[S-GC32] Volatiles in the Earth - from Surface to Deep Mantle

[SGC32-P07] Melt inclusions in zircon of the Gamano granodiorite in Ryoke Belt from Yashiro Island, Yamaguchi Prefecture, Japan

*川島 泰地¹、齊藤 哲¹、下岡 和也¹ (1.愛媛大学大学院理工学研究科)

キーワード：珪長質マグマ、地質温度圧力計、花崗岩類、ジルコン中メルト包有物、含水量

Granitoids represent unerupted felsic magmas crystalized in the crust. In this study, we estimate water contents and crystallization pressures of granitoid using melt inclusions in zircon, a ubiquitous accessory mineral in granitoids. Homogenization experiments of polymineralic inclusions hosted in zircon have been conducted for a granitoid sample from the Cretaceous Gamano granodiorite in Yashiro-jima Island, southwest Japan, using a piston-cylinder high pressure-high-temperature apparatus. Water contents of 4.2-11.3 wt% and high SiO₂ contents (76-78 wt% anhydrous basis) of homogenized melt inclusions are revealed by SEM-EDS analysis implying that they represent fractionated interstitial hydrous melts trapped in growing zircon crystals. The MagMaTaB geobarometer applied for the melt inclusions yields pressures ranging from 563 to 266 MPa (with error of ± 100 MPa), interpreted as the crystallization pressures of zircons. Comparing the results with the metamorphic pressures of adjacent metamorphic rocks previously estimated (529-420 MPa; Ikeda, 2004), the pressures obtained in this study (563-266 ± 100 MPa) are comparable to slightly lower. This suggests that zircon crystallized at various pressures during magma ascent after the Gamano granodiorite magma intruded into the host rock. Furthermore, the estimated temperature (731-706 ℃ with error of ± 25 ℃) using the MagMaTaB geothermometer) show slight increase with decreasing pressure implying reheat of magma reservoir during ascent probably induced by injection of mafic magma into magma reservoir, which is consistent with field observation of abundant mafic magmatic enclaves in the Gamano granodiorite. The pressures and water contents of the 14 melt inclusions estimated in this study plot broadly along the water solubility curve of haplogranitic melts in the P-T diagram suggesting the high water activity of the interstitial melts during zircon crystallization in the Gamano granodiorite magma.