長野市貉郷路安山岩体の鉱物組成と化学組成とくにその成因的考察

抄録

Muzina-goro is a neck-like mass of andesite, about 200m across and 70m high, intruded into the mudstone of the Ogawa formation of upper Miocene, in the western part of Nagano City. Vertical columnar joints are beautifully displayed. It is a composite mass composed of the andesite of tholeiite series (Vc type) in the interior, and of andesite of calc-alkali rock series (Vd type) in the margin (Fig. 1), and the two types gradually change to each other without a sharp boundary. By systematic sampling, it was possible to trace the course of crystallization and consequently to illustrate the genesis of the two types. Presence of phenocrystic pigeonite around augite is remarkable in the andesite of both Vc and Vd types. Although the two types are similar, small but distinct difference is observed between the two in their chemical composition as well as mineral composition. This difference can not entirely be attributed to the differentiation in situ, for the chilled margin is also composed of a similar andesite of Vd type. When the chemical composition of these various facies are plotted in the MgO-FeO+Fe₂O₃-Na₂O+K₂O diagram, they all lie within the area of calc-alkali rock series of this district. The ratio of rhombic pyroxene: total pyroxene (Rh + 100/Rh + Mo) in the groundmass varies in wide range, although these rocks are chemically much alike in the present case. Probably, the original magma was differentiated at the depth into two parts, which later successively intruded (Fig. 4). It may be postulated that high oxygen partial pressure, p₀₂, (Osborn, 1959) due to the contamination of water from the surrounding mudstone favoured the formation of calc-alkali rock series in the upper layer of the two parts separated at the depth. p₀₂ might also be higher in the margin than in the inner part when intruded in the mudstone. The wide variation of the ratio, Rh × 100/Rh + Mo, in the groundmass of calc-alkali rock series may be explained by the variation of p₀₂ in the chilled margin of these rocks. In this connection, it is noted that Fe₂O₃ content is higher in the contact part (M 2) than in the inner part (M 3), total iron oxide remaining the same.