Presentation Information
[R7-09]Granitic melt inclusions in undifferentiated basalt-hosted diopside megacrysts from the Southern Fossa Magna region
*Takashi Amagai1 (1. AIST. GSJ)
Keywords:
Granitic melt inclusion,Diopside megacrysts,Undifferentiated basalt,Hydrothermal ore deposits
Hydrothermal ore deposits in Japanese island arcs are formed by fluids derived from granitic magmas, whose original chemistry is critical for understanding ore-forming processes. However, exposed granites often lose primary signatures through differentiation and fluid separation. To address this, we investigated granitic melt inclusions trapped in phenocrysts of undifferentiated basalts that likely ascended from the mantle and captured lower crustal granitic melts.
We identified granitic melt inclusions in diopside megacrysts from Neogene basaltic dykes in the Sanogawa area (Yamanashi Prefecture) and submarine basaltic rocks related to the early Izu–Bonin arc collision in the Tanzawa area (Kanagawa Prefecture) (Amagai & Kurosawa, 2023; 2024, 2025, Abstracts of the Society of Resource Geology).
In Sanogawa, Cr-diopside megacrysts occur in ankaramitic basalts and contain fully crystallized granitic melt inclusions of quartz, plagioclase, K-feldspar, and amphibole, with minor pyrite and chalcopyrite. Trace sulfides (Ni, Zn, Cd, Pb), Au–Ag compounds, and platinum minerals were also observed. Other inclusions of chlorite and calcite were present, but no fluid inclusions.
In Tanzawa, Mg-rich basaltic rocks contain diopside and augite megacrysts with two inclusion types: microcrystalline (in diopside) and glassy (in augite). The former includes Si–K–Na-rich glass with magnetite, hornblende, and occasional chalcopyrite and pyrite. The latter consists mainly of homogeneous glass with magnetite, hornblende, and Fe-sulfides.
Trapping pressures (7–12 kb), estimated from pyroxene crystallization conditions, match depths of granitic magma generation. Average granitic melt compositions indicate metaluminous, calc-alkaline granodiorite in Sanogawa, consistent with medium-K Neogene granites. In Tanzawa, granitic melts resemble the parent magma of M-type Tanzawa tonalite. Water content was low (<1.2 wt%), while S and metal contents were comparable to or higher than those in fertile Japanese granites.
Sulfide species and their relative abundances in the melt inclusions are consistent with those in nearby Neogene vein-type deposits, suggesting that granitic melt chemistry preserved in undifferentiated basalts may provide insights into the nature of associated ore-forming systems.
We identified granitic melt inclusions in diopside megacrysts from Neogene basaltic dykes in the Sanogawa area (Yamanashi Prefecture) and submarine basaltic rocks related to the early Izu–Bonin arc collision in the Tanzawa area (Kanagawa Prefecture) (Amagai & Kurosawa, 2023; 2024, 2025, Abstracts of the Society of Resource Geology).
In Sanogawa, Cr-diopside megacrysts occur in ankaramitic basalts and contain fully crystallized granitic melt inclusions of quartz, plagioclase, K-feldspar, and amphibole, with minor pyrite and chalcopyrite. Trace sulfides (Ni, Zn, Cd, Pb), Au–Ag compounds, and platinum minerals were also observed. Other inclusions of chlorite and calcite were present, but no fluid inclusions.
In Tanzawa, Mg-rich basaltic rocks contain diopside and augite megacrysts with two inclusion types: microcrystalline (in diopside) and glassy (in augite). The former includes Si–K–Na-rich glass with magnetite, hornblende, and occasional chalcopyrite and pyrite. The latter consists mainly of homogeneous glass with magnetite, hornblende, and Fe-sulfides.
Trapping pressures (7–12 kb), estimated from pyroxene crystallization conditions, match depths of granitic magma generation. Average granitic melt compositions indicate metaluminous, calc-alkaline granodiorite in Sanogawa, consistent with medium-K Neogene granites. In Tanzawa, granitic melts resemble the parent magma of M-type Tanzawa tonalite. Water content was low (<1.2 wt%), while S and metal contents were comparable to or higher than those in fertile Japanese granites.
Sulfide species and their relative abundances in the melt inclusions are consistent with those in nearby Neogene vein-type deposits, suggesting that granitic melt chemistry preserved in undifferentiated basalts may provide insights into the nature of associated ore-forming systems.