Genetic relationship between the Cretaceous high-Sr tonalite (Itoshima mass) and trondhjemite (Fukae mass) in the central part of Saga Prefecture, northwest Kyushu: Implications for magmatic differentiation
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概要
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Cretaceous granitoids are widely exposed on the northern part of Kyushu Island, and are divided into the tonalite-granodiorite and the granite series, based on the petrographical and geochemical characteristics. The Itoshima and Fukae masses belonging to the tonalite-granodiorite series are significantly voluminous in the Cretaceous granitoids in this area. They correspond to the magnetite-series granitoids, and are characteristically high in Sr. The Itoshima mass geologically surrounds the Fukae mass. The boundary between both masses is gradational. The Itoshima mass is lithologically subdivided into ITO-and ITO-2. ITO-1 contains euhedral hornblende, and is characterized by a preferred orientation of hornblende, biotite and plagioclase. ITO-2 has the same mineral assemblage as ITO-1 but more leucocratic, and is weakly foliated. The Fukae mass shows massive leucocratic lithofacies lacking hornblende. It corresponds to trondhjemite in terms of modal composition. ITO-1 gradually changes into the Fukae mass with decrease in modal hornblende through ITO-2. The bulk chemistry of both masses forms continuous trends in the variation diagrams, suggesting magmatic differentiation, which can be modeled by the fractional crystallization mainly of hornblende + biotite + plagioclase + titanite + magnetite in the early stage, and biotite + plagioclase + magnetite in the later stage. Inferred crystallization depth of the Itoshima mass using the total-Al in hornblende geobarometer is ca 0.6 GPa, which is equivalent to mid-crustal depth. This estimation is supported by the presence of magmatic epidote. In the discrimination Sr/Y-Y diagram, the samples of the Fukae mass are plotted on the adakite field, whereas the Itoshima mass falls within the island arc field. The Fukae adakitic magma could therefore be derived from the dacitic Itoshima magma by a fractionation under the middle crust conditions.
- 一般社団法人 日本地質学会の論文
一般社団法人 日本地質学会 | 論文
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