国立科博専報，(51), pp. 53–58 , 2016 年 3 月 28 日
Mem. Natl. Mus. Nat. Sci., Tokyo, (51), pp. 53–58, March 28, 2016

Petrological Study of Cretaceous Granitoidsand Triassic Sandstones in Sado Island
Kazumi Yokoyama*, Kenichiro Tani and Yukiyasu Tsutsumi
Department of Geology and Paleontology, National Museum of Nature and Science,
4–1–1 Amakubo, Tsukuba, Ibaraki 305–0005, Japan* E-mail: yokoyama@kahaku.go.jp

Abstract. Sado Island is composed mostly of Tertiary volcanic and sedimentary rocks. PreTertiary basement rocks occurring locally on the island are Cretaceous granitoids, pre-Cretaceous
sedimentary rocks and ophiolitic rocks. Ages of the Cretaceous granitoids were obtained by
Inductively coupled plasma mass spectrometry and electron probe micro analyzer. The ages are
101–105 Ma, similar to granitoids occurring on the coastal area of south Primorye, Far East Russia.
Age patterns of the detrital monazite and zircon in the Triassic sandstones have a peak at
500 Ma. It indicates that Triassic sediments were derived from a terrane including the Khanka andJiamusi blocks in Far East Asia, as well as the sandstones from the Nabae and Yakuno groups inthe Maizuru Belt. In addition to the Maizuru Belt in the Japanese Islands, both the granitoids andsandstones are probably connected with the south Primorye in Russia.
Key words: granite, Triassic, zircon, monazite, age

Introduction
In the Sado Island, outcrops of the basement
rocks are restricted in two area: Washizaki andAkadomari areas (Fig. 1). In the Washizaki area,
an ophiolitic assemblage is observed. They areserpeninized peridotite, gabbro, metabasite,chert, limestone and clastic sediments. Itasaka etal. (1994) found Late Triassic radiolarian fromthe sedimentary sequence. Middle Permian fusulinids and Latest Triassic or Early Jurassic radiolarians were reported from this area (cf. Ichida etal., 2010). A Cretaceous granitoid body also
occurs in the area. The K–Ar age of the granitoid
was reported to be 99 Ma (MITI, 1994).
Whereas, in the Akadomari area, a mud-sandstone sequence, chert and limestone blocks arefound. Ichida et al. (2010) reported Middle
Permian fusulinids in the limestone blocks fromthe Akadomari area. Carboniferous or Permianbryozoans, Early Jurassic radiolarians and late
Middle Permian radiolarians were also reported
in this area (cf. Ichida et al., 2010).

Granitic bodies were found in four localities
from the Washizaki and Akadomari areas. K–Arages of the rocks are from 95 Ma to 103 Ma (Fig.

Fig. 1. Distribution of pre-Tertiary basement rocks
on Sado Island and sampling sites for age analyses.

Kazumi Yokoyama et al.

54

1: MITI, 1994). The Kawabe conglomerate layer
in the Aikawa Formation is composed mostly of
granitic blocks with a subordinate amount of volcanic matrix. Age of the granite was reported as
43 Ma which was obtained by the ﬁssion track
method (Ganzawa, 1982).
There are many discussions about the assignment of the basement rocks with tectonic belts inthe main land of the Japanese Islands. Komatsu
et al. (1985) thought that Sado Island is an extension of the Maizuru belt as well as the Joetsu
Belt. Ichida et al. (2010) considered that thebasement rocks are an extension of the Maizuru
Belt or ultra-Tanba Belt based on the fossils in
the limestone block.
In this study, we analyzed monazites by Electron Probe Micro Analyzer (EPMA) and zircons
by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to obtain mineral ages of pre-Cretaceous sandstones in the terrigenous sequences.
In addition to the rocks, we obtained mineralages of the Cretaceous granitoids and Tertiary
volcanic rocks to conﬁrm their ages that wereobtained by K–Ar and ﬁssion track methods. As
comparable samples, we analyzed the Triassicand Permian sandstones in the South Primorye inRussia.
Samples
Sandstone samples were collected from theintercalated sequences of mudstone and sandstone from the Washizaki and Akadomari areas(Fig. 1). From the Primorye, we had collected theTriassic and Permian sandstones. In a previous
Table 1.

study (Yokoyama et al., 2009), we analyzed Triassic sandstones from the Abrek Bay area, south
Primorye, Russia and from the Nabae andYakuno groups in the Maizuru Belt. Present samples were collected from the Vlagiostok, south
Primorye.
Granitic samples were collected from the relatively large body in the Washizaki area and fromthe Katabe conglomerate layer. The conglomerate layer belongs to the Aikawa Formation andcontains abundant granite blocks up to 2 m indiameter. The matrix of the conglomerate is brecciated volcanic rocks, showing that the granitic
body was crushed by the intrusion of the volcanic rocks. We analyzed zircons of the volcanic
rocks in the matrix to obtain the age of the volcanic intrusion. In the Tertiary sequence of Sado
Island, the lowest stratum is the Nyukawa Formation. The volcanic rock of the formation wasalso collected to obtain the age of the ﬁrst volcanic activity from the Tertiary sequence on Sado
Island.
Age results
All the samples were crushed in a stainlesssteel vessel and heavy fractions were separated
by methylene iodide. Zircon grains were handpicked from the heavy fractions. The residual
heavy fractions were cemented in a thin section
for the analyses of monazite by EPMA at theNational Museum of Nature and Science. The
age method determined by EPMA for monazite
was described in detail by Santosh et al. (2006),
and the age method determined by Lazer Abla-Ages of granitoids and Tertiary volcanic rocks on Sado Island.

Cretaceous granitoids
rock typemedium-grained granite
medium-grained granite
porphyritic granite
granodiorite

locality

sample No.

mineralmethodage (Ma)

Kawabe area
Kawabe area
Kawabe area
Washizaki area

SD03-1SD03-3SD03-2
SD04

monazitemonazitezirconzirconEPMAEPMA
LA-ICP-MS
LA-ICP-MS

101.3±3.4
101.3±3.1
103.1±1.2
105.2±1.1

zirconzirconLA-ICP-MS
LA-ICP-MS

26.4±0.5 Ma
27.5±0.6 Ma

volcanics in the Tertiary sequence (N.F.- Nyukawa F., A.F.-Aikawa F.)
andesite (N.F.)
andesite (A.F.)

Washizaki area
Kawabe area

SD17
SD12-1Granite and sandstones in Sado Islandtion Inductively Coupled Mass Spectrometer(LA-ICP-MS) for zircon was described by Tsutsumi et al. (2012). The ages of both monazite
and zircon are represented as probability diagrams in Figs. 2, 3, and 4. The age results ofgranitoids and volcanic rocks are listed in Table 1.
Sandstones: Among the many samples collectedfrom Sado Island, only one sample, SD09, fromthe Akadomari area contains detrital monazite.
The age pattern shows a bimodal pattern withpeaks at 490 Ma and 1850 Ma (Fig. 2). There is
no younger age than 450 Ma. On the other hand,the age pattern for detrital zircon has the strongest peak at around 250 Ma with subordinate

55peaks at 500 Ma and 1850 Ma (Fig. 3). The
youngest and oldest zircon ages are 216 Ma and2740 Ma, respectively.
In the Washizaki area, detrital zircons from thesample, SD16, were analyzed. The age patternshows only one peak at around 250 Ma (Fig. 3).
The youngest and oldest ages are 229 Ma and2700 Ma, respectively. Zircon with ages older
than 300 Ma are rare and sporadic. It is noteworthy that zircon with an age of around 1850 Ma isabsent, which is different from the sample SD09
from the Akadomari area.
Lower Triassic sandstones from the Abrek Bay
area, south Primorye Territory, were reported byYokoyama et al. (2009). Their age patterns haveone major peak at 508 Ma. Newly collected sam-Fig. 2. Probability distribution of ages of detritalmonazites from the sandstone, SD09, on Sado
Island. Numerical value (n) denotes the numberof analyzed monazite grains.

Fig. 3. Frequencies and probability distributions ofconcordant zircon ages from the samples, SD09
and SD16. Numerical value (n) denotes thenumber of analyzed zircon grains and concordant grains.

Fig. 4. Probability distribution of ages of detritalmonazites from the Triassic and Permian sandstones from Vladiostok, south Primorye.
Numerical value (n) denotes the number of analyzed monazite grains.

Kazumi Yokoyama et al.

56

ples from Vladivostok are Lower Triassic andPermian sandstones. Only detrital monazites
were analyzed in the samples. The age pattern ofthe Permian sandstone in Fig. 4 has only one
peak at around 509 Ma, whereas Lower Triassic
sandstone has two major peaks at 273 Ma and517 Ma.
Granitoids and volcanic rocks: As listed inTable 1, the ages of four granitic rocks wereobtained. The granitoid body in the Washizaki
area is a few Ma older than granite blocks in theKawabe conglomerate layer. They are in a narrow range from 101 Ma to 105 Ma, which isroughly consistent with K–Ar ages of 95–103 Ma
for the four small granitoid bodies reported for
Sado Island by MITI (1994).
The Tertiary volcanic sequence is widespreadalong the Sea of Japan. On Sado Island, the oldest Tertiary formation is the Nyukawa Formationthat was succeeded by the Aikawa Formation.
Many ages of the volcanic rocks in the Tertiary
formations were obtained by K–Ar and the ﬁssion track method (e.g. Ganzawa, 1982: MITI,1994). Their results show a wide range from18 Ma to 31 Ma for the Nyukawa and Aikawa
formations. It is commonly referred to that theyounger results were due to later stage thermal
events, and the present results by the LA-ICPMS method for zircon will be the most reliable
age data for the Sado Tertiary sequence. Agesfrom the Nyukawa and Aikawa formations are26.4 Ma and 27.5 Ma, respectively. They are Oligocene in age and show no clear difference.
Discussion
Basement rocks on Sado Island are widely
covered by thick Tertiary volcanic and sedimentary sequences and occur locally in the northernand southern parts of Sado Island (Fig. 1). Fossils, rarely reported from the basement rocks, arefrom the Permian to Early Jurassic. Some ofthem were found in the limestone blocks isolated
from the sequence. In this study, monazite andzircon ages were obtained from the sandstone

samples in both the Akadomari and Washizaki
areas. The age result will contribute not only toprovenance analyses but also to the depositionalages.
The sample, SD09, from the Akadomari areashows an age pattern of monazite with a strong
peak at 500 Ma. This pattern is rare in the preTertiary sandstones in the Japanese Islands(Yokoyama, 2016). The pattern was found in theTriassic sandstones from the Nabae and Yakuno
groups in the Maizuru Belt and from Triassic andPermian sandstones in the south Primorye, Russia. As discussed by Yokoyama et al. (2009),
provenances for these kinds of sandstones
include the Khanka and Jiamusi blocks or theirequivalent terrane. The sample has a peak at1850 Ma that is characteristic of the North China
Craton including the Korean Peninsula. As wellas the Triassic sandstones in the Nabae Group(Yokoyama et al., 2009), it is concluded thatthe constituents of the sandstone were derived
from both the North China Craton and KhankaJiamusi blocks, after amalgamation of the cratonand blocks during the Early Triassic.
Zircons from the same sample, SD09, show adifferent age pattern from the monazite mentioned above. The zircon pattern is composed ofa major peak at 250 Ma with subordinate peaks at500 and 1850–1900 Ma. The difference is due tothe constituent members in the provenance.
Monazite is generally derived from granitoid andhigh-temperature metamorphic rocks, whereas
zircon is derived from volcanic rock in addition
to granitoid and high-grade metamorphic rock.
The difference between zircon and monazite patterns is explained by the abundant volcanic rocks
with an age of 250 Ma in the provenance.
The sandstone sample, SD16, from theWashizaki area has a zircon pattern with almost
one peak at 250 Ma (Fig. 3). Zircons older than
300 Ma are rare in the sample. It shows that theprovenance for the sandstone was widely covered by volcanic rock with an age of around
250 Ma. The youngest zircon for the sandstone
from the Washizaki area is 226 Ma, which isslightly older that the sample from the Akado-

Granite and sandstones in Sado Island

mari area of 216 Ma．Assuming that the volcanic eruption continued during the Triassic, depositional ages of both the sandstones wereTriassic. Although the age should be reconﬁrmed
by more age analyses of sandstone or tuff layerin each area, the Triassic result is consistent withthe Late Triassic radiolarians reported from amudstone sequence with intercalation of a thinsandstone layer by Itasaka et al. (1994).
Ages of three granitic blocks from the Kawabe
conglomerate layer in the Aikawa Formation are
well concentrated in a range from 101 to 103 Ma.
The age is far older than the ﬁssion track age,
43 Ma, by Ganzawa (1982). Three granitic bodies
from the Akadomari area and one grannodiorite
body from the Washizaki area were 95 to 103 Ma
determined by the K–Ar method (MITI, 1994).
Our result for the granodiorite sample from theWashizaki area was 105 Ma. All these data fromSado Island are concentrated in a narrow range,
showing granitoids with an age of around 100 Ma
that occur widely under the Tertiary sequence in
the island. Granite blocks in the Kawabe conglomerate layer has been treated as gravel. In
addition to the narrow range of granite age, theoccurrence of huge granite blocks up to 2 m indiameter and a small amount of volcanic matrix
suggest that the Kawabe conglomerate layer wasformed by the intrusion of volcanic rocks into thegranitic region, rather than deposition as gravel
brought by a river.
Komatsu et al. (1985) suggested that Sado
Island was a northeastern extension of the Maizuru Belt through the Joetsu Belt (Fig. 1). Ichida
et al. (2010) selected the basement rocks on Sado
Island as a probable member of the Maizuru Beltor ultra-Tanba Belt. Tsutsumi et al. (2014) concluded that the granitoids in the Maizuru Belt arecorrelated in age with the granitoids in Vlagiostock, south Primorye. The rapid opening of theSea of Japan started at around 15 Ma (Otofuji &Matsuda, 1983). Before the opening, Sado Island
was probably close to the Primorye. Our data is
not enough to conclude the close connection withthe rocks in south Primorye, but the sandstones
data indicate the correlation with the coastal zone

57

of south Primorye. This will be supported by age
correlation of granitoids from Sado Island andsouth Primorye. Granitic rocks along the Sea ofJapan in the Niigata Prefecture, very close to theSado Island, have well concentrated ages of
65–68 Ma (Yokoyama et al., 2016). They are different from the granitoids on Sado Island.
Whereas, the granitoids with an age around
100 Ma occur along the coastal area of the Sea ofJapan in south Primorye (Khanchuk et al., 2008).
More detailed analyses of granitic rocks in both
Sado Island and south Primorye, Russia, havebeen left as future areas of study.
Acknowledgements
The authors are very grateful to Ms. M. Shigeoka for her help with chemical analyses andthe heavy mineral separations throughout thisstudy.
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佐渡の白亜紀花崗岩と三畳紀砂岩の岩石学的研究
横 山 一 己・谷健一郎・堤之 恭

佐渡の花崗岩質岩 4 資料の年代測定と 2 資料の砂岩中の鉱物の年代測定を行った．花崗岩質岩の年代は，100–105 Ma の狭い範囲に入り，佐渡の他地域の年代測定も同様で，佐渡の地下には，100 Ma 前後の花崗岩が広く分布しているものと推定される．三畳紀砂岩の砕屑性鉱物の年代分布は，舞鶴帯の難波江層群や夜久野層群と同様に 500 Ma にピークがあるもので，ロシアの Khanka・Jiamusi 帯を含む地帯が供給源と考えられる．しかし，この三畳紀砂岩の供給源には，250 Ma 前後の火山岩が広い分布していることも明らかにされた．