268
Proc. Japan
53.
Acad.,
54, Ser. B (1978)
[Vol.
New Fossil Discovery from the Hida
Rocks in the Unazuki Area, Central
54(B),
Metamorphic
Japan*'
By Yoshikuni HIROI,** ) Norio FUJI,**) and Yuj i OKIMURA*
** >
(Communicatedby Teiichi KoBAYASHI,
M.J. A.,June 15, 1978)
The Hida metamorphic belt is one of the main geologic provinces
in southwestern
Japan, and is important for the geologic development
of the Japanese Islands. Two conflicting opinions have been postulated concerning the age of the metamorphism
of the belt; one is that
the age is Late Paleozoic or Early Mesozoic Era suggesting the rocks
are recrystallized
Paleozoic deposits ; the other is that the Hida
metamorphic
terrain is the exposed Precambrian
basement of the
Japanese Islands inferring the belt is the product of repeated metamorphism (Hashimoto et al., 1970).
Since 1972, one of us (Y.H.) has been studying geology and
petrology of the Hida metamorphic rocks distributed in the Unazuki
area, Central Japan. The rocks exposed in the area have been divided
into two groups, Katakai-gawa
and Unazuki (Hiroi, in press).
The Katakai-gawa
group mainly consists of coarse-grained
marble and quartzo-feldspathic
gneiss which overthrust onto the Unazuki
group from the west. The hornblende age of this group is between
345 and 285 million years by K-Ar method (Ohmoto, 1964).
The Unazuki group is composed of fine-grained marble, pelitic
schist, quartzo-feldspathic
schist, and alternation of basic and psammitic-pelitic
schists in ascending order. The group shows an anticlinal structure
with a trend of axis in N-S direction and dipping
westward.
The western wing can be traced nearly continuously for the distance of 15 km, whereas the eastern one occurs as roof remnants on
the plutonic complex.
The western wing is divided into two progressive metamorphic
zones, chloritoid and staurolite, which are comparable to the intermediate-pressure
type facies series based on the mineral assemblages
of the pelitic schists (Hiroi, 1978). The eastern wing and a part of
the western wing were thermally metamorphosed
by plutons (Hiroi,
*'
Kanazawa
**'
***'
University.
Contribution
from
the
Institute
of Earth
Science
University,
New Series
No. 71.
Institute
of Earth
Science , Faculty
of Education,
Institute
of Geology
and
Mineralogy
, Faculty
, Faculty
Kanazawa
of
Science,
of Education,
University.
Hiroshima
No. 6]
New
Fossil
Generalized
from
geologic
the
map
Hida
Metamorphic
in the
Unazuk
Rocks
i Area,
269
in Japan
Central
Japan.
in press). The mineral age of the western wing ranges from 212 to
248 million years according to the Rb-Sr method (Yamaguchi and
Yanagi, 1968, 1970; Shibata et al., 1970). The Plutonic complex has
been considered as a part of Funatsu plutonics of Late Paleozoic or
Early Mesozoic Era (Kano, 1973).
We have successfully discovered smaller f oraminif era from siliceous part of the marble within the chloritoid zone of the Unazuki
group, they are : Climacammina cf. antiqua (BRADY), C. sp., Tetrataxis sp., and Bradyina sp. The geologic occurrences of the first species
are limited from the upper Mississippian to middle Pennsylvanian
(Brady,1876 ; Lee et al., 1930 ; Cummings, 1956; Okimura, 1967), and
judging from this f oraminif eral assemblage the age of the marble
may be doubtless the late Carboniferous.
The discovery of these smaller f oraminif era from the Hida meta-
270
Y. HIROI, N. FUJI,
Fig. 1. Bryozoa
sp.
Fig. 4.
(BRADY).
morphic
rock
Finally
Kobayashi,
M. Murata
and
we
Y. OKIMURA
wish
provide
the
an important
original
to
express
clue to the
54(B),
[Vol.
sp.
Fig.
gen. et sp. inlet.
Fig. 2. Bradyina
5. Climacammina
Climacammina
sp.
Fig.
would
metamorphism
and
3. Bradyina
cf. antiqua
ages
of the
Hida
rocks.
our
M. J. A., Drs. M. Sugimoto,
and H.-Y.
Ling for their
sincere
thanks
to
Dr.
Teiichi
M. Tagiri,
S. Banno,
H. Onuki,
kind advices.
We express
our
No. 6]
deep
for
New
appreciation
a grant
Fossil
from
to the
the
Hida
Ministry
Metamorphic
of Education,
Rocks
in Japan
Japanese
271
Government
in 1977.
References
Brady, H. B. (1876) : Pal. Soc. London, Monogr., 30, 68-69.
Cummings, R. H. (1956) : Micropaleontology,
2, 226-227.
Hashimoto, M., Igi, S., Seki, Y., Banno, S., and Kojima, G. (1970) : Metamorphic
facies map of Japan, scale 1: 2,000,000. Geological Survey of Japan.
Hiroi, Y. (1978) : J. Jap. Assoc. Min. Pet. Econ. Geol., 73, 90.
--:
J . Geol. Soc. Japan (in press).
Kano, T. (1973) : Ibid., 79, 407-421.
Lee, J. S., Chen, S., and Chu, S. (1930) : Acad. Sinica, Nat. Res. Inst. Geol., Mem .,
no. 9, 102-103.
Ohmoto, H. (1964) : Proc. Japan Acad., 15, 26-41.
Okimura, Y. (1967) : J. Sci. Hiroshima
Univ. Ser. C, 5, 255-266.
Shibata, K., Nozawa, T., and Wanless, R. K. (1970) : Canad. J. Earth Sci., 7,
1383-1401.
Yamaguchi,
M., and Yanagi, T. (1968) : J. Geol. Soc. Japan, 74, 91.
-(1970) : Eclogae geol. Helv., 53, 371-388 .