名古屋大学博物館報告
Bull. Nagoya Univ. Museum
No. 36, 35–51, 2021
DOI: 10.18999/bulnum.036.03

Geochemistry of the basaltic blocks in the Jurassic and Cretaceous accretionary
complexes in the Yoshino area, central Kii Peninsula, Southwest Japan
Bayart NADMID1）*, Kazuhiro TSUKADA2）, and Makoto TAKEUCHI1）
1）Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University,
Chikusa-ku, Nagoya, 464-8601, Japan
2）Nagoya University Museum, Chikusa-ku, Nagoya, 464-8601, Japan*Corresponding author

Abstract
This study describes the geochemistry of basaltic blocks in the Daifugendake and Wasabidani complexes (Chichibubelt) and the Unokawa and Takaharagawa complexes (Shimanto belt) exposed in the Yoshino area of the central Kii
Peninsula of Japan and discusses the tectonic implications. In this study, we examined 5 samples from the Chichibu belt
and 10 samples from the Shimanto belt. The samples are divided into alkaline and tholeiitic basalts. Blocks in the Chichibu
belt and the Takaharagawa complex of the Shimanto belt are determined to be alkaline basalts, and those in the Unokawa
complex of the Shimanto belt are tholeiitic basalts. Alkaline and tholeiitic basalts have been recovered from the Chichibu
and Shimanto belts throughout southwest Japan, respectively. Alkaline basalts in the Takaharagawa complex, which havenever been described in the Shimanto belt, may imply that Chichibu belt components are involved into the Shimanto beltin the Yoshino area. It is possible that alkaline basalt derived from the Chichibu belt was mixed into a “shear zone” (themélange unit) formed through thrust movement following accretion.

Introduction
Southwest Japan is divided into the Inner zone and the Outer zone by the Median tectonic line. The Outerzone consists mostly of Mesozoic accretionary complexes that are subdivided into the Sanbagawa, Chichibu, andShimanto belts (Fig. 1: Taira et al., 1988; Kurimoto, 1994). The main constituent of the Chichibu belt is Jurassic–
Cretaceous accretionary complexes, and the Shimanto belt consists mainly of Cretaceous–Paleogene accretionary
complexes. The Sanbagawa belt is dominated by metamorphosed Cretaceous accretionary complexes.
The Sanbagawa, Chichibu, and Shimanto belts are arranged from north to south in the Kanto, Shikoku, andKyushu areas; however, the rocks of the Shimanto belt contact those of the Sanbagawa belt to intersect the
Chichibu belt in the central part of the Kii Peninsula (Fig. 1: Sasaki and Isozaki, 1992). The question here is how
the tectonic differences between the Kii Peninsula and the other areas arose. Thus, the geological structure in the
Yoshino area of the Kii Peninsula would be a key factor (Fig. 1).
On the basis of lithology, the Yamato Omine Research Group (YORG, 1992) divided the rocks in the
Yoshino area into the Daifugendake, Wasabidani, and Takahara formations of the Jurassic Chichibu belt andthe Obadanigawa and Akataki formations of the Cretaceous Shimanto belt in descending order. The Takahara
Formation consists of sandstone-dominant unit and mélange unit with blocks of chert, basalt, and Torinosutype limestone which is a distinctive component of the Chichibu belt. Takeuchi (1996), on the grounds that thesandstones of the Obadanigawa Formation (YORG, 1992) and of the sandstone-dominant unit of the Takahara
Formation (YORG, 1992) are petrologically quite similar, uniﬁed them to deﬁne the Takaharagawa Formation of
the Cretaceous Shimanto belt. He considered that the blocks of limestone, chert, and basalt in the mélange unit
― 35 ―

Fig. 1

Index map showing the tectonic division of the Outer zone in southwest Japan [modiﬁed from Taira et al. (1988)].

of the Takahara Formation (YORG, 1992) were exotic blocks derived from the Jurassic Chichibu belt. Besides,
Takeuchi (1996) redeﬁned the rocks of the Shimanto belt in this area as the Unokawa, Takaharagawa, Akataki,
and Makio formations in descending order. YORG (2005) consolidated the Takahara and Wasabidani formations
into the Wasabidani complex, and redeﬁned a part of the Takaharagawa Formation (Takeuchi, 1996) as the Osako
complex of the Jurassic Chichibu belt. Opinions on the tectonic correlation of the Takaharagawa Formation(Takeuchi, 1996) are divided between Takeuchi (1996) and YORG (2005), due to barren in fossil in the sandstonedominant unit of this formation. Takeuchi (1996) noted that the primal pile-nappe structure of the Sanbagawa,
Chichibu, and Shimanto belts of this area, had been modiﬁed by faulting and folding attributed to post-Miocene
uplifting to produce the present-day structure. Ota et al. (2019) divided the Takaharagawa Formation into threetectono-stratigraphic units: a lower sandstone unit, a mixed-rock (mélange) unit (called mélange unit, hereinafter),and an upper sandstone unit. They also described middle Cretaceous detrital zircons from the lower and uppersandstone units to conﬁrm that a part of the Takaharagawa Formation is in the Shimanto belt. However, the age ofthe mélange unit has not been revealed, and the tectonic correlation of this unit is still open for discussion. Indeed,
it is possible that the mélange unit, which contains Torinosu-type limestone that is a distinctive component of the
Chichibu belt, should be assigned to the Chichibu belt (Shiida, 1962; YORG, 1992; Ishida, 2011).
The geochemistry of the basaltic rocks in the accretionary complexes of the Chichibu and Shimanto belts has
been described throughout southwest Japan. Additionally, it is recognized that the Shimanto belt includes exotic
blocks of mid-ocean ridge basalt (MORB) and in-situ MORB/island arc basalt (IAB); however, the Chichibu belt
also contains exotic blocks of MORB and oceanic island basalt (OIB) (e.g., Sugisaki et al., 1979; Miyashita and
Katsushima, 1986; Kiminami and Miyashita, 1992; Kiminami et al., 1992; Asaki et al., 1993; Yoshida et al., 1994;
Kiminami et al., 1994; Osozawa and Yoshida, 1997; Umeki and Sakakibara, 1998; Asaki and Yoshida, 1999; Asaki
et al., 1999; Kawabata and Kiminami, 1999; Nakamura et al., 2000; Sato, 2003; Ishizuka et al., 2003; Onoue― 36 ―

et al., 2004; Chinen et al., 2005; Nozaki et al., 2005; Fujinaga et al., 2006; Moriyama et al., 2007; Yamanashi
and Kashiwagi, 2010). The ﬁeld occurrence and geochemistry of basaltic blocks in these accretionary complexes
provide important information for considering the tectonic division of the Chichibu and Shimanto belts; however,
little is known about geochemistry of the basaltic blocks in Yoshino area.
This paper focuses on the geochemistry of the basaltic blocks in the Chichibu and Shimanto belts in the Yoshinoarea and discusses their tectonic settings to reﬁne the tectonic correlation of the mélange unit of the Takaharagawa
Formation in the Shimanto belt.
Geological overview of the Yoshino area
The stratigraphic division of this study follows Takeuchi (1996) and Ota et al. (2019). The study area is in
Kawakami Village, in the Yoshino area of central Kii Peninsula, and exposes Mesozoic accretionary complexesof the Chichibu (Daifugendake and Wasabidani complexes) and Shimanto belts (Unokawa, Takaharagawa,Akataki, and Makio complexes) (Fig. 2: Takeuchi, 1996; Ota et al., 2019). The lithology, structure, and age of the
Daifugendake, Wasabidani, Unokawa, and Takaharagawa complexes, which include expose basaltic rocks, aredescribed as follows (Figs. 3 and 4).
Chichibu belt
Daifugendake complex
Daifugendake complex rocks are exposed in the southern part of the study area (Figs. 2 and 4a). The
Daifugendake complex is subdivided into lower and upper units (Ota et al., 2019). The lower unit consists ofmélange that includes blocks of limestone and basaltic rocks (massive lava, hyaloclastite, and reddish tuff) in a
muddy matrix. The hyaloclastite contains abundant angular clasts in a ﬁne matrix that had been entirely alteredinto secondary minerals. Minor amounts of sandstone blocks are also included in the mélange. The upper unit
consists of piled thrust sheets of a chert–clastic sequence. The chert, which is associated with massive basaltic lava,
is white or red and fossiliferous. The basaltic lava is ﬁne-grained and shows an intersertal texture with plagioclase
laths (Fig. 5a). The chert was, in places, intruded by basalt–dolerite dikes composed mainly of euhedral/subhedral
plagioclase and hornblende, forming a subophitic–ophitic texture. The sandstone is gray to dark gray, massive or
bedded, and ﬁne-grained to course-grained. The mudstone is black or dark red and is interbedded in the sandstone
in some horizons. The sandstoneʼs bedding plane generally strikes NW and dips 20° to 75° north or south. The
Daifugendake complex is in fault contact with the Takaharagawa and Wasabidani complexes (Ota et al., 2019).
Radiolarian fossils that correlate to the Callovian–Oxfordian were reported from the mélange matrix. The chert
yields radiolarian fossils suggesting Lower Permian–Lower Triassic and Middle Triassic–Middle Jurassic (YORG,
1992, 2005). U–Pb age of the youngest detrital zircon in the sandstone samples ranges from ca. 174 Ma to 164 Ma(Shimura et al., 2019).
Wasabidani complex
Wasabidani complex rocks are exposed in the eastern portion of the study area (Fig. 2). The Wasabidani
complex consists entirely of mélange that includes blocks of basaltic rock, chert, limestone, mudstone, andsandstone in a muddy matrix (YORG, 2005; Ota et al., 2019). Most of the basaltic blocks includes clinopyroxene,
plagioclase, and hornblende phenocrysts that are 1–2 mm on the major axis, thus forming a porphyric texture. The
clinopyroxene and hornblende are generally fresh; however, plagioclase laths were partially altered into calcite and― 37 ―

Fig. 2

Simpliﬁed geological map and cross section of the Yoshino area [modiﬁed from Takeuchi (1996) and Ota et al. (2019)].

mica minerals. The groundmass has been mostly altered into secondary minerals such as chlorite and calcite (Fig.
5b). The sandstone block is gray or dark gray, massive or bedded, and ﬁne-grained to coarse-grained. Black or
dark red mudstone layers are interbedded in the sandstone in places.
This complex thrusts onto the Takaharagawa complex of the Shimanto belt (Ota et al., 2019). The limestone― 38 ―

Fig. 3

T he stratigraphic compilation of the Daifugendake and Wasabidani complexes (Chichibu belt) and Unokawa and
Takaharagawa complexes (Shimanto belt). The range of the youngest zircon U–Pb ages in the sandstone samples are also
shown. The data were from YORG (1992, 2005), Shimura et al. (2019, 2020), and Ota et al. (2019).

block yields Middle Permian fusulinaceans and Upper Triassic corals. Radiolarian fossils assigned to the
Oxfordian–Kimmeridgian were reported from the muddy matrix (YORG, 1992, 2005). Youngest detrital zircons inthe sandstone samples are ca. 162 to 154 Ma in U–Pb age (Shimura et al., 2019).
Shimanto belt
Unokawa complex
The rocks of the Unokawa complex are exposed in the northern portion of the study area (Figs. 2 and 4b). This
complex consists of mélange that includes blocks of basaltic rocks and chert in a mudstone matrix.
Basaltic rocks are composed of tuff, hyaloclastite, and pillow lava. The lava ﬂows are mostly enclosed by achilled margin, and radial joints and spherical/ellipsoidal vesicles are developed in the pillow lava (Fig. 5c). The
pillow lavas are light green to dark green or red and generally <1 m in diameter, with fractures ﬁlled with calcite.
― 39 ―

Fig. 4

Route maps of the (a) Daifugendake, (b) Unokawa, and (c) Takaharagawa complexes showing sampling localities.

The pillow lavas generally show an intersertal texture with plagioclase laths (Fig. 5d), and the vesicles are rarely
ﬁlled by chlorite and calcite to form amygdule. The hyaloclastite includes abundant angular clasts of up to 5 cm
in a ﬁner matrix that has been entirely altered into chlorite. The sandstone is ﬁne- grained to coarse- grained andmassive or bedded. The mudstone of the matrix is gray to dark gray and is generally sheared.
This complexʼs rocks are strongly sheared in the southern portion of the study area, whereas those in thenorthern part are hardly deformed. The Unokawa complex thrusts over the Takaharagawa complex of the Shimanto
belt (Takeuchi, 1996). Youngest detrital zircon in the sandstone samples are ca. 112–99 Ma in U–Pb age (Shimura
et al., 2020).
Takaharagawa complex
Takaharagawa complex rocks are exposed in the central part of the study area (Figs. 2 and 4c). The― 40 ―

Fig. 5Field occurrence and photomicrographs of basaltic rocks in the study area. (a) Photomicrograph of the massive lava in the
Daifugendake complex. (b) Photomicrograph of the massive lava in the Wasabidani complex. (c) Basaltic block with pillow
structure in the Unokawa complex. (d) Photomicrograph of pillow lava in the Unokawa complex. (e) Basaltic block withpillow structure in the Takaharagawa complex. (f) Photomicrograph of pillow lava in the Takaharagawa complex. Cpx,
clinopyroxene; Hb, hornblende.

Takaharagawa complex is divided into three units in ascending order: a lower sandstone unit, a mélange unit, andan upper sandstone unit (Ota et al., 2019). The lower and upper sandstone units mainly consist of medium-grained
to coarse-grained massive sandstone, with bedded ﬁne-grained sandstone layers intercalated in some horizons.
The mélange unit includes basaltic rocks, limestone, chert, reddish tuff, and clastic rocks in mudstone matrix.
The basaltic rocks are light to dark green and show an intersertal texture with plagioclase laths (Fig. 5e f). The
sandstone in the mélange is middle-grained wacke that contains abundant lithic fragments of felsic tuff andradiolarian chert, with subordinate amount of quartz and plagioclase (Ota et al., 2019). Scaly cleavage is developed― 41 ―

in the muddy matrix.
The bedding plane of the sandstone generally strikes NE–SW and dips 20° to 70° north or south. Triassic
conodonts and Upper Jurassic Torinosu-type fauna were found in the limestone blocks (Shiida, 1962; YORG,
1992; Ota et al., 2019). U–Pb age of the youngest detrital zircon in the sandstone samples ranges from 119 to 109
Ma in the lower sandstone unit, and from 118 to 107 Ma in the upper sandstone unit (Ota et al., 2019).
Whole-rock geochemistry
Methodology
We examined 15 basaltic rock samples (5 from the Daifugendake and Wasabidani complexes of the Chichibu
belt and 10 from the Unokawa complex and the mélange unit of the Takaharagawa complex of the Shimanto belt).
The samples were coarse crushed using a stainless steel bowl and pounder. After coarse crushing, veins andaltered parts of the samples were removed under a stereo microscope. Each analysis sample weighed >100 g.
Concentrations of major oxides were determined using X-ray ﬂuorescence (XRF: Rigaku Primus II ZSX equippedwith Rh X-ray tube, 50 kV, 60 mA) and trace element (V, Cr, Co, Ni, Zn, Ga, Rb, Sr, Y, Zr, Nb, Pb, Ba, Hf, and Ta)
and rare earth element (REE) compositions were analyzed using inductively coupled plasma–mass spectrometry(quadrupole type ICP-MS; Agilent 7700x with collision cell of He) at Nagoya University.
For the XRF analysis, we prepared glass beads by fusing mixtures of 0.5 g of powdered sample with 5.0 g oflithium tetraborate. Calibration was conducted using standard rock samples issued by the Geological Survey ofJapan. We estimated the analytical precision of major elements to be <1% for Si and approximately 3% for otherelements, except for TiO2 and MnO, which had analytical precisions of >3% when the measured level was <0.1%(Yamamoto and Morishita, 1997; Takebe and Yamamoto, 2003).
We determined trace element and REE compositions using ICP-MS with a technique based on the methodology
described by Yamamoto et al. (2005). Approximately 30 mg of each sample was digested with a mixed solutionof HF–HClO4 (2:1 by volume) at 150°C. After the acids had completely evaporated, we added 2 mL of 1.7 N HCl
to dissolve the cake. The residue was then separated by centrifugation at 12,000 rpm with a 2-mL polypropylene
tube. After centrifugation, the supernatant was transferred to another 10 mL Teﬂon beaker, and the residue wasagain fused with HF–HClO4 (2:1 by volume) at 150°C. The fused cake was then dissolved with approximately 2mL 1.7 N HCl by mild heating, and the resulting solution was centrifuged at 12,000 rpm. No residue was observed
after centrifugation in most cases. The HCl solution was then evaporated to dryness, and the fused cake was redissolved in a 2% HNO3 solution and analyzed using ICP-MS. In and Bi were used to trace ICP sensitivities, andthe In and Bi concentrations were mostly unchanged throughout the analysis. The oxide generation factor (LnO/
Ln) was determined for each 20 ppb solution and used for REE analytical data correction. In the ICP-MS analysis,
the correlation coefﬁcients (R values) of each element, calculated for ﬁve standard samples, were 0.9994–1.0000,
and the concentration relative standard deviation of the data were mostly <3%.
Geochemical description of the basaltic rocks
Table 1 lists the whole-rock compositions of the samples, and Figure 6 displays variation diagrams for analyzed
elements against SiO2. The loss of ignition was approximately <5 wt% for most samples.
The samples with SiO2 concentrations of 41–54 wt% are assigned to picrite basalt to basaltic andesite. The
examined samples are subdivided into two types based on their geochemical characteristics (Table 1): type 1(samples from the Daifugendake, Wasabidani, and Takaharagawa complexes) and type 2 (samples from the― 42 ―

Unokawa complex). Type 1 and type 2 samples have nearly identical compositions of Al2O3, Co, Ni, Zn, and Y;
however, the type 2 samples contain more TiO2, P2O5, Ga, Sr, Zr, Nb, Pb, Hf, and Ta than the type 1 samples. In
chondrite-normalized REE patterns, type 1, which is enriched in light REE, shows a declining trend toward heavy
REE, whereas type 2 shows a nearly horizontal trend.
Discussion
Basaltic rock magma types and tectonic setting
Basalt geochemistry provides evidence for volcanic activity that formed in the tectonic setting because basalt's
chemical composition varies on the basis of its origins. Many discrimination diagrams of basaltic rocks have been
proposed (e.g., Pearce, 1982). In this section, we used discrimination diagrams to discuss the tectonic setting
and magma type of basaltic rocks in the study area. Discrimination of the analyzed samples is based mainly on
diagrams using comparatively ﬂuid-immobile elements, such as Ti, P, Zr, Nb, Hf, Ta, and REE.
Type 1 and type 2 basaltic rocks are plotted in ﬁelds of alkaline and tholeiitic basalts, respectively, in the TiO2
vs. (Zr/P2O5) × 10,000 diagram (Fig. 7: Winchester and Floyd, 1976). On the basis of the MORB-normalized
multi-element patterns, type 1 basalt's clear reducing trend strongly suggests alkaline basalt (Fig. 8: Nakamura etal., 2000). The high-ﬁeld-strength elements (HFSE) and REE concentrations in type 2 basalt are nearly identical to
MORB; however, the higher concentration of large-ion lithophile elements (LILE), such as K, Rb, and Ba, than in
MORB, implies that type 2 basalt is a calc-alkaline basalt (Fig. 8: Nakamura et al., 2000).
LILE is highly mobile through secondary processes; thus, making a precise discussion using a spidergram is
difﬁcult here. The samples showed chondrite-normalized REE patterns, suggesting that type 1 basalt, which isenriched in light REE and shows a reducing trend toward heavy REE, has a strong afﬁnity to alkaline basalt, andtype 2 basalt shows a nearly horizontal trend that resembles tholeiitic basalt rather than calc-alkaline basalt (Fig.
9: e.g., Anders and Grevesse, 1989; Yamamoto et al., 2005). Taking these pieces of evidences together, it seems
probable that the type 1 and type 2 basalts are alkaline and tholeiitic basalts, respectively. Discrimination using
HFSE vs. (La/Yb)cn diagrams strongly support this view (Fig. 10: Nakamura et al., 2000). That is, it is concluded
that the basaltic rocks from the Unokawa complex of the Shimanto belt are tholeiitic basalt, whereas those fromthe other complexes are alkaline basalt.
Tectonic significance of the basaltic rocks
Tectonic blocks of MORB and in-situ MORB/IAB were found in the Shimanto belt at the Kii Peninsula andin the Shikoku, Kyushu, and Okinawa areas, and exotic blocks of MORB and OIB have been reported in the
Chichibu belt in the Kanto, Chubu, and Shikoku areas (e.g., Sugisaki et al., 1979; Miyashita and Katsushima,
1986; Kiminami and Miyashita, 1992; Kiminami et al., 1992; Asaki et al., 1993; Yoshida et al., 1994; Kiminami etal., 1994; Osozawa and Yoshida, 1997; Umeki and Sakakibara, 1998; Asaki and Yoshida, 1999; Asaki et al., 1999;
Kawabata and Kiminami, 1999; Nakamura et al., 2000; Sato, 2003; Ishizuka et al., 2003; Onoue et al., 2004;
Chinen et al., 2005; Nozaki et al., 2005; Fujinaga et al., 2006; Moriyama et al., 2007; Yamanashi and Kashiwagi,2010).
Opinions on the tectonic setting of the in-situ basaltic rocks in the Shimanto belt vary. Some authors haveinferred, on the basis of LILE and HFSE concentrations, that these rocks are alkaline/high-alkaline tholeiitic basalt
attributed to island arc type magmatism (e.g., Asaki et al., 1993; Yoshida et al., 1994; Asaki and Yoshida, 1998,
1999; Asaki et al., 1999). In contrast, others have asserted, on the basis of HFSE concentration, that the basaltic― 43 ―

― 44 ―

363
59.4
20.7
44.1
125
12.2
54.3
366
58.6
345
74.0
2.28
578
9.06
4.28

55.9
122
15.3
63.3
13.6
4.10
13.8
1.98
11.9
2.28
6.15
0.859
4.99
0.737

279
86.5
40.7
95.5
93.9
3.85
4.65
277
20.4
153
69.4
4.04
97.3
4.87
4.19

Trace elements (ppm)
V
Cr
Co
Ni
Zn
Ga
Rb
Sr
Y
Zr
Nb
Pb
Ba
Hf
Ta

Rare earth elements (ppm)
La
40.5
Ce
80.5
Pr
9.29
Nd
39.2
Sm
7.76
Eu
2.47
Gd
6.81
Tb
0.979
Dy
5.12
Ho
0.928
Er
2.06
Tm
0.225
Yb
1.29
Lu
0.150

1.05
98.9
2.41

0.59
99.2
2.80

76.8
156
17.9
68.7
12.2
3.79
9.65
1.22
7.63
1.36
3.64
0.461
2.67
0.386

96.5
36.6
18.4
28.5
144
7.20
65.0
433
34.5
218
68.9
7.81
314
5.26
3.82

0.96
97.2
7.17

15.0
13.4
0.67
4.58
7.65
4.43
2.38

16.8
13.4
0.05
1.91
3.10
4.26
3.02

P2O5
Total
LOI

45.3
2.84

51.2
3.99

10.7
0.26
8.61
10.6
3.34
0.20

2

68.2
20.3
25.5
11.3
152
11.1
38.8
471
41.0
293
122
5.86
562
7.99
7.34

0.94
98.8
5.06

17.1
13.2
0.37
7.10
3.66
4.57
1.57

47.6
2.70

85.8
172
19.8
75.8
13.8
3.50
11.6
1.50
8.99
1.66
4.27
0.538
3.17
0.484

Chichibu belt
Daifugendake
5
6

Fe2O3*
MnO
MgO
CaO
Na2O
K2O

Major elements (wt%)
SiO2
49.0
TiO2
2.55
Al2O3
13.5

Wasabidani
1

Fe2O3* is total iron as Fe2O3.

211
214
46.5
162
153
13.9
11.0
683
50.8
651
158
9.19
723
14.1
10.1

1.72
98.0
6.35

13.4
14.0
0.23
11.0
7.80
2.80
0.33

42.7
3.98

119
240
28.2
114
19.9
6.00
17.4
2.14
11.7
1.99
5.09
0.687
3.51
0.480

7

46.7
121
15.9
68.4
15.3
4.31
13.5
1.83
9.34
1.56
3.46
0.375
1.97
0.195

323
683
99.6
368
155
18.2
90.4
171
34.9
213
42.7
4.40
983
4.89
2.55

0.77
99.3
5.04

18.3
15.9
0.14
6.19
5.45
1.98
3.49

41.3
5.75

236
415
63.0
252
107
3.37
2.09
178
20.6
159
51.5
1.88
19.5
3.66
3.02

0.62
98.1
6.83

14.3
14.2
0.22
9.86
5.85
3.18
0.05

46.9
2.99

37.9
76.9
9.53
37.8
7.43
2.43
6.83
0.907
5.15
0.877
2.03
0.243
1.31
0.139

Takaharagawa
8
9

0.12
98.6
3.01

16.0
10.6
0.16
6.22
8.01
4.31
1.10

50.8
1.34

1.92
6.63
1.23
7.35
3.15
1.00
4.72
0.773
5.90
1.34
3.78
0.532
3.47
0.468

280
238
37.4
87.8
295
2.56
23.8
64.2
32.4
60.9
1.41
1.62
60.2
2.30
0.294

10

0.10
99.0
3.29

16.7
10.4
0.15
5.09
10.3
2.40
3.18

49.3
1.30

2.62
7.94
1.42
8.42
3.02
1.33
4.82
0.824
6.00
1.23
3.64
0.543
3.65
0.524

283
217
37.8
69.6
89.1
3.70
57.4
65.9
32.9
57.1
2.58
1.17
102
2.22
0.245

11

0.13
99.0
2.66

15.0
12.3
0.17
6.09
9.84
3.04
1.65

49.2
1.58

3.01
9.57
1.73
9.86
3.62
1.23
5.28
0.970
7.15
1.48
4.43
0.607
3.98
0.666

329
238
44.5
102
96.4
3.05
30.8
43.7
38.7
67.7
1.65
0.784
55.7
2.26
0.246

12

0.10
99.0
3.05

16.6
10.6
0.17
5.64
9.33
2.84
2.74

49.7
1.31

Unokawa

2.30
7.44
1.29
8.02
3.12
1.21
4.89
0.756
5.52
1.25
3.57
0.509
3.06
0.489

275
207
41.4
82.3
88.6
3.47
48.8
50.8
30.9
56.3
1.90
0.886
90.8
1.74
0.296

13

Shimanto belt

0.11
99.1
3.04

16.9
10.4
0.15
5.04
9.25
2.91
2.98

50.0
1.30

2.28
7.29
1.31
8.47
3.07
1.07
4.07
0.807
5.50
1.22
3.73
0.485
3.45
0.420

285
207
41.5
76.9
87.5
3.38
51.7
48.1
31.7
56.8
1.96
0.664
99.9
1.90
0.194

14

1.11
99.4
3.32

19.2
11.4
0.13
2.50
5.55
4.07
3.85

50.1
1.50

4.03
8.68
2.38
14.1
5.34
2.01
8.31
1.57
11.8
2.71
8.53
1.24
8.60
1.35

193
265
41.3
117
106
4.19
62.7
75.7
84.2
63.0
1.16
0.747
127
2.51
0.170

15

0.13
98.6
3.61

13.2
10.1
0.16
6.28
10.3
4.02
0.11

52.8
1.48

1.61
6.24
1.37
7.94
3.50
1.03
4.90
0.867
6.11
1.29
3.83
0.564
3.83
0.609

249
216
37.0
99.8
82.0
1.85
2.69
53.5
31.7
72.4
1.68
0.621
13.9
2.57
0.526

16

0.22
99.4
2.46

15.6
11.7
0.16
3.58
6.41
5.23
0.76

54.2
1.60

2.67
8.95
1.69
10.5
3.70
1.52
5.73
1.03
7.44
1.59
4.95
0.698
4.35
0.631

274
152
41.0
81.0
115
2.40
16.2
45.8
40.4
76.4
1.48
0.963
43.8
3.39
0.285

17

Whole-rock chemical composition of basaltic rocks in the Chichibu and Shimanto belts in the study area. The data are displayed to three signiﬁcant digits, except for MnO, K2O, and P2O5.

Complexes
Sample No.

Table 1

Fig. 6

Variation diagrams for major and trace elements against SiO2.

― 45 ―

Fig. 7

Data plot on the TiO2 vs. (Zr/P2O5) × 10,000 diagram proposed by Winchester and Floyd (1976).

rocks were generated by mid-oceanic ridge type magmatism (e.g., Miyashita and Katsushima, 1986; Kiminami
and Miyashita, 1992; Kiminami et al., 1992; Kumon and Miyake, 1993; Kiminami et al., 1994; Nakamura et al.,2000). Nakamura et al. (2000) and Moriyama et al. (2007) suggested that these basaltic rocks had their LILE
compositions largely modified through secondary processes and demonstrated, from the perspective of REE
concentrations, that the basaltic rocks have clear tholeiitic natures. In either scenario, basaltic rocks with REE
compositions that show an alkaline nature have not yet been described in the Shimanto belt.
Thus, tholeiitic basalts represented by MORB from the Unokawa complex (Shimanto belt) and alkaline
basalts included in OIB from the Daifugendake and Wasabidani complexes (Chichibu belt) agree with the resultsfrom previous studies. In contrast, basaltic rocks with an alkaline REE nature from the mélange unit of the
Takaharagawa complex, currently assigned to the Shimanto belt, do not seem to be concordant with previous
studies (Nakamura et al., 2000; Moriyama et al., 2007). The mélange unit includes blocks of Triassic limestone
and Jurassic Torinosu-type limestone, which are known as distinctive components of the Chichibu belt, in addition
to the alkaline basalts described here (Shiida, 1962; YORG, 1992; Ota et al., 2019). Additionally, Ota et al.
(2019) demonstrated that the sandstone in this unit differs in modal composition from that of the upper and lower
sandstone units, implying that the sandstone in the mélange unit is an exotic block derived from another geologic
unit. The low-angle thrusts enclosing the mélange unit cut the fold structure of the Takaharagawa complex's lower
and upper sandstone units (Fig. 2: Ota et al., 2019). Consequently, the evidence points to a thrust movement
following the folding being involved in the accretion process of the oceanic floor. The mélange unit, which isfoliated and <100 m wide, is distributed in parallel to the thrusts cutting the folding structure of the lower and― 46 ―

Fig. 8

MORB-normalized multi-element patterns of the examined samples. MORB composition normalization follows Sun and
McDonough (1989).

Fig. 9Chondrite-normalized REE patterns of the examined samples. Chondrite concentration normalization follows Anders andGrevesse (1989) and Yamamoto et al. (2005).

― 47 ―

Fig. 10

Data plot on the HFSE vs. (La/Yb)cn diagrams proposed by Nakamura et al. (2000).

upper sandstone units (Fig. 2: Ota et al., 2019). Taking these together, it can be speculated that the mélange unit
might be a “shear zone” with blocks derived from the Chichibu belt that had formed through thrust movement
following accretion. This can be confirmed when the chronological and kinematical data of the mélange unit
become available.

― 48 ―

Conclusion
Tectonic blocks of MORB and in-situ MORB/IAB were found from the Shimanto belt, and exotic blocks of
MORB and OIB have been reported from the Chichibu belt throughout southwest Japan. The Daifugendake andWasabidani complexes of the Chichibu belt in the study area yield alkaline basalt, on the other hand, the Unokawa
complex of the Shimanto belt yields tholeiitic basalt. This is in match with the previous view. The mélange unit of
the Takaharagawa complex of the Shimanto belt has alkaline basalt as well as the Torinosu-type limestone, which
both are the distinctive components of the Chichibu belt. The mélange unit might be a “shear zone” with blocks
derived from the Chichibu belt that had formed through thrust movement following accretion.
Acknowledgments
Special thank goes to Prof. K. Yamamoto and Prof. Y. Asahara at Nagoya University for technical supports
in XRF and ICP-MS analysis. We are grateful to Prof. H. Yoshida at Nagoya University for his helpful advice
and comments. We thank Dr. O. Munkhtsetseg at Mongolian University of Science and Technology for her kind
supports and advice. We are grateful to Dr. Y. Koketsu and Dr. S. Fujiwara at Nagoya University for helpful
criticism of the manuscript. We would like to thank Enago (www.enago.jp) for the English language review. This
study was supported by Higher Engineering Education Development Project of Japan International Cooperation
Agency (JICA MJEED project No. JR22B15).
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