1) Geologic and Petrographic Work

As shown in Figure I-3, the Kofu granitic body has been surveyed by many workers. However, most of the geologic and petrographic descriptions of the Kofu granitic body based on field observations are incomplete. The first geological map (scale 1: 200,000) of the district was made by Suzuki (1888), in which he assumed that the Kofu granitic body was emplaced during Mesozoic time. Ishizuka (1955) outlined the general geology of Yamanashi Prefecture by drawing a geological map (scale 1: 150,000), and reported that the intrusion of the Kofu granitic body was during Paleogene age. Katada (1956) published a geological map (scale 1: 50,000) of the southeastern part of the Kofu Basin, in which he reported that the intrusion of (biotite-)hornblende tonalite (corresponding to the Ashigawa-type granitoid as defined by Yamanashi Prefecture, 1970) was of Miocene age.

The northern part of the Kofu Basin has also been studied. T. Sato (1957) divided the granitoids into eight types, from biotite granite to biotite-bearing hornblende tonalite. Fujimoto et al. (1958) divided the northern part of the Kofu granitic body into four types based on field observations and nine chemical analyses of the rocks: Mitake-type, Ogarasutype, Tokuwa-type, and Kogarasu-type. They pointed out that Ogarasu-type granodiorite is chemically similar to Tokuwa-type granodiorite. They regarded the Mitakc-, Ogarasu-, and Tokuwa-types to have intruded in Cretaceous time, whereas the Kogarasu-type intruded in the Miocene, although this was questioned by Kawano and Ueda (1966) and others. It was pointed out that the Ashigawa-type is chemically similar to the rocks of the Tanzawa granitic body (Shibata, H. et al,, 1959). They insisted that Tokuwa-type granodiorite was intruded by Ashigawa-type tonalite 1 km north of the Himine Shrine. Morita (1970) and Shimizu and liyama (1981a), however, did not agree with those descriptions of the contact relation. The distribution maps of the granitoids reported by Ishida (1962, 1963) are extremely accurate. Although he divided the eastern part of the Kofu granitic body into five types (diorite to granodiorite), he reported them as a single rock body because of their gradual relationship. Matsubara (1966) also divided the granitoids into six types based on their petrographical characters.

Except for short descriptions, very little lithological information can be found in the literature before the report by Kato (1968a), who divided the Kofu granitic body into ten types mainly based on occurrence and petrography, although the distribution was still obscure. He stated that the Hirose-, Sampo-, and Shiodaira-types have gradual relations with each other, and that the Kanegawa- and Ashigawa-types are also intergradational. He systematically reported 79 modal compositions and 57 chemical compositions through the whole of the Kofu granitic body. Morita (1970) indicated that the K₂O/Na₂O ratios of 35 bulk granitoids increase constantly from south to north, and that the crystallization sequence of the main constituent minerals based on microscopic observations is as follows: pyroxene (cpx and scarcely opx) → plagioclase → hornblende → biotite → quartz → K-feldspar. He assumed, as did Kato (1968a), that the granitic magma of the Ashigawa-type was of a different origin from other types of the Kofu granitic body, because the chemical properties of the Ashigawa-type granitoids greatly resemble those of the granitoids of the Tanzawa granitic body, being quite different from other types in the Kofu granitic body. Morita described 17 modal compositions of Tokuwa batholith rocks, and also described contact metamorphic rocks, indicating that some of the Kobotoke Group rocks belong to the sillirnanite subfacies of the amphibolite fades, after the grouping of Miyashiro (1965).

Yamanashi Prefecture (1970) summarized the data on the Kofu granitic body and divided it into frequently used five types: Mitake-type biotite granite, Ogarasu-type biotite-horn-blende granodiorite, Tokuwa-type biotite-hornblende granodiorite, Ashigawa-type tonalite, and Kogarasu-type tonalite. Tsunoda (1971) described the lithology of small dioritic to gabbroic masses along the Fuefuki River. He reported that they have gradual relations with the host granitoids. Tsunoda et al. (1982) reported 16 modal compositions and 18 chemical compositions of the Tokuwa batholith rocks around the Sakeishi district.

2) Recent Work concerning the Age of Intrusion

The age of intrusion of the Kofu granitic body has been geologically inferred based on the effects of contact metamorphism, as will be mentioned in Chapter III, to be the Miocene by Katada (1956) and most other researchers.

Kawano and Ueda (1964) indicated the age to be 12 Ma (sample number G-82-B, hornblende-biotite granodiorite collected from Sakeishi, Enzan City), 8.7 Ma (G-85-B, hornblende-biotite granodiorite collected from Somaguchi, Enzan City), and 8.0 Ma (G-83-B, biotite granite collected from Shosenkyo, Kofu City). Because their descriptions and dating results in the text and their Table 4 are unfortunately contradictory, their report cannot be cited with authority.

Kawano and Ueda (1966) published the results of K-Ar dating of granitoids in northeastern Japan, in which they again, indicated dating results of the Kofu granitic body as follows:

12 Ma;

G-262, medium-grained biotite granodiorite collected from Shosenkyo (Mitake-type).

14 Ma;

G-282, medium-grained hornblende-biotite granodiorite collected from Kimpusan (Mitake-type).

8.0 Ma;

G-82, fine-grained hornblende-biotite granodiorite collected from Sakeishi (Tokuwa batholith).

7.3 Ma;

G-85, medium-grained hornblende-biotite granodiorite collected from. Somaguchi (Kogarasu-type).

Morita (1970) reported the results of K-Ar dating of the Tokuwa batholith rocks as measured by M. Ojima and H. Haramura of the University of Tokyo:

11 Ma;

NM65070902, granodiorite collected from Tokuwa, Mitomi Village.

12 Ma;

NM65032301, granodiorite collected from Hajikano, Yamato Village.

14 Ma;

NM66072807, tonalite ("Ashigawa-type"), collected from Tonogi, Misaka Town.

Yamanashi Prefecture (1970) gave the results of Rb-Sr dating of biotite from the socalled Ogarasu-type granodiorite (9.5 Ma) corresponding to the Tokuwa batholith and Mitake-type granite (11 Ma) as measured by I. Hayase et al. of Kyoto University.

Shimizu and Yamana (1983) noted that Rb-Sr dating gave Late Miocene ages of 10.1 (±0.4) and 10.5 (±0.1) Ma, and K. Shibata et al. (1984) also reported that the Tokuwa batholith gave Late Miocene ages of 10.1 (±0.1) and 10.8 (±0.3) Ma based on the K-Ar dating method.

The data mentioned above agree with each other, and all the data indicate that the intrusion occurred in the Middle to Late Miocene.

3) New Division of the Kofu Granitic Body

Because the contact relations among the several types of the Kofu granitic body as divided by previous workers have not yet been observed except by Fujimoto et al. (1958) and Kato (1968a), it can be said that each type has not been clearly established. Kato (1968a) reported the gradational variation between Sampo- and Hirosc-types corresponding approximately to the so-called Ogarasu- and Tokuwa-types as defined by Yamanashi Prefecture (1970). Morita (1970) and Shimizu and liyama (1981a) also pointed out the gradational variation between the so-called Tokuwa- and Ashigawa-types as defined by Yamanashi Prefecture (1970).

In summary, a series of plutonic rocks from dioritic to granitic composition are distinguished in the Kofu granitic body. It is necessary to restudy their mode of occurrence and distribution in addition to the interrelationships between ilmenite-series and magnetite-series granitoids in the Tokuwa batholith. The results of the age determinations (Kawano and Ueda, 1966; Morita, 1970; Yamanashi Prefecture, 1970; Shimizu and Yaxnana, 1983; Shibata et al., 1984) indicate that the various results (Miocene) approximately overlap within the range of experimental error.

As shown in Fig. I-3, this paper proposes a new division of the Kofu granitic body based on field observations, lithological characters, and age determinations. The Kofu granitic body is divided into following:

(1) ilmenite-series Mitake(-Shosenkyo)-type biotite granite (12 and 14 Ma).
(2) mainly granodiorite and tonalite of the Tokuwa batholith (8, 9.5, 10.1, 10.1, 10.5, 10,8, 11, 11, 12 and 14 Ma)^* and
(3) magnetite-series Kogarasu-type granodiorite (7.3 and 11 Ma).

So-called Ogarasu-, Tokuwa-, and Ashigawa-type granitoids of Yamanashi Prefecture (1970) are referred to as belonging to the single unit of Tokuwa batholith rocks in this paper. Among the above three granitoids, the Tokuwa batholith, was selected for study because both ilmenite-series and magnetite-series granitoids occur.

* This wide range is considered to be attributed to the open-ended K-Ar system, mainly owing to contamination. by country rocks.[return to the text]