An orbicular rock from Shiroishi, Miyagi Prefecture

SUZUKI Ioki, AKIZUKI Mizuhiko, NAGASE Toshiro, FUJIMAKI Hirokazu

doi:10.2465/ganko.90.124

Orbicular rock from Inusotoba, Shiroishi, Miyagi Prefecture was studied by optical and chemical methods. The orbicules, which are 5∼20 cm in diameter, are divided into two parts, — core and shell. The core is ball-shaped and dark-colored, and consists mainly of fibrous hornblende and chlorite. The shell, which surrounds core, is light-colored and consists of coarse radial anorthite crystals less than l cm long, which are elongated to a-axis. These radial crystals are characterized by micro subgrains of albite twin and parallel-growing crystals. The other type of orbicule is also known, whose shell consists of several alternate layers of mafic and felsic minerals. This latter type of orbicules is never produced together with the former radial type. The matrix, solidified among orbicules, has a heterogeneous texture, which consists of the dark-part of poikilitic hornblende and the light-part rich in euhedral plagioclase.<br> The forming environment of orbicular rocks is featured by many close-packed orbicules, the flow structure of felsic mineral assembledge in matrix and unmixing of magma including orbicules with the other magma containing plagioclase spherulites. It is considered that orbicules were formed in supercooling magma possibly due to income of another magma.

An orbicular rock from Shiroishi, Miyagi Prefecture

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