The eruptive products from Asama volcano, central Japan, often show textural heterogeneities. Especially, the pyroclastic materials of “B'-scoria fall deposit”, which erupted between the 1108 and the 1783 eruptions, show remarkable textural heterogeneities in various scale.
B' is constituted of scoria, pumice, heterogeneous scoria and some banded pumice. The bulk rock composition of these materials are andesitic (58 wt% SiO₂) to dacitic (63 wt%). These materials show remarkable textural heterogeneities such as the patchy or the banding texture in various scale. These textural heterogeneities are due to the coexistence of four petrographically different groundmass glass parts.
Petrographical features and mineralogical data suggest that the mixing of different types of magmas produced these groundmass glass parts. Furthermore, the patchy and the banding texture indicate that the four parts were mingled with one another.
These features may be interpreted by the mixing model ; magma mixing, producing heteroge-neous mixed magma (having four magma parts), and then the mingling of these four magma parts, which erupted finally and became various pyroclastic materials such as heterogeneous scoria, scoria, pumice and banded pumice.

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Rb-Sr isotopic age determinations were made on the basement rocks of the Northern subzone of the Maizuru Terrane in the western Maizuru City and Oe Town area. The rocks of the area consist mainly of granitic rocks with minor amounts of quartz-diorite, gabbro, diabase, and pelitic schist and amphibolite (Komori metamorphic rocks), being called the Northern Yakuno rocks. They are distinguished from the Southern Yakuno rocks which consist of ophiolitic complex (Yakuno ophiolite) lying in the Southern subzone of the terrane by their lithofacies and whole rock chemistry. The Northern Yakuno rocks are characterized by the abundance of K-feldspar rich granitic rocks, whereas the Southern Yakuno rocks do not contain such rocks. The Northern Yakuno rocks are thrust over the Shimomidani Formation of the Akiyoshi Terrane to the north in the area.
The Rb-Sr mineral isochron ages were determined upon six granitic rocks and one quartzdiorite. They can be divided into two age groups, the older (around 300 Ma) and the younger (around 150 Ma). The rocks give whole rock isochron ages of 327 Ma for the older group and 149 Ma for the younger one, whose initial ⁸⁷Sr/⁸⁶Sr ratios are 0.70669 and 0.70779 respectively. Whole rock isochron age for the gabbros and diabases is 163 Ma, and initial ⁸⁷Sr/⁸⁶Sr ratio is 0.70687. Nearly coincidence between whole rock and mineral isochron ages for each age group explains the existence of two stage igneous activities at those ages in this area, despite their mylonitization and recrystallization. Both whole rock chemistry and the initial ⁸⁷Sr/⁸⁶Sr ratio of those rocks indicate that the Northern Yakuno rocks was evolved as an matured crust of island-arc or continent by the late Carboniferous time. In this respect, the evolutional history of the Northern subzone is quite different from that of the Southern subzone in the terrane.

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