Geological and petrological studies on the tanzawa mountainland

In the Tanzawa mountainland, there are various types of plutonic rocks such as norite, tonalite and schistose gabbro in the plutonic mass which have been known as the Tanzawa Quartz diorite mass. The present paper described on the petrography and discussed on the petrogeny of the plutonic rocks based on the field and microscopic observations. They are summarized as follows. 1. The Tanzawa plutonic mass consists of gabbroic rocks and dioritic rocks which were formed in successive order. 2. Gabbroic rocks are composed of mylonitic hornblende gabbro, plagioclase porphyritic hornblende gabbro, schistose hornblende gabbro, metasomatic gabbro, dioritic gabbro, tonalitic gabbro, and diabasic gabbro, which are grouped into two series. Gabbro amphibolite series and hornblende gabbro series. Both of the series are originated from diabasic gabbro, but one metamorphosed to the amphibolite by mylonitization and recrystallization and other to the hornblende gabbro by the acidification, though the two courses were displayed simultaneously. 3. The dioritic rocks are composed of gabbroic diorite, normal hornblende diorite, gneissose biotite diorite, plagioclase porphyritic diorite, quartz porphyritic diorite, and tonalitic diorite, which were formed in successive order. The first two are related to the initial stage, the middle one to the upheaval stage, and the last three to the intrusion stage of the complex. These dioritics are derived from gabbroics resulting from the strong enrichment of SiO2 and Na2O. 4. Gabbro amphibolite series often gradually changes into or intercalate with amphibolites while the dioritic rocks of the intrusion stage has hornfelses around them. The other series or types of gabbroic and dioritic rocks has least relation with metamorphic rocks. They are in close strict relation to each other as main constituents of the dioritization process of gabbro in the plutonic mass.