5:15 PM - 7:15 PM
[HRE12-P11] Tonalitic melt inclusions in basaltic rock-hosted diopside megacrysts from the Tanzawa area at the Southern Fossa Magna region
Keywords:Granitic melt inclusion, Diopside, Tonalite, Porphyry copper deposit
Most hydrothermal metal ore deposits in the island arc are formed by hydrothermal fluids from granitic magma, and the chemical compositions and behaviors of the fluids are closely related to the original chemistry of the granitic magma. It is important to elucidate the chemical characteristics of primitive granitic melts that occur in the lower to upper crust. However, granitic bodies have lost their primitive chemical characteristics due to crystallization differentiation and hydrothermal fluid separation, making it difficult to obtain primary information on ore deposit from exposed granites.
We have searched for primitive granitic melt inclusions trapped in diopside megacrysts in undifferentiated basalt ascending from the upper mantle and clarified the relationship between the chemical characteristics of primitive granitic melt and surface hydrothermal ore deposits in the Sanogawa River area in the southern Fossa Magna (Amagai & Kurosawa, 2023).
In this study, we examined the chemical characteristics of the primitive magma that formed the Tanzawa tonalite (Kanagawa Prefecture), a typical M-type granite in Japan, by examining melt inclusions contained in diopside phenocrysts in basalt from the Tanzawa area. M-type granites, which have mantle-like characteristics, are associated with large porphyry copper deposits on continental margins. However, in the Tanzawa tonalite masses, they are only accompanied by a small number of vein-type copper deposits and layered iron sulfide deposits. The Tanzawa tonalite is causally related to the collision of the Izu-Ogasawara arc, which is accompanied by volcanic rocks containing high concentrations of copper, and it is possible that it has the potential to form porphyry copper deposits. On the other hand, the mineralized zone of the Tanzawa tonalite mass may have been lost due to rapid denudation. Therefore, clarifying the chemical characteristics of the primitive granitic magma may help to explain the reasons for the absence of large-scale deposits.
The Hayatogawa River area, located in the eastern part of the Tanzawa tonalite masses, is distributed with the Tanzawa Group, which consists of submarine sediments and volcanic rocks. Samples of basaltic lava and basaltic andesite dykes have been collected. The chemical composition of these basaltic rocks is indicative of low to intermediate K-series volcanic rocks and is similar to the Mg-rich basalts (ankaramitic) of the rift zone during the collision of the Izu-Ogasawara arc as described by Kawate (1997). The basalt contained large phenocrysts of diopside with thick rims of augite, and the basaltic andesite contained augite. Both phenocrysts showed compositional zoning and the cores were irregular to oval in shape. The inclusions were primary melt inclusions that distributed the cores and compositional zoning. Two types of granitic melt inclusions were identified: the microcrystalline-type, which is partially crystallized in negative crystals, and the glassy- type, which is oval in shape and composed almost entirely of glass. The microcrystalline-type was composed of partially crystallized heterogeneous residual glass, rich in silica and alkaline, and many pores. It was accompanied by magnetite, amphibole and apatite, and sometimes contained chalcopyrite and pyrite. The glassy-type is almost homogeneous and contains bubbles, magnetite and hornblende, pyrrhotite and chalcopyrite were observed. As voids and bubbles contain precipitates, it is possible that the melt was trapped prior to hydrothermal fluid separation. The diopside of the basalt contain mainly microcrystalline-types, and the augite of the basaltic andesite contain glass types.
The formation pressure of the core of diopside in the basalt was about 6-8 kb, which is close to the formation pressure of the parent magma of the Tanzawa tonalite (10 kb, Nakajima & Arima 1998). The formation pressure of the augite in the basaltic andesite was slightly lower at 3-6 kb. Therefore, the microcrystalline-type melt inclusions in the diopside are important for elucidating the characteristics of the primitive granitic magma. The estimated average chemical composition of these melt inclusions is equivalent to a low K series granodiorite of 63 wt% SiO2 and 0.4 wt% K2O. This suggests that they may have captured primitive Tanzawa tonalitic melt in the lower part of the upper crust. Compared to the primary granitic melt in the Sanogawa River area, the amount and type of sulfide minerals, and the sulfur and water contents are low. Therefore, it is estimated that the conditions were even less conducive to the formation of ore deposits than in the Sanogawa area.
We have searched for primitive granitic melt inclusions trapped in diopside megacrysts in undifferentiated basalt ascending from the upper mantle and clarified the relationship between the chemical characteristics of primitive granitic melt and surface hydrothermal ore deposits in the Sanogawa River area in the southern Fossa Magna (Amagai & Kurosawa, 2023).
In this study, we examined the chemical characteristics of the primitive magma that formed the Tanzawa tonalite (Kanagawa Prefecture), a typical M-type granite in Japan, by examining melt inclusions contained in diopside phenocrysts in basalt from the Tanzawa area. M-type granites, which have mantle-like characteristics, are associated with large porphyry copper deposits on continental margins. However, in the Tanzawa tonalite masses, they are only accompanied by a small number of vein-type copper deposits and layered iron sulfide deposits. The Tanzawa tonalite is causally related to the collision of the Izu-Ogasawara arc, which is accompanied by volcanic rocks containing high concentrations of copper, and it is possible that it has the potential to form porphyry copper deposits. On the other hand, the mineralized zone of the Tanzawa tonalite mass may have been lost due to rapid denudation. Therefore, clarifying the chemical characteristics of the primitive granitic magma may help to explain the reasons for the absence of large-scale deposits.
The Hayatogawa River area, located in the eastern part of the Tanzawa tonalite masses, is distributed with the Tanzawa Group, which consists of submarine sediments and volcanic rocks. Samples of basaltic lava and basaltic andesite dykes have been collected. The chemical composition of these basaltic rocks is indicative of low to intermediate K-series volcanic rocks and is similar to the Mg-rich basalts (ankaramitic) of the rift zone during the collision of the Izu-Ogasawara arc as described by Kawate (1997). The basalt contained large phenocrysts of diopside with thick rims of augite, and the basaltic andesite contained augite. Both phenocrysts showed compositional zoning and the cores were irregular to oval in shape. The inclusions were primary melt inclusions that distributed the cores and compositional zoning. Two types of granitic melt inclusions were identified: the microcrystalline-type, which is partially crystallized in negative crystals, and the glassy- type, which is oval in shape and composed almost entirely of glass. The microcrystalline-type was composed of partially crystallized heterogeneous residual glass, rich in silica and alkaline, and many pores. It was accompanied by magnetite, amphibole and apatite, and sometimes contained chalcopyrite and pyrite. The glassy-type is almost homogeneous and contains bubbles, magnetite and hornblende, pyrrhotite and chalcopyrite were observed. As voids and bubbles contain precipitates, it is possible that the melt was trapped prior to hydrothermal fluid separation. The diopside of the basalt contain mainly microcrystalline-types, and the augite of the basaltic andesite contain glass types.
The formation pressure of the core of diopside in the basalt was about 6-8 kb, which is close to the formation pressure of the parent magma of the Tanzawa tonalite (10 kb, Nakajima & Arima 1998). The formation pressure of the augite in the basaltic andesite was slightly lower at 3-6 kb. Therefore, the microcrystalline-type melt inclusions in the diopside are important for elucidating the characteristics of the primitive granitic magma. The estimated average chemical composition of these melt inclusions is equivalent to a low K series granodiorite of 63 wt% SiO2 and 0.4 wt% K2O. This suggests that they may have captured primitive Tanzawa tonalitic melt in the lower part of the upper crust. Compared to the primary granitic melt in the Sanogawa River area, the amount and type of sulfide minerals, and the sulfur and water contents are low. Therefore, it is estimated that the conditions were even less conducive to the formation of ore deposits than in the Sanogawa area.