Abstract
The 1991-1995 dacite of Unzen volcano contains mafic inclusions. They are usually ovoidal and 1-50 cm in diameter, constituting ca. 0.2-1 volume % of the host dacite. Nakada and Motomura (1997) showed that the bulk rock SiO2 contents of the mafic inclusions range from 50 to 60 wt%, and suggested that these mafic inclusions are derived from the marginal boundary layer of the magma chamber of Unzen volcano. In the present study, the author carried out petrographic work on the mafic inclusions, and concluded that the mafic inclusions represents quenched liquid drops of high-temperature mafic magmas in low-temperature felsic magma based on the following observations; i.e. (1) chilled margin in the mafic inclusion, (2) ubiquitous occurrence of skeletal plagioclase and hornblende in the mafic inclusions, (3) plagioclase microlites in the mafic inclusions tend to have higher Ca/(Ca+Na) ratio compared with the phenocryst plagioclase in the dacite, (4) pyroxenes in the mafic inclusions show high equilibration temperatures of ca. 1075°C. (1) Among 50 mafic inclusions, only one sample showed fined grained chilled margin against the host dacite. Grain width of plagioclase and hornblende are 0.02-0.05mm to 0.1-0.2 mm. (2) Generally, the mafic inclusions show diktytaxitic texture, consisting of elongated microlites of plagioclase and hornblende in vesicular glass. Even tabular plagioclase show skeletal distribution of Ca-rich framework with Na-rich plagioclase filling the hollow of the framework. (3) Although mineral compositions vary among mafic inclusions, Ca/(Ca+Na) ratio of plagioclase in mafic inclusions tends to be higher (up to 0.91) than that of phenocrysts in the dacite(up to 0.70). (4) One mafic inclusion contained pigeonite, augite, and orthopyroxene, giving equilibration temperature of 1075°C, where as only orthopyroxene appears in some of the mafic inclusions. A magma mixing model of Holtz et al.(submitted) indicate high-temperature end member magma has SiO2 content of 60-64 wt% with temperature of 1030°C, which is slightly high in SiO2 compared with the range of mafic inclusions. This discrepancy may be reconciled when we take the rheological behaviour of mafic magma during mafic-felsic magma interaction into consideration. Sparks and Marshall(1986) and Blake and Koyaguchi (1991) pointed out that mafic inclusion may form only if the temperature difference of two magmas is large, and the mass ratio of mafic to felsic magma is small, in which conditions the mafic magma would be congealed to form mafic inclusions. In other conditions the mafic magma remains fluidal and hybridization of the two magma proceeds, which actually took place to form the hybridized dacite of the 1991-1995 dacite of Unzen volcano.
