Character and origin of lithofacies in the conduit of Unzen volcano, Japan

Unzen, western Kyushu, Japan, is an andesitic to dacitic, polygenetic volcano that reaches an elevation of 1486 m above sea level. A 1996-m-long hole has been drilled on a slanted trajectory passing beneath the volcano, penetrating the conduit zone of the volcano at 30–150 m below sea level. Spot drill cores, totalling 75 m in length, were recovered between lengths 1582 to 1996 m of the hole. The principal lithofacies of the cores are polymict volcanic breccia (74 vol.% of total drill cores), coherent dacite (13 vol.%), coherent andesite (6 vol.%), partly brecciated coherent dacite (5 vol.%), and volcaniclastic veins (2 vol.%). The polymict volcanic breccia is poorly sorted, non-graded and made up of various clasts of andesite/dacite composition, 1–120 cm across, in a matrix of andesite/dacite fragments, up to 5 mm across. The clasts are subangular to subrounded, non-vesicular to vesicular, and contain 62–66 wt.% SiO 2. This facies is interpreted as forming the subvertical body of a diatreme, and to have been produced by fragmentation of vent–conduit wall rocks by explosive eruptions and associated gravitational failure. The coherent dacite (SiO 2 = 66–67 wt.%) is uniform to flow-banded and commonly has chilled margins. The dacite is porphyritic containing phenocrysts of plagioclase, hornblende, biotite and minor quartz in a non-vesicular groundmass. This facies is interpreted as representing dykes that have intruded into the polymict volcanic breccia. The coherent andesite (SiO 2 = 59 wt.%) and partly brecciated coherent dacite (SiO 2 = 69 wt.%) are massive to fractured, vesicular and porphyritic. These facies are interpreted to be lavas extruded during the old stage (500–300 ka) of the evolution of the Unzen volcano. The volcaniclastic veins occur within all the lithofacies described and range from 0.1 to 250 mm wide. The veins consist of volcanic lithic and mineral fragments up to several millimetres across, and are inferred to have formed by injection of high-temperature fluid and entrained particles into temporarily opened fractures. Below 30–150 m sea level the conduit zone is at least 350 m wide in the north–south direction, and consists of polymict volcanic breccia and east–west-striking subvertical dykes. Both single dykes, 4–8 m wide, and composite dykes, 26 – 44 m wide, are present. Numerous volcaniclastic veins intrude both the breccia and dykes.