Abstract
Paleogene intrusive rocks are distributed in the eastern part of the San'in province, Southwest Japan. They are called the Namariyama Intrusives. They are tonalitic to granitic in composition and are commonly characterized by porphyritic texture. They are easily discriminated from the Late Cretaceous Imbi Intrusives in the same province by means of the characteristic porphyritic texture. The authors will describe the geology, petrography and mineral chemistry of the Yubarako granophyre intrusion, one of the largest massif of the Namariyama Intrusives, in this paper. The Yubarako granophyre intrusion occupies an area of about 67 km2 and crops out as a gourd-shaped body in the northern part of Okayama Prefecture. It consists largely of the leucocratic porphyritic rocks, that is, granophyre, granite porphyry and fine-grained granophyre. They are granitic in composition, commonly including phenocrysts of quartz, plagioclase and biotite. The texture of groundmass is, however, gradually changing from the granophyre, granite porphyry to the fine-grained granophyre. They are distributed from center to margin in this order. It is concluded that varying cooling condition in the intrusion gave rise to this textural variation. The emplacement of the leucocratic porphyritic rocks was preceded by the formation of small bodies of the granodiorite porphyry at the margin of the Yubarako granophyre intrusion, and was succeeded by the injection of many thin aplitic dikes in the southern part of the intrusion. Compared with the Late Cretaceous plutonic rocks in the San'in province, the Yubarako granophyre intrusion is characterized by lower K2O/Na2O in whole rock composition, and by higher Mg/Fe ratio in biotite, lower Or/Ab ration in potassium feldspar.
