Inner Zone Of Southwest Japan Research Articles

SHRIMP zircon and EPMA monazite dating of granitic rocks from the Maizuru terrane, southwest Japan: Correlation with East Asian Paleozoic terranes and geological implications

Abstract The Maizuru terrane, distributed in the Inner Zone of southwest Japan, is divided into three subzones (Northern, Central and Southern), each with distinct lithological associations. In clear contrast with the Southern zone consisting of the Yakuno ophiolite, the Northern zone is subdivided into the western and eastern bodies by a high‐angle fault, recognized mainly by the presence of deformed granitic rocks and pelitic gneiss. This association suggests an affinity with a mature continental block; this is supported by the mode of occurrence, and petrological and isotopic data. Newly obtained sensitive high mass‐resolution ion microprobe (SHRIMP) zircon U–Pb ages reveal the intrusion ages of 424 ± 16 and 405 ± 18 Ma (Siluro–Devonian) for the granites from the western body, and 249 ± 10 and 243 ± 19 Ma (Permo–Triassic) for the granodiorites from the eastern body. The granites in the western body also show inherited zircon ages of around 580 and 765 Ma. In addition, electron probe microanalysis (EPMA) monazite U–Th–total Pb dating gives around 475–460 Ma. The age of intrusion, inherited ages, mode of occurrence, and geological setting of the Siluro–Devonian granites of the Northern zone all show similarities with those of the Khanka Massif, southern Primoye, Russia, and the Hikami granitic rocks of the South Kitakami terrane, Northeast Japan. We propose that both the Siluro–Devonian and Permo–Triassic granitic rocks of the Northern zone are likely to have been juxtaposed through the Triassic–Late Jurassic dextral strike‐slip movement, and to have originated from the Khanka Massif and the Hida terrane, respectively. This study strongly supports the importance of the strike‐slip movement as a mechanism causing the structural rearrangement of the Paleozoic–Mesozoic terranes in the Japanese Islands, as well as in East Asia.

Early Cretaceous adakitic magmatism in the Inner Zone of Southwest Japan and its implications for tectonics(<Special issue>Origin of Cretaceous adakitic granitoids in the Japanese Islands)

Early Cretaceous adakitic magmatism in the Inner Zone of Southwest Japan and its implications for tectonics(<Special issue>Origin of Cretaceous adakitic granitoids in the Japanese Islands)

K-Ar age of high magnesium andesite dikes at the Toki River in Gifu Prefecture, central Japan.

A high-magnesium andesite dike swarm intruding into the Jurassic mélange of the Mino-Tamba Belt and the Cretaceous Toki Granite is exposed along the Toki River in central Japan. At the time of intrusion, the direction of the maximum horizontal compressive stress was E-W in this area, as indicated by the predominant direction of the dike swarm. New K-Ar age dating suggests that the intrusive age of the dike swarm is 60-70 Ma. We point out of possibility that high-magnesium andesite magmatism occurred at 60-70 Ma in the eastern part of Inner Zone of Southwest Japan.

A new concept on tectonic correlation between Korea, China and Japan: Histories from the late Proterozoic to Cretaceous

The Dabie–Sulu collision belt in China extends to the Hongseong–Odesan belt in Korea while the Okcheon metamorphic belt in Korea is considered as an extension of the Nanhua rift within the South China block. The Hongseong–Odesan belt divides Korea's Gyeonggi massif into northern and southern portions. The southern Gyeonggi massif and the Yeongnam massif are correlated with China's Yangtze and Cathaysia blocks, respectively, while the northern Gyeonggi massif is part of the southern margin of the North China block. The southern and northern Gyeonggi massifs rifted from the Rodinia supercontinent during the Neoproterozoic, to form the borders of the South China and North China blocks, respectively. Subduction commenced along the southern and eastern borders of the North China block in the Ordovician and continued until a Triassic collision between the North China and South China blocks. While subduction was occurring on the margin of the North China block, high-P/T metamorphic belts and accretionary complexes developed along the inner zone of southwest Japan from the Ordovician to the Permian. During the subduction, the Hida belt in Japan grew as a continental margin or continental arc. Collision between the North and South China blocks began in Korea during the Permian (290–260 Ma), and propagated westwards until the Late Triassic (230–210 Ma) creating the sinistral TanLu fault in China and the dextral fault in the Hida and Hida marginal belt in Japan. Phanerozoic subduction and collision along the southern and western borders of the North China block led to formation of the Qinling–Dabie–Sulu–Hongseong–Hida–Yanji belt.

Provenance of Paleozoic–Mesozoic sedimentary rocks in the Inner Zone of Southwest Japan: An evaluation based on Nd model ages

Nd model ages using depleted mantle (TDM) values for the sedimentary rocks in the Inner Zone of the SW Japan and western area of Tanakura Tectonic Line in the NE Japan allow classification into five categories: 2.6–2.45, 2.3–2.05, 1.9–1.55, 1.45–1.25, and 1.2–0.85 Ga. The provenance of each terrane/belt/district is interpreted on the basis of the TDMs, 147Sm / 144Nd vs. 143Nd / 144Nd relation, Nd isotopic evolution of the source rocks in East China and U–Pb zircon ages. The provenance of 2.6–1.8 Ga rocks, which are reported from Hida–Oki and Renge belts and Kamiaso conglomerates, is inferred to be the Sino–Korean Craton (SKC). The 2.3–1.55 Ga rocks, mostly from Ryoke, Mino and Ashio belts, are originally related with the SKC and/or Yangtze Craton (YC). The provenances of the sedimentary rocks with 1.45–0.85 Ga, from the Suo belt, Higo and some districts in the Mino and Ashio belts, are different from the SKC and YC. Especially, the Higo with 1.2–0.85 Ga is considered as a fragment of collision zone in East China. Akiyoshi belt probably belongs to the youngest age category of 1.2–0.85 Ga. Some metasedimentary rocks from the Ryoke belt have extremely high 147Sm / 144Nd and 143Nd / 144Nd ratios, whose main components are probably derived from mafic igneous rocks within the Ryoke belt itself and from the adjacent Tamba belt.

PERMIAN RADIOLARIAN FAUNAS FROM CHERT IN THE KHABAROVSK COMPLEX, FAR EAST RUSSIA, AND THE AGE OF EACH LITHOLOGIC UNIT OF THE KHABAROVSK COMPLEX

The Khabarovsk Complex, a Jurassic accretionary complex distributed in and around the Khabarovsk city area, Far East Russia, comprises mélange and schist facies. From the review of previous studies including Russian papers, the lithology and age of the constituent rocks of the mélange facies can be summarized as follows: Upper Paleozoic basic volcanic rocks, mainly pillow lava, and altered gabbro, Upper Carboniferous to Upper Permian fusulinoidean-bearing limestone associated with tuff, Lower and Middle Jurassic siliceous mudstone, Upper Jurassic tuffaceous mudstone, uppermost Jurassic carbonate concretions embedded in mudstone, and age-unknown sandstone. Newly found sequences of limestone-chert and of basalt-chert in the mélange facies crop out along the Amur River in the Khabarovsk city area. A chert sample of the limestone-chert sequence contains Albaillella aff. asymmetrica and Pseudoalbaillella aff. lomentaria, and a chert sample of the basalt-chert sequence includes Follicucullus monacanthus, Follicucullus porrectus, and Pseudoalbaillella cf. yanaharaensis. The radiolarian assemblages from the limestone-chert and basalt-chert sequences have a maximum age of middle Early Permian and late Middle Permian, respectively, overlapping the time of deposition of the fusulinoidean-bearing limestone. The co-occurrence of chert and limestone indicates that the fusulinoidean-bearing limestone was formed on a basaltic topographic high in a pelagic ocean whereas the radiolarians accumulated in a deeper part. Limestone debris occasionally flowed into the depositional site of the radiolarian chert. Although the Khabarovsk Complex was simply considered as a northern extension of the Mino–Tamba Belt of the Inner Zone of southwest Japan, we propose a new correlation based on the lithologic associations. The mélange facies of the Khabarovsk Complex is correlative with one of the Kasugano, Funabuseyama, Nabi, and Yabuhara Formations in the Mino–Tamba Belt, whereas the schist facies is correlative with the Hikami Formation of the Ultra–Tamba Belt.

Geochemistry of adakitic quartz diorite in the Yamizo Mountains, central Japan: Implications for Early Cretaceous adakitic magmatism in the inner zone of southwest Japan

Abstract Small‐volume plutons of Early to Late Cretaceous ages are widely distributed in the Yamizo Mountains, central Japan. These plutons consist predominantly of granitoids, classified into hornblende gabbro, quartz diorite, hornblende–biotite granodiorite and coarse‐grained biotite granite. The quartz diorite (52–64 wt% of SiO2) is characterized by a high Sr content (606–769 p.p.m.) associated with a low Y (13–27 p.p.m.) and heavy rare earth element content (Yb content of 1.19–2.13 p.p.m.). On the Sr/Y versus Y diagram, this rock type mainly plots in the adakite and Archean high‐Al tonalite, trondhjemite and granodiorite (TTG) field. Together with its initial Sr isotopic ratios, which range from 0.7038 to 0.7046, these data suggest that quartz diorite originated as slab melts. However, geochemical calculations assuming either eclogite or garnet amphibolite as the source material do not support this suggestion. Instead, the chemical compositions of quartz diorite are better explained by the fractional crystallization of hornblende, plagioclase and biotite from a primitive, basaltic melt in a magma chamber. In this case, the formation of the associated hornblende gabbro can also be explained by the accumulation of hornblende and plagioclase. Adakitic rocks of Early Cretaceous ages have also been reported in the Tamba Belt of the inner zone of southwest Japan, located ca 500 km west of the Yamizo Mountains. These rocks can be correlated to the adakitic rocks in the Yamizo Mountains based on the geology, petrography, geochemistry and radiometric ages. Therefore, we propose the possibility that the Early Cretaceous adakitic rocks in the inner zone of southwest Japan were produced by fractional crystallization from basaltic arc magmas generated by a partial melting of metasomatized wedge mantle peridotite.

Asymmetric diagenetic changes in a half-graben basin: the Kanmon Group (Lower Cretaceous), SW Japan

The Kanmon Group (Lower Cretaceous) is a nonmarine sedimentary succession in the Inner Zone of south-west Japan. It was deposited in a basin having a half-graben structure with major depocenters on the west side of the bounding fault. The degree of some diagenetic changes of the Kanmon Group sandstones is largely dependent on the depositional position in an asymmetric basin, i.e., the distance from the basin-bounding fault. Illite crystallinity values and compaction of Kanmon Group sediments in the vicinity of the boundary fault are much lower (more thermally altered) and more severe, respectively, than those away from it. In addition, calcite phases near the fault are more diverse and complex. These results indicate that the boundary fault system strongly influenced diagenetic changes in the Kanmon Basin. The basin-bounding fault is considered to have opened episodically by tensional stress caused by oblique subduction of the proto-Pacific plates (Izanagi and Kula plates) throughout basin evolution, and it was used as a pathway for diagenetic fluids, causing more extensive and advanced mineralogical alterations.

Internal and permeability structures of Pre-Cretaceous nappe boundaries in the Inner Zone of Southwest Japan

福井県小浜市下加斗片地域に露出している丹波帯と超丹波帯のナップ境界断層帯の内部構造と浸透率構造を調べた．その結果，（1）両地質体は約3 ｍのカタクレーサイト帯を介して完全に固着して接しており，変形は幅0.2～3 cmの面状カタクレーサイトに集中していること，（2）断層帯では，石英や方解石，リモナイト脈，炭質物の濃集層のほか，多量の層状珪酸塩鉱物が晶出しており，流体を媒介とした活発な物質移動があったこと，（3） 本ナップ境界では，上盤に低い浸透率を示す砂岩層，下盤および断層帯ではそれより1～3桁高い浸透率を示す礫質泥岩層およびカタクレーサイト帯が分布していることがわかった．このような浸透率構造によって，断層帯内部に高間隙水圧が保持され，ナップの大規模な衝上運動が可能になったと考えられる．

Lithofacies and eruption ages of Late Cretaceous caldera volcanoes in the Himeji–Yamasaki district, southwest Japan: Implications for ancient large‐scale felsic arc volcanism

Abstract The Himeji–Yamasaki region in the Inner Zone of southwest Japan is underlain mainly by Late Cretaceous volcanic rocks called the Ikuno Group or the Hiromine and Aioi Groups. A new stratigraphic and geochronological study shows that the volcanic rocks in this area consist of 15 eroded caldera volcanoes between 82 and 65 Ma; they are, in order of decreasing age, the Hiromine, Hoden, Ibo, Okawachi, Seppikosan, Hayashida, Shinokubi, Fukusaki, Kurooyama, Ise, Fukadanigawa, Nagusayama, Matobayama, Yumesaki and Mineyama Formations. These calderas vary in diameter from 1 to 20 km and are bounded by steep unconformities; they coalesce and overlap each other. The individual caldera fills are composed mainly of single voluminous pyroclastic flow deposits, which are often interleaved with debris avalanche deposits and occasionally underlie lacustrine deposits. The intracaldera pyroclastic flow deposits are made up of massive, welded or non‐welded tuff breccia to lapilli tuff, and are characterized by their great thickness. The debris avalanche deposits are ill‐sorted breccia, generated by the collapse of the caldera wall toward the caldera floor during the pyroclastic‐flow eruption. The large calderas that are more than 10 km in diameter contain original values of approximately 100 km3 of intracaldera pyroclastic flow deposits. These large calderas are similar to the well‐known Valles‐type calderas in their dimensions, although it is uncertain whether their caldera floors are coherent plates or incoherent pieces. Conversely, the small calderas have diatreme‐like subsurface structures. The variety of the caldera volcanoes in this area is caused by the difference in the volume of caldera‐forming pyroclastic eruptions, as the large and small calderas coexisted. The caldera‐forming eruption rates in Late Cretaceous southwest Japan, including the studied area, were similar to those in late Cenozoic central Andes and northeast Honshu arc, Japan, but obviously smaller than those of late Cenozoic intracratonic caldera clusters in western North America and the Quaternary extensional volcanic arcs in Taupo, New Zealand. The widespread Late Cretaceous felsic igneous rocks in southwest Japan were generated by a long‐term accumulation of low‐rate granitic magmatism at the eastern margin of the Eurasian Plate.

Palaeogeographic constraints on the tectonic evolution of the Maizuru Terrane of Southwest Japan to the eastern continental margin of South China during the Permian and Triassic

The Maizuru Terrane in the Inner Zone of Southwest Japan consists of weakly metamorphosed ophiolitic complexes and Permian and Triassic strata. The Permian Maizuru Group is 1500–3000 m thick and consists mainly of mudstone, sandstone, and alternating beds of sandstone and mudstone. It is subdivided into lower, middle, and upper formations, and is overlain with angular unconformity by the Lower–Middle Triassic Yakuno Group. The Maizuru Group is characterised by the occurrence of the Capitanian Lepidolina kumaensis fauna in conglomerates of the middle and upper formations, and the Wuchiapingian Codonofusiella– Colaniella and Changhsingian Palaeofusulina– Colaniella faunas in lenticular limestones and conglomerates of the upper formation. Pre-Capitanian foraminifera are contained in limestone clasts of conglomerates of the upper formation and the basal part of the Yakuno Group. Examination on the stratigraphy, lithology, foraminiferal fauna, and Late Palaeozoic to Mesozoic tectonics in and around the Maizuru Terrane leads to the following conclusions. Capitanian and Lopingian limestone blocks and clasts were originally deposited on the continental shelf, and were subsequently redeposited on the deeper continental slope. Almost all of the pre-Capitanian limestone clasts were derived from the Akiyoshi Seamount accreted by the early Late Permian, except for those containing Verella prolixa and Pseudostaffella praegorski which were reworked from the shallow continental shelf. The Maizuru Terrane originated on the active continental margin located close to South China during Permian–Triassic time, but the depositional environment of the Permian and Triassic strata rapidly changed from deeper continental slope to shallow continental shelf.

鉱化花崗岩特性（I）: 西南日本内帯のモリブデンとタングステン鉱床生成区

花崗岩類の化学的性質と関連鉱床との成因的関係を追及する目的で，Mo鉱床生成区と Sn-W鉱床生成区，さらに不毛区から採取した47試料にていて偏光蛍光エックス線解析法で分析した。Mo鉱床生成区の花崗岩類はAl2O3，アルミナ飽和指数(ASI)，Ga ，K2O，Rbで最も低い。苦鉄質成分ではV，Cr，Zn，Coは鉄の2価，3価の規制を受けて，その含有量が変化する．微量成分としてのMoはMo鉱床生成区で高く，不毛区で最低である。従って，これは同鉱床探査の岩石地化学探査の指示元素として使いうる．WはSn-W 鉱床生成区で高く，以外にもMo鉱床生成区でも高い。微量のW, Snは，Sn-W鉱床生成区における鉱床探査指示元素として使いうる． 今回の分析試料は1969年代に湿式分析（ロ－レン・スミス法）したものであり，そのため両分析方法による比較を試みた。相関係数が低いものは，MnO (0.83)， P2O5(0.85)，Al2O3(0.87)， Na2O(0.88)で認められた．両分析法の一致線からの隔たりが大きいものは，P2O5(0.79)， MnO(0.83)，K2O(1.15) であり，湿式分析法のカリウムが低くでる．一方，CaO とSiO2 さらには原子吸光法による微量Snについては，良い一致性を示した．

西南日本内帯の白亜紀−古第三紀花崗岩類の成因に対する酸素同位体組成からの束縛条件

西南日本内帯の白亜紀－古第三紀花崗岩類の成因を明らかにするために全岩130試料，石英7試料の酸素同位体組成（SMOWに対するδ18O値）を測定した．その値は磁鉄鉱系花崗岩類では6 ‰以上，チタン鉄鉱系では9 ‰以上の場合，二次的変化を受けていない，初生的な値と判断される．δ18O値は磁鉄鉱系で低く，チタン鉄鉱系で高い．磁鉄鉱系の山陰－白川帯では，白川地域で5.9～8.1 ‰, 奥丹後－鳥取東部で6.9～10.6 ‰，三朝－上斉原地域で6.0～8.2 ‰である．一方，チタン鉄鉱系花崗岩類では，山陽－苗木帯の土岐花崗岩体（無鉱化）で9.2～9.8 ‰，タングステン鉱化を伴う苗木花崗岩体で7.4～8.1 ‰であるが，京都の大谷鉱山岩株では逆に11.7～12.8と高い．近畿－中国地方のチタン鉄鉱系花崗岩類は一般に7.3～10.8 ‰の値を持つが，山口県東部のみが11.6～12.0 ‰と例外的に高い．この花崗岩類はその北方のタングステンスカルン鉱床と関係している可能性がある．領家帯の花崗岩類は一般に高いδ18O値を持つ．中部地方のI帯のIタイプ花崗岩類は9.0～10.9 ‰，IIとIII帯のIタイプ花崗岩類が9.1～12.1 ‰，Sタイプ花崗岩類が10.5～12.5 ‰である． 測定値を八丈島の第四紀火山岩類のトレンドに合わせてSiO2 70 %に規格化したδ18O値を求めて広域的分布を見ると，磁鉄鉱系が主として分布する日本海側で低い値(< 8 ‰ δ18O)が多く，全般に高い(> 8 ‰ δ18O)山陽－苗木帯と領家帯で幾つかの極大が認められる（第７図）．δ18O値 －87Sr/86Sr初生比図上で磁鉄鉱系花崗岩類はδ18O値が低く，87Sr/86Sr初生比が中間的な領域を占める．この事実はこれら花崗岩類が大陸地殻中－下部の火成岩類に主たる起源を有することを示している．他方，チタン鉄鉱系は両同位体比とも一般に高い領域に分布し，大部分は領家変成岩類の平均値と第四紀玄武岩を結んだ領域を占める．この事実は，岩体中に苦鉄質アンクラーヴが多いことと合わせ て，Iタイプ花崗岩類はマントルからの苦鉄質マグマと地殻深部まで折り畳まれたであろう付加体を起源物質としたことを示している．堆積岩源物質の比率は一般には20～30 %と考えられる．一方，Sタイプ花崗岩類は苦鉄質包有物を含まず，堆積岩源物質50 %程度で酸素同位体的に均質化した大陸地殻物質の部分溶融で生じたものと考えられる．白川・土岐・奥丹後などの87Sr/86Sr初生比が高い地域の深部には古期基盤の存在が予想される.

Eruptive system of the Irohazaka Welded Tuffs and correlation to the Late Cretaceous igneous activity of the Inner Zone of Southwest Japan

Eruptive system of the Irohazaka Welded Tuffs and correlation to the Late Cretaceous igneous activity of the Inner Zone of Southwest Japan

Diagenesis of the Lower Cretaceous Kanmon Group sandstones, SW Japan

The Kanmon Group (Lower Cretaceous) is a non-marine sequence in the Inner Zone of southwest Japan and is divided into the lower Wakino (lacustrine) and the upper Shimonoseki (fluvial) subgroups. Major diagenetic changes in this group are compaction, iron-oxide cementation, calcite cementation and grain replacement, quartz overgrowth and pore-fill cementation, illite authigenesis, chlorite pore-fill cementation and grain replacement, albitization of feldspar, and grain replacement by pyrite. Two subgroups of the Kanmon Group present no significant differences in general diagenetic features, paragenetic sequence, or the degree of diagenetic changes despite differences in depositional environments (lacustrine vs. fluvial) and stratigraphic positions. However, some differences are recognized in the content and chemistry of authigenic minerals caused by different sandstone framework compositions. The content of authigenic clay minerals is higher in sandstones of the Shimonoseki Subgroup containing abundant volcanic rock fragments. In addition, the composition of chlorite, the most abundant authigenic clay mineral in Kanmon sandstones, is Mg-rich in the volcanoclastic Shimonoseki sandstones, compared to an Fe-rich variety in Wakino sandstones. The original sandstone composition played a significant role in pore-water composition and diagenetic reactions. The Wakino sandstones lost most of its porosity by compaction, whereas Shimonoseki sandstones are only compacted in the vicinity of the basin-bounding fault. The weakly compacted Shimonoseki sandstones, instead, were largely cemented by pore-filling calcite during early diagenesis; cementation prevented compaction during further burial. The Kanmon Group sediments were heated to about 300 °C based on illite crystallinity values.

Sources and Depositional Environments of Some Permian and Triassic Cherts: Significance of Rb‐Sr and Sm‐Nd Isotopic and REE Abundance Data

Rb‐Sr and Sm‐Nd isotopic data, rare earth element (REE ) abundances, and major‐element compositions are reported for the Triassic cherts of the Mino Terrane in the Inner Zone of southwest Japan and for the Permian and Triassic cherts of the Sambosan Terrane in the Outer Zone of southwest Japan. Rb‐Sr isotopic data of the Mino and Sambosan cherts define separate isochron lines, and the Rb‐Sr ages of ca. 210 and 240 Ma reflect the chert deposition and the end of the subsequent chemical diagenesis during which amorphous silica was transformed into quartz. It is concluded that the Rb‐Sr isotopic system of the cherts becomes essentially homogenized among biogenic silica, detrital components and hydrogeneous components during deposition, and the subsequent chemical diagenesis before lithification due to high mobility of Rb and Sr. The Sr initial ratio (0.71363) of the Triassic Mino cherts is clearly higher than those (0.7079 and 0.7068) of the Triassic and Permian Sambosan cherts, which are close to the estimated oceanic Sr ratios of 0.7066–0.7081 in the Permian and Triassic. On the other hand, Sm‐Nd isotopic data do not define isochron lines. This might be due to incomplete homogenization of the system among biogenic silica, detrital components and hydrogeneous components during deposition, and chemical diagenesis because of low mobility of REE. Initial Sr and Nd isotopic ratios of the Mino cherts can be interpreted by mixing typical continental crustal rocks, represented by aeolian loess compositions, with a small amount of Precambrian rocks (<10%), while those of the Sambosan cherts can be explained by mixing continental crustal rocks with oceanic volcanic rocks such as MORB (mid‐ocean ridge basalt). In the mixing model between Chinese loess and MORB, a proportion of loess is ca. 30%–60% for the Middle Permian Sambosan cherts and ca. 60%–90% for the Triassic Sambosan cherts. Al2O3/(Al2O3 + Fe 2O3) ratios and Ce anomalies in the REE patterns also show a higher contribution of continental components in the Triassic Sambosan and Mino cherts than the Permian Sambosan cherts, suggesting deposition in a marginal sea or on the continental shelf and slope for the Triassic Sambosan and Mino cherts. Our study shows that the geochemical and isotopic features of the Mino and Sambosan cherts essentially retain the nature of their formation and subsequent chemical diagenesis before lithification. Alteration or metamorphic effects during subduction and accretion processes cannot be found in the chemical and isotopic features of these cherts. Our result requires the reexamination of the previous paleogeographical reconstructions of the Japanese Islands as well as the relationship between the Mino and Sambosan Terranes in Permian and Triassic time.

A correlation of accretionary complexes of southern Sikhote-Alin of Russia and the Inner Zone of Southwest Japan

This paper briefly describes the geology of southern Sikhote-Alin of Russia and the Inner zone of Southwest Japan, and presents a new correlation model in which the Samarka terrane (sensu stricto), Udeka Formation, Sebuchar Formation and Kalinovka ophiolite in Sikhote-Alin are considered as northern extensions of the Mino-Tamba terrane, Hikami Formation, Kozuki Formation, and Yakuno ophiolite in Southwest Japan, respectively. This correlation is based on the similarities in lithology, age, faunal assemblage, and structural relationship between them. The Samarka and Mino-Tamba terranes consists of Jurassic accretionary complexes including Permian greenstone-limestone complex, Permian to Lower Jurassic radiolarian bedded chert, Jurassic clastic rocks, and Jurassic melanges. The Udeka and Hikami formations are composed mainly of greenish gray sandstone with minor shale intercalations yielding Permian radiolarians. The Sebuchar and Kozuki formations are characterized by basalts accompanied with shale, limestone and chert. The limestone includes Carboniferous fusulinaceans, while Permian radiolarians occur in the chert. The Kalinovka and Yakuno ophiolites consist of a series of ultramafic to mafic igneous rocks with an ophiolitic succession. Since the ages of these ophiolites have not been clearly established, we correlate them on the basis of their lithology and tectonic positions. These units form a stack of nappes from the lower to upper horizons in the following order: Mino-Tamba terrane, Hikami Formation, Kozuki Formation, and Yakuno ophiolite in Japan, and Samarka terrane (sensu stricto), Udeka Formation, Sebuchar Formation, and Kalinovka ophiolite in Russia. This correlation supports the reconstruction model of Japan before the opening of the Sea of Japan proposed by Yamakita and Otoh (1999).

Geochemical characteristics of Carboniferous greenstones in the Inner Zone of Southwest Japan

Abstract Greenstones, representing remnants of paleo‐oceanic crust, occur in Permian and Jurassic accretionary complexes of the Inner Zone in the Southwestern Japan arc. The formation age of most of the greenstones is early Carboniferous, based on fossil ages for overlying limestones and Sm‐Nd isotope ages of the greenstones themselves. The geochemistry of such greenstones is similar to those of present‐day oceanic islands. Greenstones of the Permian accretionary complex (Akiyoshi belt) are alkalic and tholeiitic in composition. Some alkali basalts show peculiar features from an EM‐1 mantle source, such as the Gough Island and Tristan da Chunha basalts in the South Atlantic. Greenstones of the Jurassic accretionary complex (Tamba belt) are also alkali and tholeiitic basalts with both basalt types in the northern part of the Tamba belt coming from strongly depleted characters similar to a mid‐ocean ridge basalt source mantle. The variable geochemistry of the oceanic basalts is explained by hypothesis on existence of a Carboniferous mantle plume below the spreading ridge which divides the Farallon and Izanagi plates. The Akiyoshi belt seamounts and/or oceanic islands of the Farallon plate and Tamba belt seamounts and/or oceanic islands of the Izanagi plate formed simultaneously by the upwelling of the thermal plume. Some part of the Akiyoshi belt basalts originated locally from an EM‐1 mantle source, while basalts from the northern parts of the Tamba belt have a normal‐type mid‐ocean ridge basalt (N‐MORB) source component. Existence of an N‐MORB signature is consistent with the presence of a spreading center in a Carboniferous ‘Pacific Ocean’ that caused separation of the Farallon and Izanagi plates. Disparity in accretion ages of the basaltic rocks in the Permian and Jurassic may have been caused by differences in the relative motion of the two plates.

Provenance of sandstones from the Lower Cretaceous Sasayama Group, Inner Zone of Southwest Japan

Abstract Modal analysis and phase chemistry of sandstones of the Lower Cretaceous Sasayama Group, Southwest Japan, indicate that the detritus were derived from heterogeneous sources that include medium- to high-grade metamorphic, mafic–ultramafic, and acidic magmatic rocks. Garnet geochemistry suggests their derivation from epidote-amphibolite to granulite facies metamorphic rocks. Sphene and chlorite geochemistry suggests their derivation from mafic rocks. Chemical compositions of chromian spinel indicate input from Alpine-type peridotite. The chemistry of the chromian spinel further suggests that the Alpine-type peridotite source was most probably formed under an oceanic back-arc basin setting rather than mid-oceanic ridge setting. Chemical compositions of chromian magnetite indicate that the Alpine-type peridotite provenance was subjected to metamorphism of up to the lower amphibolite facies prior to erosion and transport. The metamorphic and granitic rocks of the Hida–Oki Terrane, ophiolitic rocks of the Sangun–Renge Terrane, and rhyolitic rocks from uncertain terrane (Akiyoshi?) are most probably the major source of detritus to the Sasayama basin. The basin is most likely a strike–slip basin that formed along the eastern margin of the Asian continent.

Geochemistry of Lower Cretaceous sediments, Inner Zone of Southwest Japan. Constraints on provenance and tectonic environment.

A geochemical study was carried out on Lower Cretaceous sedimentary rocks of the Wakino Subgroup, Kenseki Formation, and Sasayama Group, distributed in the Inner Zone of Southwest Japan. The chemical characteristics of the Lower Cretaceous sediments indicate that these rocks are immature first-order sediments derived from igneous and/or meta-igneous rocks of predominantly felsic composition. The sediments from the Kenseki Formation and the Sasayama Group, however, show high Cr and Ni abundances, suggesting a significant contribution of detritus from ultramafic rocks. Weathering at the source areas was moderate. The high Th/U ratios (mostly > 3.8), negative Eu anomalies (Eu/Eu∗ between 0.67 and 0.93) and Th/Sc ratios (mostly between 0.5 and 1) of the Lower Cretaceous sediments suggest their derivation dominantly from an old upper crust with minor amounts of young arc-derived detritus. The major, trace and rare earth element compositions imply that deposition took place in an active continental margin environment. The small amounts of young arc-derived material in the sediments support the inference by other workers that arc magmatism was not so prominent in Southwest Japan during the early Cretaceous.