17:15 〜 19:15
[SGC37-P04] Water contents and pressures estimated from melt inclusions in zircon of the Cretaceous Habu granitoid pluton, San-yo Belt, southwest Japan.
キーワード：土生岩体、花崗岩、ジルコン中メルト包有物
Melt water contents and crystallization pressures of granitic magmas are fundamental to unraveling the tectonic evolution of orogenic belts. Recently, Taniwaki et al. (2023, Lithos) and Kawashima et al. (2024, JMPS) proposed a new approach to estimating melt water contents and crystallization pressures of granitoid magmas using melt inclusions in zircon, a ubiquitous accessory mineral in granitoids. In this study, we newly estimated water contents of melt inclusions in zircon and applied melt geobarometer to a shallow-level intrusion of the Cretaceous Habu granitoid pluton in the San-yo Belt, southwest Japan. We further compare our results with the data reported from a deeper-level intrusion of the Cretaceous Gamano granodiorite in the Ryole Belt, southwest Japan (Kawashima et al., 2024, JMPS).
The samples studied are a hornblende-biotite-bearing granodiorite sample from the outer domain of the Habu pluton and biotite-bearing granite sample from the inner domain of the Habu pluton. Zircon grains (mostly 200–50 µm in length) contain rounded inclusions of variable size (mostly 3–10 µm) comprising quartz, alkali feldspar, and plagioclase. These polymineralic quartz-felspathic inclusions are interpreted as crystallized melt inclusions trapped in zircon during crystal growth within a granitic melt. Homogenization experiments of the polymineralic inclusions in zircon have been conducted under high-pressure using a piston-cylinder high-pressure–high-temperature apparatus. Major element analyses of the homogenized (glass) inclusions are carried out using a SEM-EDS after the homogenization experiments. The homogenized compositions of the inclusions have higher SiO2 contents (76–78 wt%) than the whole-rock SiO2 contents (outer domain: 64 wt%, inner domain: 73 wt%) of the zircon-separated samples. We also estimated water contents of melt inclusions from SEM-EDS analysis (cf., Kawashima et al., 2024, JMPS) which resulted in 6.8–9.8 wt%. The results imply that the melt inclusions in zircon represent fractionated hydrous interstitial melts trapped in growing zircon crystals, which is consistent with the microscopic observations of zircon occurrences at the grain boundaries between major minerals or the rim domain in the biotite. The melt inclusions have peraluminous compositions (alumina saturation index: A.S.I. = 1.06–1.13).
We tested the melt geobarometer of Yang et al. (2022, CMP) which considers the effect of the excess Al on the barometry because the melt compositions obtained in this study are slightly peraluminous (A.S.I. = 1.06–1.19). The calculated pressures (433–198 MPa with error of ± 120 MPa) based on Yang et al. (2022, CMP) probably represent the pressures of zircon crystallization from peraluminous interstitial melt in the Habu granitoid magma. The lowest pressure (i.e., 198 ± 120 MPa) among the calculated pressures was then considered as the pressure of final magma emplacement, consistent with the pressure conditions of contact metamorphism (~100–300 MPa) in the study area (Ikeda, 2004, CMP). Assuming an average crustal density of 2.65 g/cm3, the emplacement depth of the Habu pluton (198 ± 120 MPa) can be estimated as 7.6 (± 4.6) km.
Recently, Kawashima et al. (2024, JMPS) have reported the melt inclusions in zircon with fractionated peraluminous compositions (SiO2 = 76–78 wt% and A.S.I. = 1.10–1.27) from the Cretaceous Gamano granodiorite (at ~35 km south of the Habu pluton) of the Yashiro-jima Island in the Ryoke Belt. It has been previously suggested that the emplacement of the Gamano granodiorite was contemporaneous with peak metamorphism. The melt geobarometer of Yang et al. (2022, CMP) yields 761–226 MPa (with error of ± 120 MPa) from the Gamano melt inclusions. The calculated pressures for the Gamano granodiorite extend to those lower than the metamorphic pressures in the Yashiro-jima Island (~690–420 MPa) (Ikeda, 2004, CMP). The results imply that the Gamano magma continued to ascend to shallow levels after the initial intrusion into the deep crust contemporaneous with the peak metamorphism.
The samples studied are a hornblende-biotite-bearing granodiorite sample from the outer domain of the Habu pluton and biotite-bearing granite sample from the inner domain of the Habu pluton. Zircon grains (mostly 200–50 µm in length) contain rounded inclusions of variable size (mostly 3–10 µm) comprising quartz, alkali feldspar, and plagioclase. These polymineralic quartz-felspathic inclusions are interpreted as crystallized melt inclusions trapped in zircon during crystal growth within a granitic melt. Homogenization experiments of the polymineralic inclusions in zircon have been conducted under high-pressure using a piston-cylinder high-pressure–high-temperature apparatus. Major element analyses of the homogenized (glass) inclusions are carried out using a SEM-EDS after the homogenization experiments. The homogenized compositions of the inclusions have higher SiO2 contents (76–78 wt%) than the whole-rock SiO2 contents (outer domain: 64 wt%, inner domain: 73 wt%) of the zircon-separated samples. We also estimated water contents of melt inclusions from SEM-EDS analysis (cf., Kawashima et al., 2024, JMPS) which resulted in 6.8–9.8 wt%. The results imply that the melt inclusions in zircon represent fractionated hydrous interstitial melts trapped in growing zircon crystals, which is consistent with the microscopic observations of zircon occurrences at the grain boundaries between major minerals or the rim domain in the biotite. The melt inclusions have peraluminous compositions (alumina saturation index: A.S.I. = 1.06–1.13).
We tested the melt geobarometer of Yang et al. (2022, CMP) which considers the effect of the excess Al on the barometry because the melt compositions obtained in this study are slightly peraluminous (A.S.I. = 1.06–1.19). The calculated pressures (433–198 MPa with error of ± 120 MPa) based on Yang et al. (2022, CMP) probably represent the pressures of zircon crystallization from peraluminous interstitial melt in the Habu granitoid magma. The lowest pressure (i.e., 198 ± 120 MPa) among the calculated pressures was then considered as the pressure of final magma emplacement, consistent with the pressure conditions of contact metamorphism (~100–300 MPa) in the study area (Ikeda, 2004, CMP). Assuming an average crustal density of 2.65 g/cm3, the emplacement depth of the Habu pluton (198 ± 120 MPa) can be estimated as 7.6 (± 4.6) km.
Recently, Kawashima et al. (2024, JMPS) have reported the melt inclusions in zircon with fractionated peraluminous compositions (SiO2 = 76–78 wt% and A.S.I. = 1.10–1.27) from the Cretaceous Gamano granodiorite (at ~35 km south of the Habu pluton) of the Yashiro-jima Island in the Ryoke Belt. It has been previously suggested that the emplacement of the Gamano granodiorite was contemporaneous with peak metamorphism. The melt geobarometer of Yang et al. (2022, CMP) yields 761–226 MPa (with error of ± 120 MPa) from the Gamano melt inclusions. The calculated pressures for the Gamano granodiorite extend to those lower than the metamorphic pressures in the Yashiro-jima Island (~690–420 MPa) (Ikeda, 2004, CMP). The results imply that the Gamano magma continued to ascend to shallow levels after the initial intrusion into the deep crust contemporaneous with the peak metamorphism.