Geological relationship between Anyui ... - Wiley Online Library

Island Arc (2008) 17, 502–516

Thematic Articles Geological relationship between Anyui Metamorphic Complex and Samarka terrane, Far East Russia SATORU KOJIMA,1* KAZUHIRO TSUKADA,2 SHIGERU OTOH,3 SATOSHI YAMAKITA,4 MASAYUKI EHIRO,5 CHEIKHNA DIA,1 GALINA LEONTIEVNA KIRILLOVA,6 VLADIMIR AKIMOVICH DYMOVICH7 AND LYUDMILA PETROVNA EICHWALD7 1

Department of Civil Engineering, Gifu University, Gifu 501-1193, Japan (email: [email protected]), 2Nagoya University Museum, Nagoya 464-8601, Japan, 3Graduate School of Science and Engineering, University of Toyama, Toyama 930-8555, Japan, 4Department of Geology, Faculty of Education and Culture, Miyazaki University, Miyazaki 889-2192, Japan, 5The Tohoku University Museum, Sendai 980-8578, Japan, 6Institute of Tectonics and Geophysics, Far Eastern Branch of the Russian Academy of Sciences, Khabarovsk 680063, Russia, and 7State Enterprise Dalgeophysics, Khabarovsk 680000, Russia

Abstract The Anyui Metamorphic Complex (AMC) of Cretaceous age is composed of metachert, schist, gneiss, migmatite and ultramafic rocks, and forms a dome structure within the northernmost part of the Jurassic accretionary complex of the Samarka terrane. The two adjacent geological units are bounded by a fault, but the gradual changes of grain size and crystallinity index of quartz in chert and metachert of the Samarka terrane and the AMC, together with the gradual lithological change, indicate that at least parts of the AMC are metamorphic equivalents of the Samarka rocks. Radiolarian fossils from siliceous mudstone of the Samarka terrane indicates Tithonian age (uppermost Jurassic), and hence, form a slightly later accretion. This signifies that the accretionary complex in the study area is one of the youngest tectonostratigraphic units of the Samarka terrane. The relationship between the Samarka terrane and AMC, as well as their ages and lithologies, are similar to those of the Tamba–Mino–Ashio terrane and Ryoke Metamorphic Complex in southwest Japan. In both areas the lower (younger) part of the Jurassic accretionary complexes were intruded and metamorphosed by Late Cretaceous granitic magma. Crustal development of the Pacific-type orogen has been achieved by the cycle of: (i) accretion of oceanic materials and turbidites derived from the continent; and (ii) granitic intrusion by the next subduction and accretion events, accompanied by formation of high T/P metamorphic complexes. Key words: Anyui Metamorphic Complex, crystallinity index, Jurassic accretionary complex, low-grade metamorphism, radiolaria, Samarka terrane, Sikhote–Alin.

INTRODUCTION Recent geological, tectonic, and micropaleontological studies have clarified that the pre-Cenozoic geotectonic units in the Sikhote–Alin Mountains consist mainly of Precambrian microcontinents, Paleozoic–Mesozoic accretionary-complex terranes, and Cretaceous turbidite basins (Kojima *Correspondence. Received 28 April 2007; accepted for publication 28 August 2008. © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd

1989; Natal’in 1993; Khanchuk et al. 1996; Kojima & Kametaka 2000; Kojima et al. 2000; Khanchuk 2001; Ishiwatari & Tsujimori 2003; Ernst et al. 2007). The Samarka terrane (Fig. 1), considered to be a Jurassic accretionary-complex terrane, is composed of volcanic and sedimentary rocks such as Late Paleozoic basalt, limestone and chert; Triassic to Jurassic chert; Jurassic clastic rocks; and Jurassic mélanges with blocks of the older rocks. The Samarka terrane formed as a part of oceanic plates, and/or was accumulated on the plates and doi:10.1111/j.1440-1738.2008.00644.x

Origin of Anyui metamorphic complex

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(i) Martyniouk et al. (1991) regard the complex as Precambrian or Early–Middle Paleozoic basement of the Mesozoic formations of the Samarka terrane; and (ii) Faure et al. (1995) and Natal’In et al. (1995) consider the unit as a microcontinent collided with Asia in Aptian time. Based on an east–west geological traverse of the Sikhote–Alin Mountains about 49°N and observation of the lithology of the northernmost part of the Samarka terrane and the southern part of the AMC (Fig. 2), this paper: (i) presents the radiolarian ages of siliceous rocks in the Samarka terrane; (ii) proposes metamorphic continuity between the Samarka terrane and the AMC based on the quartz grain size and crystallinity index (CI) of chert and metachert; and (iii) presents considerations on the relationship between these two geological units, origin of the AMC, and the similarity between the AMC in the Sikhote–Alin Mountains and the Ryoke Metamorphic Complex in southwest Japan.

GEOLOGICAL SETTING

Fig. 1 Index map showing the distribution of Jurassic to Early Cretaceous accretionary complexes (1), Jurassic accretionary complexes (2), and Cretaceous high T/P metamorphic rocks (3) in Japan and Northeast Asia (modified from Isozaki & Maruyama 1991; Natal’in 1993; Isozaki 1996). Cretaceous metamorphic rocks in Japan are considered to be metamorphic equivalents of the Jurassic accretionary complexes. Open square shows the area of Figure 2. Ab, Abukuma Belt; As, Ashio terrane; B, Badzhal terrane; C, Chichibu terrane; CSF, Central Sikhote–Alin Fault; ISTL, Itoigawa–Shizuoka Tectonic Line; K, North Kitakami terrane; M, Mino terrane; MTL, Median Tectonic Line; N, Nadanhada terrane; R, Ryoke Belt; S, Samarka terrane; Tb, Tamba terrane; Th, Taukha terrane.

then subducted and accreted along the continental margin of East Asia. This terrane occupies the central area of the Sikhote–Alin Mountains ranging from north of Vladivostok to northeast of Khabarovsk and west of the Central Sikhote–Alin Fault. Similar accretionary complexes to the east of the Central Sikhote–Alin Fault, which are studied in this paper, are also regarded as part of the Samarka terrane (Natal’in 1993; Khanchuk et al. 1996). The Anyui (or Anuy) Metamorphic Complex (AMC) occurs in the northernmost part of the Samarka terrane. There are two interpretations of the origin of this metamorphic unit:

The southern part of the Sikhote–Alin Mountains are underlain by two principal tectonostratigraphic units: the Khanka superterrane composed of Precambrian continental basement covered by thick Lower Paleozoic sedimentary formations, and the Sikhote–Alin superterrane made up mainly of accretionary complexes (Khanchuk et al. 1996). The Sikhote–Alin superterrane is subdivided into the Samarka terrane (Jurassic accretionary complex), Taukha terrane (Early Cretaceous accretionary complex), Zhuravlevka terrane (Early Cretaceous turbidite basin) and Kema terrane (Aptian–Albian island arc). Natal’in (1993) distinguished the narrow Kiselevka–Manoma terrane (Early Cretaceous accretionary complex) on the western margin of the Samarka terrane in the northern part of the Sikhote–Alin Mountains. The Udeka Formation and the Sebuchar Formation, which are regarded as Permian accretionary complexes by Kojima et al. (2000), are also part of the Sikhote–Alin superterrane. The Samarka and Zhuravlevka terranes occur along the traverse route from west to east (Fig. 2). The boundary between the two terranes is a fault, although several Triassic chert sheets most probably derived from the Samarka terrane are sandwiched between the turbidite formations of the Zhuravlevka terrane near the boundary (Fig. 2). The AMC occurs within the Samarka terrane. The © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd

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Fig. 2 (a) Geological map, and (b) geological cross-section of the Anyui area (modified from Faure et al. 1995 and Vaskin et al. 2007). The map area is shown in Figure 1. C.S.F., Central Sikhote–Alin Fault. Locality of the sample numbered as 1 for grain size and crystallinity index measurements is about 25 km west of the Central Sikhote–Alin Fault. Numbers 1–14 correspond to localities in Table 1.

Central Sikhote–Alin Fault, one of the major strike–slip faults in East Asia, cuts the western part of the Samarka terrane. To the west of the Samarka terrane is the Early Cretaceous Kiselevka–Manoma terrane (Natal’in 1993; © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd

Kirillova & Khanchuk 2002), about which Popova et al. (1999) and Philippov (2001) conducted detailed radiolarian biostratigraphical studies in an area approximately 15 km west of the Central Sikhote–Alin Fault.

Origin of Anyui metamorphic complex SAMARKA TERRANE

Vaskin et al. (2007) divided the Samarka terrane in this region into the western Khor–Tormasu zone and the eastern Anyui zone on the basis of the difference in lithology and age, although the boundary between the two zones is not clear (Fig. 2). The AMC is distributed in the Anyui zone. Each zone is then subdivided into several tectonostratigraphic units also on the basis of lithology and age, all of which are unconformably covered by Berriasian–Valanginian turbidites rich in molluscan fossils like Buchia. The geological descriptions below are mostly based on Vaskin et al. (2007). The Yackchi, Tomchi, Tormasu, Hasami, and Kadadi units occur in the Khor–Tormasu zone (Fig. 2). These units are composed of basalt, chert, siliceous mudstone, mudstone, sandstone, conglomerate, and mélanges. Late Permian to Late Triassic conodonts and radiolarians were found from the chert, while Middle to Late Jurassic radiolarians were reported from the clastic rocks. Late Triassic bivalves like Halobia sp. and Monotis ochotica, and Early to Middle Jurassic molluscs were also found in the siltstone and sandstone. The accretionary complexes in the Anyui zone are subdivided into the Dzhaur, Sanga, and Svetlorechensky units. These three units are composed of chert, siltstone, mudstone, sandstone, basalt, and limestone. Volokhin et al. (2000, 2003) made detailed conodont biostratigraphical studies on the chert formations of the Dzhaur unit and revealed that the Triassic succession more than 200 m in thickness ranges in age from the middle Anisian to early Norian. Middle Jurassic radiolarians and several species of Buchia indicating Tithonian age (Late Jurassic) were also found from these units. ANYUI METAMORPHIC COMPLEX (AMC)

The AMC forms a dome structure (~10 km ¥ 20 km); the complex is tectonically covered by the rocks of the Samarka terrane (Fig. 2). The metamorphic complex had been considered to be Precambrian or Early to Middle Paleozoic in age (Martyniouk et al. 1991). However, Faure et al. (1995) and Natal’in et al. (1995) measured Ar–Ar ages (cooling ages) of biotite, muscovite, and amphibole and showed minimum ages of two stages of metamorphism and deformation as 110 and 73–58 Ma. According to Faure et al. (1995), the AMC is subdivided into four tectonostratigraphic units. These are, from lower to upper: (i) metasandstone–

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phyllite unit; (ii) mica schist–gneiss–quartzite unit; (iii) migmatite unit; and (iv) ophiolitic nappe unit (Fig. 2). Faure et al. (1995) discussed that units (i)–(iii) were part of a microcontinent onto which unit (iv) was obducted in Late Jurassic–Berriasian time, and then the units collided with Asia in Aptian time. All units were affected by the two metamorphic and deformational events described above. The AMC along our traverse route (Fig. 2) consists of metachert, schist, and gneiss, and seems to be part of units (ii) and (iv). We could not examine the rocks of units (i) and (iii) because of lack of exposure. Most of the schists are psammitic, and the gneiss characteristically includes lenses of metachert. Samples analyzed in this study seem to be metachert overlying the ophiolitic rocks of unit (iv), and metachert of unit (ii) described as quartzite by Faure et al. (1995). Gradual change in metamorphic intensity can be observed in the field from the unmetamorphosed bedded chert, shale, and mélanges of the Samarka terrane to the highly metamorphosed metachert, mica schist, and gneiss with quartzite blocks of the AMC. This observation suggests that the protoliths of the AMC could be the rocks of the Samarka terrane. ZHURAVLEVKA TERRANE

The most complete Early Cretaceous terrigenous sequences in the Sikhote–Alin Mountains are observed in the Zhuravlevka terrane, ranging from late Berriasian to middle Albian in age, and are approximately 12 km in thickness (Kirillova 2002). Rocks of the Zhuravlevka terrane are composed mostly of turbidites, which yield Early Cretaceous molluscan fossils. They are intensely folded and faulted, and sandwich the slivers of chert probably derived from the Samarka terrane near its boundary (Figs 2,3). CRETACEOUS TO PALEOGENE GRANITE

All the pre-Middle Cretaceous rocks in the study area are intruded by Middle Cretaceous to Paleogene granitic rocks, and are unconformably covered by Late Cretaceous volcanic rocks, both of which are widely distributed in the central part of the Sikhote–Alin Mountains. Faure et al. (1995) described the granitic rocks in Anyui area as aluminous and characteristically including muscovite, cordierite, and garnet. They have numerous xenoliths of sedimentary rocks, hornfels, metamorphic rocks, cataclastic and amphibolized gabbro, and © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd

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Fig. 3 Geological sketch map of the Triassic chert at locality L46. Although the chert formations are folded, faulted, and intruded by the felsic dike, radiolarian ages are younging to the northwest. The locality of this outcrop is shown in Figure 2.

Fig. 4 Triassic conodonts and radiolarians obtained from chert in the Anyui area. Sample localities are shown in Figures 2 and 3.

serpentinite. Faure et al. (1995) also reported biotite Ar–Ar age of 106.8 Ma on the granite, which is concordant with the first metamorphic and deformational event of the AMC.

AGE OF CHERT AND SILICEOUS MUDSTONE We collected nineteen chert and five siliceous mudstone samples from the Samarka terrane distributed along the traverse. Among these, eleven chert and three siliceous mudstone samples yield well-preserved radiolarian and conodont fossils (Figs 2,3). The siliceous rocks and the related sandstone formations represent a chert–clastics sequence: the radiolarian chert at the bottom © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd

grades upward (northwestward) into siliceous mudstone and then turbidite. Although the Triassic part of the sequence was studied in detail by Volokhin et al. (2000, 2003), the age of the upper part of the sequence has not been determined in the study area. AGE OF CHERT

Some of the chert formations include conodonts which indicate pre-Jurassic age, and most of the cherts contain radiolarian fossils. They are usually poorly preserved due to the thermal effects of Cretaceous–Paleogene granitic rocks. Conodonts from sample L21, however, can be identified as Norigondolella navicula of early Norian age (Fig. 4).


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Fig. 5 Representative Triassic radiolarian and conodont fossils obtained from chert in the Anyui area. 1, Archaeosemantis sp. (L46-G); 2, Hozmadia sp. (L46-G); 3, Hozmadia sp. (L46-A); 4, Pentactinocarpus fusiformis (L46-C); 5, Eptingium manfredi (L46-D); 6, Spinotriassocampe annulata (L46-C); 7, Yeharaia elegans group (L46-E); 8, 9, Oertlispongus diacanthus (L46-G); 10, Pantanellium (?) virgeum (L46-G); 11, Pantanellium (?) virgeum (L46-A); 12, 13, Pseudostylosphaera japonica group (L46-C); 14, Pseudostylosphaera sp. (L46-E); 15, Triassocampe sp. (L46-F); 16, 17, Triassocampe deweveri (L46-C); 18, Triassocampe sp. (L46-C); 19, Spinotriassocampe sp. cf. S. hungarica group (L46-B); 20, Neogondollella bulgarica (L46-A). Scale bar for image 1, 0.041 mm; 2, 0.046 mm; 3, 6, 8, 10, 11, 16, 17, 18 and 20, 0.067 mm; 4, 5, 0.11 mm; 7, 0.051 mm; 9, 15, 0.081 mm; 12, 13 and 14, 0.1 mm; 19, 0.054 mm.

The thrust sheet of chert at L46 (Figs 2,3) intercalated by the Lower Cretaceous turbidite formations of the Zhuravlevka terrane includes well-preserved Triassic radiolarians. Radiolarian assemblages of the samples L46-G and L46-A are characterized by the occurrence of Oertlispongus diacanthus, Pantanellium (?) virgeum, and Hozmadia spp. (Figs 4,5), which were reported from the lower Anisian (Sashida 1991; Sugiyama 1992, 1997; Sashida et al. 1998). Samples L46-F, B, C, D and E range in age from the middle Anisian to middle Ladinian, since the occurrence of Triassocampe deweveri from these samples is limited in this interval (Sugiyama 1997). Pentactinocarpus fusiformis from sample L46-C is known to occur in the lower Ladinian, and the Yeharaia elegans group from sample L46-E was reported from the lower to middle Ladinian (Dumitrica 1978; Sugiyama 1997). The chert formations at the locality of L46, although folded and faulted, are generally younging northwestward from the lower Anisian

to lower or middle Ladinian, and the facing and younging trend is consistent with that of the Samarka terrane. Although the cherts are distributed in the Zhuravlevka terrane, the lithological and age similarities with those in the Samarka terrane indicate they are members of the Samarka terrane. Part of the ocean plate stratigraphy of the Samarka terrane can be reconstructed by chert formation data at locality L46. The chert sample L35-C yields Acanthocircus sp. cf. A. suboblongus and Parahsuum sp. (Figs 6,7); the former species is indicative of the Middle to Late Jurassic and the latter species is common in the Early to Middle Jurassic (Baumgartner et al. 1995). The age of this sample is most probably Middle Jurassic. AGE OF SILICEOUS MUDSTONE

Preservation of radiolarians from the siliceous mudstone of the Samarka terrane is also poor, but © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd

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Fig. 6 Jurassic radiolarians obtained from chert and siliceous mudstone in the Anyui area. Sample localities are shown in Figure 2. ch, chert; sm, siliceous mudstone.

Mirifusus sp., the genus characteristic of the Upper Jurassic to Lower Cretaceous, is identified from sample L20Z-A (Figs 6,7). The radiolarian assemblage of sample L35-A is most diverse (Figs 6,7), and includes Archaeodictyomitra apiarium (middle Callovian to early Aptian), Cinguloturris cylindra (early Tithonian to late Valanginian), Eycyrtidiellum pyramis (Tithonian), E. ptyctum (latest Bajocian to early Tithonian), Pseudodictyomitra primitiva (late Bathonian to late Tithonian), and Xitus gifuensis (late Kimmeridgian to early Valanginian). The age range of each species is after Baumgartner et al. (1995). The age of sample L35-A is most probably Tithonian on the basis of the biostratigraphical studies by Matsuoka and Yao (1986), Matsuoka (1995), and Baumgartner et al. (1995). Although the age of sample L35-B is difficult to determine precisely due to the poor diversity and preservation of the radiolarians, the fossils suggest a similar age to that of sample L35-A. © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd

QUARTZ GRAIN SIZE OF CHERT AND METACHERT In order to confirm metamorphic continuity from the rocks of the Samarka terrane to the AMC as suggested by field observation, we measured quartz grain size and the crystallinity index of metachert and chert, which are indicative of degree of thermal metamorphism. In the study area, the exposure condition is fairly poor and it can be difficult to find continuous rock exposures other than chert and metachert, which are more resistant against weathering than other rocks. We therefore chose siliceous rocks instead of shale and basalt commonly used for metamorphic grade analysis. We collected 14 siliceous rock samples along the traverse (Fig. 2) for grain size and crystallinity index measurements. Most of the rocks are white, gray, greenish-gray or light purple radiolarian bedded chert and bedded metachert, although sample 03R14-2 (Table 1) is a metachert block in


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Fig. 7 Representative Jurassic radiolarian fossils obtained from chert and siliceous mudstone in the Anyui area. 1, Acanthocircus sp. cf. A. suboblongus (L35-C); 2, 3, Parahsuum sp. (L35-C); 4, 5, Mirifusus sp. (L20Z-A); 6, 7, Pseudodictyomitra primitiva (L35-A); 8, Eucyrtidiellum ptyctum (L35-A); 9, 10, Eucyrtidiellum pyramis (L35-A); 11, Xitus gifuensis (L35-A); 12, Cinguloturris cylindra (L35-A); 13, Amphipyndax sp. (L35-B); 14, Archaeodictyomitra apiarium (L35-A). Scale bar for image 1, 0.13 mm; 2, 3, 0.15 mm; 4, 0.24 mm; 5, 0.25 mm; 6, 7 and 10, 0.083 mm; 8, 9, 0.074 mm; 11, 12 and 14, 0.1 mm; 12, 0.17 mm.

Table 1 Grain size and crystallinity index of quartz in chert and metachert Locality† 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Sample No.

GS (SD) (mm)

CI (n, SD)

SK03092901B SK03100402C 03R3 03R69 03R70 03R9 03R12-1 03R14-2 03R23 03R24 SK03100107B SK03100202A SK03100206C SK03100310B

C C 0.19 (0.15) 0.10 (0.08) 0.17 (0.10) 0.07 (0.03) 0.08 (0.03) 0.35 (0.20) 0.03 (0.01) 0.03 (0.01) C C C C

2.3 (3, 0.02) 8.1 (3, 0.16) 9.1 (3, 0.21) 9.5 (5, 0.24) 9.8 (3, 0.11) 9.4 (3, 0.24) 9.4 (3, 0.11) 10.1 (5, 0.15) 8.4 (3, 0.20) 9.1 (3, 0.19) 8.4 (3, 0.20) 8.5 (3, 0.19) 3.7 (3, 0.06) 7.5 (3, 0.10)

† Localities 1–6 and 11–14 are in the Samarka terrane; 7–10, Anyui Metamorphic Complex (Fig. 2). C, cryptocrystalline chert; CI, average crystallinity index of quartz normalized by standard quartz; GS, average grain size of quartz; n, number of measurements; SD, standard deviation.

gneiss most probably originated from the chert block in mélange.

PROCEDURE FOR DETERMINING QUARTZ GRAIN SIZE

We made thin-sections of the siliceous rocks and measured lengths of the long and short axes of 100 quartz grains under an optical microscope. Quartz grains on a 1 mm ¥ 1 mm grid were selected for measurement using a mechanical stage. The magnification was converted according to the mean quartz grain size of the samples (Fig. 8), but the same magnification was used for the measurement of one sample. The averages of the geometric means of the lengths of the long and short axes, and the standard deviations are shown in Table 1. Chert samples from localities 1, 2, and 11–14 are composed of cryptocrystalline quartz except for the radiolarian tests included (Fig. 8). It is impossible to measure grain size under the optical © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd

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Fig. 8 Thin-section photomicrographs of chert and metachert (crossed-polars). (a) SK03100310B at locality 14, (b) 03R12-1 at locality 7, (c) 03R70 at locality 5. Scale bars, 1 mm.

Fig. 9 (a) Averages of quartz grain size (䊊) and crystallinity index (䉬) of chert and metachert of the Samarka terrane and the AMC collected along the traverse shown in Figure 2. Grain size of cryptocrystalline quartz is shown as zero. Distance is measured along the traverse, and (b) histograms showing size distribution of 100 quartz grains of chert and metachert. Horizontal and vertical axes indicate the quartz-grain size and the numbers of the grains, respectively. The metachert at locality 8 includes three grains more than 1 mm in size: 1.14, 1.19, and 1.32 mm.

microscope; thus, grain sizes are regarded as zero on Figure 9. ANALYTICAL RESULTS

The cherts more than 10 km from the boundaries between the Samarka terrane and the AMC are all © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd

composed of cryptocrystalline quartz. Other cherts consist of quartz grains larger than 0.03 mm average diameter. Although the grain sizes of chert and metachert from the Samarka terrane near the western boundary with the AMC vary between 0.07 and 0.19 mm, they are not discrete at the boundary: 0.07 mm on the Samarka side and 0.08 mm on


the Anyui side (Table 1 and Fig. 9). This means it is impossible to determine the boundary between the two geological units by the quartz grain size, which is an indicator of intensity of thermal metamorphism. CRYSTALLINITY INDEX OF CHERT AND METACHERT The crystallization process of amorphous silica has been studied mainly from an experimental point of view. Crystallinity of quartz is considered to be controlled by temperature and chemical environment; if the chemical environment is homogeneous, crystallinity is determined mainly by temperature. Murata and Norman (1976) proposed a crystallinity index (CI) of quartz based on the degree of resolution of the (212) X-ray reflection at 1.3820 Å. Recently, Gocmez and Haber (2004, 2005) identified correlations between crystallinity determined by the Murata and Norman (1976) X-ray diffraction method, and those determined by the diffuse reflectance infrared Fourier transform spectroscopy and full-width-at-halfmaximum methods. Gocmez and Haber (2004, 2005) showed good correlations between these methods, indicating the validity of X-ray diffraction. According to Murata and Norman (1976), well-crystallized quartz shows CI of 8.0–10.0, but poorly crystallized quartz has CI of 1.0–3.0. This indicates the potential application of the CI to the study of metamorphic grades of quartzose rocks.

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peak; and b = height of the (212) peak at 67.74°2q – background. In order to express CI on a familiar scale of 10, the determined value of completely euhedral quartz should be raised to 10.0 by a scaling factor. In this study, euhedral quartz from the Naegi pegmatite, central Japan, is used for calculation of the scaling factor. The determined value of the euhedral quartz is 7.74 (2s = 0.24) and scaling factor is approximately 1.29. ANALYTICAL RESULTS

The CI of chert and metachert ranged widely from 2.29 (SK03092901B) to 10.14 (03R14-2); examples are shown in Figure 10. The highest value, slightly greater than the CI of standard, was obtained from the metachert near the western boundary between the Samarka terrane and the AMC, and the values of CI from the Samarka terrane within 10 km of the boundary are not much different from those in the AMC (Fig. 9). In addition, the very high values of CI (>8.14) from the Samarka terrane within 30 km of the boundary clearly indicate that the AMC and surrounding Samarka terrane can not be divided from each other from the viewpoint of crystallinity of quartz in chert. Although samples SK03092901B and SK03100206C with low values of CI (2.29 and 3.70, respectively) yield many radiolarians, no fossils were found in other samples.

PROCEDURE FOR DETERMINING CRYSTALLINITY

The same samples used for grain size measurement were used for CI measurement. The samples were crushed into sand-size grains with a hammer and then ground in a flint ball-mill for 20–30 min. In characterizing the bulk crystallinity of chert, care was taken to exclude vein materials. Samples were mounted in a conventional glass holder before X-ray diffraction (MultiFlex, Rigaku, Tokyo, Japan) at Nagoya University Museum. The interval of 65–70°2q was run at 0.25°2q/min with Ni-filtered copper radiation (40 kV and 20 mA, divergence slit 1°, scattering slit 1°, receiving slit 0.3 mm). All samples were measured three to five times until the standard deviation was calculated to be below 0.25°. The precision of the method is 0.50°2q. The CI is shown as the following equation.

CI = 10 × F × a b where F, scaling factor; a = -height of the (212) peak at 67.74°2q - height of bottom of the next

DISCUSSION CONTINUITY OF METAMORPHIC GRADES BETWEEN SAMARKA TERRANE AND AMC

In the field it is observed that the unmetamorphosed radiolarian bedded chert of the Samarka terrane is graded into the metachert of the AMC. The metachert, mica schist, and gneiss with quartzite blocks of the AMC could be the metamorphic equivalents of the chert, shale, and mélanges of the Samarka terrane. Moreover, the quartz-grain size and CI of chert and metachert continuously change across the boundaries between the two geological units, as indicated in the previous section. These lines of evidence indicate that the rocks of the Samarka terrane and the AMC were originally formed as a single accretionary complex before metamorphism, and the metamorphism affecting the rocks in this area is the same series of thermal events. Ui and Mizukami © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd

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Fig. 10 Examples of X-ray diffraction measurements of chert and metachert from the Anyui area and the standard from the Naegi pegmatite, central Japan.

(1994) examined the effects of granite intrusion on quartz grain size and CI of chert in central Japan, and reported that a low value of CI (4.8) suddenly changed to a high value near 10 in the low-grade metamorphic zone at about 350°C, and that quartz grain size is more sensitive at high temperature and drastically increases near 450°C. Ui and Mizukami’s (1994) results are consistent with our measurements that the cherts beyond 10 km from the Samarka–Anyui boundaries consist of cryptocrystalline quartz, whereas the CI values of the cherts are as high as 8. OCEAN PLATE STRATIGRAPHY OF SAMARKA TERRANE

The chert of the Samarka terrane in the study area yields Triassic conodonts and Middle Triassic to Middle Jurassic radiolarians but the siliceous mudstone includes Latest Jurassic radiolarians (Figs 4–7). Matsuda and Isozaki (1991) studied Jurassic accretionary complexes in the Mino terrane, central Japan, and indicated that they have a stratigraphic succession starting from ocean floor basalt, through pelagic radiolarian chert and hemipelagic siliceous mudstone, capped by trench-fill turbidite; these workers called the sequence ocean plate stratigraphy. The age of accretion is estimated to be just after the age of © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd

the uppermost datable unit of the ocean plate stratigraphy, which is in most cases age of the siliceous mudstone, because the turbidite usually includes no age-diagnostic fossils. The siliceous mudstone of the study area yields latest Jurassic radiolarians, indicating the age of accretion of the Samarka terrane in this area is just after the latest Jurassic. Ages of accretion indicated by the ages of siliceous mudstones in the Samarka terrane range from the Early to Late Jurassic (Khanchuk et al. 1996; Fig. 11). Since the siliceous mudstone of the Samarka terrane in this area is latest Jurassic (Tithonian) in age, the accretionary complex is one of the youngest tectonostratigraphic units in the Samarka terrane. Oceanic sediments together with the continent-derived clastic cover formations were subducted underneath the continental plate and accreted below the already-accreted tectonostratigraphic units. Accretionary ages of the thrustbounded sedimentary packages are younging from top to bottom (Matsuda & Isozaki 1991; Kimura & Hori 1993). Gradual change from the CI values of chert and metachert of the Samarka terrane to those of the AMC, and lithological similarity between these two geological units suggest that they formed as a single tectonostratigraphic unit before thermal metamorphism. As


Fig. 11 Summary of age and lithology of the accretionary complexes in the Anyui area reconstructed on the basis of the radiolarian ages (right). Also shown are the standard geological columns of the Mino terrane in Japan and the Samarka terrane in Far East Russia (left, after Khanchuk et al. 1996; Sano & Kojima 2000). Radiometric ages are after Gradstein et al. (2004). 1, sandstone and shale; 2, siliceous mudstone; 3, chert; 4, Permo–Triassic boundary black shale; 5, limestone; 6, basalt; K, Cretaceous; C, Carboniferous; U, Upper; M, Middle; L, Lower.

the metasedimentary rocks of the AMC tectonically underlie the rocks of Samarka terrane (Fig. 2), the age of accretion of the original rocks of the AMC is probably younger than that of the Samarka terrane. CRETACEOUS METAMORPHISM IN SIKHOTE–ALIN MOUNTAINS AND JAPAN

The Tamba–Mino–Ashio terrane in Japan is composed of Jurassic accretionary complexes and is considered to be a southern extension of the Samarka terrane before the opening of the Sea of

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Fig. 12 Map showing the continuation of Permian to earliest Cretaceous accretionary complexes and Late Cretaceous metamorphic complexes in Japan and Northeast Asia before opening of the Sea of Japan (modified from Kojima et al. 2000; Yamakita & Otoh 2000).

Japan (Kojima 1989; Kojima et al. 2000; Fig. 12). The Ryoke Belt formed on the southeast of the Tamba–Mino–Ashio terrane before the opening of the Sea of Japan (Fig. 12), and is comprised of Late Cretaceous metamorphic and granitic rocks; the former are much smaller in volume than the latter (Nakajima 1994). The metamorphic rocks are composed of metachert, metasandstone, granitic gneiss, amphibolite, and migmatite, and the abundant biotite and hornblende K–Ar and monazite chemical Th–U–total Pb isochron (CHIME) © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd

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ages for metamorphism and plutonism ranging from 70 to 100 Ma indicate a series of Late Cretaceous metamorphic events (Nakajima 1994; Suzuki et al. 1994; Suzuki 2005). The rocks of the Tamba– Mino–Ashio terrane are at most localities in faultcontact with the Ryoke Metamorphic Complex at the southeastern margin of the terrane, but their gradational changes are confirmed in some areas. Moreover, the crystallinity of illite in the pelitic rocks continuously increases across the boundary between the Tamba–Mino–Ashio terrane and the Ryoke Belt (Otsuka & Watanabe 1992). These lines of evidence clearly indicate that the rocks of the Ryoke Belt are the metamorphic equivalents of the Jurassic accretionary complexes in the Tamba– Mino–Ashio terrane (Nakajima 1994). The relationship between the Tamba–Mino– Ashio terrane and the Ryoke Metamorphic Complex is similar to that between the Samarka terrane and the AMC. Both the Ryoke and AMC are similar in lithology to the Jurassic accretionary complexes, and were metamorphosed in Late Cretaceous time together with the Jurassic complexes. We propose that they are metamorphic equivalents of the Jurassic accretionary complexes. Faure et al. (1995) regarded units (i) to (iii) of the AMC as an exotic microcontinent. We could investigate only part of unit (ii) in this study, which is also similar in lithology to the rocks of the Samarka terrane. Although we could not investigate units (i) and (iii), we suggest the possibility that all units are metamorphic equivalents of Jurassic accretionary complexes. The Ryoke Metamorphic Complex, and at least part of the AMC, were originally part of the Jurassic accretionary complexes, and metamorphosed by high T/P crustal conditions and huge amounts of granitic intrusion into the lower (younger) part of the Jurassic accretionary complexes. Crustal development of the Pacific-type orogen has been achieved by the cycle of: (i) accretion of oceanic materials and turbidites derived from the continent; and (ii) granitic intrusion by the next subduction and accretion events, accompanied by the formation of high T/P metamorphic complexes.

CONCLUSIONS The conclusions of this study are summarized as follows. 1. Since the radiolarian fossils from siliceous mudstone of the Samarka terrane are uppermost Jurassic (Tithonian) in age, the geologic © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Asia Pty Ltd

unit is one of the youngest accretionary complexes of the Samarka terrane. 2. Quartz grain sizes and CI values of chert and metachert of the Samarka terrane and the AMC, together with the similarity in lithology between the two geological units, indicate that at least parts of the Anyui rocks are the metamorphic equivalents of the Samarka rocks. 3. The ages and lithologies of the Samarka terrane and the AMC are similar to those of the Tamba– Mino–Ashio terrane and the Ryoke Metamorphic Complex in southwest Japan. The lower (younger) part of the Jurassic accretionary complexes were intruded and metamorphosed by Late Cretaceous granitic magma.

ACKNOWLEDGEMENTS This study was financially supported in part by Grants-in-Aid from the Japan Society for the Promotion of Science (B13573008, C13640457). We thank T. Itoh, M. Turbin and S.A. Medvedeva for assistance during this project. The manuscript was greatly improved by the constructive reviews of T. Tsujimori, H. Hara, A. Ishiwatari, K. Hisada, and A. Khanchuk.

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