Journal of Earth Science, Vol. 26, No. 2, p. 259–272, April 2015 Printed in China DOI: 10.1007/s12583-015-0531-1 ISSN 1674-487X Depositional Environment of Cherts of the Sikhote-Alin Region (Russia Far East): Evidence from Major, Trace and Rare Earth Elements Geochemistry Igor’ V. Kemkin*1, 2, Raisa A. Kemkina1, 2 1. Far East Federal University, Vladivostok 690950, Russia 2. Far East Geological Institute, Far Eastern Branch of Russian Academy of Sciences, Vladivostok 690022, Russia ABSTRACT: The first data of geochemical study of the Benevka Section cherty rocks belonging to the Taukha terrane of the Sikhote-Alin Late Jurassic–Early Cretaceous accretionary prism, Russia Far East are presented. These data demonstrate essential distinctions of major, trace and rare earth element concentrations in different parts of the measured stratigraphic section. The lower chert horizons exhibit high Fe2O3 and MnO contents, low concentrations of Al2O3 and TiO2, relatively high V/Y ratio, and extremely low value of negative Ce anomaly. In contrast the upper horizons composed of clayey cherts and siliceous mudstones are characterized by high Al2O3, TiO2 and K2O contents, low Fe2O3 and MnO values, low V/Y ratio, and slightly negative Ce anomaly. In the middle part of the Benevka Section, in which cherts gradually changed to clayey cherts, intermediate geochemical characteristics are present. Based on these data obtained the depositional environments correspond to proximal to the spreding ridge, open-ocean and near continental margin regimes were successfully reconstructed from bottom to top of the Benevka Section, that indicate that significant horizontal movement took place of the sea-floor, on which the cherts were deposited. KEY WORDS: depositional environment, cherts geochemistry, Sikhote-Alin. 0 INTRODUCTION Radiolarian bedded cherts are a widespread lithology within the sedimentary formations of the Mesozoic accretionary complexes of the Sikhote-Alin region. The bedded chert units are 20–100 m thick and occur in tectonic slices up to 40 km in strike length. These tectonic slices alternate with slices of terrigenous rocks, and to a lesser extent with slices of basalts, limestones, and mélange formations (Kemkin, 2006; Kemkin and Fhilippov, 2002; Kemkin and Kemkina, 2000, etc.). Gradual lithologic transitions have been described from bedded cherts to clay-chert varieties and to siliceous-clay formations (siliceous mudstones) finally to terrigenous rocks through intermediate varieties (Kemkin et al., 1999; Kemkin and Kemkina, 1998; Kemkin and Roudenko, 1998; Kemkin et al., 1997; Kemkin and Golozoubov, 1996, etc.). On lithological grounds, and also taking into account the age of rocks, the bedded cherts, siliceous-clays and terrigenous formations have been identified as tectonically dispersed fragments of the former sedimentary cover of a paleo-oceanic plate which were deposited at different distances from the seafloor spreading center (Kemkin and Fhilippov, 2001; Kemkin et al., 2001, etc.). Such chert-terrigenous sequences are known as “oceanic plate *Corresponding author: [email protected] © China University of Geosciences and Springer-Verlag Berlin Heidelberg 2015 Manuscript received October 18, 2014. Manuscript accepted March 15, 2015. stratigraphy sequences” (Matsuda and Isozaki, 1991; Isozaki et al., 1990; Berger and Winterer, 1974, etc.), i.e., deposits which have accumulated on an oceanic plate during its drift from the mid-ocean ridge spreading center to the subduction zone. Matsuda and Isozaki (1991) have convincingly proved on an example of chert-terrigenous formations of the Kiso River Section (Inuyama area, Mino-Tamba belt, SW Japan) that tectonic alternation of bedded cherts and coarse-grained terrigenous clastics is a secondary feature that was caused by duplexing and underplating in the subduction zone. Imbricate-thrusted structure of bedded cherts and clastic rocks of the Mino-Tamba belt these researchers have explained by the tectonic rearrangement of a coherent sedimentary succession in normal stratigraphic order caused by the subduction of the Paleo-Pacific plate under the Asian continental plate. The presence of paleo-oceanic formations (Permian, Triassic and Jurassic bedded cherts, Middle Paleozoic MORB-type basalts, Late Paleozoic and Triassic limestones) at the Sikhote-Alin folded belt structure is also interpreted as a result of subduction of the Paleo-Pacific oceanic plate (also known as the Izanagi plate) during Mesozoic and partial accretion of fragments of its 1st and 2nd layers to the eastern margin of the Paleo-Asian continent (Kemkin, 2006; Khanchuk and Kemkin, 2003, etc.). The lithostratigraphy and the faunas in the bedded cherts indicate that they have formed in a pelagic depositional environment. They represent typical planktonogenic (radiolarian) deposits lacking in terrigenous impurities. Their maximum thicknesses within the tectonic slices does not exceed 100 m, and the age range covers a time interval of 70–100 Ma. The Kemkin, I. V., Kemkina, R. A., 2015. Depositional Environment of Cherts of the Sikhote-Alin Region (Russia Far East). Journal of Earth Science, 26(2): 259–272. doi:10.1007/s12583-015-0531-1 260 low rate of sedimentation (about 1–1.5 mm per 1 000 years) and sedimentary contact with underlying oceanic tholeiites of MORB-type occasionally observed in individual tectonic slices indicate that bedded cherts have been accumulated in areas of paleo-ocean basins portions significantly distant from continents on the sea bottom deeper than the carbonate compensation depth. Some questions remain to be answered concerning the specific facies conditions of bedded cherts formation, and depositional environments of chert accumulation. This article presents the initial data of a geochemical study of the bedded cherts of the Taukha terrane (Late Jurassic–Early Cretaceous accretionary prism of the Sikhote-Alin, see Fig. 1) including the distribution of rare-earth elements (REE) in the cherty rocks and the calculated value of the cerium (Ce) anomaly which is a reliable indicator of the depositional environment of seafloor deposits (Kametaka et al., 2005; Kato et al., 2002, 1998; Murray, 1994; Murray et al., 1991, 1990; Shimizu and Masuda, 1977). Another important aim of this study is to demonstrate, using the obtained geochemical data, additional evidences of formation of the Sikhote-Alin bedded cherts at an open ocean (pelagic) depositional environment rather than continental margin sea basin. Some geologists studying the Sikhote-Alin geology are opposed to the pelagic origin of bedded cherts in this region (e.g., Kazachenko et al., 2012; Volokhin and Mikhaylik, 2008; Volokhin et al., 2003, etc.). They believe that co-occurrence of bedded cherts and terrigenous (clastic) rocks such as sandstones, gravelstones, siltstones, silty mudstones indicate the continental margin sea basin environment, because the latter rock types are derived from continental source with sialic crust and were deposited on the continental slope and immediately adjacent seafloor. 1 THE DISTRIBUTION OF REE IN MARINE SEDIMENTS The main mechanism of REE incorporation in marine sediment is adsorption from sea water, and in continental margin environments direct inclusion of metalliferous and terrigeneous particulate matter with an inherited REE pattern (Murray et al., 1991, 1990). At first sight, it would seem, that the sediments deposited in the continental margin basins should be characterized by relatively high total contents of REE. However, according to the available data (Dubinin, 2006; Murray et al., 1991, 1990; Toyoda et al., 1990; Elderfield and Greaves, 1982; Elderfield et al., 1981) actual total contents of REE in continental margin deposits is much less, than in those of the open-ocean basins, because the high sedimentation rate and quick burial limit seawater exposure time, restricting the potential of the sediments to adsorb REE (Murray et al., 1990; Ruhlin and Owen, 1986). At the same time, in spite of relative increase of total REE content toward the open-oceanic environments, the behavior of cerium (Ce) shows an opposite tendency. The relative Ce depletion of open-ocean sediments is explained by the oxidation of Ce3+ in oxygen enriched mid-ocean water to tetravalent insoluble phase (Ce4+) and thus the preferential removal of Ce from the water column. In contrast, in continental margin waters enriched with dissolved organic carbon, the reducing conditions inhibit oxida- Igor’ V. Kemkin and Raisa A. Kemkina tion of Се. Such chemical behavior of Ce in waters of different ocean portions explain the relatively high Ce contents in continental margin sediments, and correspondingly relatively low Ce content in open-ocean sediments. So the open-ocean deposits are characterized by well-developed negative Ce anomalies, while those from the continental margin environments contain no or only slight anomalies of Ce. Negative Ce anomalies and positive Ce anomalies are defined as values of less and greater than 1, respectively. In addition, sediments deposited near spreading ridges demonstrate more extreme negative Ce anomalies, because the seawater in these areas is enriched by metalliferous particulates originating from ridge hydrothermal plume. The Fe and Mn oxides-hydroxides scavenge Ce from the water column causing very low Ce content in seawater, and correspondingly low Ce contents in sediments deposited close to ocean ridges. On the basis of published data on REE distribution in modern seafloor sediments, as well as in ancient cherts and siliceous-clayey rocks, Murray et al. (1991, 1990) have calculated average values for the Се anomaly of the oceanic sediment facies zones. For example, the value of the Се anomaly for sediments deposited within 400 km of spreading ridges ranges from 0.10 to 0.36, with an average of 0.29 (extremely low Се anomaly). The open-ocean seafloor sediments Ce anomaly ranges from 0.23 to 1.06 (average value 0.55), and those from continental margin environment-commonly 0.90–1.20 (mean quantity=1), i.e., slightly negative and, in most cases, a positive Се anomaly. In addition, Murray (1994) has proposed an alternative criterion for measuring of Ce relative behavior, and correspondingly for identification of the paleo-oceanic environment of seafloor sediment accumulation (e.g., continental margin, open-ocean, and ridge-proximal depositional regimes). This criterion is the NASC (North American Shale Composite)-normalized La/Ce ratio (i.e., Lan/Cen), which does not depends upon the age or the location of the sediments. Murray (1994) calculated the average values of Lan/Cen ratio in continental margin cherts as ~1, and 3.5 in the ridge-proximal cherts, while open-ocean cherts are characterized by intermediate Lan/Cen values. Thus, taking into consideration the above and that REE are not affected by post-burial diagenesis (Chen et al., 2006; Murray, 1994), it follows, that the REE analysis of marine sediments is a useful tool for the determination of their depositional environment. 2 GEOLOGICAL BACKGROUND AND STUDIED SECTION For the purpose of specifying the facies conditions of formation of cherty rocks accreated and their depositional environment, geochemical study has been made of the bedded cherts and siliceous-clayey formation in the lower structural unit of the Sikhote-Alin Taukha terrane. The Taukha terrane is located in the southern part of the Sikhote-Alin region, Far East of Russia (6 in Fig. 1). It represents a fragment of Late Jurassic–Early Cretaceous accretionary prism, which was formed as a result of a consecutive accretion of the Paleo-Pacific oceanic plate sedimentary formations (fragments of paleoguyots and sedimentary covers of the abyssal plain) to the eastern Paleo-Asian Depositional Environment of Cherts of the Sikhote-Alin Region (Russia Far East) 261 Figure 1. The tectonic scheme of the Sikhote-Alin region and adjoining areas (after Khanchuk, 2006 with additions). 1. Early Paleozoic continental blocks: Bureya (BR)-Jiamusi (JM)-Khanka (KH) superterrane and Sibirian Craton (SB); 2. fragments of an Early Paleozoic continental margin-Sergeevka terrane (SR); 3. Permian–Triassic accretionary prizm-Dzhagdy-Kerbin (DK), Nilan (NL), Galam (GL), Laoelin-Gradek (LG) terranes; 4. Jurassic turbidite basin-Ulban and Un’ya-Bom terranes (UL); 5. Jurassic accretionary prism-Samarka (SM), Nadanhada-Bikin (NB), Khabarovsk (KH) and Badzhal (BD) terranes; 6. Tithonian-Hauterivian accretionary prism-Taukha terrane (TU); 7. Early Cretaceous turbidite basin-Zhuravlevka-Amur terrane (Zh-A); 8. Hauterivian-Albian island arc-Kema terrane (KM); 9. Hauterivian-Albian accretionary prism-Kiselevka-Manoma terrane (KS); 10. Early Paleozoic passive continental margin-Ayan terrane (AY); 11. left-lateral strike-slip faults; 12. thrusts. Faults: Lm. Limourchansky; Pk. Paoukansky; Kk. Koukansky; Kr. Koursky; MFA. Misha-Fushung-Alchan; Nh. Naolihe; Dh. Dahezhen; Ar. Arsen’evsky; CSA. Central Sikhote-Alin; Fr. Fourmanovsky; Zp. Zapadno-Primorsky. 262 continental margin (Kemkin, 2006; Khanchuk and Kemkin, 2003; Kemkin and Kemkina, 2000, 1998; Khanchuk et al., 1989a). It is distributed in a strip of about 60 km in width stretched in northeast direction along the north-western coast of the Sea of Japan (Fig. 2) from the mouth of the Kievka River up to the mouth of the Dzhigitovka River. According to available data (Kemkin, 2006; Kemkin and Kemkina, 2000), the terrane consists of three successive tectono-stratigraphic units which are similar in lithology and structure, but differ in age of the rocks (Fig. 2). Each unit is composed of marine deposits (mainly bedded cherts and lesser amounts of limestones) in the lower parts that gradually change upward along the section to terrigenous rocks of continental margin origin. In the upper part, terrigenous rocks are replaced by subduction mélange formations that represent chaotic deposits composed of siltstone matrix containing variously sized and aged blocks and fragments. The lithologic composition and age of these blocks and fragments indicate that they are derived from directly overlying tectono-stratigraphic units (Kemkin, 2006). The lower unit (Erdagou Formation) is represented by Late Jurassic to Early Cretaceous (Berriasian) bedded cherts and clayey cherts underlain by Middle Jurassic (Callovian) basalts (Erdagou Suite) and overlain by Berriasian-Valanginian siltstone and sandstone deposits (Silinka Suite) (Kemkin, 2006; Kemkin and Kemkina, 2000; Bersenev, 1969). The bedded cherts together with basalts are about 150 m thick, whereas the terrigenous rocks are estimated at 500 m thick. The siltstone and sandstone deposits conformably and gradually replace bedded cherts through a sequence of siliceous mudstone and mudstone. Valanginian-Hauterinian mélange formations also conformably overlie the siltstone and sandstone deposits (Kemkin et al., 1997). Blocks and fragments of this mélange consist of Middle to Late Triassic limestone, high-titanium alkaline basalt, Triassic and Jurassic bedded chert, and Middle to Late Triassic and Early Cretaceous terrigenous rocks. The blocks and fragments vary widely in size: from several millimeters to tens of centimeters to several tens of meters. Age and lithologic composition of these blocks (excluding Triassic terrigenous rocks) indicate that they are derived from overlying Gorbousha Formation. The blocks and fragments of Triassic shallow-marine terrigenous rocks containing the macrofauna fossils (Monotis), are, most likely, derived from the continental margin, under which the Taukha prism was formed. The mélange formations range from 100 to 400 m thick in different areas. The middle unit (Gorbousha Formation) consists of Middle to Late Triassic limestone (Tetyukha Suite) with high-titanium alkaline basalts at the base (400–500 m thick) that are interpreted as fragments of paleoguyots (Khanchuk et al., 1989b). Early Triassic to Late Jurassic bedded cherts and clayey cherts (about 100 m thick) gradually change to Late Tithonian-Berriasian siltstone and sandstone deposits (Gorbousha Suite), and then to Berriasian-Valanginian mélange (Kemkin et al., 1999; Kemkin and Kemkina, 1998; Bragin, 1991). Blocks and fragments of this mélange consist of Devonian, Carboniferous and Early Permian limestone and basalt, Carboniferous, Permian, Triassic and Middle Jurassic bedded Igor’ V. Kemkin and Raisa A. Kemkina chert, and Late Jurassic terrigenous rocks (sandstone and siltstone). In different slices the terrigenous rocks are 350–700 m thick. The thickness of the mélange formation is equivalent to that of the Erdagou Formation. The upper unit (Skalistorechenka Formation) is composed of Late Devonian to Early Permian limestone (Skalistorechenka Suite) associated with high-titanium alkaline basalts (about 400 m thick) that are also interpreted as fragments of paleoguyots and Carboniferous to Middle Jurassic bedded cherts overlapped by Late Jurassic siltstone and sandstone deposits (Pantovyi Creek Suite) (Kemkin, 2006; Kemkin and Kemkina, 2000; Khanchuk et al., 1989b). The thickness of the bedded chert and clastic deposits is not clear because of their fragmented outcrop. The investigated chert-terrigenous sequences (so-called oceanic plate stratigraphy sequences) belonging to a lower structural unit, is located on the right bank of the Benevka River, 6 km upstream from its mouth. Here (see Fig. 3), in an unnamed canyon-like creek (fourth right inflow of the Benevka River), basalts, bedded cherts and terrigenous rocks are exposed. The base of the section is composed of massive dark green to black-green basaltic lava flows with pillow structures. These rocks represent weakly porphyritic basalts with intersertal or transitive intersertal-ophitic textures of the groundmass. Rare phenocrysts are presented by fine grains of plagioclase and clinopyroxene (augite). In some samples the relict grains of olivine completely replaced by pseudomorphs of chlorite are observed. The groundmass of basalts consists of isomorphic tablets of augite and blades of plagioclase, dust-like (powdered) ore minerals and chloritizated volcanic glass in interstices. Plagioclases are completely albitized and pyroxenes in different degree are replaced by amphiboles of tremolit-aktinolite series. Besides that, threadlike veinlets of epidote and also calcite are often observed in groundmass of basalts. Along with massive basalts the amygdaloidal dark green, sealing-wax and variegated pillow-lavas are widespread. They are characterized by hyalopilitic, hyaloid, hyalopilitic and variolitic textures of the groundmass. The amygdalas are usually filled by calcite, and rarely by chlorite, quartz or chalcedony. The porphyritic grains in amygdaloidal basalts are usually represented by clinopyroxene and plagioclase, but sometimes among the phenocrysts olivine is prevails which completely is replaced with secondary minerals. Based on both petrochemical characteristics and contents and distribution of iron group elements, the basalts of the given section are the typical basalts of the second oceanic layer formed in the middle-oceanic ridges (Simanenko et al. 1999). They are characterized by low contents of SiO2, K2O, TiO2 and high contents of MgO, showing that they correspond to MORB tholeiites. On the AFM diagram, they are located in a field of abyssal tholeiites (Simanenko et al. 1999). The value of alkali-calcium index (Na+K)/Ca of the abovementioned basalts is equal 0.4–2.2 that also corresponds to the basalts of tholeiitic series. The FeO*/MgO-SiO2 ratio of these basalts also indicates that they are MORB tholeiites. The basalts are overlain by bedded cherts. A microscopic study of the basalt-chert boundary shows that the contact Depositional Environment of Cherts of the Sikhote-Alin Region (Russia Far East) 263 Figure 2. Schematic geological map of the southeast part of the Sikhote-Alin area, combined with a generalized section of the Taukha terrane and tectono-stratigraphic complexes (after Kemkin and Kemkina, 2000 with additions). 1-2-terranes: 1. Zhuravlevka-Amur; 2. Taukha; 3. Late Cretaceous volcanites; 4. Late Cretaceous granitoids; 5. faults; 6. border between different-aged tectono-stratigraphic units; 7. tholeiitic basalts; 8. high-titanium alkaline basalts; 9. cherts; 10. limestones; 11. turbidites; 12. subduction mélange; 13. character of contact between various lithogenic types of rocks: sedimentary (a), unstated (b). Er. Erdagou; Gr. Gorbousha; Sk. Skalistorechenka formations (or tectono-stratigraphic complexes). 264 Igor’ V. Kemkin and Raisa A. Kemkina Figure 3. Structure of the chert-terrigenous sequence on the right bank of the Benevka River (after Kemkin and Taketani, 2008 with additions). 1. Suduction mélange; 2. basalts and cataclastic basalts of the Erdagou suite; 3. bedded cherts; 4. clayey cherts; 5. siliceous mudstones; 6. mudstones; 7. siltstones; 8. alternation of sandstones and siltstones; 9. localities of radiolarian and geochemical samples; 10. faults. Depositional Environment of Cherts of the Sikhote-Alin Region (Russia Far East) between them is sedimentary. The bedded cherts (let’s name them Unit 1 for future discussion) are apparently about 17 m thick and their colors within this interval vary repeatedly from cherry-pink and reddish-brown to yellowish-green and greenish-grey. The bedded cherts smoothly and gradually pass to clayey cherts (Unit 2) of pinkish-brown and cherry-violet shades, which also are gradually replaced by siliceous mudstones (Unit 3) of the similar color. The clayey cherts are about 33 m thick, and the siliceous mudstones about 7 m thick. The overlying siliceous mudstones in turn gradually change into light gray mudstones (about 3 m thick) and, further, to grey and dark grey siltstones, which is about 11 m thick. The siltstones are replaced by rhythmical alternating siltstones and fine-grained sandstones (turbidite). The data of biostratigraphic research of this section are presented in (Kemkin and Taketani, 2008). In this paper we exhibit the results of geochemical study of above mentioned rocks. 3 MATERIALS AND METHODS For geochemical study we used duplicates of the cherty rock samples that were originally collected for radiolarian analyses. The rock samples were crushed into small fragments, then rock fragments were carefully hand-picked under the binocular microscope to avoid contaminations of altered and vein materials, and then were pulverized and passed through standard sieve for geochemical analysis. Major-element compositions (excepting SiO2 and Н2О-) were determined by atomic emission spectroscopy with inductively coupled plasma mass spectrometer iCAP 6500Duo (Thermo Scientific Corporation, USA). SiO2 and Н2О- contents were determined using a standard weighting (gravimetric) method described in (Popov and Stolyarova, 1974). Trace element and REE abundances were measured by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500C, Agilent Technologies, USA). Details of the analytical methods of both atomic emission spectroscopy and mass spectrometry are given in Barcelo (2003). All geochemical analyses were carried out at the laboratory of Analytical Chemistry of the Far Eastern Geological Institute, Far Eastern Branch of Russian Academy of Sciences. The precision of the analyses was generally 2%–5% for major oxides, and 5%–10% for trace and REE elements. REE abundances were normalized to North American Shale Composite (NASC) with normalized values as proposed by Gromet et al. (1984). Cerium anomalies (Ce/Ce*) were calculated from Ce/Ce*=(Cesampl/CeNASC)/(0.5(Lasampl/LaNASC)+0.5 (Prsampl/PrNASC)) (e.g., Murray et al., 1990). 4 RESULTS 4.1 Major Elements Major and trace elements contents, REE abundances, values of Ce anomalies and Lan/Cen ratio, and ratios of some major element are presented in Tables 1 and 2. It is clear to see that SiO2 contents (83.10 wt.%–66.73 wt.%) in the cherty rocks of the Benevka Section are slightly less than in pure cherts. The lowest values are observed in the siliceous mudstones of Unit 3, and the highest values in the bedded cherts of Unit 1. The clayey cherts of Unit 2 exhibit intermediate SiO2 265 contents. The lower SiO2 values is caused by high concentrations of other major oxides such as Fe2O3 and Al2O3. The contents of these two oxides show a complete mirror image of each other in stratigraphic section. In bands of bedded cherts-clayey cherts- siliceous mudstones Fe2O3 concentrations vary from 13.41 wt.% to 6.86 wt.%. Contrariwise, behavior of Al2O3 has an opposite tendency, and ranges from 3.32 wt.% to 14.90 wt.%. In other words, Al2O3 content increases progressively up the stratigraphic section, whereas the concentration of Fe2O3 inversely decreases in this direction. TiO2 contents show the same stratigraphic variations as Al2O3, and MnO concentrations resemble that of Fe2O3. The MnO contents of Unit 1 of the Benevka Section are much higher (3.21 wt.%) than those of Unit 3 (0.09 wt.%) which show an abrupt decrease in MnO up-section. The concentrations of TiO2 in Unit 1 are five time lower (0.11 wt.%) than those in Unit 2 (0.62 wt.%). The contents of other major oxides are very low and are mostly below 1 wt.% in Unit 1 and Unit 2, and only in Unit 3 MgO, K2O and Na2O contents make up 1.5 wt.% –3 wt.%. A number of researchers (e.g., Zhang et al., 2006; Murray, 1994; Taylor and McLennan, 1985) have reported that since some chemical elements (namely Al, Ti, Fe and some others) are relatively immobile during post-depositional chemical processes including diagenesis and weathering, their relative contents in seawater sediments (and specially in the cherty rocks) can be used for depositional environment determination. For example, high contents of Al, Ti and K (i.e., components of aluminosilicate mineral phases) are caused by terrigenous input (detrital materials), while high Fe and Mn contents are an indicator of hydrothermal metalliferous flux influence. Using the differences in concentrations of above mentioned chemical elements for various sedimentation regimes, Murray (1994) has suggested several plots of major element ratios in cherts for distinguishing continental margin and spreading ridgeproximal depositional environment. On these plots the cherty rocks of the Benevka Section subdivide clearly into three groups (Fig. 4). The first group that is represented by bedded cherts of the lower horizons of Unit 1 of the Benevka Section, which overlie tholeiitic basalts, falls within the field of spreading ridge-proximal depositional environment. A second group, which is located within the continental margin field, is formed by samples of siliceous mudstones of Unit 3 and clayey cherts of Unit 2. The samples from the upper horizons of Unit 1 fall within the intermediate field between first and second groups, and, thus, correspond to the open-oceanic depositional environment. 4.2 Trace Elements Geochemical study of the chert sequences of the Franciscan Complex and Monterey Group (Murray et al., 1991) have shown that cherts forming near spreading ridges and within open-ocean basins are characterized by significantly high contents of V, low contents of Y and consequently higher V/Y ratios than cherts from continental margin. Our geochemical data on the Benevka Section cherty rocks demonstrate relatively high concentrations of V in bedded cherts of Unit 1 and siliceous mudstones of Unit 3, 266 Igor’ V. Kemkin and Raisa A. Kemkina Table 1 Major (wt.%) and trace elements (mg/kg) composition of the Benevka Section cherty rocks Elements SiO2 (wt.%) TiO2 Al2O3 Fe2O3(tot) MnO MgO CaO Na2O K2O P2O5 Н2О LOI ∑ (total) Li mg/kg Be Sc V Cr Co Ni Cu Zn Ga As Rb Sr Y Zr Nb Cd Sn Cs Ba Hf Ta Tl Pb Th U Al2O3/(Al2O3 +Fe2O3) Fe2O3/TiO2 Fe2O3/ (100-SiO2) Al2O3/ (100-SiO2) V/Y Samples Be-15/1 Be-15b/1 Be-15c/1 Be-14/1 Be-12/1 Be-10/1 Be-8/1 Be-6/1 Be-4/1 76.20 0.32 4.09 13.41 1.39 0.87 0.41 0.24 0.60 0.07 0.60 1.70 99.90 6.93 0.70 5.8 112.7 81.8 9.21 98.49 178.19 47.32 3.11 4.45 11.76 41.4 4.68 77.72 4.56 0.01 0.20 1.38 591 2.33 0.42 0.18 22.8 2.65 0.68 76.27 0.11 3.46 10.62 3.21 1.09 0.88 0.12 0.37 0.06 0.03 3.57 99.78 21.04 0.42 4.55 78.6 53.1 21.80 168.04 73.14 103.28 5.49 5.81 12.81 41.65 6.51 5.16 1.92 0.04 2.24 1.70 611.5 1.16 0.20 0.18 24.62 2.03 0.35 83.10 0.13 3.32 9.84 0.58 0.45 0.44 0.59 0.73 0.05 0.22 0.18 99.64 8.71 0.70 3.4 31.15 69.65 13.88 60.39 69.74 38.02 4.62 8.87 27.22 52.6 4.51 30.88 2.35 0.04 17.06 1.42 1 057 1.10 0.24 0.24 29.54 2.21 0.58 82.92 0.24 5.85 7.13 0.29 0.54 0.32 0.81 1.40 0.07 0.10 0.90 100.57 7.19 0.66 7 38.3 54.6 10.04 42.31 46.75 31.52 7.06 11.88 40.20 56.3 4.32 51.53 3.80 0.03 3.20 2.49 556 1.90 0.44 0.42 3.09 3.38 0.51 82.40 0.21 5.04 7.35 0.19 0.85 0.57 0.74 1.27 0.02 0.80 1.00 100.43 6.65 0.55 4.9 49.15 58.35 6.09 32.63 29.77 26.21 3.95 1.80 29.14 61.95 2.09 43.33 3.56 0.04 2.50 2.82 620.8 1.74 0.29 0.22 9.77 2.38 0.49 70.93 0.46 9.90 6.14 0.08 0.97 0.12 1.10 2.53 0.05 0.95 6.30 99.52 12.07 0.76 9.9 73.65 56.75 7.71 33.30 28.32 40.15 11.32 6.83 75.79 46.65 7.98 54.97 7.73 0.03 3.10 5.16 577 2.28 0.50 0.62 13.70 7.96 1.40 69.61 0.62 13.05 6.03 0.09 1.61 0.20 1.41 3.40 0.07 0.80 3.60 100.50 19.80 1.78 14.1 93.9 66.85 8.85 32.97 23.82 60.03 16.19 6.11 109.59 60.15 10.89 140.57 12.74 0.16 4.11 9.52 954 5.83 1.27 0.85 16.31 10.88 1.93 67.09 0.62 14.66 7.01 0.12 2.21 0.53 1.51 3.42 0.07 0.00 3.20 100.42 34.24 2.21 13.3 107.4 74.4 14.96 46.35 35.77 84.08 19.48 3.82 116.59 70.85 11.55 149.23 12.60 0.07 3.13 10.31 659 5.70 1.36 0.62 14.68 9.84 1.82 66.73 0.61 14.90 6.86 0.09 2.05 0.63 2.02 3.04 0.09 0.00 3.20 100.22 37.33 2.71 13.7 94.9 94.7 13.22 53.39 50.34 87.63 20.04 3.34 112.29 86.05 14.88 166.88 13.36 0.06 9.15 10.81 556.5 6.70 1.52 0.59 22.35 12.19 1.87 0.23 0.25 0.25 0.45 0.40 0.62 0.68 0.67 0.69 42.10 94.11 77.09 29.72 34.86 13.30 9.69 11.28 11.16 0.56 0.45 0.58 0.42 0.42 0.21 0.20 0.21 0.21 0.17 0.15 0.20 0.34 0.29 0.34 0.43 0.45 0.45 24.08 12.07 6.91 8.87 23.52 9.23 8.62 9.30 6.38 whereas behavior of Y contents corresponds to those reported by Murray et al. (1991). On the whole V/Y ratios are in compliance with Murray’s data (Murray et al., 1991). The values of V/Y ratio in bedded cherts and clayey cherts range from 8.87 to 24.08, while in siliceous mudstones the ratio is 6.37–9.29. 4.3 Rare Earth Elements As mentioned above, REE contents, and particularly Ce/Ce* and the NASC (North American Shale Composite) normalized Lan/Cen ratio, are the best criteria for determination of depositional environment of the cherty rocks. The measured values of REE concentrations of the Benevka Section cherty Depositional Environment of Cherts of the Sikhote-Alin Region (Russia Far East) 267 Figure 4. Plots of Fe2O3/TiO2 vs. A12O3/(A12O3+Fe2O3) and Fe2O3/(100-SiO2) vs. A12O3/(100-SiO2) for the Benevka Section cherty rocks. Fields of ridge, pelagic, and continental margin are from Murray (1994). rocks are given in Table 2. These data show that REE contents in our cherty rock samples are significantly low in comparison to NASC values. Total REE concentrations (ΣREE) of different cherty rock varieties range from 40.36 to 161.20 mg/kg, whereas ΣREE of NASC makes up 172.61 mg/kg (Gromet et al., 1984). The largest concentrations among the REE are of La, Ce and Nd, while contents of others REE are very low. A certain regularity in Ce distribution is also observed. Ce content increases progressively from bedded cherts through clayey cherts to siliceous mudstones. Correspondingly, the Ce/Ce* values decrease progressively up the Benevka Section. In other words, the value of Ce anomaly changes stratigraphically. A stratigraphic variation of REE patterns normalized to NASC is presented in Fig. 5. It is evident that extremely low Ce/Ce* values occur in the low part of Unit 1 of the Benevka Section, namely in the low horizons of bedded cherts. The Ce/Ce* value of these low horizons (sample Be-15/1) is 0.34 identical to those of lowermost horizons (first 7 m) of the Early Jurassic Franciscan cherts (Marin Headlands Section), which were deposited in the spreading ridge-proximal depositional environment (Murray et al., 1991, 1990). The upper horizons of bedded cherts of Unit 1 and lower horizons of clayey cherts of Unit 2 exhibit slightly less, but also well-developed negative Се anomalies (0.69–0.85) that are very close to those of next 40 m of the Marin Headlands Section cherts accumulated in pelagic (open-ocean) depositional environment (Murray et al., 1991, 1990). And finally, slight Ce anomalies (0.91–0.94) were revealed in the upper horizons of clayey cherts of Unit 2 and in siliceous mudstones of Unit 3 (see Table 2 and Fig. 5). Such low values of Ce anomalies are also very similar to those of reported by Murray et al. (1991, 1990) for cherts of the uppermost portion of the Marin Headlands Section, which were deposited in a continental margin regime according their interpretation. 5 DISCUSSION Thus, taking into account the values of Ce anomaly in the Benevka Section cherty rocks, it is possible to conclude that they were formed in an oceanic depositional environment, but in different paleo-ocean locations. The lowermost part of the Benevka Section which is composed of bedded cherts of Unit 1 (namely first 4 m) was deposited in near spreading ridge-proximal depositional environment, because the Ce/Ce* ratio in sample Be-15/1 located 4 m from the basalt-chert contact is 0.34. The next 3 meters of bedded cherts of Unit 1 lying up-stratigraphic section most likely accumulated in a transition zone between spreading ridge-proximal and open-ocean depositional environment. This interpretation is based on the values of Се anomalies in samples Be-15b/1 and Be-15c/1 (Ce/Ce*=0.49 and 0.69 respectively). Although the plots of major element ratios in these samples of Fe2O3/TiO2 against A12O3/(A12O3+Fe2O3) and Fe2O3/100-SiO2 versus A12O3/(100-SiO2)(see Fig. 4), fall into the area of the spreading ridge-proximal depositional environment, these values probably caused by high Fe2O3 contents reflecting significant influence of metalliferous hydrothermal plumes from the ridge. We have used the Al-Fe-Mn diagram proposed by Adachi et al. (1986) for checking this idea. The Al-Fe-Mn diagram is valuable tool providing information on the Fe and Mn oxides-hydroxides hydrothermal component relative to the detrital component enriched in Al. According to Adachi et al. (1986) Fe and Mn enrichment in cherty rocks is usually caused by hydrothermal fluids, whereas Al enrichment is related to involvement of terrigenous materials. On the Al-Fe-Mn diagram (Fig. 6), these two samples (Be-15b/1 and Be-15c/1) plot in the upper part of hydrothermal field, indicating on relatively high hydrothermal input. In contrast to the lower horizons of bedded cherts of Unit 1, the values of negative Се anomaly of upper horizons of bedded cherts of Unit 1 and low horizons of clayey cherts of Unit 2 composing next 18 m of the Benevka Section are essentially less and change progressively from 0.75 (sample Be-14/1) through 0.85 (Be-12/1) to 0.87 (Be-10/1). This allows us to reconstruct successive open-ocean and transition zone between open-ocean and near continental margin depositional environments for this portion of the measured section. The same interpretation follows from diagrammes of major element ratios (Fig. 4), where the above mentioned samples fall into the intermediate area between the spreading ridge-proximal and near continental margin depositional environments. For the last 12 m of cherty rocks section (i.e., its uppermost portion), where gradual sedimentary transition from 268 Igor’ V. Kemkin and Raisa A. Kemkina Table 2 Elements La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ∑ (total) Ce/Ce* Lan/Cen La/Lu Rare earth element composition of the Benevka Section cherty rocks (mg/kg) Samples Be-15/1 Be-15b/1 Be-15c/1 Be-14/1 Be-12/1 Be-10/1 Be-8/1 Be-6/1 Be-4/1 20.77 16.49 5.10 19.85 3.36 0.65 2.53 0.31 1.61 0.30 0.79 0.12 0.78 0.10 72.76 0.34 2.87 207.70 11.49 13.95 3.25 13.05 2.57 0.63 2.26 0.34 1.82 0.36 0.96 0.14 0.83 0.11 51.76 0.49 1.88 104.45 9.09 13.98 2.12 8.38 1.77 0.42 1.65 0.23 1.19 0.21 0.59 0.08 0.58 0.07 40.36 0.69 1.48 129.86 16.13 22.69 2.55 9.93 1.97 0.43 1.79 0.22 1.11 0.21 0.58 0.08 0.50 0.07 58.26 0.75 1.62 230.43 11.11 17.48 1.69 6.22 1.17 0.25 1.05 0.13 0.61 0.11 0.27 0.04 0.29 0.04 40.46 0.85 1.45 277.75 23.86 46.66 5.62 20.66 3.80 0.78 3.42 0.39 2.00 0.35 1.09 0.17 1.13 0.15 110.08 0.87 1.17 159.07 32.28 71.68 8.41 30.92 5.28 1.12 4.52 0.54 2.74 0.50 1.40 0.21 1.41 0.19 161.20 0.94 1.03 169.89 22.28 49.78 6.30 23.56 4.39 0.88 3.91 0.47 2.41 0.46 1.32 0.19 1.32 0.18 117.45 0.91 1.02 123.78 26.54 59.08 7.23 26.48 4.54 0.85 4.08 0.52 2.95 0.58 1.67 0.24 1.64 0.22 136.62 0.92 1.02 120.64 Figure 5. REE patterns of the Benevka Section cherty rocks normalized to NASC. Depositional Environment of Cherts of the Sikhote-Alin Region (Russia Far East) Figure 6. Location of the Benevka Section cherty rock samples on the AI-Fe-Mn diagram. clayey cherts of Unit 2 to siliceous mudstones of Unit 3 takes place, on the basis of their geochemistry these rocks were deposited near the continental margin. The cherty rocks of this stratigraphic interval are characterized by slightly-developed Ce anomaly (Ce/Ce* makes up 0.91–0.94, see samples Be-8/1, Be-6/1, and Be-4/1 in Table 2). On the plot of Fe2O3/TiO2 against A12O3/(A12O3+Fe2O3) these samples fall into the overlapping zone of areas of open-ocean and near continental margin depositional environments, and on the plot of Fe2O3/(100-SiO2) versus A12O3/(100-SiO2) they are located directly in the area of near continental margin regime (see Fig. 4). More evidence of the different positions of the chert-bearing rocks in the different oceanic depositional environments is provided by the plot of Lan/Cen versus Al2O3/(Al2O3+Fe2O3) proposed by Murray (1994). On this plot our cherty rock samples are subdivided into three groups (Fig. 7). Two of these groups coincide respectively with the open-ocean regime (samples Be-14/1, Be-12/1, Be-10/1) and the frontier zone between open-ocean and continental margin depositional environments (samples Be-8/1, Be-6/1, Be-4/1). The third group (samples Be-15/1, Be-15b/1, Be-15c/1) has Al2O3/(Al2O3+Fe2O3) ratios of 0.23–0.25, is similar to those of cherts forming in spreading ridge-proximal regime. The relatively low La contents in bedded cherts of the lower horizons of Unit 1 of the measured section cause the Lan/Cen values to be less than those reported by Murray (1994) for cherts forming in near ridge regime. For this reason these samples fall outside the area of spreading ridge-proximal depositional environment and are located slightly away from it. Anyway, three distinctive depositional regimes are clearly established by using this plot for the Benevka cherty rocks. The geochemical data of the extreme members of cherty succession of the Benevka Section (i.e., bedded cherts and siliceous mudstones) show that they differ in both the contents of major oxides and concentrations of REE that is a result of their sedimentation in various depositional environments. In case the depositional environment of accumulation of silica and clay-rich sediments is a same (for example, for bedded cherts in which the thin cherty-clayey (shale) bands (1–3 mm) alternated with layers of pure chert), the contents of major oxides 269 Figure 7. Plots of NASC normalized Lan/Cen vs. A12O3/(A12O3+Fe2O3) for the Benevka Section cherty rocks. Fields of ridge, pelagic, and continental margin are from Murray (1994). (mainly Al2O3 and SiO2) are different, but concentrations of REE and Ce/Ce* values are identical (e.g., Fig. 3 in Murray et al., 1991; see Fig. 2 in Murray et al., 1990). It is necessary to note, that in a number of publications (e.g., Kakuwa and Matsumoto, 2006; Kato et al., 2006; MacLeod and Irving, 1996; German and Elderfield, 1990; Liu et al., 1988; Wright et al., 1987; Liu and Schmitt, 1984) the value of Се anomaly is used for another purpose, namely as paleo-redox indicator for evaluating of oxic or anoxic conditions of marine sediments accumulation. The well-developed negative Се anomaly indicates oxic seawater conditions, whereas no or slightly negative Се anomaly correspond to reduced water condition. In this connection the following may be said. The concentration of Се in sea water depends not only on concentration of the dissolved oxygen, but also on quantity of suspended particles represented by detrital material, organic matter, oxides-hydroxides of Mn and Fe and some others (e.g., Sholkovitz et al., 1994), which promote to removal of Се from seawater column, because surface coatings of these inorganic and biogenic particles preferentially adsorb light REEs, and particularly Ce. In this case, it seems difficult to extract precise information on the redox conditions of sediment accumulation (especially for ancient deep-water sediments) using only cerium anomaly without additional geological and sedimentological criteria of evidence. On the other hand, there were many paleo-redox studies using data on concentration of so-called redox-sensitive elements (V, Cu, Ni, Zn, Mo, U, Cd and some others) in deep-water sediments (Tribovillard et al., 2006; Lyons et al., 2003; Morford and Emerson, 1999; Thomson et al., 1993; Calvert and Pedersen, 1993; Emerson and Huested, 1991). The enrichment factors of redox-sensitive trace elements are commonly used to estimate if these elements are relatively enriched or depleted. Enrichment factors (EF) usually is calculated as EF element X=X/Al sample/ X/Alaverage shale. If EFX is greater than 1, then element X is enriched and, if EFX is less than 1, it is depleted. The high enrichment factors of redox-sensitive metals are used by these researchers to support the notion that such rocks formed under 270 Igor’ V. Kemkin and Raisa A. Kemkina anoxic bottom water conditions. The absence of enrichments of this group of metals strongly suggests that sedimentation occurred under oxygenated bottom water conditions. For the purpose to evaluate paleo-redox conditions of the Benevka Section cherty rocks accumulation we also have used obtained concentrations of some redox-sensitive elements, and namely their enrichment factors. Figure 8 visually demonstrates the absence of enrichments of such redox-sensitive metals as V, Co, Ni, Zn, Cd and U in the most of cherty rock samples of the Benevka Section that indicate oxic seawater conditions during their accumulation. Only lowermost part of bedded cherts of the Unit 1 exhibit insignificant enrichments of V, Co, Ni and Zn, that may be caused by either suboxic deep-water conditions in near spreading ridge depositional environment or influence of metalliferous hydrothermal fluids. Thus, major, trace and rare earth element geochemical data discussed above clearly indicate that the Benevka Section cherty rocks were formed successively and continuously in the three different depositional environments corresponding at the beginning to the near spreading ridge regime, then open-ocean basin, and, finally, near continental margin environment. It is evident that the successive changing of depositional environments during silicic sediment accumulation was caused by seafloor spreading and oceanic plate drift, which moved stratigraphic section through different oceanic facies zones. The results of geochemical study together with data on age of the cherty rocks and rate of their sedimentation allow to assess, with certain reservations, the size of paleobasin in which the Benevka Section cherty rocks have been accumulated. For example, the part of chert section fixing transition from bedded cherts to clayey cherts (interval between samples of Be-12/1 and Be-11) is about 4 m. According to radiolarian analysis data, accumulation of this part of chert section took approximately 2 Ma (Kemkin and Taketani, 2008). Consequently the chert accumulation rate was about 2 mm per 1 000 years. Taking into account the value of Ce anomalies in sample Be-15/1, it can be argued that first 4 m of chert section have been accumulated during movement of an oceanic plate on a distance of 400 km from spreading center. Assuming that the rate of chert sedimentation did not change significantly, these 4 m were also accumulated during 2 Ma. That is, an oceanic plate has moved to 400 km for 2 Ma. This means that the speed of movement of an oceanic plate was about 20 cm/yr that, incidentally, is consistent with data of Engebretson et al. (1985) for the Early Cretaceous. Total time of the Benevka Section chert accumulation according to radiolarian analysis data is 14–15 Ma (Kemkin and Taketani, 2008). Consequently, paleo-oceanic plate has passed from spreading center to subduction zones (continental margin) a distance of about 3 000 km (that is approximately 1/3 of width of the modern Pacific Ocean on the beam from Tokyo to Los Angeles). The whole width of paleobasin was, correspondingly, twice more. Thus, the Benevka Section cherty rock geochemical data allow also to conclude that there was no any small Triassic-Jurassic continental margin sea within the Sikhote-Alin region, where cherts and terrigenous sediments were accumulated. Multiple alternation of clastic rocks and cherts in the Sikhote-Alin structure is resulted from consecutive accretion of fragments of the former sedimentary cover of a paleo-oceanic plate which were deposited at different distances from the seafloor spreading center. 6 CONCLUSION This geochemical study of the Benevka Section cherty rocks has revealed distinct major, trace and rare earth element concentrations in various parts of the measured stratigraphic section. The lower horizons are characterized by bedded cherts exhibiting high Fe2O3 and MnO contents, low concentrations of Al2O3 and TiO2, relatively high V/Y ratio, and extremely low Ce/Ce* values. In contrast the upper horizons composed of clayey cherts and siliceous mudstones are characterized by high Al2O3, TiO2 and K2O contents, low Fe2O3 and MnO values, low V/Y ratio, and slightly negative Ce anomalies. In the middle part of the Benevka Section, in which bedded cherts gradually replaced by clayey cherts, intermediate geochemical characteristics are present. Using the data obtained, depositional environments corresponding to the spreading ridge-proximal, open-ocean and near continental margin regimes have been reconstructed from bottom to top of the Benevka Section that indicate a significant horizontal movement of the sea-floor, on which chert accumulation took place. Thus, the geochemical characteristics of the cherty rocks studied here provided helpful criteria for determination of their depositional environments, and, on the other hand, they are direct evidence of tectonic incorporation (by means of subduction-accretion processes) of these bedded cherts into the structure of the Sikhote-Alin area which in turn, is very important for the understanding of the geodynamic evolution of other regions composed by chert-terrigenous sequences. Figure 8. Enrichment factor (EF) of some redox-sensitive elements of the Benevka Section cherty rocks. Depositional Environment of Cherts of the Sikhote-Alin Region (Russia Far East) ACKNOWLEDGMENTS This study was financially supported by the President of the Russian Federation (No. 1159.2014.5) and the Far Eastern Branch of the Russian Academy of Sciences (No. 15-1-2-013-2). REFERENCES CITED Adachi, M., Yamamoto, K., Sugisaki, R., 1986. Hydrothermal Chert and Associated Siliceous Rocks from the Northern Pacific: Their Geological Significance and Indication of Ocean Ridge Activity. Sediment Geol, 47: 125–148 Barcelo, D., 2003. Comprehensive Analytical Chemistry. Volume XLI. Elsevier Science. Amsterdam, the Netherlands. 1286 Berger, W. H., Winterer, E. L., 1974. Plate Stratigraphy and Fluctuating Carbonate Line. In: Hsu, K. J., Jehkyns, H., eds., Pelagic Sediments on Land and under the Sea. International Association of Sedimentologists, Special Publication, 1: 11–48 Bersenev, I. I., 1969. 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