, indicating its formation by biotite breakdown melting in the presence of the C-O-H fluid [9]. In order to investigate spatial distribution of such C-O-H-fluid-present partial melting in the Aoyama area, we observed 206 pelitic migmatite samples and found that 35 samples collected widely from the migmatite zone (Grt-Crd zone) contain C-O-H fluid inclusions enclosed in garnet. Most of the samples which contain C-O-H fluid inclusions are mainly composed of garnet (Grt), biotite (Bt), K-feldspar (Kfs), plagioclase (Pl), and quartz (Qtz). Some samples lack Kfs and/or Bt. One sample consists of Grt, Crd, Bt, Pl, and Qtz. Among these samples, we examined two diatexites collected from the diatexite-dominant part of the Grt-Crd zone in detail.
The diatexite samples Y07C1 and Y39A [11] mainly consist of Grt, Bt, Pl, and Qtz. Garnet exhibits xenomorphic shape and Bt+Pl intergrowth replaces the rim. Abundant ilmenite inclusions are enclosed in the garnet. Euhedral plagioclase and fluid inclusions are enclosed in almost the same domain of the garnet. The fluid inclusions have both fluid (CH4±CO2), and solid phases (carbonate minerals). Following [4], we consider that the solid phases are secondary daughter minerals formed through a post-entrapment reaction. Therefore, this texture indicates that C-O-H-fluid was present during the garnet-forming partial melting. Minor rutile (~10 µm in diameter) is also enclosed in the garnet in sample Y39A. The Zr-in-rutile geothermometer [12] applied to the rutile inclusion yielded preliminary temperature estimate of 717 oC assuming 5 kbar pressure condition previously estimated for the Grt-Crd zone [e.g., 7]. This temperature estimate is consistent with the biotite-consuming continuous melting reaction to form garnet [13].
As documented above, we confirmed for the first time that the biotite-consuming incongruent melting to form garnet in the presence of C-O-H fluid [9] prevailed in the migmatite zone (i.e., Grt-Crd zone) of the Aoyama area. Future studies in other metamorphic belts are required to confirm whether the fluid-present incongruent melting is a common phenomenon in the low-P/T type metamorphic belts.
References
1. Sawyer et al., 2011. Elements. 7, 229-234.
2. Brown, 2013. Geol. Soc. America Bull. 125, 1079–1113.
3. Cesare et al., 2009. Geology. 37, 627–630.
4. Carvalho et al., 2019. J. Metam. Geol. 37, 951–975.
5. Miyashiro, 1965. Iwanami Shoten Publishers, (in Japanese).
6. Brown,1998. J. Metam. Geol. 16, 3-22.
7. Kawakami, 2001. J. Metam. Geol. 19, 61–75.
8. Kawakami and Ikeda, 2003. Contrib. Mineral. Petrol. 145, 131-150.
9. Yoshimoto and Kawakami, 2021. JpGU. Abstr. SMP25-06.
10. Hiroi et al., 1995. Proc. NIPR Symp. Antarct. Geosci. 8, 107-120.
11. Kawakami, 1999. Unpublished Master’s Thesis submitted to Kyoto Univ.
12. Tomkins et al., 2007. J. Metam. Geol. 25, 703–713.
13. Spear et al., 1999. Contrib. Mineral. Petrol. 134, 17–32.
The Ryoke metamorphic belt is a Cretaceous low-P/T type metamorphic belt distributed for ~ 800 km in SW Japan [e.g., 5]. Partial melting in the migmatite zones of the Ryoke metamorphic belt has been explained by dehydration melting reactions consuming biotite [6, 7, 8]. However, [9] reported C-O-H fluid inclusions enclosed in peritectic garnet from a diatexite sample (B81) from the metatexite-dominant part of the Grt-Crd zone of the Aoyama area (Ryoke metamorphic belt, SW Japan). The outer rim of the garnet encloses abundant ilmenite and rare euhedral plagioclase inclusions (textural indicator of partial melting, e.g., [10]) in addition to the C-O-H fluid inclusions, indicating its formation by biotite breakdown melting in the presence of the C-O-H fluid [9]. In order to investigate spatial distribution of such C-O-H-fluid-present partial melting in the Aoyama area, we observed 206 pelitic migmatite samples and found that 35 samples collected widely from the migmatite zone (Grt-Crd zone) contain C-O-H fluid inclusions enclosed in garnet. Most of the samples which contain C-O-H fluid inclusions are mainly composed of garnet (Grt), biotite (Bt), K-feldspar (Kfs), plagioclase (Pl), and quartz (Qtz). Some samples lack Kfs and/or Bt. One sample consists of Grt, Crd, Bt, Pl, and Qtz. Among these samples, we examined two diatexites collected from the diatexite-dominant part of the Grt-Crd zone in detail.
The diatexite samples Y07C1 and Y39A [11] mainly consist of Grt, Bt, Pl, and Qtz. Garnet exhibits xenomorphic shape and Bt+Pl intergrowth replaces the rim. Abundant ilmenite inclusions are enclosed in the garnet. Euhedral plagioclase and fluid inclusions are enclosed in almost the same domain of the garnet. The fluid inclusions have both fluid (CH4±CO2), and solid phases (carbonate minerals). Following [4], we consider that the solid phases are secondary daughter minerals formed through a post-entrapment reaction. Therefore, this texture indicates that C-O-H-fluid was present during the garnet-forming partial melting. Minor rutile (~10 µm in diameter) is also enclosed in the garnet in sample Y39A. The Zr-in-rutile geothermometer [12] applied to the rutile inclusion yielded preliminary temperature estimate of 717 oC assuming 5 kbar pressure condition previously estimated for the Grt-Crd zone [e.g., 7]. This temperature estimate is consistent with the biotite-consuming continuous melting reaction to form garnet [13].
As documented above, we confirmed for the first time that the biotite-consuming incongruent melting to form garnet in the presence of C-O-H fluid [9] prevailed in the migmatite zone (i.e., Grt-Crd zone) of the Aoyama area. Future studies in other metamorphic belts are required to confirm whether the fluid-present incongruent melting is a common phenomenon in the low-P/T type metamorphic belts.
References
1. Sawyer et al., 2011. Elements. 7, 229-234.
2. Brown, 2013. Geol. Soc. America Bull. 125, 1079–1113.
3. Cesare et al., 2009. Geology. 37, 627–630.
4. Carvalho et al., 2019. J. Metam. Geol. 37, 951–975.
5. Miyashiro, 1965. Iwanami Shoten Publishers, (in Japanese).
6. Brown,1998. J. Metam. Geol. 16, 3-22.
7. Kawakami, 2001. J. Metam. Geol. 19, 61–75.
8. Kawakami and Ikeda, 2003. Contrib. Mineral. Petrol. 145, 131-150.
9. Yoshimoto and Kawakami, 2021. JpGU. Abstr. SMP25-06.
10. Hiroi et al., 1995. Proc. NIPR Symp. Antarct. Geosci. 8, 107-120.
11. Kawakami, 1999. Unpublished Master’s Thesis submitted to Kyoto Univ.
12. Tomkins et al., 2007. J. Metam. Geol. 25, 703–713.
13. Spear et al., 1999. Contrib. Mineral. Petrol. 134, 17–32.