Ryoke Zone of the Toyone-Mura Area, Aichi Prefecture, Japan*

J. Japan. Assoc. Min. Petr. Econ. Geol. 69. 52-74, 1974

PETROLOGY OF THE GABBROIC ROCKS IN THE RYOKE ZONE OF THE TOYONE-MURA AREA, AICHI PREFECTURE, JAPAN*

TOSHIO KUTSUKAKE

Department of Geology and Mineralogy, Faculty of Science, Kyoto University, Kyoto, Japan

have genetical relations such as co-magma INTRODUCTION tic ones to the Ryoke granites or not, and In the Ryoke zone, gabbroic rocks occur the nature of the metamorphism, they have sporadically as small masses in both gra suffered, are still to be elucidated. nites and metamorphic rocks. These have During his work on the Ryoke meta been named "biotite-quartz diorite" (Dbq) morphic rocks of the Toyone-mura area, the in the Geological Map 1:75,000 of the Geol. author's attention was called to these pro Surv. of Japan (Notomi, 1924), but they blems of the gabbroic rocks. In this paper, are generally pyroxene-bearing hornblende their mode of occurrence, petrography, che gabbros and/or norites, frequently accom mistry and mineralogy are described and panied by cortlandite. their genesis and metamorphism will be The gabbroic rocks in the Ryoke zone discussed in some detail. have been studied by several authors (Yoshi mura, 1940; Yagi, 1944; Yoshizawa, 1949, GEOLOGICAL OUTLINE 1950, 1951, 1952; Suwa, 1961; Soma, 1963; Toyone-mura area occupies the south Kutsukake, 1973a). However, whether they eastern part of the Ryoke zone in central

Fig. 1. Geological map of the Toyone-mura area. Legend 1: Alluvium, 2: Debris, 3: Terrace deposits, 4: Basalt and andesite, 5: Dacite, 6: Rhyolite, 7: Tertiary sediments, 8: Sambagawa schists, 9: Busetsu granite, 10: Inagawa granite, 11: Mitsuhashi granite, 12: Tenryukyo granite, 13: Kamihara quartz diorite, 14: Fine grained biotite granodiorite, 15: Mylonite, 16: "Halleflinta," 17: Gabbro, 18: Metadiabasic rock, 19: Hornfels, 20: Micaschist and quartzschist, 21: Gneiss, 22: Marble , 23: Metamorphic rock derived from "Schalstein," 24: Fault. M. T. L.: Median Tectonic Line , I. S.T. L.: Itoigawa - Shizuoka Tectonic Line. Locality name As: Asakusa, Ch: Chausu-yama, Fu: Futto, Ha: Hanare-yama , Ho: Hongo, Hy: Hiyosawa, Ib: Ichibara, In: Inoshikori, Ka: Kami-Awashiro, Ko: Kobayashi, Kt: Kakino taira, Md: Midashi, Mk: Makinoshima, Ms: Misono, Ni: Niino-toge, Ns: Nakashitara , Od: O dachi, Oh: Ohata, Oi: Oiwa-dake, Os: Osawa, Oz: Ozasa-yama, Sk: Shimo-Kurokawa , So: Sogawa, Su: Sakauba, Tk: Tashika, Ts: Tsugawa, Tw: Tawagane-toge, Uk: Urakawa, Ur: Ure , Zn: Zinno-yama.

* Presented in Part at the 80th Annual Meeting of the Geological Society of Japan , held in Sendai, on April 4, 1973 (Kutsukake, 1973b). (Manuscript received, October 3, 1973) Petrology of the gabbroic rocks in the Ryoke zone of the Toyone-mura area 53 54 T. Kutsukake

Japan. Preliminary report of geology of Three gabbroic masses are found, one this area has been presented (Kutsukake, of which is in metamorphic rocks and other 1970). two are in granites. Conveniently they are In the southeastern part of the area, called the Inoshikori-, Tashika-, and Ohata granitic rocks (the Tenryukyo granite) oc mass respectively. cupy the wide extention and in the central MODE OF OCCURRENCE and western parts, metamorphic rocks ex ceed granitic rocks in extent. Especially (1) The Inoshikori mass in the central part, the basic metamorphic This mass is the largest of three. It rocks, which are metadiabasic rocks differ occurs in gneiss, and is intruded by medium ent from the gabbros of the present subject, grained gneissose biotite granodiorite, a occur as large mass. Geological map is member of the Kamihara quartz diorite, shown in Fig. 1. which is one of the oldest granitic rocks in Based on the mineralogical variations in the Ryoke zone in central Japan (Ryoke pelitic rocks, it is inferred that the meta Research Group, 1972). And the mass is morphic grade increases to the centre from accompanied by fine-grained diabasic rocks both the eastern and western sides. The of xenolithic form. eastern limit part belongs to the cordierite The mass occupies an area of approxi zone, the neighbouring eastern and the mately two square kilometres. western parts to the first silimanite zone, and the central part to the second silliman ite zone, defined by Hayama (1960, 1964) respectively.

Fig. 2. Route-map along the road from Sakauba to Inoshikori. 1. Tertiary dyke rock, 2: Medium-grained gneis sose biotite granodiorite, 3: Metadiabasic rock, Fig. 3. Route-map along the road from 4. Micaschist and quartzschist, 5. Gneiss, 6. Tashika to Shin-toyone dam. Aplite, 7. Quartz gabbro, 8. Hornblende gabbro, 1. Tenryidkyo granite, 2. Kamihara quartz 9. Olivine-hornblende norite and hornblende no diorite, 3. Metadiabase, 4. Quartzofelds rite, 10. Olivine norite. pathic gneiss, 5. Hornblende gabbro. Petrology of the gabbroic rocks in the Ryoke zone of the Toyone-mura area 55

Route-map along the road from Saka grained plagioclase-porphyritic biotite gra uba to Inoshikori is shown in Fig. 2. nodiorite intruded into the mass. Moreover, a biotite granite dyke of 50cm width occurs.

(2) The Tashika mass Route-map across the centre is shown It is composed of several separate bodi in Fig. 4. es, occurring between the Kamihara quartz PETROGRAPHY diorite and the Tenryukyo granite. It occurs as dyke-foim in the quartzofeldspa (1) The Inoshikori mass thic gneiss, including the latter's blocks as This mass is mainly composed of the xenolith. The rock is homogeneous horn following rocks: blende gabbro throughout the bodies. 1) Olivine norite, Route-map along the road from Tashika 2) Olivine-hornblende norite, to the Shin-toyone dam is shown in Fig. 3. 3) Norite, 4) Hornblende norite,

(3) The Ohata mass 5) Hornblende gabbro, It occurs in the Tenryukyo granite with 6) Quartz gabbro, and 700m•~1000m area, mainly composed of 7) Metadiabasic rock.

pyroxene-hoinblende gabbro and hornblende Of these, the last one is xenolithic and

gabbro. Fine-grained quartz diorite occupi also found in the surrounding gneissose es the inner portion of the mass, and fine granodiorite as xenolith. Other rock types gradually grade into one another, and no sharp boundary is detectable between them. Hornblende norite is the most predominant rock type in the mass. Olivine-hornblende norite and hornblende gabbro occur spora dically in hornblende norite. Olivine norite occupies the western limit part and quartz gabbro the eastern limit part of the mass respectively. Norite is found at the southern margin of the mass. 1) Olivine norite (Fig. 5 A) The rock is medium-grained and dark greenish in colour. In thin slice, it shows hypidiomorphic-poikilitic texture. It is mainly composed of plagioclase, hypersthene, olivine, hornblende, augite and

Fig. 4. Route-map cross the centre of the cummingtonite. Ohata mass. Plagioclase and olivine are enclosed by 1. Tertiary sediments, 2. Tenryukyo granite, hypersthene and augite which in turn are 3. Aplitic biotite granite, 4. Fine-grained plagio clase-porphyritic biotite granodiorite, 5. Fine-gr enclosed by hornblende and cummingtonite. ained biotitequartz diorite, 6. Medium-grained Between olivine and plagioclase, pale green augite-hornblende gabbro, 7. Medium-grained hornblende-spinel association is found. hornblende gabbro, 8. Medium-grained two pyro xene-hornblende gabbro. Plagioclase is idiomorphic and free from 56 T. Kutsukake

Fig. 5. Photomicrographs of the gabbroic rocks. (Cross-nicols) A. Olivine norite, Specimen No. 73072304, Inoshikori mass. B. Olivine-hornblende norite, Specimen No. 73030601, ditto. C. Norite, Specimen No. 70092005, ditto. D. Hornblende norite, Specimen No. 72090810, ditto. E. Intensely metamorphosed gabbro, Specimen No. 72090807, ditto. F. Hornblende gabbro, Specimen No. 73072303A, Tashika mass.Ol . olivine, Hy. hypersthene, BHo. brown hornblende, Bi: biotite, Mg. magnetite, Sp. spinel, Pl. plagioclase, Ch. chlorite Petrology of the gabbroic rocks in the Ryoke zone of the Toyone-mura area 57

zoning, showing polysynthetic twinning. n1 ƒÀ =1.704 , ƒÁ=1.707, (-) 2V=58.5•K. Rim =1.578 on cleavage flake (An 94) . Olivine hypersthene around olivine gives (-) 2V=

is rounded crystal, along its crack numer 56.5•K. Augite is small in quantity, and

ous dusty opaque minerals are developed. always associated with hypersthene, partly

ƒÁ =1.764, (-) 2V=77•K. Hypersthene, with replaced by hornblende. ƒÀ=1.684, (+) 2V

exsolution lamellae parallel to (100) plane, =53.5•K, c•ÈZ=43•K. Hornblende is poikilitic,

is hypidiomorphic and frequently embraced enclosing plagioclase and hypersthene. It

by cummingtonite, sometimes by horn has exsolution lamellae of cummingtonite

blende. ƒ¿=1.699, ƒÀ=1.710, ƒÁ=1.712, (-) parallel to (001). It is pleochroic with X= 2V=50•K. Augite occurs usually intergrown very pale yellow, Y=yellowish brown, Z=

with hypersthene, and is embraced by brown. ƒ¿=1.658, ƒÀ=1.674, ƒÁ=1.682, c•ÈZ=

brown hornblende. ƒÀ=1.692, c•ÈZ=39•K, (+) 19•K, (-) 2V=80•K. Cummingtonite occurs as

2V=41•K. Cummingtonite is embraced by two fashions: one is associated with horn

hornblende with common b-axis. Horn blende in parallel intergrowth, and has

blende occurs as poikilitic crystal enclosing exsolution lamellae of hornblende. It in

other minerals. Pleochroism is as follows: cludes hypersthene poikilitically. The other

X=pale yellow, Y=yellow brown, Z= is replacing hypersthene. The former has

brown. a=1.652, ƒÀ=1.664, ƒÁ=1.676, c•ÈZ= ƒÁ =1 .678, c•ÈZ=19•K, (+) 2V=82•K, and the

18•K, (-) 2V=77•K. Hornblende, associated latter c•ÈZ=18.5•K, (+) 2V=80•K. Biotite

with spinel, is pale green in colour and has occurs also as two fashions; one is deve ill-form. ƒÁ=1.665, (-) 2V=88•K. Spinel is loped around iron ore, the other is replac

small grain and deep green (pleonaste). ing hornblende. The latter is pleochroic

Minor constituents: Magnetite, ilmenite, with X=yellow, Y=Z=orange brown. ƒÁ=

apatite and biotite . 1.643. Quartz occurs in small quantity as 2) Olivine-hornblende noiite (Fig. 5 B) interstitial crystal.

This rock is medium-grained and brown Minor constituents: Magnetite, ilmen

ish grey in colour. In this slice, it shows ite, apatite and zircon.

hypidiomorphic-poikilitic texture. It is 3) Norite (Fig. 5 C)

mainly composed of plagioclase, hyper It is fine-grained and dark greenish in

sthene, cummingtonite, augite and olivine colour. In thin slice, it shows ophitic text

with small amount of biotite and quartz. ture. It is mainly composed of plagioclase,

Plagioclase occurs as hypidiomorphic hypersthene and augite. Amphiboles around

tabular crystal, polysynthetically twinned pyroxenes are also found. and slightly zoned. nl on cleavage flake Plagioclase is lath-shaped and polysyn

1.562-1.565 (An 64-70). Olivine has round thetically twinned, sometimes weak zoning

ed outline and is altered to iddingsite (?) with n1=1.567-1.572 on cleavage flake (An

from the margin. (-) 2V=80•K. It is always 74-83). Hypersthene, with exsolution lamel

surrounded by a corona of hypersthene, sug lae parallel to (100) plane, is hypidiomorphic

gesting their reaction-relation. Hypersthene, and long prismatic. ƒ¿=1.697, ƒÀ=1.709, with exsolution lamellae of augite parallel ƒÁ =1.712, (-) 2V=49•K. Augite is small in

to (100) plane, having tendency to elongate quantity and is associated with hypersthene. along c-axis. It is frequently replaced by ƒÀ =1.689, c•ÈZ=39•K, (+) 2V=54•K. Pale brown

cummingtonite from the rim. ƒ¿=1.694, hornblende embraces augite, replacing the 58 T. Kutsukake

latter. It is pleochroic with X=pale yellow, composed of plagioclase, hornblende, cum

Y=pale yellow brown, Z=light brown. mingtonite and biotite with small amount

ƒ¿ =1.645, ƒÀ=1.656, ƒÁ=1.667, c•ÈZ=17•K, (-) of quartz.

2V=88•K. Cummingtonite occurs embracing Plagioclase is tabular, showing poly

hypersthene, and replaces the latter. ƒÁ= synthetic lamellar twinning and weak zon

1.676, c^Z=17.5•K, (+) 2V=82•K. ing with n1=1.563-1.567 on cleavage flake

Minor constituents: Magnetite and il (An 66-73). Hornblende is hypidiomorphic menite (abundant), biotite (fringes around and pleochroic with X=pale yellow, Y=

iron ore), and apatite. yellow brown, Z=brown, and ƒÁ=1.681, (-) 4) Hornblende norite (Fig. 5 D) 2V=80•K, c•ÈZ=17•K. It frequently changes

It is medium-grained and dark grey to greenish brown hornblende from the

ish in colour. In thin slice, it shows hypi margin. Cummingtonite is always inter

diomorphic-granular texture. It is mainly grown with and usually enclosed by horn composed of plagioclase, hypersthene, horn blende. ƒÁ=1.677, (+) 2V=86•K, c•ÈZ=20•K.

blende, cummingtonite and biotite with Biotite is irregular flakes and associated

fair amount of quartz. with amphibole. It is pleochroic with X=

Plagioclase is hypidiomorphic-tabular, pale yellow, Y=Z=yellowish brown. showing polysynthetic lamellar twinning and Minor constituents: Magnetite, ilmen

zoning with n1=1.567-1.570 (An 74-80). ite and apatite.

Hypersthene, with exsolution lamellae, occurs 6) Quartz gabbro

as hypidiomorphic prism, embraced by cum It is medium-grained and greyish in

mingtonite, which in turn is enclosed by colour. Under the microscope, it shows

hornblende. ƒ¿=1.695, ƒÀ=1.706, ƒÁ=1.710, hypidiomorphic-granular texture. It is

(-) 2V=54•K. Hornblende is irregular-shaped mainly composed of plagioclase, hornblende, and poikilitic, enclosing hypersthene and biotite and quartz with small amount of

cummingtonite. It is pleochroic with X= potash feldspar.

pale yellow, Y=brown, Z=brown. d=1,653, Plagioclase is hypidiomorphic- to xeno ƒÀ =1.667, ƒÁ=1.679, c•ÈZ=19•K, (-) 2V= morphic-tabular, showing polysynthetic

84•K. Cummingtonite usually occurs inter twinning and weak zoning with n1=1.556-

grown with and enclosed by hornblende, 1.560 on cleavage flake (An 52-60). Horn and frequently embraces hypersthene. ƒ¿= blende is xenomorphic. It is pleochroic with

1.647, ƒÀ=1.660, ƒÁ=1.675, (+) 2V=88•K, c•ÈZ X=pale yellow, Y=yellow brown, Z=bro

=19•K. Biotite is associated with other mafic wn, and ƒÁ=1.683, (-) 2V=75•K, c•ÈZ=18•K.

minerals, partly replacing the latter, and Biotite is elongated flake, and flocks toge

also occurs around iron ore. It is pleoch ther with hornblende. It is pleochroic with

roic with X=yellow, Y=Z=deep brown, X=brown, Y=Z=dark brown, and ƒÁ=

and ƒÁ=1.647. 1652. Quartz is interstitial and shows un

Minor constituents: Magnetite, ilmen dulatory extinction. Potash feldspar is also

ite, apatite and zircon. interstitial and small in quantity.

5) Hornblende gabbro Minor constituents: Apatite and magne

It is medium-grained and dark grey tite.

ish in colour. In thin slice, it shows hypi 7) Metadiabasic rock

diomorphic-granular texture. It is mainly It is fine-grained, dark greenish to Petrology of the gabbroic rocks in the Ryoke zone of the Toyone-mura area 59

Table 1. Optical properties of some constituent minerals of fine-grained metadiabasic rocks of the Inoshikori mass.

1. Specimen No. 72090801, 2. Specimen No. 72090805 3. Specimen No. 72090808, 4. Specimen No. 72090817

black rock. Under the microscope, it ex of metamorphic minerals, which are various hibits ophitic to granoblastic texture. Es hornblendes, actinolite, chlorite, prehnite,

sential minerals are plagioclase, hornblende, sericite and calcite as common occurrences. cummingtonite, biotite and magnetite with Sometimes, cummingtonite and anthophyl small amount of quartz. lite are found. Plagioclase is lath-shaped and polysyn Optical properties of some of them are thetically twinned. Sometimes, larger ones given here; are present, core of which is sericitized. Actinolite ƒÁ=1.659, (-) 2V=78•K, c•ÈZ Hornblende is short prismatic to granular. =13•K. (Specimen No. 72090804) Cummingtonite usually occurs intergrown Chlorite ƒÁ=1.613, (+) 2V=10•K (Ditto) with hornblende with common crystallo Prehnite ƒÁ=1.648, (+) 2V=66.5•K

graphic axes. Biotite is ill-formed flake, (Ditto) sometimes larger one is present. Apatite is (2) The Tashika mass a common accessory. This mass is exclusively composed of Optical properties of the constituent biotite-cummingtonite-hornblende gabbro. minerals are tabulated (Table 1). (Fig. 5F) Intensely metamorphosed rocks. (Fig. It is medium-grained and greyish brown 5 E) in colour. In thin slice, it shows hypidio

It is very difficult to determine their morphic-granular texture. It is mainly com original rocks of the intensely metamorphos posed of plagioclase, hornblende, cumming ed rocks. Such rocks are mainly composed tonite and biotite with small amount of 60 T. Kutsukake

quartz. Plagioclase is hypidiomorphic-tabular, Plagioclase is hypidiomorphic-tabular showing polysynthetic lamellar twinning and

and shows polysynthetic twinning. It is weakly zoning with n1=1.568-1.575 on

zoned; basic core is mantled by acid rim cleavage flake (An 76-89). Augite is irregu

with distinct boundary. The core has n1= lar-formed and always embraced by horn

1.555-1.559 on cleavage flake (An 51-59). blende. ƒÀ=1.686, (+) 2V=50•K, c•ÈZ=39•K.

The rim has n1=1.550-1.553 on cleavage It is replaced by greenish brown hornblende

flake (An 43-47). Hornblende is ill-formed from the margin and/or spottedly. Hyper

and ragged crystal. It usually changes to sthene is also irregular-shaped and frequently

greenish brown one from the margin. Pleo replaced by cummingtonite. ƒÁ=1.711, (-) chroism is as follows: X=pale yellow, Y= 2V=52•K. Hornblende is poikilitic, enclos

pale yellow brown, Z=brown. ƒ¿=1.653, ing plagioclase, augite and iron ore crystals.

ƒÀ =1.668, ƒÁ=1.678, (-) 2V=73.5•K, c•ÈZ=15•K. Hornblende replaces parts of augite. It is Greenish brown rim gives ƒÁ=1.675, (-) 2V pleochroic with X=pale brownish yellow, =80•K and c•ÈZ=19•K . It is pleochroic with X Y=yellowish brown, Z=deep yellowish

=pale yellow, Y=pale greenish brown, Z= brown, and has ƒ¿=1.661, ƒÀ=1.675, ƒÁ=

pale brown with greenish tint. Cumming 1.682, (-) 2V=71.5•K and c•ÈZ=17•K. Cumming

tonite is almost always in intergrowth with tonite occurs enclosed in hornblende with

hornblende. It sometimes occurs in the core common crystallographic axes.

of hornblende. It has ƒÁ=1.676, (+) 2V= Minor constituents: Magnetite, ilmen

80•K and c•ÈZ=19•K. Biotite is tabular flakes, ite, biotite and apatite.

frequently associated with amphibole. It is 2) Augite-hornblende gabbro

pleochroic with X=yellow, Y=Z=reddish It is medium-grained gneissose rock brown, and gives ƒÁ=1.654. Quartz is small with dark greyish colour. Under the micro

in quantity. and interstitial. scope, it shows hypidiomorphic- to xenomor

Minor constituents: Magnetite, ilmen phic-granular texture. It is mainly compos ite and apatite. ed of plagioclase, hornblende and augite.

(3) The Ohata mass Plagioclase is hypidiomorphic- to xeno

This mass is composed of the following morphic-tabular, showing polysynthetic rock types: lamellar twinning and slight zoning with

1) Two pyroxene-hornblende gabbro, n1=1.560-1.567 on cleavage flake (An 63-

2) Augite-hornblende gabbro, and 74). Augite is scarcely found but very rarely

3) Hornblende gabbro. occurs in the core of hornblende crystals.

Of three, the last occupies large part of Hornblende is hypidiomorphic and poikili

the mass. tic. It is pleochroic with X=pale yellow , Y 1) Two pyroxene-hornblende gabbro =pale yellowish brown , Z=pale brown, and

It is medium-grained black rock. Under ƒÁ =1.679, (-) 2V=81•K , c•ÈZ=15•K. It changes the microscope, it shows hypidiomorphic to greenish brown hornblende from the

granular texture. It is heterogeneous due to margin. It is pleochroic with X=pale

the quantitative variation of the constituent yellow, Y=pale brownish green, Z=brown minerals. It is mainly composed of plagio ish green, and ƒÁ=1.667, (-) 2V=78•K , c•ÈZ= clase, hornblende, cummingtonite, hyper 18•K. Furthermore, it changes to actinolite , sthene and augite. which has ƒÁ=1.662, (-) 2V=78•K and c•ÈZ= Petrology of the gabbroic rocks in the Ryoke zone of the Toyone-mura area 61

12•K. It is medium-grained and greyish in

Minor constituents: Biotite, quartz, colour. In thin slice, it shows hypidiomor magnetite, ilmenite and apatite. phic-granular texture. It is mainly compos 3) Hornblende gabbro ed of plagioclase, hornblende, cummingto

Table 2. Chemical compositions and C. I. P. W. norms of the gabbroic and metadiabasic rocks of the Inoshikori mass.

Anal. T. Kutsukake 1. Specimen No. 72090813, Olivine norite. 2. Specimen No. 70092005, Norite. 3. Specimen No. 72090810, Hornblende norite. 4. Specimen No. 72090804, Hornblende gabbro. 5. Specimen No. 72090807, Intensely metamorphosed gabbro. 6. Specimen No. 72090812B, Fine-grained metadiabasic rock. 7. Specimen No. 72090805, Ditto. 62 T. Kutsukake

nite and biotite with small amount of ite and apatite. quartz. CHEMISTRY Plagioclase is hypidiomorphic- to xeno

morphic-tabular, showing polysynthetic Chemical analyses of five gabbros and

lamellar twinning and zoning with n1= two metadiabasic rocks from the Inoshi

1.557-1.571 on cleavage flake (An 55-81).

Hornblende is irregular-shaped and ragged

crystal, forming clots. It is pleochroic with X=pale yellow, Y=yellow, Z=pale brown,

and ƒÁ=1.677, (-) 2V=84•K, c•ÈZ=21•K. It is

replaced by green hornblende from the

margin. Green hornblende is pleochroic with

X=pale yellow, Y=yellow green, Z=dirty

yellow green, and ƒÁ=1.670, (-) 2V=74•K, c•ÈZ=17•K. Cummingtonite occurs almost al

ways in parallel intergrowth with horn

blende, enclosed by the latter. Biotite is

small in amount and interstitial with horn

blende. Quartz is small in amount and

interstitial. Fig. 7. Normative feldspar diagram. Minor constituents: Magnetite, ilmen Gd. Hornblende diabase dyke near Guadalupe igneous complex, representative of the origi nal magma of the complex, after Best (1963). SM. Average San Marcos gabbro, after Miller (1937). H. Aphyric hypersthenic rock series, after Kuno (1968). Sk. Skaergaard liquid trend. The other symbols are as in Fig. 6.

Fig. 6. AFM-giagram. 1. gabbroic rock, 2. metadiabasic rock. SM. Average San Marcos gabbro, after Miller (1937). Gd. Guadalupe igneous complex, after Best (1963). GH-GF. Garabal Hill - Gren Fyne igneous complex, after Nockolds (1941), cited from Hess (1960). Fig. 8. Normative pyroxene diagram. Sk. Skaergaard liquid trend. Symbols are as in Fig. 7. Petrology of the gabbroic rocks in the Ryoke zone of the Toyone-mura area 63 kori mass are shown in Table 2, with cal Garabal Hill-Gren Fyne igneous complex culated C. I.P. W. norms. (Nockolds, 1941). The average San Marcos These rocks show narrow range of SiO2 gabbro (Miller, 1937) is plotted near to content (44-49 wt.%), and of iron content . these. On the contrary, MgO content is fairly In the normative feldspar diagram (Fig. variable. 7), they are plotted near An-corner. To Numbers 2 and 5 are very rich in Al2O3 compare with other igneous suites except (ca. 20%). This reflects the fact thet they the San Marcos gabbro, they are remark have high contents of calcic plagioclase. ably rich in An-component. Intensely metamorphosed specimen, In the normative pyroxene diagram such as number 5, is rich in water. (Fig. 8), they are in the area near En-Fs They are plotted in small area in AFM join. This accords with the fact that their diagram (Fig. 6). Fractionation can not pyroxenes are Ca-poor orthorhombic ones. be detected on this diagram. For comparison, the trends of three MINERALOGY typical differentiated igneous complexes are Here, only primary minerals are de shown. These gabbros are richer in iron scribed, and the description of metamorphic than other calc-alkaline suites, such as minerals will be given on later pages. Guadalupe igneous complex (Best, 1963) and (1) Olivine

Table 3. Chemical compositions and optical properties of hypersthenes in gabbros of the Inoshikori mass.

1. Specimen No. 70092005, Norite, EPMA anal. T. Maruyama. 2. Specimen No. 72090810, Hornblende norite, Chem. anal. T. Kutsukake. 64 T. Kutsukake

Table 4. Optical properties and compositions estimated therefrom of hypersthenes.

1. Specimen No. 72090813, Inoshikori mass. 2. Specimen No. 70092005, Ditto. 3. Specimen No. 72090810, Ditto. 4. Specimen No. 73030601, Ditto. 5. Specimen No. 71032406, Ohata mass.

It is found in olivine norite and olivine the exsolution lamellae are parallel to (100). hornblende norite. It is enclosed by augite Two analyses of hypersthene are shown and hypersthene in olivine norite. In olivine in Table 3. Optical properties are summariz hornblende norite, it is surrounded by re ed in Table 4. action rim of hypersthene. (3) Augite Chemical composition estimated from It is present in small quantity in some optical properties is Fa43in olivine norite, and rock types. It is embraced by hornblende, Fa41in olivine-hornblende norite respectively. suggesting their reaction-relation. It is also (2) Hypeysthene intergrown with hypersthene, suggesting It is one of the important minerals in their parallel crystallization. Sometimes it these gabbroic rocks. It qualifies as the has exsolution lamellae parallel to (001) Bushveld type (cf. Hess, 1960, p. 23) in that plane.

Table 5. Chemical composition of augite in norite (Specimen No. 70092005)

EPMA anal. T. Maruyama ƒÀ =1.689, (+) 2V=54•K , c•ÈZ=39•K. Petrology of the gabbroic rocks in the Ryoke zone of the Toyone-mura area 65

Table 6. Optical properties and compositions estimated therefrom of augites.

1. Specimen No. 72090813, Inoshikori mass. 2. Specimen No. 70092005, Ditto. 3. Specimen No. 73030601, Ditto. 4. Specimen No. 71032406, Ohata mass.

7. These figures suggest the similarity in

chemical composition.

(5) Cummingtonite Cummingtonite is not common to ig

neous rocks. However, their mode of occur

rence suggests that some of them must have

crystallized from magma. It never occurs

as separate individual crystal. It is always

embraced by hornblende and often encloses

hypersthene poikilitically. In that case, the

reaction between hypersthene and cumming Fig. 9. Plots of pyroxenesin Di-Hd-En-Fs tonite can not be recognized. Such a mode system. of occurrence is very similar to those of the 1. Chemicalanalysis, 2. EPMA analysis, 3. En mass of the Tabito igneous complex in Estimated from optics, 4. Skaergaard trend, 5. Tie lines connectingcoexisting pyroxene in the Abukuma plateau (Shido, 1958; Onuki rocks. and Kato, 1971). Optical properties and the Mg•~100/

Analysis of an augite is shown in Table (Mg+Fe+2) ratios estimated from them are 5. Optical properties are summarized in summarized in Table 8. The chemical com

Table 6. position of cummingtonite may also be fairly Chemical compositions of hypersthene constant in different rock types. and augite are plotted in Di-Hd-En-Fs (6) Biotite diagram (Fig. 9). It is found in almost all the rock types, (4) Hornblende and its mode of occurrence is as follows: 1) Hornblendes are brown variety it is frequently associated with hornblende, throughout all the rock types. It generally sometimes replacing the latter, 2) it fringes shows poikilitic habit, enclosing other min around magnetite crystals. As to the form erals, such as pyroxene. er, it is very difficult to determine it to be Optical properties of hornblende of igneous or metamorphic. several rock types are summarized in Table Optical properties of some of them are 66 T. Kutsukake

Table 7. Optical properties of brown hornblendes.

1. Specimen No. 72090813, Inoshikori mass. 2. Specimen No. 72090810, Ditto. 3. Specimen No. 72090812A, Ditto. 4. Specimen No. 73030601, Ditto. 5. Specimen No. 73072303A, Tashika mass. 6. Specimen No. 71032406, Ohata mass. 7. Specimen No. 71032401, Ditto. 8. Specimen No. 71032404, Ditto.

Table 8. Optical properties and chemical compositions estimated therefrom of cummingtonites.

1. Specimen No. 72090810, Inoshikori mass. 2. Specimen No. 73030601, Ditto. 3. Specimen No. 72090812A, Ditto. 4. Specimen No. 73072303A, Tashika mass.

Table 9. Optical properties of biotites. mens under the reflected microscope, mag netite and ilmenite were detected for all the rock types. Magnetite is irregular-shaped grains, almost always has thin exsolution lamellae of ilmenite. Ilmenite is always associated with mag 1. SpecimenNo. 73030601,Inoshikori mass. 2. SpecimenNo. 72090810,Ditto. netite in irregular patch except in exsolu 3. SpecimenNo. 72090812A,Ditto. tion lamellae (Fig. 10). This suggests that 4. SpecimenNo. 72090802,Ditto. 5. SpecimenNo. 73072303A,Tashika mass. they have crystallized in equilibrium. (8) Plagioclase summarized in Table 9. Plagioclase of these gabbros is very (7) Magnetite and ilmenite calcic; it reaches up to An 94 in olivine By the examination of polished speci norite, and even in the most acid type , Petrology of the gabbroic rocks in the Ryoke zone of the Toyone-mura area 67

Table 10. Optical properties of metamorphic hornblendes.

1. Pale brown hornblende, Specimen No. 70092005, Inoshikori mass. 2. Greenish brown hornblende, Specimen No. 73072303A, Tashika mass. 3. Brownish green hornblende, Specimen No. 71032406, Ohata mass. 4. Ditto, Specimen No. 71032401, Ditto. 5. Green hornblende, Specimen No. 73072304, Inoshikori mass. 6. Ditto, Specimen No. 71032404, Ohata mass. 7. Bluish green hornblende, Specimen No. 72090813, Inoshikori mass. 8. Ditto, Specimen No. 72090809, Ditto.

know the nature of metamorphism itself as well. Details of metamorphicchanges of minerals on each mass are as follows: (1) Inoshikori mass The effectsof metamorphismare detect able all over the mass. However,its degree is considerably variable, even in an out crop; nearly non-metamorphicpart grades into intensely metamorphosed one within Fig. 10. Sketch of magnetite and ilmenite in several decades of centimetres. Intensely reflected light. metamorphosed rocks are now composed A. Specimen No. 70092005, norite. B. Specimen No. 72090810, hornblende norite. almost of the metamorphicminerals. C. Specimen No. 72090804, hornblende gabbro. The metamorphic changes of the min erals observed under the microscopeare as follows: quartz gabbro, it is limited to labradorite. Generally, compositional range in zon Hyperstheneis replaced completely or ing is not large, and their zoning pattern is partly by cummingtonite,which is frequent normal. ly polysynthetically twinned or has the Compositional range of plagioclase of shape of a bundle of fibers. Along the mar each rock type is summarized in Fig. 11. gin of cummingtonite, green hornblende develops. METAMORPHISM Augite is replaced by pale brown horn These gabbroic rocks have suffered blende or greenish brown hornblende from metamorphism more or less. To treat these the margin and/or spottedly. Furtermore, rocks as problems of igneous petrology, it is greenish brown hornblende changes to green very important to clean away its effects, to one. 68 T. Kutsukake

Fig. 11. Composition of plagioclase in each rock type.

Brown hornblende (magmatic) changes long prismatic or fibrous, cutting other to greenish brown hornblende from the minerals. margin, and the latter to green hornblende. (2) Tashika mass These metamorphic hornblendes change In this mass, the metamorphic changes to bluish green hornblende, which in turn is are comparatively simple. replaced by actinolite. Brown hornblende changes to greenish Moreover, actinolite is replaced by brown one from the margin. chlorite with calcite. Plagioclase is fringed by clear acid rim Biotite persists well to change to chlor with distinct boundary. Rim is composi ite at the later stage. tions An 43-47. Plagioclase is gradually replaced by (3) Ohata mass sericite and calcite. However, it is difficult The metamorphic changes are very to distinguish this change from the ordinary similar to those of the Inoshikori mass. alteration change. However, so intensely metamorphosed rocks As already described, in olivine norite such in the Inoshikori mass do not occur. spinel (pleonaste)-hornblende association is Hypersthene is replaced by cumming found between olivine and anorthite. The tonite, which in turn changes to green same feature has been reported on the gab hornblende from the margin. broic rocks in other areas of the Ryoke zone Augite is replaced by greenish brown (Yoshimura, 1940; Suwa, 1961). hornblende in irregular form. In the intensely metamorphosed rocks, Brown hornblende (magmatic) changes the main constituents are bluish green horn to greenish brown one from the margin, and blende, actinolite, chlorite, prehnite, cal the latter also to green one. Green horn cite and sericite, of which last two are blende is gradually replaced by actinolite. mainly derived from plagioclase. Some The summing up of the metamorphic times anthophyllite is developed, which is changes of minerals is shown in Fig. 12. Petrology of the gabbroic rocks in the Ryoke zone of the Toyone-mura area 69

Fig. 12. Metamorphic changes of minerals.

lite series (Deer et al., 1963). The changes of hornblendes with in creasing metamorphism is generally as fol lows: bluish green to green and brown, as reported by Miyashiro (1958) and Shido (1958) in the central Abukuma plateau, as well as in other metamorphic terrains (Miya shiro, 1968). In the case of the Toyone-mura gab bros, the reverse change is observed, and this may suggest the retrogressive metamor phism from the higher amphibolite facies to Fig. 13. Correlation between index of refraction the lower amphibolite or still lower facies. (ƒÁ) and optical angle ((-)2V) of calci Metamorphicminerals, formed at differ ferous amphiboles of the gabbroic rocks in the Toyone-mura area. ent stages in the course of the retrogressive 1. magmatic brown hornblende, 2. pale brown metamorphism, occur together. Therefore, hornblende, 3. brownish green hornblende, 4. some of them would not be equilibrium one green hornblende, 5. bluish green hornblende, 6. actinolite. another. The later stage metamorphic changes

Metamorphism is conspicuously reveal are very similar to those of hydrothermal ed in the changes of amphiboles. Optical alteration, and the introduction of water and carbon dioxide is suggested by such a properties of several typical metamorphic hornblendes are summarized in Table 10. reaction:

The correlation between index of refraction

(ƒÁ) and optical angle ((-)2V) of them is shown in Fig. 13. Metamorphic hornblendes have lower index of refraction than those of igneous ones. They are rather clearly sepa ?? This accords with the fact that the rated from the igneous field in the diagram. intensely metamorphosed rocks, e.g., no. 5 Analysis of a metamorphic hornblende of Table 2, is very rich in water. is made with the result in Table 11. This The metamorphism, above mentioned, composition is near to tremolite-ferroactino would correspond to the retrogressive stage 70 T. Kutsukake

Table 11. Chemical composition and optical properties of hornblende in norite (Specimen No. 70092005).

EPMA anal. T. Maruyama of the Ryoke regional metamorphism. rocks to the present gabbros, and regarded The basalt intercalated in sedimentary their mineral assemblages as equilibrium metamorphic rocks about 1500m from the paragenesis of the granulite facies in the Inoshikori mass, is now typical amphibolite, Ryoke regional metamorphism. However, mainly composed of plagioclase and horn some of the minerals, such as olivine and blende. On the contrary, the present gabbros pyroxenes, would be relicts of magmatic are not completely metamorphosed to am origin, so the "granulite facies" condition in phibolite. This fact would suggest that the the Ryoke regional metamorphism is quite gabbros have not past the same metamor doubtful. phic history as the surrounding metamor As to the Ohata mass, occurring in the phic rocks, that is to say, their intrusions granite, it is possible to attribute the meta took place not at so earlier stage but nearly morphic changes to the effect by the granite. at the peak or more later stage of the Ryoke Their mineralogical changes are very simi regional metamorphism. lar to those as reported by Yoshizawa In his paper on the Ryoke metamorphic (1952) in the ase of granitization. rocks of the Mitsue-mura area of Kii penin sula, Suwa (1961) described the similar Petrology of the gabbroic rocks in the Ryoke zone of the Toyone-mura area 71

the crystallization have to be calcic as above GENETICAL CONSIDERATIONS mentioned. Moreover, zoning is absent in 1) Crystallization of the Inoshikori gabbroic anorthite and the other rock types with such magma. a calcic plagioclase are also absent. These In view of the textural relations of the features suggest that the anorthite crystals minerals under the microscope, the order were removed from the magma so as to of their crystallization should be summariz escape from reaction with the latter and ed as in Fig. 14. This crystallization se accumulated to form olivine norite. quence is common to calcalkaline plutonic Also, in the San Marcos gabbro of rocks, such as Guadalupe igneous complex southern California, "olivine rocks" bear (Best, 1963), except plagioclase too An-rich highly calcic plagioclases of compositions and the presence of cummingtonite. An 92-95. The calcic magma to precipi tate such plagioclase would have been derived from the norite magma by some special differentiation process (Miller, 1937). As to the Inoshikori gabbro, the similar process of magmatic differentiation have to be assumed. As already mentioned, each mafic minerals has rather narrow range of compo sition, and plagioclase of each rock type also has narrow compositional range. These facts would suggest that the separation of minerals from the magma took place very appreciably. The presence of various mafic minerals, from olivine to biotite, of discontinous reac

Fig. 14. Crystallization sequence of minerals tion series (Bowen, 1928) in a rock type, and the mineral parageneses of several such as olivine norite, is one of the charac rock types of the Inoshikori mass. teristic features of these gabbroic rocks. Of course, the magma must have be Mineral parageneses of the principal come so hydrous as to precipitate the hy rock types are also shown in the same drous minerals, such as hornblende, cum figure. mingtonite and biotite, from some stage of As already mentioned, all of the rock the crystallization. types grade into one another, therefore, they Whether the crystallization of the obviously have been derived from a single magma took place in situ or beneath the parent magma by a systematic process of present position is uncertain so far. How differentiation. ever, the contact effects by the gabbro mass Plagioclase in olivine norite is of com to the surrounding metamorphic rocks are position An 94, which would be in equili absent. The temperature of the magma in brium with a liquid of the composition An the earlier stage of the crystallization must 85. Thus, the magma in the early stage of have been higher than that of the Ryoke 72 T. Kutsukake regional metamorphism of amphibolite faci brought to the present position in relation es. Therefore, the intrusion would have took to the tectonic movement at the time of the place as the state close to solid rather than Ryoke regional metamorphism. As the as liquid. Ohata mass occurs in the Tenryukyo gra 2) The relations to the Ryoke regional nite, it may be possible to be caught by metamorphism. the latter and brought up to the present The similar rocks to these gabbros have position. been found to occur in other areas of the The ultimate origin of these gabbros Ryoke zone, and some of them have been may be attributed to the basement of the reported. Ryoke zone, that is, the "basaltic" layer, They have a variety in their petrogra where the magmas would have been generat phical, mineralogical and chemical charac ed. And the upheaval of geotherm at the ters. However, they have some features time of the regional metamorphism would common to all the masses. One of those is have some connections with the formation their mode of occurrence; they occur exclu of the magmas. The uplift and the crystal sively in the highest grade part of the Ryoke lization of the magmas may have took place regional metamorphism, that is, the silli in relation to the tectonic movements of the manite zone (Hayama, 1960, 1964) or zone Ryoke regional metamorphism. IV of Suwa (1961). And they have zonal arrangements, as suggested by Yoshizawa SUMMARY (1949) in the Kinki district. This is clearly seen in the geological map of the Ryoke In the Ryoke zone of the Toyone-mura zone of central Japan (Yamada et al., 1974), area, three masses of gabbroic rocks occur. as well as in that of the Kinki district They are named Inoshikori-, Tashika-, and (Yoshizawa et al., 1966). Especially, in the Ohata mass respectively. The first mass is Kinki district two zones of their arrange composed of several rock types, ranging ments, nearly parallel, can be recognized. from olivine norite to quartz gabbro through These facts would suggest that they norite and hornblende norite. They would have some genetical relations to the Ryoke be formed through the fractional crystalli regional metamorphism. zation of the noritic magma. The second On the other hand, as pointed out in mass is composed of hornblende gabbro of another paper (Kutsukake, 1973a), some of dyke-like form. The last mass is composed the hornblende gabbros in the Ryoke zone, of pyroxene-hornblende gabbro and horn which are concordant to the surrounding blende gabbro, accompanied by some other granites may have some genetical relations rocks. It occurs in the Tenryukyo granite. to them. However, the present gabbros These gabbroic masses suffered retro would have not direct genetical relations, gressive metamorphism from higher amphi such as co-magmatic, to the Ryoke granites. bolite facies to still lower facies. This may In view of its mode of occurrence, the correspond to the retrogressive stage of the Tashika mass would have intruded as Ryoke regional metamorphism. magma. However, the Inoshikori mass and In view of their mode of occurrence, it the Ohata mass may have not intruded as is suggested that they have genetical rela magma. The Inoshikori mass could be tions to the Ryoke regional metamorphism Petrology of the gabbroic rocks in the Ryoke zone of the Toyone-mura area 73

rather than to the Ryoke granites. & Sons: New York. Kutsukake, T. (1970), On the Ryoke granitic and Whether they have intruded as magma metamorphic rocks in the Toyone-mura area, or solid-state, except the Tashika mass, is Aichi Prefecture, Japan. Mem. Fac. Sci.,

not certain so far. Kyoto Univ., Ser. Geol. & Miner., 37, 133- 160.

ACKNOWLEDGMENTS •\ (1973a), Structure and petrography of the Hatsuse basin in the Ryoke zone of the The author would like to express his Kinki district, Japan. Jour. Japan. Assoc. Min. Petr. Econ. Geol., 68, 37-57. sincere thanks to Dr. I. Nakayama of Kyoto •\ (1973b), The gabbroic rocks in the Ryoke University for his constant encouragement zone of the Toyone-mura area, Aichi Prefec and critical reading of the manuscript. He ture. (Abstr.) Program 80th Ann. Mtg. Geol.

is also indebted to Dr. Y. Hayama of Tokyo Soc. Japan. (Sendai), 211, (in Japanese). Miller, F. S. (1937), Petrology of the San Marcos Nogyo Daigaku (Tokyo Agricultural Univer gabbros, southern California. Bull. Geol. Soc. sity) for his critical reading of the manu Amer., 48, 1397-1425.

script and invaluable comments. Miyashiro, A. (1958), Regional metamorphism of the Gosaisyo-Takanuki district in the central Messrs. K. Kurokawa and R. Takita, Abukuma plateau. Jour. Fac. Sci., Univ. both of Kyoto University, read the manu Tokyo, Sec. II, 11, 219-272.

script and gave him useful suggestions. Mr. •\(1968), Metamorphism of mafic rocks. in "Basalts" 2 , (Ed. H. H. Hess), Wiley & Sons: T. Kano helped him in preparing the photo New York. micrographs of thin sections. Dr. T. Maru Nockolds, S. R. (1941), The Garabal Hill - Gren Fyne igneous complex. Quart. Jour. Geol. Soc. yama of Akita University kindly analyzed London, 96, 451-511. some minerals using EPMA. The author Notomi, S. (1924), Geological map of the Sidara, records his hearty thanks to these persons. scale 1:75,000 and its explanatory text. 49p. Geol. Surv. Japan. (in Japanese with English abstr.). REFERENCES Onuki, H. and Kato, Y. (1971), Some gabbroic Best, M. G. (1963), Petrology of the Guadalupe rocks of the Tabito plutonic complex in the igneous complex, southwestern Sierra Nevada Abukuma plateau. Sci. Rep., Tohoku Univ., foothills, California. Jour. Petr., 4, 223-259.•\ 3rd. Ser., 11, 113-123.

(1967), Compositions and crystallizations Ryoke Research Group (1972). The mutual rela of mafic minerals in the Guadalupe igneous tions of the granitic rocks of the Ryoke complex, California. Amer. Min., 52, 436- metamorphic belt in central Japan. Earth 474. Science (Chikyil Kagaku), 26, 205-216, (in

Bowen, N. L. (1928), The evolution of the igneous Japanese with English abstr.). rocks. Princeton Univ. Press: Princeton, N. Shido, F. (1958), Plutonic and metamorphic rocks

J. of the Nakoso and Iritono districts in the Deer, W. A., Howie, R. A. and Zussman, J. (1963), central Abukuma plateau. Jour. Fac. Sci., Rock-forming minerals, Vol. 2, Chain silicates. Univ. Tokyo, Sec. II, 11, 131-217. Longmans: London. Soma, T. (1963), The Ikoma gabbro complex.

Hayama, Y. (1960), Geology of the Ryoke meta Jour. Geol. Soc. Japan, 69, 211-218, (in Japan morphic belt in the Komagane district, Nagano ese with English abstr.). Pref., Japan. Jour. Geol. Soc. Japan, 66, 87- Suwa, K. (1961), Petrological and geological studi 101. es on the Ryoke metamorphic belt. Jour.

•\ (1964), Progressive metamorphism of Earth Sci., Nagoya Univ., 9, 224-303. pelitic and psammitic rocks in the Komagane Yagi, K. (1944), Petrochemical study of ultrabasic district, Nagano Pref., central Japan. Jour. rocks, (Part 1), Cortlandite from Osafuji-mura, Fac. Sci., Univ. Tokyo, Sec. II, 15, 321-369. Nagano Prefecture. Jour. Japan. Assoc. Min. Hess, H. H. (1960), Stillwater igneous complex, Petr. Econ. Geol., 32, 52-63, (in Japanese). Montana. Geol. Soc. Amer. Mem. 80. Yamada, N., Katada, M., Hayama, Y., Yamada, Kuno, H. (1968), Differentiation of basalt mag T., Nakai, Y. Kutsukake, T., Suwa, K. and mas. in "Basalts" 2, (Ed. H. H. Hess), Wiley Miyakawa, K. (1974), Geological map of the 74 T. Kutsukake

Ryoke zone in central Japan, scale 1:200,000. •\ (1951), An example of the granitization Geol. Surv. Japan. of the ultrabasic rock in Ryoke-zone. ibid., Yoshimura, T. (1940), Spinel-bearing gabbroic No. 3, 16-19, (in Japanese).

rocks at Kajishima, Ehime Prefecture, Japan. •\(1952), The metagabbroic rocks bearing Jour. Geol. Soc. Japan, 47, 265-269, 297-305 ferriferous minerals in the Ryoke zone, Japan. (in Japanese). Mem. Coll. Sci., Univ. Kyoto, Ser. B, 20, 55- Yoshizawa, H. (1949), On the norites having zonal 68. arrangement in the Ryoke zone of the south •\ Nakajima, W. and Ishizaka, K. (1966), west Japan. Earth Science (Chikyd Kagaku), The Ryoke metamorphic zone of the Kinki No. 1, 11-16, (in Japanese). district, southwest Japan: Accomplishment of

•\ (1950), On the norite from Shizuki, Awaji a regional geological map. Mem. Fac. Sci., shima. Science of the Earth (Chigaku), No. Kyoto Univ., Ser. B, 32, 437-454. 2, 33-35 (in Japanese).

愛知県豊根村地域領家帯のはんれい岩類

沓掛俊夫

表記地域の領家帯には, 3つのはんれい岩の小岩体が分布する。それらをそれぞれ,猪古里岩体・田鹿岩体および大畑岩体と呼ぶ。猪古里岩体は,片麻岩中に産し,中部地方領家帯最古期の花崗岩類のひとつである神原石英閃緑岩により貫かれる。田鹿岩体は,神原石英閃緑岩と天竜峡花崗岩との間に発達し,石英長石質片麻岩を岩脈状に貫くいくつかの小岩体よりなる。大畑岩体は,天竜峡花崗岩中に産し,他の数種の岩石種を伴う。猪古里岩体は,かんらん石ノーライト,ノーライト,角閃石ノーライト,角閃はんれい岩および石英はんれい岩などよりなり,これらは,ひとつのノーライト質マグマから,結晶分化作用で生じたものと考えられる。これらのはんれい岩類は,角閃岩相からより低度の変成相にわたる下降変成作用を受けている。この変成作用は,領家変成作用の下降期に対応するものであろう。これらのはんれい岩類の迸入は,領家変成作用のそう早くない時期,すなわち,最盛期ないしそれ以後に行われたと考えられる。また,これらのはんれい岩類が,マグマとして現在位置に貫入したかどうかは,田鹿岩体を除いては,いまのところ不明である。これらのはんれい岩類に類似した岩石は,領家帯のあちこちに見られるが,分布が高変成度域に限られ,また帯状配列を示すことから,領家変成作用と成因的に何かしら関係を有することが予想される。

地名 Asakusa浅草 Chausu-yama茶臼山 Futto古戸

Hanare-yama離山 Hiyosawa日余沢 Hongo本郷 Ichibara市原 Inoshikori猪古里 Kakinotaira柿ノ平

Kami-Awashiro上粟代 Kobayashi小林 Makinoshima牧ノ島 Midashi見出 Misono御園 Naka-Shitara中設楽

Niino-toge新野峠 Odachi大立 Ohata大畑 Oiwa-dake大岩岳 Osawa大沢 Ozasa-yama大笹山

Sakauba坂宇場 Shimo-Kurokawa下黒川 Sogawa 曾川 Tashika田鹿 Tawagane-toge田和金峠 Tsugawa津川

Urakawa浦川 U〓e宇連 Zinno-yama神野山