ARTICLE

Memoir of the Fukui Prefectural Dinosaur Museum 12: 35–78 (2013)© by the Fukui Prefectural Dinosaur Museum

DETRITAL ZIRCON GEOCHRONOLOGY OF THE SILURIAN–LOWER CRETACEOUS
CONTINUOUS SUCCESSION OF THE SOUTH KITAKAMI BELT, NORTHEAST JAPAN
Hiroyuki OKAWA 1 , Masanori SHIMOJO 2 , Yuji ORIHASHI 3 , Koshi YAMAMOTO 4 ,Takafumi HIRATA 5 , Shin-ichi SANO 6 , Yasuo ISHIZAKI 1 , Yoshikazu KOUCHI 1 ,Shuichi YANAI 7 and Shigeru OTOH 1
Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan
3
Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
4
Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
5
Graduate School of Science, Kyoto University, Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
6
Fukui Prefectural Dinosaur Museum, 51-11 Terao, Muroko, Katsuyama, Fukui 911-8601, Japan
7
Japan Geocommunications Co., Ltd., 2-10 Yotsuya, Shinjuku-ku, Tokyo 160-0004, Japan1

2

ABSTRACT
U-Pb analyses of more than 1,000 single detrital zircons from 16 formations of the Silurian–Lower Cretaceous
continuous succession of the South Kitakami Belt (SKB), Northeast Japan, provide a detrital zircon reference forthe complex continental-margin orogen of Japan. As a result, three tectonic phases were discriminated. Siluro–
Devonian sandstone samples contain many syn-sedimentary zircons and 36.5‒48.0% of Precambrian zircons
scattering between 700 Ma and 3,000 Ma, suggesting that they were deposited along an active continental margin of
East Gondwana. Permian–Early Jurassic sandstone samples contain virtually no Precambrian zircons, suggesting thatthey were deposited along the active margin of an oceanic island arc. Middle Jurassic–Early Cretaceous sandstone
samples contain many 300‒170 Ma zircons and up to 28% of Paleoproterozoic (around 1,850 Ma) zircons but no
Neoproterozoic zircons. Moreover, the zircons during the magmatic hiatus in Korea (158‒110 Ma) were detected
only in one Lower Cretaceous sandstone sample. The age distribution suggests that the Paleoproterozoic zircons in
the Middle Jurassic–Lower Cretaceous sandstone of the SKB were most likely supplied from a Paleoproterozoic
orogen in the North China Block. Thus, the South Kitakami Paleoland, which accumulated the continuous successionof the SKB was born along a margin of Gondwana in the Silurian–Devonian, rifted from the continent and driftedin the Tethys ocean as an oceanic island arc in the Permian–Early Jurassic, and finally amalgamated along an active
continental margin where detrital zircons of the North China Block were supplied in the Middle Jurassic.
Key words: U-Pb age, detrital zircon, LA-ICPMS, South Kitakami Belt, Northeast Japan, Gondwana
大川泰幸・下條将徳・折橋裕二・山本鋼志・平田岳史・佐野晋一・石﨑泰男・高地吉一・柳井修一・大藤 茂（2013）東北日本，南部北上帯のシルル～前期白亜紀連続層序における砕屑性ジルコン年代分布の推移．福井県立恐竜博物館紀要 12：35–78．
南部北上帯の浅海成シルル～下部白亜系連続層序から 16 層を選び，砕屑性ジルコンのウラン－鉛年代を測定した結果，日本列島の標準となる砕屑性ジルコン年代分布の推移が示された．①シルル～下部石炭系は 1500–750 Ma のジルコンを特徴的に含む多峰型年代分布をなし，新原生代ジルコンを産するゴン
ドワナ大陸北東縁からのジルコン供給を示唆する．②ペルム～下部ジュラ系は，いずれもほぼ堆積時ジルコンのみからなる単峰型年代分布をなす．③中部ジュラ～下部白亜系は，北中国地塊から供給されたと見られる古原生代（1850 Ma 付近）ジルコンを含む二峰型年代分布をなす．以上より，本連続層序を堆積した南部北上古陸は，①シルル～前期石炭紀に位置したゴンドワナ大陸北東部の大陸縁から，②ペ
ルム～前期ジュラ紀には分離してテチス海中の海洋性島弧として挙動し，③中期ジュラ紀には，北中国地塊からジルコンが供給される大陸縁に癒合したと見られる．

Received August 2, 2013. Accepted November 4, 2013.
Corresponding author—Shigeru OTOH
E-mail: shige@sci.u-toyama.ac.jp35

H. OKAWA ET AL.

36

INTRODUCTION
This paper aims (1) to introduce temporal transition of the
detrital zircon age distributions recorded in the Silurian–Lower
Cretaceous continuous succession of the South Kitakami Belt
(SKB), Northeast Japan, and (2) to discuss the evolutionary
history of the South Kitakami Paleoland, which accumulated thecontinuous succession of the SKB.
Recent progress in analytical technique has enabled rapid andexact U-Pb isotopic age determination of zircons using sensitive
high-resolution ion-microprobe (SHRIMP) or inductively
coupled plasma-mass spectrometry with laser ablation sampling(LA-ICPMS) (e.g., Compston, 1996; Kosler and Sylvester,2003). Detrital zircon age distribution reflects the changes ofprovenance, paleogeography, and tectonic setting, and can bea powerful tool for inferring the plate tectonic evolution of acomplex orogen like the Japanese Islands (e.g., Gehrels et al.,
1995; Darby and Gehrels, 2006). To make such an inference,
we have to know the temporal change of detrital zircon age
distribution of each crustal block (or terrane) that makes up theorogen, and compare the data with the reference age distribution
for the major crustal blocks in the world (e.g., Soreghan andGehrels, 2000; Darby and Gehrels, 2006 and references therein).
Such comparison enables us to know the origin and tectonic
history of each crustal block in the orogen. The study of detrital
zircon geochronology in Japan (Tsutsumi et al., 2000, 2003,
2006, 2009, 2011, 2012; Aoki et al., 2007, 2012; Otoh et al.,2010), however, is still local and is not comprehensive.
The SKB is the best target for a comprehensive study of
detrital zircon geochronology in Japan, because it retains a 350Myr continuous succession of shallow-marine to terrestrial beds
formed during the Silurian to the Early Cretaceous (Kawamura
et al., 1990; Mori et al., 1992; Ehiro and Kanisawa, 1999). The
continuous succession of the SKB has already been studied as astandard succession of lithostratigraphy and paleobiogeography.
It helps to estimate the transition of tectonic setting and affinities
with the other crustal blocks or terranes of the Japanese Islands
and East Asia (e.g., Kato, 1990; Nakamura and Tazawa, 1990;
Otoh and Yanai, 1996; Ehiro and Kanisawa, 1999). The detrital
zircon geochronological data of the SKB presented in this study
will be another set of reference data. They can be much more
useful than the paleobiogeographical data because even fossilfree sandstones of an accretionary prism or a metamorphic belt
contain zircons, enabling to compare the SKB with any otherbelts but ultra-high-temperature metamorphic belts.
GEOLOGIC SETTINGThe SKB consists mainly of basement rocks and overlying
Ordovician to Early Cretaceous strata (Figs. 1 and 2). The
outline of the basement rocks and the continuous succession we
studied is described below.

Basement rocks
The basement rocks of the SKB are the Hayachine Complex(Ehiro et al., 1988) and its equivalents and the Hikami Granite(Murata et al., 1974).
The Hayachine Complex and its equivalents fringe
northeastern to western boundary of the SKB (Fig. 1). They
consist mainly of ultramafic to mafic rocks with small amounts
of tonalite–trondhjemite–granodiorite (TTG). According to
the petrological studies of Ozawa (1983, 1984) and Mori et al.
(1992), they are fragments of volcanic-arc lithosphere. K-Ar
hornblende ages of 421‒484 Ma and a U-Pb zircon age of 462
Ma were reported from gabbro and tonalite, respectively (Ozawa
et al., 1988; Shibata and Ozawa, 1992; Shimojo et al., 2010).
The Hikami Granite mainly exposes in the mid-easternpart of the SKB (Fig. 1). The Hikami Granite consists ofmassive to schistose granite, granodiorite, and tonalite, partly
including blocks of gneissose metamorphic rocks (Tsubonosawa
Metamorphic Rocks). Petrochemical studies suggested thatthe Hikami Granite is calc-alkaline, volcanic-arc granitoid(Kobayashi et al., 2000).
The Hikami Granite is unconformably overlain by the Silurian
Kawauchi Formation (Murata et al., 1974, 1982), mentioned
below, and has a SHRIMP U-Pb zircon age of 442 Ma(corresponding to the Late Ordovician; Watanabe et al., 1995).
Shimojo et al. (2010), on the other hand, reported LA-ICPMS,
U-Pb zircon ages of 416‒403 Ma from four samples, suggestingthat the granite at least partly forms Devonian intrusive bodies.
Silurian–Devonian strata
The Silurian to Devonian strata of the SKB crop out with thebasement rocks (Fig. 1). They consist mainly of siliciclastic tovolcaniclastic rocks with intercalations of felsic to mafic tuff andlimestone, suggesting that they are deposits of a shallow-marine
environment along an active continental margin.
In the northeastern part of the SKB, the Yakushigawa
Formation overlies the Hayachine Complex and consists ofinterbedded basaltic volcaniclastic rocks, quartz-feldspathic
sandstones, and felsic tuffs in the lower part, and of shalesin the upper part. The shales of the Odagoe Formation,
which yield a Silurian brachiopod Trimerella sp., overlie theYakushigawa Formation (Ehiro et al., 1986). In the northern
marginal part of the SKB to the south of Morioka (Fig. 1),
the Hayachine Complex is associated with the Nameirizawa
and Orikabetoge formations. The Nameirizawa Formation islithologically correlated with the Yakushigawa Formation,
whereas the Orikabetoge Formation consists of clastic rocks
with orthoquartzite clasts (Okami et al., 1984) and yields
Silurian fossils such as Halysites kuraokaensis and Encrinurussp. (Kawamura et al., 1984; Yamazaki et al., 1984). In the
Nagasaka area, western part of the SKB (Fig. 1), the Upper
Devonian Tobigamori Formation overlies the Ohachimori
Amphibolite, an equivalent of the Hayachine Complex, having
K-Ar hornblende ages of 479‒424 Ma (Kanisawa et al., 1992;

DETRITAL ZIRCON GEOCHRONOLOGY OF THE SOUTH KITAKAMI BELTFIGURE 1. Geologic map of the South Kitakami Belt showing the sampling locations. Modified after Onuki (1981), Ehiro (1989), and Sasaki(2003). Abbreviations—Is.: Island, M: distribution of the Miyako Group.

37

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H. OKAWA ET AL.

Sasaki et al., 1997). The Tobigamori Formation consists ofthree members. The Lower Member consists of interbedded
tuffaceous sandstone and shale yielding no fossils. The Middle
Member consists of red conglomerate, sandstone, and shale,
whereas the Upper Member consists mainly of sandy shale. The
two members yield abundant brachiopod and plant fossils suchas Cyrtospirifer tobigamoriensis and Leptophloeum rhombicum,
and are correlated with the Famennian (Upper Devonian;
Tachibana, 1950, 1952; Noda and Tachibana, 1959). Ehiro
and Takaizumi (1992), on the other hand, found a Tournaisianammonoid, Protocanites sp., from a float of the uppermost
part of the Tobigamori Formation. In the Hikoroichi area, mideastern part of the SKB (Fig. 1), Siluro–Devonian Kawauchi,
Ono, and Nakazato formations, in ascending order, lie on top of
the Hikami Granite.
The Siluro–Devonian fauna and flora of the SKB haveaffinities with those of coeval northern East Gondwana: i.e.,present-day Australia, South China, and the southern part of theCentral Asian Orogenic Belt (CAOB). For example, tabulate
corals from the Kawauchi Formation, such as Schedohalysites
and Falsicatenipora, are abundant in the coeval strata ofAustralia and South China (e.g., Hamada, 1960; Kato, 1990),
and the Eifelian brachiopod fauna from the Nakazato Formation
has affinities with the coeval fauna from the CAOB in Inner
Mongolia, China (Tazawa and Chen, 2001). Moreover, the Upper
Devonian flora Leptophloeum of the Tobigamori Formationcommonly occurs in the coeval strata of Australia, South China,
CAOB, and the Imjingang Belt of North Korea (e.g., Kimura,
1987; Om et al., 1996; Tazawa et al., 2006).
Carboniferous strata
The distribution of the Carboniferous strata in the SKB ismuch wider than that of the Silurian to Devonian strata (Fig. 1).
The Lower Carboniferous in the SKB is rich in felsic and mafic
volcanic and pyroclastic rocks, whereas the Upper Carboniferous
is rich in carbonate rocks (Kawamura and Kawamura, 1989a).
Kawamura and Kawamura (1989b) and Kawamura et al. (1990)
regarded that the Lower Carboniferous volcanic and pyroclastic
rocks indicate the bimodal volcanism related to intra-arc rifting.
In the Nagasaka area, the Karaumedate Formation overlies
the Tobigamori Formation, and is composed of interbedded
sandstone and mudstone, felsic tuff, and calcareous sandstone(Kawamura and Kawamura, 1989a). The lower part of the
Karaumedate Formation yields Tournaisian brachiopods,
whereas the upper part yields Visean rugosa corals such as
Kueichouphyllum sp. and Dibunophyllum sp., and brachiopods
such as Productus giganteus. In the Hikoroichi area, the Lower
Carboniferous Hikoroichi, Onimaru, and Lower to Upper
Carboniferous Nagaiwa formations, in ascending order, overlies
the Devonian Nakasato Formation. The Hikoroichi Formation,
consisting mostly of felsic tuff and tuffaceous clastic rocks(Kawamura and Kawamura, 1989a), yields various rugosa
corals such as Amygdalophyllum etheridgei, and brachiopods
such as the Rotaia-Marginatia-Syringothyris assemblage andSchizophoria resupinata, whereas the Onimaru Formation yields
Late Visean rugosa corals such as Kueichouphyllum glacile,
Yuanophyllum kansuense, and Diphyphyllum hochangpingense.
Two Carboniferous faunal provinces have been discriminatedin the Eurasian realm: the northern province characterized by
the rugosa coral Gangamophyllum and the southern province
characterized by Kueichouphyllum. The northern province
includes present-day northern Siberia, whereas the southern
province includes present-day South China, Indochina, andnorthern Australia (Liao, 1990). Moreover, the mixed fauna of
the northern and southern provinces has been recognized in the
CAOB. Lower Carboniferous fauna of the SKB still has affinities
with that of coeval northern East Gondwana. For example, therugosa coral Amygdalophyllum from the Hikoroichi Formation
is abundant in Australia (southern province). Occurrence of
Kueichouphyllum and absence of Gangamophyllum in the
Onimaru Formation also suggest an affinity with the southern
province. The Rotaia-Marginatia-Syringothyris brachiopod
assemblage, on the other hand, indicates an affinity with the
CAOB (Tazawa, 1996). Late Carboniferous fauna from the
SKB has various boreal elements (e.g., Kato, 1990), probably
indicating the influence of global cooling at that time.
Permian to Middle Triassic strata
Permian strata
Permian strata in the SKB were subdivided, in ascendingorder, into the Sakamotozawan, Kanokuran, and Toyoman
series (Minato et al., 1978), although we do not follow thislocal chronostratigraphic division and will call these “series” as“groups”. The Permian strata consist mostly of shallow marine
epiclastic rocks and limestone, with small amounts of felsic tointermediate tuff in the Sakamotozawan Group.
The Sakamotozawan Group includes the Sakamotozawa
Formation in the Hikoroichi–Setamai area, the Notsuchi
Formation in the Nagasaka area, and the Nishikori Formation in
the Toyoma area to the south of Nagasaka (Fig. 1). They consist
mostly of sandstone, mudstone, and interbedded sandstone andmudstone, with some limestone beds particularly in the middle
horizon (Kanmera and Mikami, 1965; Saito, 1966; Ehiro, 1989).
The Sakamotozawan Group yields fusulinids such as Zellia,
Monodiexodina, and Pseudofusulina (Kanmera and Mikami,1965). Among them, genus Monodiexodina characterizes the
Monodiexodina territory (Ishii et al., 1985), which includes
the CAOB in Tarim, northeastern China, and Primorye insoutheastern Russia (Ozawa, 1987). Moreover, the Nishikori(or Rodai) Formation yields the Maiya Flora consisting ofGigantopteris, Taeniopteris, and Sphenophyllum, common withthe coeval strata in the Cathaysian Floristic Province in China
and Korea (Asama, 1985).
The Kanokuran Group includes the Kanokura Formationin the Setamai area, the upper part of the Notsuchi Formationand the Usuginu Conglomerates in the Nagasaka area, theTenjinnoki Formation and the Yamazaki Conglomerates in the
Toyoma area, and the Iwaizaki Limestones in the Motoyoshi

DETRITAL ZIRCON GEOCHRONOLOGY OF THE SOUTH KITAKAMI BELT

39

FIGURE 2. Stratigraphic division of the pre-Aptian sequences of the South Kitakami Belt showing the lithofacies and sampling horizons. Numericalages for all systems are taken from International Commission on Stratigraphy (2013). Abbreviations—Cg.: Conglomerates, E: Erathem, Fm.: Formation,
G: Group, K–O: Karakuwa–Oshika Subbelt, N. China: North China, O: Ofunato Subbelt, S: System, Sr: Series, S–H: Shizugawa–Hashiura Subbelt, T:
Tectonic setting, TTG: Tonalite–trondhjemite–granodiorite.

40

H. OKAWA ET AL.

area to the southeast of Nagasaka (Figs. 1 and 2). They consist
generally of mudstone, sandstone, and conglomerate, with someinterlayers of limestone. The conglomerate in the Kanokuran andoverlying Toyoman groups is collectively called the Usuginutype Conglomerate with granitic clasts of 300‒280 Ma (LAICPMS U-Pb ages; Okawa, unpublished data). The Usuginutype Conglomerate is particularly thick in the Kanokuran Groupin the Nagasaka and Toyoma areas. The Kanokuran Group
yields Roadian to Capitanian fusulinids of the Monodiexodina
matsubaishi, Colania kotsuboensis, and Lepidolina multiseptata
zones (Choi, 1973). The Kanokuran Group also yields suchammonoid genera as Timorites, Paraceltites, and Cibolites, andbrachiopods such as Leptodus nobilis and Spiriferellina cristata.
Monodiexodina matsubaishi and Spiriferellina cristata indicatesa faunal affinity with Mongolia and the southern margin ofSiberia, whereas the ammonoids and Leptodus nobilis show thesimilarity with the Tethyan realm such as the South China Block(Ehiro, 1998; Tazawa, 1991, 2001).
The Toyoman Group includes the Kowaragi Formation in
the Setamai–Karakuwa area and the Toyoma Formation in the
Nagasaka, Toyoma, and Motoyoshi areas (Figs. 1 and 2). They
consist mostly of black mudstone with some interlayers ofsandstone, limestone, and conglomerate. The black mudstone
bears strong slaty cleavage, particularly along the western
limb of synclines (Sasaki, 2001, 2003), and partly contains
carbonate and phosphate nodules (Kanisawa and Ehiro, 1986).
The Toyoman Group yields Wuchiapingian to Changhsingianammonoids such as Araxoceras sp. and Paratirolites compressus(Murata and Bando, 1975; Ehiro, 1996). Among themAraxoceras is a typical genus of the Tethys Ocean (Bando et al.,1987).
Lower to Middle Triassic strata
Lower to Middle Triassic strata of the SKB are collectively
called the Inai Group (Fig. 2). The Inai Group is distributed in
the southeastern part of the SKB and consists of two sedimentary
cycles: the Hiraiso and Osawa formations constitute the firstcycle, whereas the Fukkoshi and Isatomae formations constitutethe second cycle (Onuki and Bando, 1959). The Hiraiso
Formation consists of an upward fining sequence, beginningwith basal conglomerate and coarse calcareous sandstone,
overlain by interbedded sandstone and mudstone. Rare felsic tuff
layers are intercalated in the lower part of the formation. The
Hiraiso Formation yields Pleuromeia flora, which commonlyoccurs from the North China Block and the southern part of
the CAOB (Kimura, 1987). The Hiraiso Formation also yields
bivalves such as Eumorphotis nipponicus and “Pecten” aff.
ussuricus. The Olenekian Osawa Formation consists mainlyof calcareous mudstone, with some intercalations of sandstoneand submarine sliding deposits (Kamada, 1983). The formation
yields Utatsusaurus hataii, one of the earliest ichthyosaur
fossils in the world. The Osawa Formation also yields abundantammonoids of the Columbites-Subcolumbites fauna (Bando andShimoyama, 1974), which is concentrated in the coeval strata in
the Tethyan region (Bando et al., 1987). The Fukkoshi Formationconsists mainly of bedded sandstone, with subordinate amountof mudstone. The formation yields Anisian ammonoids such as
Bolatonites cf. kitakamicus, Hollandites spp., and Rikuzenites
nobilis (Shimizu, 1930; Yabe, 1949). The generic compositionof the Anisian ammonoids from the SKB is the Pacific–Tethyan
type (Ehiro, 1998) although they contain some common species
with the coeval ammonoids from Primorye and Kolyma ofeastern Russia (Nakazawa, 1991). The Fukkoshi Formation also
yields brachiopods such as Spiriferina and Terebratula (Ichikawa,1951). The Isatomae Formation consists of laminated muddy
sandstone and mudstone, intercalated with some sandstone beds.
The formation also yields Anisian Pacific–Tethyan ammonoids
such as Hollandites japonicus, “Danubites” naumanni, andBolatonites kitakamicus (Shimizu, 1930; Onuki and Bando,
1959; Bando, 1964; Ehiro, 1998).
Upper Triassic to Lower Cretaceous strata
Upper Triassic to Lower Cretaceous strata are distributed in
the southeastern part of the SKB. They occur in three subbelts
along the axes of three major synclines: the Shizugawa–
Hashiura, Karakuwa–Oshika, and Ofunato subbelts from west
to east (e.g., Yamashita, 1957; Takizawa, 1977, 1985; Fig.
1). The succession and thickness of the Mesozoic strata inthe three subbelts substantially differ from each other. Sasaki(2003) reported that the regional strain is concentrated along
the western limb of the major synclines and concluded that the
major synclines are conical synclines with subvertical rotation
axes and were formed through sinistral shearing along the highstrain zones (i.e., their western limbs at present).
Upper Triassic strata
Upper Triassic strata of the SKB are collectively called
the Saragai Group (Fig. 2), which occurs in the Shizugawa–
Hashiura and Ofunato subbelts but is absent in the Karakuwa–
Oshika Subbelt (Fig. 1). The Saragai Group in the northern partof the Shizugawa–Hashiura Subbelt consists of the Shindate andChonomori formations (Onuki and Bando, 1958), whereas thegroup in the Ofunato Subbelt is called the Myojinmae Formation(Kanagawa and Ando, 1983). The Shindate Formation consists
mainly of massive feldspathic sandstone with subordinate
amounts of mudstone, granule conglomerate, felsic tuff, andrare carbonaceous mudstone. The Carnian–Norian Chonomori
Formation, overlying the Shindate Formation, consists ofinterbedded micaceous sandstone and mudstone. The Chonomori
Formation is characterized by a rich Monotis fauna, which
belongs to the Arcto-Pacific Realm (Kobayashi and Tamura,
1983; Tamura, 1987) and consists of M. scutiformis, M. ochotica,
and M. zabaikalica (Nakazawa, 1964; Ando, 1987). The
Myojinmae Formation in the Ofunato Subbelt consists mainly oftuff, with some andesite lava, tuffaceous sandstone, and volcanic
conglomerate. A tuff clast in the conglomerate yields Monotis
ochotica (Kanagawa and Ando, 1983).
Lower to lower Middle Jurassic strata
Lower to lower Middle Jurassic strata of the SKB is called the
Shizugawa Group and occurs only in the Shizugawa–Hashiura
Subbelt (Fig. 2). The Shizugawa Group in the type locality,

DETRITAL ZIRCON GEOCHRONOLOGY OF THE SOUTH KITAKAMI BELT

northern part of the Shizugawa–Hashiura Subbelt, consists of the
Niranohama and Hosoura formations, in ascending order (Inai,
1939). The Niranohama Formation consists of brackish-water
black mudstone and trigoniid-bearing coarse sandstone. The
former yields parallic bivalves such as Bakevellia, Burmesia, andGeratrigonia, whereas the latter is characterized by abundantoccurrence of parallic bivalves (Trigonia and Vaugonia) andbelemnites, together with middle to late Hettangian ammonoids
such as Alsatites (or Yebisites) onoderai (Matsumoto, 1956;
Hayami, 1961; Sato and Westermann, 1991; Iba et al., 2012).
The Hosoura Formation consists mostly of laminated sandy
mudstone and yields Sinemurian to Aalenian ammonoids (Sato,
1957, 1962; Takahashi, 1969; Sato and Westermann, 1991).
Many ammonoid and bivalve species from the Shizugawa Groupare endemic and have not been found in other regions of East
Asia (Hayami, 1990).
Middle Jurassic to Lower Cretaceous strata in the
Shizugawa–Hashiura Subbelt
The Middle Jurassic to Lower Cretaceous strata in the
Shizugawa–Hashiura Subbelt consists of the Hashiura andJusanhama groups, in ascending order (Fig. 2). The HashiuraGroup in the northern part of the subbelt is subdivided into
the Aratozaki, Arato, and Sodenohama formations (Mabuti,
1933; Matsumoto, 1953), whereas the strata correlative with
the Aratozaki and Arato formations are called the Nakahara
and Nagao formations, respectively, in the southern part of thesubbelt (Mori, 1949; Kase, 1979). The Aratozaki Formationconsists mainly of coarse quartz-feldspathic sandstone with someintercalations of conglomerate, and yields marine bivalves such
as Inoceramus morii and Vaugonia yokoyamai (Hayami, 1961).
The Arato Formation consists mainly of bedded black mudstone
with interbedded mudstone and sandstone in its basal part. The
Arato and Nagao formations yield abundant ammonoids such as
Bajocian Stephanoceras hashiuraense and Cadomites bandoi,
Callovian Kepplerites mabutii, and Oxfordian–Kimmeridgian
Kranaosphinctes cf. matsushimai and Taramelliceras sp.
(Sato, 1962; Takahashi, 1969; Kase, 1979). Among these,
Kepplerites is a typical boreal genus, whereas Kranaosphinctes
is a Tethys–Pacific genus (Bando et al., 1987). Kase (1979)
also reported from the uppermost part of the Nagao Formationa poorly-preserved ammonoid belonging to Olcostephanidae
or Berriasselidae, and suggested that the horizon may be of
Tithonian or younger age. The Sodenohama Formation consistsof massive sandstone and interbedded sandstone and mudstone,
and yields Kimmeridgian (Takahashi, 1969) or Tithonian
ammonoids (Matsumoto, 1953). The Jusanhama Group occurs
only in the southern part of the subbelt and is subdivided into
Yoshihama, Tategami, and Tsukihama formations, in ascendingorder (Mori, 1949; Kase, 1979). The Yoshihama and Tsukihama
formations consist mostly of quartz-feldspathic sandstone,
whereas the Tategami Formation consists of interbedded
quartz-feldspathic sandstone and bituminous mudstone (Kase,
1979). Endemic species of such bivalve genus as Filosina andProtocardia occur in the Tategami Formation (Hayami, 1960),
and Tashiro and Kozai (1989) pointed out that Protocardia

41

characterizes the Nankai Fauna, a southern Tethyan fauna
occurring restrictively in the Kurosegawa and Southern Chichibu
belts of the Outer Zone of Southwest Japan. Considering theage of the underlying Hashiura Group, the Jusanhama Group is
likely of Tithonian–Early Cretaceous age.
Middle Jurassic to Lower Cretaceous strata in the
Karakuwa area, northern part of the Karakuwa–Oshika
Subbelt
The Middle Jurassic to Lower Cretaceous strata in the
Karakuwa area, northern part of the Karakuwa–Oshika Subbelt,
are the Karakuwa and Oshima groups, in ascending order(Fig. 2). The Karakuwa Group is subdivided into the Kosaba,
Tsunakizaka, Ishiwaritoge, Mone, Kogoshio, and Isokusa
formations in ascending order (Shiida, 1940; Hayami, 1961),
whereas the Oshima Group consists of the Kanaegaura andYokonuma formations, in ascending order (Onuki, 1969).
Middle Jurassic to Lower Cretaceous strata in the Oshika
area, southern part of the Karakuwa–Oshika Subbelt
The Middle Jurassic to Lower Cretaceous strata in the
Oshika area, southern part of the Karakuwa–Oshika Subbelt,
are the Oshika Group (Onuki, 1956) and Yamadori Formation(Inai and Takahashi, 1940), in ascending order (Fig. 2). The
Oshika Group is subdivided into the Tsukinoura, Oginohama
and Ayukawa formations, in ascending order (Takizawa et al.,
1974; Takizawa, 1985). The Tsukinoura Formation consistsof the lower sandstone and upper mudstone members. The
upper part of the lower member yields ammonoids such as
Stephanoceras cf. plicatissimum and Normannites (Itinsaites) sp.
and is correlated with the Otoites sauzei and/or Stephanoceras
humphriesianum zones of the European Middle Bajocian(Sato, 1972). The member also yields rich bivalves such as
Trigonia sumiyagura and Vaugonia kodaijimensis (Hayami,1961). The Oginohama Formation, consisting of sandstone
and interbedded sandstone and mudstone with some layers ofconglomerate, is subdivided into the Kitsunezaki Sandstone
and Shale, Makinohama Sandstone, Kozumi Shale and Fukiura
Shale and Sandstone members, in ascending order (Takizawa
et al., 1974). The upper part of the Kozumi Shale Member
yields late Oxfordian ammonoids such as Perisphinctes(Perisphinctes) ozikaensis and Perisphinctes (Kranaosphinctes).
cf. matsushimai and early Kimmeridgian ammonoids suchas Discosphinctes cf. kiritaniensis, Lithacoceras onukii, andAulacostephanus (Pararasenia) sp. (Fukada, 1950; Sato, 1962;
Takahashi, 1969). The Fukiura Shale and Sandstone Member
yields Tithonian ammonoids such as Virgatosphinctes aff.
communis and Aulacosphinctoides? sp. (Takahashi, 1969;
Takizawa et al., 1974). Further, abundant plant fossils belonging
to the Ryoseki Flora occur from the upper part of each member(Kimura and Ohana, 1989). The Ayukawa Formation, consistingof quartz-feldspathic sandstone and mudstone, is subdivided
into the Kiyosaki Sandstone, Kobitawatashi Sandstone andShale, Futawatashi Shale, and Domeki Sandstone members, inascending order (Takizawa et al., 1974). The lower part of the
Kobitawatashi Sandstone Member yields Berriasian ammonoids
such as Berriasella sp. (Takizawa, 1970), whereas the upper

H. OKAWA ET AL.

42

part of the member and the upper part of the Futawatashi Shale
Member yield Valanginian ammonoids such as Thurmanniceras
cf. isokusense, Kilianella sp., and Lyticoceras sp. (Takizawa,
1970; Obata, 1988). The Yamadori Formation consists ofandesitic to dacitic pyroclastic rocks and overlying basaltic lava
and pyroclastic rocks (Takizawa et al., 1974).
Lower Cretaceous strata of the Ofunato Subbelt
The Ofunato Subbelt is mostly occupied by the Lower
Cretaceous Ofunato Group, which is subdivided into the
Hakoneyama, Funagawara, Hijochi, Kobosoura, and Takonoura
formations, in ascending order (Onuki and Mori, 1961; Fig.
2). Among them, the Hakoneyama Formation, consisting
mostly of volcanic conglomerate, has been interpreted to be asouthern extension of the Upper Triassic Myojinmae Formation(Kanagawa and Ando, 1983).
SAMPLE DESCRIPTIONS
We studied the following 16 sandstone samples and examined
their provenances from the age-distribution of detrital zircons.
Here follow the descriptions of studied samples summarized inFig. 3.
Silurian Nameirizawa Formation (Sample 08331-9;
N39°32’55.8”, E141°20’20.2”)
Sample 08331-9 of the Silurian Nameirizawa Formation
was collected from the middle part of the formation along theNameirizawa River, Hanamaki City, Iwate Prefecture (Fig.
1). The sandstone sample was of medium to fine feldspathic
wacke, with the matrix volume of a little more than 15%. The
sandstone was angular and ill-sorted. The zircon grains were
mostly abraded and anhedral, having columnar shapes withthe longer dimension of 90‒180 µm and the shorter dimensionof 50‒100 µm. Most of the zircons showed oscillatory zoningin cathodoluminescence (CL) images, a common feature of
igneous zircons (Corfu et al., 2003), although few zircons were
homogeneous or had metamorphic rim.
Silurian Yakushigawa Formation (Sample 08331-3;
N39°32’06.8”, E141°37’30.5”)
Sample 08331-3 of the Silurian Yakushigawa Formation wascollected from the lower part of the formation along the upperstream of the Yakushigawa River, Miyako City, Iwate Prefecture(Fig. 1). The sandstone sample was of angular and ill-sorted,
fine feldspathic wake. Although quartz veins with the widthof 1 mm or less sparsely cut the sample, no zircons have been
microscopically detected in it. More than half of the zircon
grains we collected were abraded and the others were euhedral.
The zircon grains generally had columnar shapes with the longer
dimension of 50‒200 µm and shorter dimension of 50‒90 µm.
Most of the zircons showed oscillatory zoning in CL images
although few zircons were homogeneous or had a detritus core.

Devonian Tobigamori Formation (Sample 08429-5;
N39°04’02.0”, E141°14’37.0”)
Sample 08429-5 of the Tobigamori Formation was collectedfrom the Lower Member of the formation along the Natsuyama
Logging Road, Ichinoseki City, Iwate Prefecture (Fig. 1). The
sandstone sample was of very ill-sorted, angular, medium
lithic wacke. Two thirds of the zircon grains we collected were
euhedral and the others were abraded. Most of the zircon grains
had columnar shape with the longer dimension of 70‒220μm and the shorter dimension of 50‒100 μm. Most of thezircons showed oscillatory zoning in CL images although few
zircons had metamorphic rim and few abraded zircons were
homogeneous.
Lower Carboniferous Karaumedate Formation (Sample
120611-2; N39°0’25.10”, E141°15’57.06”)
Sample 120611-2 of the Lower Carboniferous Karaumedate
Formation was collected approximately 50 m above the baseof the formation, 1 km to the east of Mt. Karaumedateyama,
Ichinoseki City, Iwate Prefecture (Fig. 1). The sandstone sample
was of angular and ill-sorted, fine to medium lithic wacke.
Nearly half of the zircon grains we collected were colorless andthe others were brown. 80% of the zircon grains were euhedral
and had columnar shapes with the longer dimension of 70‒200μm, the shorter dimension of 40‒100 μm, and aspect ratioof 1.5‒2.5. The other zircon grains, all brown colored, wereabraded and had anhedral shapes. Larger zircon grains tended
to have inclusions and microcracks. Most of the zircons showedoscillatory zoning in CL images.
Lower Permian Nishikori Formation (Sample 120611-8;
N38°41’14.17”, E141°17’24.39”)
Sample 120611-8 of the Lower Permian Nishikori Formation
was collected 20 m below the top of the formation along thelower stream of the Kitakamigawa River, Tome City, Miyagi
Prefecture (Fig. 1). The sandstone sample was of ill-sorted,
rounded to sub-rounded, medium- to coarse-grained lithic
sandstone. The lithic fragments were mostly of volcanic rocks
having plagioclase phenocrysts, with few polycrystalline quartz
grains. Most of the zircon grains we collected were euhedral andcolorless, having columnar shapes with the longer dimensionof 70‒400 μm, the shorter dimension of 40‒200 μm, and the
aspect ratio of 1.5‒2. Most of them showed oscillatory zoningin CL images, and larger zircon grains tended to contain many
inclusions.
Upper Permian Toyoma Formation (Sample 101001-1;
N38°48’02.3”, E141°33’04.0”)
Sample 101001-1 of the Upper Permian Toyoma Formation
was collected from the uppermost part of the formation along
the Maehama Coast, Kesennuma City, Miyagi Prefecture (Fig. 1).

DETRITAL ZIRCON GEOCHRONOLOGY OF THE SOUTH KITAKAMI BELT

43

FIGURE 3. Diagram summarizing the sample description. Abbreviations—F: feldspars, L: lithic fragments, Q: single quartz.

The sandstone sample was of ill-sorted, sub-angular, and fineto medium-grained lithic arenite. The zircon grains we collected
were mostly euhedral and not abraded, among which 80%
were colorless and the others were brown. The zircon grains
had columnar shapes with the longer dimension of 50‒180μm, the shorter dimension of 20‒80 μm, and the aspect ratio of
1.5‒2.5. Most of them showed oscillatory zoning in CL images,
and larger zircon grains tended to contain many inclusions andmicrocracks.
Lower Triassic Osawa Formation (Sample 120612-3;
N38°31’56.7”, E141°32’2.43”)
Sample 120612-3 of the Lower Triassic Osawa Formationof the Inai Group was collected from the middle part of theformation on the east side of the Arahama Beach, Ishinomaki
City, Miyagi Prefecture (Fig. 1). The sandstone sample was ofwell-sorted, sub-angular to sub-rounded, and fine- to mediumgrained lithic arenite. The zircon grains we collected were
mostly euhedral to subhedral and colorless, having columnar
shapes with the longer dimension of 70‒180 μm, the shorter
dimension of 40‒110 μm, and the aspect ratio of 1.7‒2.5. All ofthem showed oscillatory zoning in CL images, and larger zircon
grains tended to contain many inclusions and microcracks.
Middle Triassic Fukkoshi Formation (Sample 120613-2;
N38°45’28.5”, E141°31’39.19”)
Sample 120613-2 of the Lower Triassic Fukkoshi Formation

was collected from the middle part of the formation along the
Kesaiso Coast, Kesennuma City, Miyagi Prefecture (Fig. 1). The
sampling horizon was 120 m below the top of the formation.
The sandstone sample was of moderately-sorted, angular to
sub-angular, and medium- to coarse-grained lithic arenite. The
collected zircon grains were euhedral to subhedral and colorless,having columnar shapes with the longer dimension of 150‒300
μm, the shorter dimension of 70‒200 μm, and the aspect ratio of
1.2‒2.0. Most of them showed oscillatory zoning in CL images,
and few zircon grains contained inclusions and/or microcracks.
Middle Triassic Isatomae Formation (Sample 120612-7;
N38°42’48.27”, E141°31’25.91”)
Sample 120612-7 of the Middle Triassic Isatomae Formationof the Inai Group was collected from the middle part of theformation along the coast on the northeast of Cape Bentenzaki,
Minamisanriku Town, Miyagi Prefecture (Fig. 1). The samplinghorizon was a little more than 500 m below the base of the Upper
Triassic Saragai Group. The sandstone sample was of poorlyto moderately-sorted, angular to sub-angular, and mediumgrained lithic arenite. The Isatomae sandstone is characterized
by the lower content of volcanic-rock fragments and inclusion of
K-feldspar grains. The zircon grains we collected were euhedral
to subhedral and virtually not abraded, among which 70% werecolorless and the others were brown. The zircon grains hadcolumnar shapes with the longer dimension of 120‒220 μm,
the shorter dimension of 70‒150 μm, and the aspect ratio of
1.5‒2.5. Most of them showed oscillatory zoning in CL images,

44

H. OKAWA ET AL.

and larger zircon grains tended to contain many inclusions andmicrocracks.
Upper Triassic Shindate Formation (Sample 120612-8;
N38°42’33.27”, E141°30’36.32”)
Sample 120612-8 of the Upper Triassic Shindate Formationof the Saragai Group was collected from the horizon several
meters below the top of the formation near the bottom of theSaragaizaka Slope, Minamisanriku Town, Miyagi Prefecture(Fig. 1). The Shindate Formation at this location, conformably
lying beneath the Carnian–Norian Chonomori Formation,is probably of Carnian age. The sandstone sample was ofmoderately- to well-sorted, angular, and fine lithic arenite. The
zircon grains we collected were mostly euhedral to subhedraland virtually not abraded, among which 90% were colorless andthe others were brown. The zircon grains had columnar shapeswith the longer dimension of 60‒200 μm, the shorter dimensionof 30‒120 μm, and the aspect ratio of 1.5‒2.5. Most of themshowed oscillatory zoning in CL images, and larger zircon grainstended to contain many inclusions and microcracks.
Lower Jurassic Niranohama Formation (Sample 101001-2;
N38°41’35.3”, E141°30’05.0”)
Sample 101001-2 of the Lower Jurassic Niranohama
Formation of the Shizugawa Group was collected from the upper
part of the formation (Fig. 1), the Niranohama or Hoinyashiki
sandstone of Kobayashi and Mori (1955) and Takahashi (1969).
The Niranohama Sandstone at this location is probably of Middle
Hettangian age, because an ammonoid of this age, Alsatites(Yebisites) onoderai, was reported from the same sandstone
close to this location (Matsumoto, 1956). The sandstone sample
was of well-sorted, sub-angular, and very fine- to fine-grained
feldspathic arenite. Most of the zircon grains we collected were
euhedral and colorless, having columnar shapes with the longer
dimension of 100‒600 μm, the shorter dimension of 30‒150 μm,
and the aspect ratio of 1.5‒5.0. Most of them showed oscillatory
zoning in CL images, and larger zircon grains tended to containmany inclusions and microcracks.
Middle Jurassic Aratozaki Formation (Sample 120612-4;
N38°41’46.3”, E141°29’54.7”)
Sample 120612-4 of the Middle Jurassic Aratozaki Formationof the Hashiura Group was collected from the horizon
approximately 10 m above the base of the formation (Fig. 1) andis probably of Aalenian–Bajocian age. The sandstone sample
was of ill-sorted, sub-angular, and fine- to medium-grained
lithic arenite. The zircon grains we collected were mostly
euhedral, among which 80% were colorless and the others werebrown. The zircon grains had columnar shapes with the longer
dimension of 50‒180 μm, the shorter dimension of 20‒80 μm,
and the aspect ratio of 1.5‒3.0. Most of them showed oscillatory
zoning in CL images, and many zircon grains contained

inclusions and microcracks.
Upper Jurassic Sodenohama Formation (Sample 101001-3;
N38°40’22.9”, E141°28’05.2”)
Sample 101001-3 of the Upper Jurassic Sodenohama
Formation of the Hashiura Group was collected from the middle
part of the formation on the coast near the Sodenohama Beach, 2
km to ESE from the center of the Minamisanriku Town, Miyagi
Prefecture (Fig. 1). The Sodenohama Formation at this location
is probably of Kimmeridgian age (Takahashi, 1969). The
sandstone sample was of moderately-sorted, sub-angular to subrounded, and fine-grained lithic arenite. The zircon grains wecollected were mostly euhedral to anhedral and colorless, havingcolumnar shapes with the longer dimension of 180‒700 μm, theshorter dimension of 100‒200 μm, and the aspect ratio of 1.5‒3.0.
Most of them showed oscillatory zoning in CL images, andmany zircon grains contained inclusions and microcracks.
Upper Jurassic Oginohama Formation (Sample 100416-5;
N38°18’16.3”, E141°29’50.2”)
Sample 100416-5 of the Upper Jurassic Oginohama
Formation of the Oshika Group was collected from the
Fukiura Shale and Sandstone Member at the eastern end of the
Kukunarihama Beach, Ishinomaki City, Miyagi Prefecture (Fig.
1), and is probably of Tithonian age. The sandstone sample
was of ill-sorted, sub-angular, and fine- to medium-grained
lithic arenite. The lithic fragments were mostly volcanic-rockfragments with minor polycrystalline quartz grains. The zircon
grains we collected were euhedral or subhedral and virtually
not abraded, among which 80% were colorless and the others
were brown. The zircon grains had columnar shapes with thelonger dimension of 120‒600 μm, the shorter dimension of120‒200 μm, and the aspect ratio of 1.5‒2.5. All of them showedoscillatory zoning in CL images and contained inclusions andmicrocracks.
Lower Cretaceous Yoshihama Formation (Sample 101002-1;
N38°34’25.4”, E141°26’52.9”)
Sample 101002-1 of the Lower Cretaceous Yoshihama
Formation of the Jusanhama Group was collected from the upper
part of the formation at Jusanhama-Tsukihama, Ishinomaki
City, Miyagi Prefecture (Fig. 1). The sandstone sample was ofwell-sorted, sub-angular, and fine-grained feldspathic arenite.
The zircon grains we collected were mostly euhedral, among
which 90% were colorless and the others were brown. The
zircon grains had columnar shapes with the longer dimensionof 100‒500 μm, the shorter dimension of 80‒200 μm, and the
aspect ratio of 1.0‒2.5. Most of them showed oscillatory zoningin CL images, and approximately 20% of these zircon grains,larger ones in particular, contained inclusions and microcracks.

DETRITAL ZIRCON GEOCHRONOLOGY OF THE SOUTH KITAKAMI BELT

45

FIGURE 4. Cathodoluminescence images of some zircons from the Domeki Sandstone Member of the Ayukawa Formation, Oshika Group (sample
100416-4) .

Lower Cretaceous Ayukawa Formation (Sample 100416-4;
N38°17’29.8”, E141°30’36.8”)
Sample 100416-4 of the Lower Cretaceous Ayukawa
Formation of the Oshika Group was collected from the Domeki
Sandstone Member at the southeastern end of Ayukawa Port
(Fig. 1), and must be of Valanginian or younger age. The
sandstone sample was of ill-sorted, angular, and very coarse- tocoarse-grained lithic wacke. The lithic fragments were mainly
polycrystalline quartz grains with some volcanic-rock fragments.
The zircon grains we collected were mostly euhedral, among
which 90% were colorless and the others were brown. The
zircon grains had columnar shapes with the longer dimension of60‒180 μm, the shorter dimension of 30‒100 μm, and the aspectratio of 1.0‒2.5. Most of them showed oscillatory zoning in CL
images (Fig. 3), and approximately 20% of these zircon grains,larger ones in particular, contained inclusions and microcracks.
ANALYTICAL METHOD
The zircon samples for analyses were prepared in accordancewith the procedures described in Kawagoe et al. (2012). The
measurement was carried out on laser ablation inductively
coupled plasma mass spectrometers (LA-ICPMS) equipped inthe (1) Department of Earth and Planetary Sciences, Graduate
School of Science and Engineering, Tokyo Institute ofTechnology (TITech; former Hirata Laboratory), (2) Earthquake
Research Institute of the University of Tokyo (ERI), and (3)Graduate School of Environmental Studies, Nagoya University(NU).
The ICPMS instrument equipped in TITech was a Thermo
Electron VG Plasma Quad 2 quadropole-based ICPMS appliedwith a chicane-type ion lens system and connected with a
MicroLas GeoLas 200CQ laser ablation system, which utilizes
193 nm wave-length ArF excimer laser (Iizuka and Hirata,
2004). The measurement conditions were as follows: theablation pit size of 16‒32 μm, energy density of 7‒8 J/cm-2, andpulse repetition rate of 5‒10 Hz. The analyses were carried out
in a peak-jumping mode and the peaks of 202 Hg, 206 Pb, 207Pb,
208
Pb, 232 Th, and 238 U were monitored. Data were acquired insequences of 30 analyses, consisting of an analysis of gas blank,
4 NIST (National Institute of Standards and Technology, U.S.A.)
SRM 610 glass standard, 4 standard zircon (91500 zircon withthe 206Pb/238U age of 1062.4 ± 0.4 Ma; Wiedenbeck et al., 1995),
1 gas blank, 10 unknown, 4 SRM 610 standard, 4 91500 zircon,
and 1 gas blank.
The ICPMS instrument equipped in ERI was a Thermo
Elemental Plasma Quad 3 quadropole-based ICPMS connectedwith a New Wave UP-213 LA system, which used the frequency
quintupled Nd-YAG 213-nm wavelength (Orihashi et al., 2008).
The measurement conditions were as follows: the ablation
pit size of 30 μm, energy density of 11‒13 J/cm-2, and pulse
repetition rate of 10 Hz. The analyses were carried out in a peakjumping mode and the peaks of 202Hg, 206Pb, 207Pb, 208Pb, 232 Th,
and 238U were monitored. Data were acquired in sequences of 28
analyses, consisting of 5 analyses of gas blank, 4 SRM 610 glass
standard, 1 standard zircon (91500 zircon), 9 unknown, 4 SRM
610 standard, and 5 gas blank.
The ICPMS instrument equipped in NU was an Agilent 7700x
quadropole-based ICPMS connected with a New Wave Research
NWR-213-type LA system, which used the frequency quintupled
Nd-YAG 213-nm wavelength. The measurement conditions,
optimized to reduce matrix effects, were as follows: energy
density of 11.7 J/cm-2, pulse repetition rate of 10 Hz, pre-ablation
time of 8 s, ablation time of 10 s, and the ablation pit size of
25 μm (Kouchi et al., 2012). The analyses were carried out ina peak-jumping mode and the peaks of 202Hg, 206Pb, 207Pb, 208Pb,
232
Th, and 238U were monitored. Data were acquired in the same
sequences with the ERI system.
Analytical bias among three laboratories was tested by using
OD-3 zircon standard. The bias was within the range of theiranalytical errors (Iwano et al., 2013) and is neglected in thefollowing discussion.

H. OKAWA ET AL.

46

FIGURE 5. Analytical data of detrital zircons from sandstone of the Silurian Nameirizawa Formation (sample 08331-9). a, Concordia diagram for all data;b, Concordia diagram for 850‒350 Ma data set; c, Probability density plot and histogram. Open (blue) circles in the concordia diagrams from Fig. 5 to Fig.
20 show the analytical data for discordant grains. Abbreviations(Figs. 5–20)—Fm.: Formation, N: total number of analyses.

FIGURE 6. Analytical data of detrital zircons from sandstone of the Silurian Yakushigawa Formation (sample 08331-3). a, Concordia diagram for all data;b, Concordia diagram for 850‒350 Ma data set; c, Probability density plot and histogram.

RESULTSWe sampled an outer part (rim or mantle) of collected zircon
grains with the laser ablation technique, and analyzed with an
ICPMS. After the analyses we first distinguished age clusters
on a concordia diagram. Then we chose grains with the % conc
value (100•(206Pb/ 238U age)/(207Pb/ 235U age)) between 90 and 110
and drew a probability density plot and a histogram with thedata interval of 50 Myr (206Pb/ 238U age). The data processing was

carried out using the Isoplot 3.70 software (Ludwig, 2008). Herefollow the results of our analyses.
Silurian Nameirizawa Formation (Sample 08331-9)
We obtained 100 analyses from 97 zircon grains collectedfrom sample 08331-9 of the Silurian Nameirizawa Formation in
TITech; we sampled the outer and inner parts of 3 zircon grains.
Detrital zircons were divided into 5 age groups on the concordia

DETRITAL ZIRCON GEOCHRONOLOGY OF THE SOUTH KITAKAMI BELT

47

FIGURE 7. Analytical data of detrital zircons from sandstone of the Upper Devonian Tobigamori Formation (sample 08429-5). a, Concordia diagram forall data; b, Concordia diagram for 1600‒300 Ma data set; c, Probability density plot and histogram.

FIGURE 8. Analytical data of detrital zircons from sandstone of the Lower Carboniferous Karaumedate Formation (sample 120611-2). a, Concordia
diagram for all data; b, Concordia diagram for 640‒320 Ma data set; c, Probability density plot and histogram.

diagram (Fig. 5a, b): 698‒403 Ma (73%), 1087‒945 Ma (4%),
1390‒1111 Ma (7%), 1620‒1402 Ma (5%), and 2955‒2642
Ma (2%). We further chose 85 concordant grains with the %conc value between 90 and 110 and drew a probability density
plot and a histogram with the data interval of 50 Myr (206Pb/238U
age; Fig. 5c). The histogram showed a multimodal pattern withthe youngest concordant age of 416 ± 13 Ma (2σ) and %Pc of
36.5. The youngest peak on the probability density plot was 432
Ma. The Th/U ratio of each analysis was 0.22‒2.22 and fell in

the range of igneous zircon, Th/U>0.1 (Rubatto and Hermann,
2003).
Silurian Yakushigawa Formation (Sample 08331-3)
We obtained 100 analyses from 100 zircon grains collectedfrom sample 08331-3 of the Silurian Yakushigawa Formation in
TITech. Detrital zircons were divided into 5 age groups on theconcordia diagram (Fig. 6a, b): 868‒385 Ma (65%), 1232‒845

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H. OKAWA ET AL.

FIGURE 9. Analytical data of detrital zircons from sandstone of the Lower Permian Nishikori Formation (sample 120611-8). a, Concordia diagram for all
data; b, Probability density plot and histogram.

Ma (13%), 1899‒1220 Ma (13%), 2238‒1845 Ma (4%), and2891‒2398 Ma (5%). All of the 100 grains had the % conc
value between 90 and 110. The histogram of the 206Pb/238U agesof 100 concordant grains showed a multimodal pattern with theyoungest concordant age of 398 ± 13 Ma and %Pc of 48.0. The
youngest peak on the probability density plot was 438 Ma (Fig.
6c). The Th/U ratio of each analysis was 0.11‒1.77 and fell in
the range of igneous zircon.
Devonian Tobigamori Formation (Sample 08429-5)
We obtained 37 analyses from 37 zircon grains collected fromsample 08429-5 of the Upper Devonian Tobigamori Formation
in TITech. Detrital zircons were divided into 4 age groups on theconcordia diagram (Fig. 7a, b): 503‒367 Ma (59%), 673‒589
Ma (8%), 1220‒1033 Ma (11%), and 1449‒1322 Ma (5%). The
histogram of the 206Pb/ 238U ages of 37 concordant grains showeda multimodal pattern with the youngest concordant age of 386 ±19 Ma and %Pc of 40.5. The youngest peak on the probability
density plot was 424 Ma (Fig. 7c). The Th/U ratio of eachanalysis was 0.20‒2.15 and fell in the range of igneous zircon.
Lower Carboniferous Karaumedate Formation (Sample
120611-2)
We obtained, in ERI, 78 analyses from 78 zircon grains
collected from sample 120611-2 of the Lower Carboniferous
Karaumedate Formation. Detrital zircons were divided into 3
age groups on the concordia diagram (Fig. 8a, b): 389‒341 Ma(58%), 470‒405 Ma (22%), and 1451‒1365 Ma (7%). The
histogram of the 206Pb/238U ages of 73 concordant grains showeda multimodal pattern with the youngest concordant age of 348.9± 7.8 Ma and %Pc of 19.2. The youngest peak on the probability
density plot was 360 Ma (Fig. 8c). The Th/U ratio of eachanalysis was 0.23‒1.20 and fell in the range of igneous zircon.
Lower Permian Nishikori Formation (Sample 120611-8)
We obtained 77 analyses from 77 zircon grains collected fromsample 120611-8 of the Lower Permian Nishikori Formation in
NU. Detrital zircons formed a single cluster on the concordia
diagram at 324‒255 Ma (100%; Fig. 9a). The histogram of
the 206Pb/238U ages of 57 concordant grains showed a unimodal
pattern with the youngest concordant age of 263.0 ± 8.4 Ma and%Pc of 0. The peak on the probability density plot was 279 Ma(Fig. 9b). The Th/U ratio of each analysis was 0.31‒1.16 and fellin the range of igneous zircon.
Upper Permian Toyoma Formation (Sample 101001-1)
We obtained 70 analyses from 70 zircon grains collected fromsample 101001-1 of the Upper Permian Toyoma Formation in
ERI (24 grains) and NU (46 grains). Detrital zircons are divided
into 3 age groups on the concordia diagram (Fig. 10a, b):374‒224 Ma (78%), 468‒445 Ma (3%), and 530‒497 Ma (5%).
The histogram of the 206Pb/ 238U ages of 59 concordant grains
showed a quasi-unimodal pattern with the youngest concordant
age of 227.8 ± 4.3 Ma and %Pc of 8.5. The youngest peak on theprobability density plot was 249 Ma (Fig. 10c). The Th/U ratioof each analysis was 0.19‒1.79 and fell in the range of igneous
zircon.

DETRITAL ZIRCON GEOCHRONOLOGY OF THE SOUTH KITAKAMI BELT

49

FIGURE 11. Analytical data of detrital zircons from sandstone of the Lower Triassic Osawa Formation of the Inai Group (sample 120612-3). a, Concordia
diagram for all data; b, Probability density plot and histogram.

FIGURE 10. Analytical data of detrital zircons from sandstone of the Upper Permian Toyoma Formation (sample 101001-1). a, Concordia diagram for all
data; b, Concordia diagram for 560‒200 Ma data set; c, Probability density plot and histogram.

Lower Triassic Osawa Formation (Sample 120612-3)

analysis was 0.42‒1.09 and fell in the range of igneous zircon.

We obtained 132 analyses from 132 zircon grains collectedfrom sample 120612-3 of the Lower Triassic Osawa Formationof the Inai Group in NU. Detrital zircons formed a single clusteron the concordia diagram at 324‒243 Ma (100%; Fig. 11a). The
histogram of the 206Pb/ 238U ages of 114 concordant grains showeda unimodal pattern with the youngest concordant age of 248.6± 5.7 Ma and %Pc of 0. The youngest peak on the probability
density plot was 266 Ma (Fig. 11b). The Th/U ratio of each

Middle Triassic Fukkoshi Formation (Sample 120613-2)
We obtained 107 analyses from 107 zircon grains collectedfrom sample 120613-2 of the Middle Triassic Fukkoshi
Formation of the Inai Group in NU. Detrital zircons formed asingle cluster on the concordia diagram at 336‒230 Ma (100%;
Fig. 12a). The histogram of the 206Pb/ 238U ages of 90 concordant
grains showed a unimodal pattern with the youngest concordant

50

H. OKAWA ET AL.

FIGURE 12. Analytical data of detrital zircons from sandstone of the Middle Triassic Fukkoshi Formation of the Inai Group (sample 120613-2). a,
Concordia diagram for all data; b, Probability density plot and histogram.

FIGURE 13. Analytical data of detrital zircons from sandstone of the Middle Triassic Isatomae Formation of the Inai Group (sample 130612-7). a,
Concordia diagram for all data; b, Probability density plot and histogram.

age of 240 ± 10 Ma and %Pc of 0. The youngest peak on theprobability density plot was 263 Ma (Fig. 12b). The Th/U ratioof each analysis was 0.3‒0.9 and fell in the range of igneous
zircon.
Middle Triassic Isatomae Formation (Sample 120612-7)
We obtained 86 analyses from 86 zircon grains collected fromsample 1230612-7 of the Middle Triassic Isatomae Formationof the Inai Group in NU. Detrital zircons formed a single clusteron the concordia diagram at 310‒223 Ma (100%; Fig. 13a). The
histogram of the 206Pb/ 238U ages of 68 concordant grains showeda unimodal pattern with the youngest concordant age of 230.0± 5.1 Ma and %Pc of 0. The youngest peak on the probability
density plot was 251 Ma (Fig. 13b). The Th/U ratio of eachanalysis was 0.37‒1.16 and fell in the range of igneous zircon.

DETRITAL ZIRCON GEOCHRONOLOGY OF THE SOUTH KITAKAMI BELT

51

FIGURE 14. Analytical data of detrital zircons from sandstone of the Upper Triassic Shindate Formation of the Saragai Group (sample 120612-8). a,
Concordia diagram for all data; b, Probability density plot and histogram.

FIGURE 15. Analytical data of detrital zircons from sandstone of the Lower Jurassic Niranohama Formation of the Shizugawa Group (sample 101001-2). a,
Concordia diagram for all data; b, Probability density plot and histogram.

Upper Triassic Shindate Formation (Sample 120612-8)
We obtained 121 analyses from 121 zircon grains collectedfrom sample 120612-8 of the Upper Triassic Shindate Formationof the Saragai Group in NU. Detrital zircons formed a single
cluster on the concordia diagram at 296‒186 Ma (100%; Fig.
14a). The histogram of the 206Pb/ 238U ages of 90 concordant
grains showed a unimodal pattern with the youngest concordant
age of 195.1 ± 9.6 Ma and %Pc of 0. The youngest peak on theprobability density plot was 222 Ma (Fig. 14b). The Th/U ratioof each analysis was 0.39‒1.33 and fell in the range of igneous
zircon.
Lower–Middle Jurassic Niranohama Formation (Sample
101001-2)
We obtained 72 analyses from 72 zircon grains collected fromsample 101001-2 of the Lower Jurassic Niranohama Formation

52

H. OKAWA ET AL.

FIGURE 16. Analytical data of detrital zircons from sandstone of the Middle Jurassic Aratozaki Formation of the Hashiura Group (sample 120612-4). a,
Concordia diagram for all data; b, Concordia diagram for 550–150 Ma data set; c, Probability density plot and histogram.

FIGURE 17. Analytical data of detrital zircons from sandstone of the Upper Jurassic Sodenohama Formation of the Hashiura Group (sample 101001-3). a,
Concordia diagram for all data; b, Concordia diagram for 300–150 Ma data set; c, Probability density plot and histogram.

of the Shizugawa Group in ERI. Detrital zircons formed a single
cluster on the concordia diagram at 288‒234 Ma (100%; Fig.
15a). The histogram of the 206Pb/ 238U ages of 68 concordant
grains showed a unimodal pattern with the youngest concordant
age of 237.6 ± 4.0 Ma and %Pc of 0. The youngest peak on theprobability density plot was 256 Ma (Fig. 15b). The Th/U ratioof each analysis was 0.26‒1.28 and fell in the range of igneous
zircon.

Middle Jurassic Aratozaki Formation (Sample 120612-4)
We obtained 85 analyses from 85 grains collected from sample
120612-4 of the lower Middle Jurassic Aratozaki Formationof the Hashiura Group in NU. Detrital zircons are divided into
3 age groups on the concordia diagram (Fig. 16a, b): 216‒161
Ma (30%, 321‒223 Ma (57%), and 2124‒1789 Ma (10%). The
histogram of the 206Pb/ 238U ages of 67 concordant grains showeda bimodal pattern with the youngest concordant age of 166.4 ±DETRITAL ZIRCON GEOCHRONOLOGY OF THE SOUTH KITAKAMI BELT

53

FIGURE 18. Analytical data of detrital zircons from sandstone of the Upper Jurassic Fukiura Shale and Sandstone Member of the Oginohama Formation,
Oshika Group (sample 100416-5). a, Concordia diagram for all data; b, Concordia diagram for 340–150 Ma data set; c, Probability density plot andhistogram.

FIGURE 19. Analytical data of detrital zircons from sandstone of the Lower Cretaceous Yoshihama Formation of the Jusanhama Group (sample 1010012). a, Concordia diagram for all data; b, Concordia diagram for 310–150 Ma data set; c, Probability density plot and histogram.

5.4 Ma and %Pc of 13.4. The youngest peak on the probability
density plot was 182 Ma (Fig. 16c). The Th/U ratio of eachanalysis was 0.10‒1.03 and fell in the range of igneous zircon.
Upper Jurassic Sodenohama Formation (Sample 101001-3)
We obtained 37 analyses from 37 zircon grains collectedfrom sample 101001-3 of the Upper Jurassic (Kimmeridgian)Sodenohama Formation of the Hashiura Group in ERI. Detrital

zircons are divided into 3 age groups on the concordia diagram(Fig. 17a, b): 231‒164 Ma (70%), 264‒252 Ma (5%), and1865‒1749 Ma (5%). The histogram of the 206Pb/238U ages of 33
concordant grains showed a bimodal pattern with the youngest
concordant age of 166.5 ± 3.0 Ma and %Pc of 7.7. The youngest
peak on the probability density plot was 170 Ma (Fig. 17c). The
Th/U ratio of each analysis was 0.18‒1.49 and fell in the rangeof igneous zircon.

54

H. OKAWA ET AL.

FIGURE 20. Analytical data of detrital zircons from sandstone of the Lower Cretaceous Domeki Sandstone Member of the Ayukawa Formation, Oshika
Group (sample 100416-4). a, Concordia diagram for all data; b, Concordia diagram for 420‒120 Ma data set; c, Probability density plot and histogram.

Upper Jurassic Oginohama Formation (Sample 100416-5)
We obtained, in ERI, 61 analyses from 61 zircon grains
collected from sample 120416-5 of the Tithonian Fukiura Shaleand Sandstone Member of the Oginohama Formation, Karakuwa
Group. Detrital zircons are divided into 5 age groups on theconcordia diagram (Fig. 18a, b): 191‒157 Ma (35%), 267‒208
Ma (37%), 1645‒1562 Ma (4%), 1831‒1671 Ma (11%), and2030‒1881 Ma (7%). The histogram of the 206Pb/238U ages of 54
concordant grains showed a bimodal pattern with the youngest
concordant age of 161.7 ± 5.2 Ma and %Pc of 27.8. The
youngest peak on the probability density plot was 174 Ma (Fig.
18c). The Th/U ratio of each analysis was 0.15‒1.14 and fell in
the range of igneous zircon.
Lower Cretaceous Yoshihama Formation (Sample 101002-1)
We obtained 31 analyses from 31 zircon grains collected fromsample 101002-1 of the Lower Cretaceous Yoshihama Formationof the Jusanhama Group in ERI. Detrital zircons are divided into
3 age groups on the concordia diagram (Fig. 19a, b): 165‒156
Ma (10%), 235‒167 Ma (74%), and 256‒242 Ma (10%). The
histogram of the 206Pb/238U ages of 21 concordant grains showeda bimodal pattern with the youngest concordant age of 159.6 ±3.9 Ma and %Pc of 9.5. The youngest peak on the probability
density plot was 160 Ma (Fig. 19c). The Th/U ratio of eachanalysis was 0.15‒1.43 and fell in the range of igneous zircon.
Lower Cretaceous Ayukawa Formation (Sample 100416-4)
We obtained, in ERI, 105 analyses from 105 zircon grains
collected from sample 100416-4 of the Valanginian or younger

Domeki Sandstone Member of the Ayukawa Formation, Oshika
Group. Detrital zircons are divided into 4 age groups on theconcordia diagram (Fig. 20a, b): 147‒120 Ma (36%), 268‒149
Ma (44%), 369‒303 Ma (6%), and 1840‒1451 Ma (10%). The
histogram of the 206Pb/238U ages of 70 concordant grains showeda bimodal pattern with the youngest concordant age of 125.9 ±6.3 Ma and %Pc of 15.3. The youngest peak on the probability
density plot was 133 Ma. The Th/U ratio of each analysis was
0.12‒1.99 and fell in the range of igneous zircon (Fig. 20c).
DISCUSSION
Comparison of the new U-Pb ages and the age of depositionof the studied samples
The accuracy of the U-Pb isotopic ratios obtained with the
ICPMS instruments is guaranteed by comparing the weighted
mean of several tens of measurements of a standard zircon andthe published ID-TIMS (isotope dilution-thermal ionization
mass spectrometry) or SHRIMP data for the same zircon. The
weighted mean shows good agreement with the publishedisotopic ratio within ± 2% (e.g., Orihashi et al., 2008). Hencethe weighted mean of the youngest age cluster, which is usuallyclose to the youngest peak age in the probability density plot,
is a good measure of the depositional age, provided that synsedimentary volcanism in the hinterland supplied certain amountof igneous zircons to the measured sample. Figure 21 compares,for each sample, the youngest peak age in the probability density
plot and the biostratigraphical age-range, i.e., the age-range
previously inferred from stratigraphy and index fossils. For
all samples except sample 101001-1 (Upper Permian Toyoma
Formation), the youngest peak age falls in the biostratigraphicalDETRITAL ZIRCON GEOCHRONOLOGY OF THE SOUTH KITAKAMI BELT

55

FIGURE 21. Diagram comparing the youngest peak age in the probability density plot (PDP; ordinate) and biostratigraphical age-range (abscissa). The
age-range of the magmatic hiatus in Korea (158‒110 Ma) and a possible magmatic hiatus during the Triassic and the Early Jurassic are also shown.
Abbreviations—F.: Formation, O: Ordovician, Sil: Silurian.

age-range or older than it. Thus we are convinced that the resultsof our measurement are mostly concordant with the litho- andbiostratigraphy of the SKB.
The youngest peak ages of the following formations are
significantly older than the biostratigraphical ages: Middle
Triassic to Lower Jurassic formations (Osawa, Fukkoshi,
and Niranohama formations) and Middle Jurassic to Lower
Cretaceous formations (Aratozaki, Sodenohama, Oginohama,and Yoshihama formations; Fig. 21). The fact suggests that
there were no significant syn-sedimentary volcanism in Middle
Triassic–Early Jurassic times and Middle Jurassic–Early
Cretaceous times. The latter interval falls within the magmatic
hiatus in Korea, 158‒110 Ma (Sagong et al., 2005), and likely
indicates its influence to the South Kitakami Paleoland.
Three tectonic stages of the South Kitakami Paleoland
Provenance analysis based on detrital zircon ages has been
carried out in various parts of the world including eastern Asia

(e.g., Darby and Gehrels, 2006; Rojas-Agramonte et al., 2011;
Yao et al., 2011, 2012; Diwu et al., 2012). According to thesestudies, the sand and sandstones of the North China Block arecharacterized by the abundance of 2.5 Ga and 1.85 Ga zircons
and absence or very rare occurrence of Neoproterozoic zircons(Darby and Gehrels, 2006; Diwu et al., 2012; Choi et al., 2013).
2.5 Ga was the age of a major tectonothermal event associatedwith the crustal growth of the North China Block (Diwu et al.,2012). 1.85 Ga was the age of crustal assembly in the North
China Block associated with the formation of the supercontinent
Columbia (e.g., Rogers and Santosh, 2002; Zhao et al., 2004).
Grenvillian tectonothermal event (1250‒980 Ma) related to
the formation of the supercontinent Rodinia was not recordedin the North China Block, which was isolated following the
breakup of Rodinia (Yin and Nie, 1996). Zircons formed during
the Grenvillian tectonothermal event are well preserved in thesand and sandstones of the South China Block (Yangtze andCathaysia blocks), Australia, and some blocks in the CAOB
including the Tarim Block (e.g., Rino et al., 2008; Iizuka et al.,56

H. OKAWA ET AL.

FIGURE 22. Histograms showing the age distributions of detrital zircon grains of all sandstone samples described in this study. The horizontal axes arefor the age of zircon grains (best estimate in Ma) and the vertical axes are for the number of grains. Abbreviations—Fm.: Formation, N: total number ofanalyses, YP: age of the youngest peak in the probability density plot, YZ: age of the youngest zircon.

DETRITAL ZIRCON GEOCHRONOLOGY OF THE SOUTH KITAKAMI BELT

57

FIGURE 23 Plate reconstruction maps showing the position of the South Kitakami Paleoland at each period. The base reconstruction maps are taken from
Lawver et al. (2009). a, 400 Ma (Early Devonian); b, 300 Ma (end Carboniferous); c, 220 Ma (Late Triassic); d, 140 Ma (end Jurassic). Abbreviations—
An: Angara Craton, Au: Australia, NCB: North China Block, SCB: South China Block, SKP: South Kitakami Paleoland.

2010; Rojas-Agramonte et al., 2011; Yao et al., 2011, 2012;
Diwu et al., 2012; Choi et al., 2013). All of these blocks wereassembled in the northern part of East Gondwana during Early
to Middle Paleozoic times (e.g., Scotese and McKerrow, 1990;
Turner, 2010; Metcalfe, 2011).
By comparing the age distribution of detrital zircons of the
SKB (Fig. 22) with that of Australia and continental blocks ineastern Asia, three stages of tectonic development have been
discriminated of the SKB. From the following paragraph, we
combine our new data with previous geological studies andpresent our model for the tectonic development.
Silurian–Early Carboniferous: Magmatic arc in thenorthern margin of East Gondwana
The age distribution of detrital zircons from the Siluro–
Devonian sandstone of the SKB is characterized by more than
50% of syn-sedimentary zircons, i.e., zircons of ca. 500 Ma
to the age of sedimentation, and relatively high proportion ofPrecambrian zircons (%Pc = 36.5‒48.0). The abundance of synsedimentary detrital zircons, along with the abundant pyroclastic
and volcaniclastic rocks in the Siluro–Devonian strata in the
SKB, indicates an igneous activity in the provenance. Moreover
the relatively high %Pc value suggests that the Siluro–Devoniansandstone was deposited in front of a continental magmatic arc
with Precambrian basement rocks. The Precambrian detrital
zircons on the concordia diagram shows several small clusters
ranging in age from Neoarchean to Neoproterozoic. The
presence of Neoarchean to Neoproterozoic zircons including

those of Grenvillian times excludes the North China Block fromthe candidates of the provenance. Considering the facts that the
Siluro–Devonian corals, brachiopods, and plants of the SKB
have affinities with those of Australia, South China, and thesouthern part of the CAOB (e.g., Hamada, 1960; Kato, 1990;
Tazawa and Chen, 2001; Kimura, 1987; Tazawa et al., 2006),
and that these blocks constituted northern East Gondwana in the
Middle Paleozoic, the Siluro–Devonian sandstone must havebeen deposited along the northern margin of East Gondwana(Fig. 23a). Although Tazawa and Chen (2001) and Tazawa etal. (2006) demonstrated that the SKB was located in the easternextension of the southern part of the CAOB (or the Tienshan–
Xinganling Belt) along the northern margin of the North China
Block in the Devonian, the absence of 1.85 Ga zircons in the
Devonian Tobigamori Formation denies their idea. Isozaki et al.
(2010), on the other hand, stated that the Japanese Islands grew
along the margin of an oceanic island arc originated from theophiolite obduction within an oceanic plate (Paleo-Pacific plate).
The Hayachine complex may have been a part of the obducted
ophiolite that forms an oceanic island arc. However, the oceanicisland arc, if existed, must have collided with the northern
East Gondwana by the Silurian. The inclusion of some 40% ofPrecambrian zircons in the Siluro–Devonian sandstone of the
SKB cannot be explained with the oceanic-island-arc setting,because Precambrian zircons are generally concentrated in thecontinental crust.
The Early Carboniferous sandstone of the Karaumedate

58

H. OKAWA ET AL.

Formation shows similar pattern of detrital zircon age
distribution with the Siluro–Devonian sandstone although the%Pc value is significantly lower (19.2). The lower %Pc value
likely indicate the commencement of the intra-arc rifting,mentioned in the next paragraph, and the decrease of the area ofthe hinterland with Precambrian rocks.
Permian–Early Jurassic: Oceanic island arc in the Tethys
Ocean
Bimodal volcanic activity is recorded in the Lower
Carboniferous sequence of the SKB and has been assumed toindicate intra-arc rifting (Kawamura et al., 1990). The Permian–
Lower Jurassic sandstones that overlie the Carboniferous
bimodal volcanic and pyroclastic rocks contain virtually no
Precambrian zircons. The result is in contrast with the detrital
zircon age distribution of coeval supracontinental strata of
Korea (Pyeongan Supergroup on the Yeongnam Massif) that
contain more than 80% of Paleoproterozoic zircons and show astrong affinity with the North China Block (Lee et al., 2012a).
The absence of Precambrian zircons in the Permian–Lower
Jurassic sandstones indicates that they were deposited along themargin of an oceanic island or microcontinent apart from a large
continental block. The inclusion of syn-sedimentary igneous
zircons in the Lower Permian Nishikori Formation (Fig. 21)
suggests that the oceanic island or microcontinent had evolved
to an active oceanic island arc by the Early Permian. Thus weinterpret that the South Kitakami Paleoland was rifted from anactive margin of East Gondwana in the Early Carboniferous
and drifted as an oceanic island arc in the Tethys Ocean fromthe Early Permian (Fig. 23b). The unimodal age distribution of
detrital zircons (centered at 280‒250 Ma) in the Permian–Lower
Jurassic sandstones indicates that the land surface of the oceanicisland arc was mostly occupied by Permian igneous rocks.
However, the detection of some Precambrian zircons from the
Toyoma Formation (sample 101001-1; %Pc = 8.5) indicates thata certain amount of Precambrian basement rocks were exposedin the South Kitakami Paleoland.
The Siluro–Devonian faunal and floral affinity between theSouth Kitakami Paleoland and Australia disappeared in the
Carboniferous; i.e., the South Kitakami Paleoland was in thetropical to subtropical Cathaysia floristic province, whereas
Australia moved southward as a part of Gondwana to theGondwana floristic province of the south polar region and partly
covered with the continental ice sheet. The paleobiogeographicalcontrast between the SKB and Australia is concordant with therifting model (Ehiro and Kanisawa, 1999). We suggest that therifting was related to the clockwise rotation of Gondwana in
Carboniferous–Permian times, which finally collided with the
Laurussia or Euramerica continent to form a collision zone of
the Variscan orogen in Europe, northwestern Africa, and eastern
North America (Fig. 23b).
The South Kitakami Paleoland during the Carboniferous–
Permian was paleobiogeographically allied to the South China
or Indochina block (corals, fusulinids, and ammonoids; e.g.,
Minato and Kato, 1965; Nakazawa, 1991; Ozawa, 1987; Ehiro,
1998), the North China Block (plants; e.g., Asama, 1985), or

the CAOB along the northern and eastern margins of the North
China Block at present (brachiopods; Tazawa, 1991, 2001).
These studies indicated that the South Kitakami Paleoland wasin the Tethyan realm (e.g., Ehiro, 1998), with some brachiopod
genera indicating the mixture of boreal elements (e.g., Tazawa,
1991). Although we have a tentative idea that the South
Kitakami Paleoland lay in the northern part of the Tethyan
realm, between the continental blocks of CAOB and South
China, and in the same climate zone with the North China Block,our detrital zircon data cannot indicate the exact position of theSouth Kitakami Paleoland in the Carboniferous–Permian. Our
new data can only indicate that the South Kitakami Paleoland
was not along the margin of a large continental block (Fig. 23c,d).
The boreal or arctic elements gradually increased in the
Triassic strata. For example, the Anisian ammonoids containsome common species with the coeval ammonoids from
Primorye and Kolyma (Nakazawa, 1991), and the Late Triassic
Monotis fauna belongs to the Arcto-Pacific Realm (Kobayashi
and Tamura, 1983; Tamura, 1987). Faunal connection between
the SKB and the Angara Craton seems to have been strengthened
through the Triassic. The Lower Jurassic Shizugawa Group ischaracterized by endemic species of ammonoids and bivalves(Hayami, 1990). The fact is concordant with our oceanicisland-arc model, but we have to evaluate the influence of mass
extinction across the Triassic–Jurassic boundary.
Middle Jurassic–Early Cretaceous: Amalgamation with
the North China Block
The age distribution of detrital zircons from the Middle
Jurassic–Early Cretaceous sandstone of the SKB is characterizedby more than 70% of syn-sedimentary zircons along with small
amounts of Paleoproterozoic zircons (%Pc = 7.7‒27.8), andabsence of Neoproterozoic zircons. Although syn-sedimentary
zircons are abundant, both the youngest zircon age and the
youngest peak age in the probability density plot of the Upper
Jurassic Sodenohama and Oginohama formations and of theLower Cretaceous Yoshihama Formation are significantly
younger than the age of sedimentation. Zircons younger than 160
Ma were not detected from the three formations. We interpretthat the magmatic hiatus in South Korea (Sagong et al., 2005)
gave an influence to the age composition of the detrital zircons
in these formations. The absence of zircons youger than 160 Ma
together with the absence of Neoproterozoic zircons strongly
indicate that the South Kitakami Paleoland was along the marginof the North China Block during the sedimentation of the
Aratozaki, Sodenohama, Oginohama, and Yoshihama formations(Fig. 23d). However the proportion of Paleoproterozoic zircons
in these formations is significantly lower than that of thesandstone on the North China Block (e.g., the Jangsan Formation
and Pyeongan Supergroup in South Korea; Lee et al., 2012a, b).
Moreover the Ryoseki-type flora that flourished on South China,
Indochina, and the Malay Peninsula in Late Jurassic to Early
Cretaceous times occur from the Oginohama Formation. Hence
we interpret that the South Kitakami Paleoland was a little faraway from the Paleoproterozoic orogens in the North China

DETRITAL ZIRCON GEOCHRONOLOGY OF THE SOUTH KITAKAMI BELT

Block, e.g., the Jiao–Liao–Ji Belt in the eastern part of the North
China Block (Zhao et al., 2005), in the Middle Jurassic–Early
Cretaceous. The Lower Cretaceous Ayukawa Formation (sample
100416-4) contains many zircons in the period of the magmatic
hiatus in South Korea, i.e., from 158 Ma to 110 Ma (Sagong etal., 2005). Considering the occurrence of the Ryoseki-type flora
from the underlying Oginohama Formation (Kimura and Ohana,1989), we interpret that the 150‒130 Ma zircons in the Ayukawa
sandstone came from the Jurassic–Cretaceous wide magmatic
province in the Cathaysia Block (Li and Li, 2007), although their
possible origin from the coeval metamorphic core complexes
in the North China Block and CAOB (Davis et al., 1996, 2001;
Wang et al., 2004) cannot be ruled out.
CONCLUSIONS
We carried out U-Pb analyses of more than 1,000 single
detrital zircons from 16 formations of the Silurian–Early
Cretaceous continuous succession of the South Kitakami Belt,
Northeast Japan. The data set provides a detrital zircon reference
for the complex continental-margin orogen of Japan for the
first time. The results and interpretations can be summarized asfollows.
1. Siluro–Devonian sandstone samples contain many synsedimentary zircons and 36.5‒48.0% of Precambrian zircons,
scattering in age between 700 Ma and 3000 Ma, suggestingthat they were deposited along an active continental margin ofnorthern East Gondwana.
2. Permian–Lower Jurassic sandstone samples contain virtuallyno Precambrian zircons, suggesting that they were depositedalong the active margin of an oceanic island arc. From
biostratigraphical evidence, the South Kitakami Paleoland
seems to have drifted northward in the Tethyan realm between
the continental blocks of CAOB (north) and South China(south) where boreal brachiopods and bivalves sometimes
reached.
3. Middle Jurassic–Lower Cretaceous sandstone samples containmany 300‒170 Ma zircons and up to 28% of Paleoproterozoic(around 1,850 Ma) zircons but no Neoproterozoic zircons.
Moreover the zircons during the magmatic hiatus in Korea(158‒110 Ma) were detected only in one Early Cretaceous
sandstone sample. The age distribution suggests that the
Paleoproterozoic zircons in the Middle Jurassic–Lower
Cretaceous sandstone of the SKB were most likely supplied
from a Paleoproterozoic orogen in the North China Block.
4. The South Kitakami Paleoland, which accumulated thecontinuous succession of the South Kitakami Belt, was thusborn along a margin of Gondwana in the Silurian–Devonian,rifted from the continent and drifted in the Tethys ocean as
an oceanic island arc in the Permian–Early Jurassic, andfinally amalgamated along an active continental margin where
detrital zircons of the North China Block were supplied in the
Middle Jurassic.

59

ACKNOWLEDGMENTS
We would like to express our sincere gratitude to Emeritus
Professor Kenji Konishi of Kanazawa University for his
encouragement to submit our work to this issue; and to Professor
Masaaki Shimizu and Associate Professor Kenji Kashiwagiof the University of Toyama for various discussions andinstructions during the course of this study. Critical reviews of
the submitted manuscript by Professor Yasufumi Iryu of Tohoku
University, Professor Keitaro Kunugiza of the University of
Toyama, Emeritus Professor Kenji Konishi, and Dr. Hiroto
Ichishima of the Fukui Prefectural Dinosaur Museum are
greatly appreciated. This study was supported by the Earthquake
Research Institute (University of Tokyo) cooperative research
program 2013-G-04, JSPS KAKENHI Grant Number 25400484,
and the discretionary budget of the President of the University of
Toyama.
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Aoki, K., Y. Isozaki, S. Yamamoto, K. Maki, T. Yokoyama and
T. Hirata. 2012. Tectonic erosion in a Pacific-type orogeny:
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in Japanesein Japanese with English abstract
in Korean

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＜ 地名・地層名 ＞
Arahama Beach ………………… 荒浜
Arato Formation ……………… 荒砥層
Aratozaki Formation ……… 荒砥崎層
Arisu Formation ……………… 有住層
Ayukawa Formation ………… 鮎川層
Ayukawa Port ………………… 鮎川港
Cape Bentenzaki …………… 弁天崎
Chonomori Formation …… 長の森層
Domeki Sandstone Member …………
……………………ドウメキ砂岩部層
Fukiura Shale and Sandstone Member …
…………………福貴浦頁岩砂岩部層
Fukkoshi Formation ………… 風越層
Funagawara Formation …… 船河原層
Futawatashi Shale Member ……………
…………………………長渡頁岩部層
Hakoneyama Formation …… 箱根山層
Hanamaki City ……………… 花巻市
Hashiura ………………………… 橋浦
Hayachine Complex … 早池峰複合岩類
Hijochi Formation ………… 飛定地層
Hikami Granite ………… 氷上花崗岩
Hikoroichi …………………… 日頃市
Hiraiso Formation …………… 平磯層
Hoinyashiki ……………… 法印屋敷
Hosoura Formation ………… 細浦層
Ichinoseki City ……………… 一関市
Inai Group ………………… 稲井層群
Isatomae Formation ……… 伊里前層
Ishinomaki City ……………… 石巻市
Ishiwaritoge Formation ……石割峠層
Isokusa Formation …………… 磯草層
Iwaizaki Limestones 岩井崎石灰岩層
Iwate Prefecture …………… 岩手県
Jusanhama Group ……… 十三浜層群
Kamaishi City ……………… 釜石市
Kanaegaura Formation ……… 鼎浦層
Kanokura Formation ………… 叶倉層
Karakuwa ……………………… 唐桑
Karaumedate Formation …… 唐梅館層
Kawauchi Formation ………… 川内層
Kesaiso Coast …………… 今朝磯海岸
Kesennuma City ………… 気仙沼市

Kitakamigawa River ………… 北上川
Kitsunezaki Sandstone and Shale Member………………… 狐崎砂岩頁岩部層
Kiyosaki Sandstone Member …………
…………………………清崎砂岩部層
Kobitawatashi Sandstone and Shale Member…………………小長渡砂岩頁岩部層
Kobosoura Formation ……… 小細浦層
Kogoshio Formation ……… 小々汐層
Kosaba Formation …………… 小鯖層
Kowaragi Formation ……… 小原木層
Kozumi Shale Member 小積頁岩部層
Kukunarihama Beach ……… 十三成浜
Kurosegawa ………………… 黒瀬川
Maehama Coast …………… 前浜海岸
Makinohama Sandstone Member ……
………………………牧の浜砂岩部層
Minamisanriku Town ……… 南三陸町
Miyagi Prefecture …………… 宮城県
Miyako City ………………… 宮古市
Miyamori ……………………… 宮守Mone Formation ……………… 舞根層
Monobegawa Group ……物部川層群
Morioka City ………………… 盛岡市
Motoyoshi ……………………… 本吉
Mt. Hayachinesan ………… 早池峰山
Mt. Hikamisan ……………… 氷上山
Mt. Karaumedateyama …… 唐梅館山
Myojinmae Formation …… 明神前層
Nagaiwa Formation ………… 長岩層
Nagao Formation …………… 長尾層
Nagasaka ……………………… 長坂
Nakahara Formation ………… 中原層
Nakazato Formation ………… 中里層
Nameirizawa Formation… 名目入沢層
Nameirizawa River ……… 名目入沢
Natsuyama Logging Road … 夏山林道
Niranohama Formation …… 韮の浜層
Nishikori Formation ………… 錦織層
Northern Chichibu Belt … 北部秩父帯
Notsuchi Formation ………… 野土層
Odagoe Formation ………… 小田越層
Odaira Formation …………… 大平層

Ofunato Group ………… 大船渡層群
Oginohama Formation …… 荻の浜層
Ohachimori Amphibolite ……………
…………………………大鉢森角閃岩
Onimaru Formation …………鬼丸層
Ono Formation ……………… 大野層
Orikabetoge Formation …… 折壁峠層
Osawa Formation …………… 大沢層
Oshika Group …………… 牡鹿層群
Oshima Group …………… 大島層群
Rodai Formation …………… 楼台層
Ryoseki ………………………… 領石
Sakamotozawa Formation … 坂本沢層
Saragai Group …………… 皿貝層群
Saragaizaka Slope …………… 皿貝坂
Sendai City ………………… 仙台市
Setamai ……………………… 世田米
Shindate Formation ………… 新館層
Shittakasawa Formation … 尻高沢層
Shizugawa …………………… 志津川
Sodenohama Beach ………… 袖の浜South Kitakami Belt（SKB）…………
……………………………南部北上帯
Southern Chichibu Belt … 南部秩父帯
Southwest Japan ………… 西南日本
Takonoura Formation …… 蛸浦層群
Tategami Formation ………… 立神層
Tenjinnoki Formation … 天神ノ木層
Tobigamori Formation …… 鳶ヶ森層
Tome City …………………… 登米市
Torinosu-type Limestone ……………
…………………………鳥巣式石灰岩
Toyoma Formation ……………登米層
Tsukihama Formation …………月浜層
Tsunakizaka Formation …… 綱木坂層
Tsukinoura Formation …… 月の浦層
Usuginu-type Conglomerate …………
……………………………薄衣式礫岩
Yakushigawa River ………… 薬師川
Yamadori Formation ………… 山鳥層
Yamazaki Conglomerates 山崎礫岩層
Yokonuma Formation ……… 横沼層
Yoshihama Formation ……… 吉浜層

TABLE 1. U-Pb isotopic data for zircons from sandstone of the South Kitakami Belt. All errors are 2σ. % conc = 100•( 206Pb/ 238U age)/( 207Pb/ 235U age) is a measure of concordance between
206
Pb/238U and 207Pb/235U ages. Analyses shown in italics are discordant and are not included in the probability density plot and histogram.

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