Gozaisyo-Takanuki District in Central Abukuma Plateau, Northeastern Japan (Fig., 1) is composed of various kinds of metamorphic and igneous rocks., The northeastern half of the area consists mainly of rather low-grade metamorphic rocks such as green schist, actinolite-green schist, phyllite, fine-grained biotite schist etc., while in the southwestern half, igneous rocks such as granites and quartz-diorites become predominent, being closely associated with high-grade metamorphic rocks which consist of amphibolite, gneissose amphibolite, biotite schist and gneiss (including injection-gneiss) etc., Namitaki, where the amphibolite dykes under consideration are found, is situated in the northern part of the district and is consisted of quartz-diorite and schistose biotite-granite, the latter intruding in the former., As seen in Figs., 2-3, the amphibolite dykes clearly cut the schistose granite and the quartz-diorite xenoliths in it., The directions of the elongation and schistosity of the amphibolite dykes are NW-SE, or NNW-SSE, and are fairly consistent with those of the schistosity of the granite and the arrangement of the quartz-diorite xenoliths., The dykes are grayish green, fine-grained rocks, with siight schistosity, and are mainly composed of plagioclase and hornblende, with small amounts of biotite and quartz (Fig., 4-5., )., Their volume percentages are shown in Table 1., Most of the plagioclases are small granular crystals, but sometimes they show more or less lath-shaped appearance., Judged from their refractive index, they belong generally to andesine-oligoclase, but occasianally have decomposed basic cores., Rarely large porphyritic plagioclases exist, some of which show polysynthetic twinning (Fig., 6-8., )., Hornblende occurs also as small granular crystals, with X-pale yellow, Y-light yellowish green, Z-green, and shows slight parallel orientation., They sometimes flock together, forming large spots which are often elongated in the direction of the schistosity., Biotite forms small flakes, with X-light yellow, Y=Z-yellowish brown to brown, and show parallel orientation, though irregular in distribution., Quartz appears in small-interstitial crystals, which are also irregularly distributed., Judging from their relict textures and mineralogical compositions, the original rocks of the amphibolite dykes might probably be the basic igneous rocks such as dolerites or diabases., After their intrusion into granite and quartz-diorite, they are amphibolitized and at the same time, slightly granitized by the regional metamorphism caused by the orogenetic movement and the concomitant invasion of granitic rocks, which took place probably in late-Palacozoic or early Mesozoic time., The frequent occurencc of ragged and mosaic plagioclases in the schistose granite and quartz-diorite shows that they were also subjected to the simultaneous metamorphism (Fig., 9-12., )., Accordingly we can distinguish the older and the younger granites in the present district., If we consider the granites without the metamorphic textures mentioned above to be the younger granites, most of the granites of this district belong to this group., Five years ago, Dr., K., SUGI suggested the existence of the older and the younger granites in the present district from the following facts: i) that the diaphthoritic rocks derived from high-grade metamorphic exist in the low-grade zone of this district., ii) that these diaphthorites again gradually raise their metamorphic grade., iii) that among them are found those which are considered to be derived from injection-gneisses., The conclusions drawn in the present papar, as to the granites of this district, are fairly agreeable with Dr., K., SUGI's suggestions mentioned above., If the younger granites of this district are late Palaeozoic or early Mesozoic as have been believed, the older granite must be of still more ancient date.,
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Jan 1, 1970
J Latham + 1
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