Abstract
The southern subzone of the Maizuru zone is occupied by an ophiolite suite composed of such members as harzburgite, dunite, pyroxenite, gabbro, and basalt in ascending order. Pillow structure is not developed in the basalts, which is frequently interbedded with mudstone. Distinct sheeted dike complex is absent, and several quartz diorite bodies are present in the lower part of the basalt member. The gabbro member exhibits strong metamorphic characteristics, and mineral parageneses in the ultramafic members are also metamorphic. The upper three members are distributed symmetrically along the axis of a possible isoclinal syncline, but the lower two members occur only in its southern flank. Maximum thickness of the exposed sequence is estimated to be about 6 km. Layering features are pervasive through lower three members, and cryptic layering units have been recognized in the dunite member. Systematic change in the composition of olivine in these members provides another evidence to conclude that these members were originally formed by the igneous fractionation. On the other hand, the composition and its variation trend of the basic rocks indicate ocean floor affinity of the magma from which they were formed. Ferric-poor spinels found in quenched lava flows in the basalt member not only confirm this affinity, but also suggest comagmatic relationship between the mafic and ultramafic members. The ophiolite formed by such igneous processes subsequently suffered metamorphism. Preliminary analysis of mineral parageneses show that basic rocks in the upper, middle and lower parts of the basalt member belong to the prehnite-pumpellyite, greenschist, and epidote amphibolite facies respectively, and that the underlying gabbro member belongs to the amphibolite facies. Furthermore, medium pressure granulite facies condition was attained below the gabbro member. Such a metamorphism may have taken place under a high thermal gradient near the site of the igneous formation of the ophiolite. Thus, it is concluded that the Yakuno ophiolite may be an on-land remnant of an oceanic crustmantle constructed at sediment-available sea floor. This conclusion necessarily requires a new dynamic model involving the formation and deformation of this oceanic crust-mantle to explain the development of the Permo-Triassic orogeny of inner southwest Japan.
