10:15 〜 10:30
[SMP25-06] C-O-H fluid was present during garnet-forming partial melting reaction in a pelitic migmatite from the Ryoke belt, Japan
キーワード：変成作用、部分溶融、地殻流体
Partial melting of the crustal rocks has long been discussed based on the scheme of two end member concepts of vapor-present melting vs dehydration melting. Common textural characteristic of the incongruent melting is a coexistence of peritectic mineral(s) and leucosome, and this has long been considered as an evidence of dehydration melting. Existence of primary melt inclusions such as nanogranites in peritectic mineral(s) is also good evidence of incongruent melting. Recently, however, coexistence of melt inclusions and fluid inclusions are found from peritectic garnet in partially molten rocks (e.g., Carvalho et al., 2019), evidencing that fluid phase coexisted with partial melts when the peritectic garnet was formed. In order to investigate the presence/absence of such fluid phase during partial melting of the middle to lower crust beneath a volcanic arc, we carried out petrographic observation of pelitic migmatites from the Cretaceous Ryoke metamorphic belt (Aoyama area, SW Japan).
The sample used in this study is a pelitic diatexite that occur in the metatexite-dominant area of the Grt-Crd zone of the Aoyama area. The sample mainly consists of biotite + garnet + plagioclase + quartz, and minor K-feldspar and muscovite are also present in the matrix. The garnet is almost homogeneous in terms of major elements except for the grain margin in contact with biotite. Therefore, we utilized discontinuous zonings in terms of P and Y to define the isochronous surface within the garnet during its growth. As a result, garnet is divided into inner core, outer core, mantle, inner rim and outer rim. Inner and outer cores show higher Y concentration compared to other domains, and prograde xenotime is included in the outer core. In the mantle and the inner rim, sillimanite needles are included. Muscovite, plagioclase, K-feldspar are included in the vicinity of the sillimanite in the inner rim. The outer rim has abundant ilmenite inclusions and rare euhedral plagioclase inclusion indicative of partial melting. Fluid inclusions are seen throughout the garnet grain except for the inner rim. These fluid inclusions are composed of both fluid and solid phases. The fluid phase is mainly composed of CH4 or CO2 + CH4, and the solid phases are siderite, chlorite, and clay minerals. Such a mode of occurrence of CH4-bearing COH fluid inclusions is interpreted as a result of post-entrapment reaction between original COH fluid inclusion and host garnet that resulted in the formation of secondary daughter minerals (Carvalho et al., 2019).
Based on pieces of observation described above, the garnet-forming process is explained as follows. (1) High Y concentration and presence of prograde xenotime inclusion and COH fluid inclusion in the outer core of the garnet suggests that the garnet core grew at around 530-570oC (Kawakami et al., 2019) in the presence of COH fluid. (2) Sillimanite, muscovite, plagioclase and K-feldspar in the inner rim suggests that the inner rim grew at around the muscovite breakdown reaction Ms + Ab + Qtz = Sil + Kfs + melt. (3) The COH fluid inclusions, abundant ilmenite and rare euhedral plagioclase enclosed in the outer rim of garnet indicate that the biotite breakdown melting in the presence of COH fluid was responsible for the formation of the outer rim of garnet. Therefore, this study revealed that COH fluid was present during the biotite breakdown melting reaction to form garnet in the middle crust of the upper-amphibolite to granulite facies Ryoke metamorphic belt. Presence of fluid phase can be common even during the partial melting of graphite-bearing pelitic lithology that produces peritectic mineral(s).
The sample used in this study is a pelitic diatexite that occur in the metatexite-dominant area of the Grt-Crd zone of the Aoyama area. The sample mainly consists of biotite + garnet + plagioclase + quartz, and minor K-feldspar and muscovite are also present in the matrix. The garnet is almost homogeneous in terms of major elements except for the grain margin in contact with biotite. Therefore, we utilized discontinuous zonings in terms of P and Y to define the isochronous surface within the garnet during its growth. As a result, garnet is divided into inner core, outer core, mantle, inner rim and outer rim. Inner and outer cores show higher Y concentration compared to other domains, and prograde xenotime is included in the outer core. In the mantle and the inner rim, sillimanite needles are included. Muscovite, plagioclase, K-feldspar are included in the vicinity of the sillimanite in the inner rim. The outer rim has abundant ilmenite inclusions and rare euhedral plagioclase inclusion indicative of partial melting. Fluid inclusions are seen throughout the garnet grain except for the inner rim. These fluid inclusions are composed of both fluid and solid phases. The fluid phase is mainly composed of CH4 or CO2 + CH4, and the solid phases are siderite, chlorite, and clay minerals. Such a mode of occurrence of CH4-bearing COH fluid inclusions is interpreted as a result of post-entrapment reaction between original COH fluid inclusion and host garnet that resulted in the formation of secondary daughter minerals (Carvalho et al., 2019).
Based on pieces of observation described above, the garnet-forming process is explained as follows. (1) High Y concentration and presence of prograde xenotime inclusion and COH fluid inclusion in the outer core of the garnet suggests that the garnet core grew at around 530-570oC (Kawakami et al., 2019) in the presence of COH fluid. (2) Sillimanite, muscovite, plagioclase and K-feldspar in the inner rim suggests that the inner rim grew at around the muscovite breakdown reaction Ms + Ab + Qtz = Sil + Kfs + melt. (3) The COH fluid inclusions, abundant ilmenite and rare euhedral plagioclase enclosed in the outer rim of garnet indicate that the biotite breakdown melting in the presence of COH fluid was responsible for the formation of the outer rim of garnet. Therefore, this study revealed that COH fluid was present during the biotite breakdown melting reaction to form garnet in the middle crust of the upper-amphibolite to granulite facies Ryoke metamorphic belt. Presence of fluid phase can be common even during the partial melting of graphite-bearing pelitic lithology that produces peritectic mineral(s).