Deformation history and processes during accretion of seamounts in subduction zones: The example of the Durkan Complex (Makran,SE Iran)

The Durkan Complex is a tectonic element of the Makran Accretionary Prism (SE Iran) that includes fragments of Late Cretaceous seamounts. In this paper, the results of map- to micro-scale structural studies of the western Durkan Complex are presented with the aim to describe its structural and tectono-metamorphic evolution. The Durkan Complex consists of several tectonic units bordered by mainly NNW-striking thrusts. Three main deformation phases (D₁, D₂, and D₃) are distinguished and likely occurred from the Late Cretaceous to the Miocene–Pliocene. D₁ is characterized by sub-isoclinal to close and W-verging folds associated with an axial plane foliation and shear zone along the fold limbs. This phase records the accretion of fragments of the seamount within the Makran at blueschist facies metamorphic conditions (160–300 °C and 0.6 – 1.2 GPa). D₂ is characterized by open to close folds with sub-horizontal axial plane that likely developed during the exhumation of previously accreted seamount fragments. An upper Paleocene – Eocene siliciclastic succession unconformably sealed the D₁ and D₂ structures and is, in turn, deformed by W-verging thrust faults typical of D₃. The latter likely testifies for a Miocene – Pliocene tectonic reworking of the accreted seamount fragments with the activation of out of sequence thrusts. Our results shed light on the mechanism of accretion of seamount materials in the accretionary prisms, suggesting that seamount slope successions favour the localization and propagation of the basal décollement. This study further confirms that the physiography of the subducting plates plays a significant role in the tectonic evolution of the subduction complexes.     

Stratigraphy and structure of the central East Sakhalin accretionary wedge (Eastern Russia)

The East Sakhalin accretionary wedge is a part of the Cretaceous-Paleogene accretionary system, which developed on the eastern Asian margin in response to subduction of the Pacific oceanic plates. Its formation was related to the evolution of the Early Cretaceous Kem-Samarga island volcanic arc and Late Cretaceous-Paleogene East Sikhote Alin continental-margin volcanic belt. The structure, litho-, and biostratigraphy of the accretionary wedge were investigated in the central part of the East Sakhalin Mountains along two profiles approximately 40 km long crossing the Nabil and Rymnik zones. The general structure of the examined part of the accretionary wedge represents a system of numerous east-vergent tectonic slices. These tectonic slices. tens to hundreds of meters thick. are composed of various siliciclastic rocks, which were formed at the convergent plate boundary, and subordinate oceanic pelagic cherts and basalts, and hemipelagic siliceous and tuffaceous-siliceous mudstones. The siliciclastic deposits include trench-fill mudstones and turbidites and draping sediments. The structure of the accretionary wedge was presumably formed owing to off-scraping and tectonic underplating. The off-scraped and tectonically underplated fragments were probably tectonically juxtaposed along out-of-sequence thrusts with draping deposits. The radiolarian fauna was used to constrain the ages of rocks and time of the accretion episodes in different parts of the accretionary wedge. The defined radiolarian assemblages were correlated with the radiolarian scale for the Tethyan region using the method of unitary associations. In the Nabil zone, the age of pelagic sediments is estimated to have lasted from the Late Jurassic to Early Cretaceous (Barremian); that of hemipelagic sediments, from the early Aptian to middle Albian; and trench-fill and draping deposits of the accretionary complex date back to the middle-late Albian. In the Rymnik zone, the respective ages of cherts, hemipelagic sediments, and trench facies with draping deposits have been determined as Late Jurassic to Early Cretaceous (middle Albian), middle Aptian-middle Cenomanian, and middle-late Cenomanian. East of the rear toward the frontal parts of the accretionary wedge, stratigraphic boundaries between sediments of different lithology become successively younger. Timing of accretion episodes is based on the age of trench-fill and draping sediments of the accretionary wedge. The accretion occurred in a period lasting from the terminal Aptian to the middle Albian in the western part of the Nabil zone and in the middle Cenomanian in the eastern part of the Rymnik zone. The western part of the Nabil zone accreted synchronously with the Kiselevka-Manoma accretionary wedge located westerward on the continent. These accretionary wedges presumably formed along a single convergent plate margin, with the Sakhalin accretionary system located to the south of the Kiselevka-Manoma terrane in the Albian.     

New age data on the deposits of the Kiselevka-Manoma accretionary complex based on radiolarian fossils

The Kiselevka-Manoma Complex, the youngest accretionary complex in the Russian Far East, is composed of Jurassic-Lower Cretaceous pelagic and hemipelagic oceanic deposits. The radiolarian biostratigraphic study made it possible to refine the stratigraphy of the upper portion of the siliceous sediments from the northeastern fragment of this accretionary complex in the vicinity of the Kiselevka settlement in the Lower Amur region. The transition from pelagic siliceous to hemipelagic siliceous-clayey sedimentation was established within the interval from the Late Barremian to the Middle Aptian in different parts of the complex. The age of the accretion of the oceanic rocks is defined as postmiddle Aptian.     

U–Pb zircon dating and nature of metavolcanics and metarkoses from the Monte Grighini Unit: new insights on Late Ordovician magmatism in the Variscan belt in Sardinia, Italy

In the external units of the Sardinian Variscides Nappe Zone, volcanic and volcanoclastic successions of Middle Ordovician age follow Lower Paleozoic calc-alkaline magmatism developed at the northern Gondwana margin. We present geochemical and zircon U–Pb isotopic data for the Truzzulla Formation, a low-to-medium-grade metamorphic volcanic–volcanoclastic succession belonging to the Monte Grighini Unit, the deepest unit in the Nappe Zone. Geochemical and radiometric data allow us to define a Late Ordovician (Katian) magmatic (volcanic) event of calc-alkaline affinity. These new data, in conjunction with previously published data, indicate that in the Sardinian Variscides, the age of Lower Paleozoic Andean-type calc-alkaline magmatism spans from Middle to Late Ordovician. Moreover, the age distribution of calc-alkaline volcanics and volcanoclastic rocks in the Nappe Zone is consistent with a diachronous development of Middle–Late Ordovician Andean-type magmatic arc through the portion of the northern Gondwanian margin now represented by the Sardinian Variscides. This reconstruction of the Sardinian Variscides reflects the complex magmatic and tectonic evolution of the northern margin of Gondwana in the Lower Paleozoic.     

Geochemical characteristics, Ar–Ar ages and original tectonic setting of the Band-e-Zeyarat/Dar Anar ophiolite, Makran accretionary prism, S.E. Iran

The Makran accretionary prism in southeastern Iran contains extensive Mesozoic zones of melange and large intact ophiolites, representing remnants of the Tethys oceanic crust that was subducted beneath Eurasia. To the north of the Makran accretionary prism lies the Jaz Murian depression which is a subduction-related back-arc basin. The Band-e-Zeyarat/Dar Anar ophiolite is one of the ophiolite complexes; it is located on the west side of the Makran accretionary prism and Jaz Murian depression, and is bounded by two major fault systems. The principal rock units of this complex are a gabbro sequence which includes low- and high-level gabbros, an extensive sheeted diabase dike sequence, late intrusive rocks which consist largely of trondhjemites and diorites, and volcanic rocks which are largely pillow basalts interbedded with pelagic sedimentary rocks, including radiolarian chert. Chondrite- and primitive-mantle-normalized incompatible trace element data and age-corrected Nd, Pb, and Sr isotopic data indicate that the Band-e-Zeyarat/Dar Anar ophiolite was derived from a midocean ridge basalt-like mantle source. The isotopic data also reveal that the source for basalts was Indian-Ocean-type mantle. Based on the rare earth element (REE) data and small isotopic range, all the rocks from the Band-e-Zeyarat/Dar Anar ophiolite are cogenetic and were derived by fractionation from melts with a composition similar to average E-MORB; fractionation was controlled by the removal of clinopyroxene, hornblende and plagioclase. Three ⁴⁰Ar–³⁹Ar plateau ages of 140.7±2.2, 142.9±3.5 and 141.7±1.0 Ma, and five previously published K–Ar ages ranging from 121±4 to 146±5 Ma for the hornblende gabbros suggest that rocks from this ophiolite were formed during the Late Jurassic–Early Cretaceous. Plate reconstructions suggest that the rocks of this complex appear to be approximately contemporaneous with the Masirah ophiolite which has crystallization age of (150 Ma). Like Masirah, the rocks from the Band-e-Zeyarat/Dar Anar ophiolite complex represent southern Tethyan ocean crust that was formed distinctly earlier than crust preserved in the 90–100 Ma Bitlis-Zagros ophiolites (including the Samail ophiolite).     

Composition and formation settings of the siliceous-volcanogenic complexes of the Nizhneussuriisk segment,Kiselevka-Manoma terrane,West Sikhote Alin

This paper reports new data on the mineralogical-petrographical composition, genetic types of sedimentary rocks, and geochemical features of the volcanic rocks of the Snarsky area of the Nizhneussuriisk segment of the Kiselevka-Manoma accretionary complex, which is developed on the right bank of the lower reaches of the Ussuri River. The Middle Jurassic-Aptian sedimentary rocks of the area are represented by pelagic radiolarian cherts, semipelagic siliceous-clayey deposits, subordinate shallow limestones, volcanomictic conglomerates, and turbidite sandstones. The basalts of the area are represented by alkaline basalts resembling within-plate ocean-island basalts and N-MORB-type tholeiitic basalts. Rock associations of two physicogeographical and geodynamic settings are distinguished: the pelagic setting with within-plate alkaline volcanic rocks and the hemipelagic one with volcanic edifices of spreading basalts.     

Upper Silurian and Devonian pelagic deep-water radiolarian chert from the Khangai–Khentei belt of Central Mongolia: Evidence for Middle Paleozoic subduction–accretion activity in the Central Asian Orogenic Belt

Recent mapping projects undertaken in Central Mongolia have revealed the widespread occurrence of radiolarian chert within a Paleozoic accretionary complex. We present the results of the first detailed tectonostratigraphic and radiolarian biostratigraphic investigations of the Gorkhi Formation in the Khangai–Khentei belt of the Central Asian Orogenic Belt.The Gorkhi Formation consists of sandstone shale, alternating sandstone and shale of turbidite affinity and chert with small amounts of siliceous shale, basalt, limestone, and clast-bearing mudstone. Radiolarian chert that is completely devoid of terrigenous clastic material is commonly associated with underlying basalt (sedimentary contact) and with conformably overlying siliceous shale and turbidite deposits. The tectonic stacking of basalt–chert and chert–turbidite successions is the most remarkable structural feature of the formation.The recovery of moderately well-preserved radiolarians and conodonts from red chert led to the recognition of four radiolarian assemblages that have a combined age range from the latest Silurian (Pridolian) to the Late Devonian (Frasnian). No age control exists for the siliceous shale, shale, and sandstone, although they are considered to be latest Devonian or slightly younger on the basis of stratigraphic relationships with underlying chert.The Gorkhi Formation has previously been interpreted as a thick sedimentary basin deposit overlying an unexposed Archean–Neoproterozoic basement; however, the stratigraphy within individual tectonic slices clearly corresponds to that of an ocean plate stratigraphy of an accretionary complex generated by the trenchward movement of an oceanic plate. From the lowermost to uppermost units, the stratigraphy comprises ocean floor basalt, pelagic deep-water radiolarian chert, hemipelagic siliceous shale, and terrigenous turbidite deposits. The biostratigraphic data obtained in the present study provide corroborating evidence for the existence of an extensive deep-water ocean that enabled the continuous sedimentation of pelagic chert over a period of nearly 50 million years. These data, together with structural data characterized by tectonic repetition of the stratigraphy, indicate that these rocks formed as an accretionary wedge along an active continental margin, possibly that of the Angara Craton. The mid-oceanic chert was probably deposited in the Northern Hemisphere portion of the Paleo–Pacific Ocean that faced the Angara Craton and the North China–Tarim blocks. Thus, we propose that subduction–accretion processes along the Paleo–Pacific rim played an important role in the accretionary growth of the active continental margin of the Angara Craton, directly influencing the evolution of the Central Asian Orogenic Belt.     

Geochemical discrimination of metabasalt rocks of the Fan–Karategin transitional blueschist/greenschist belt, South Tianshan, Tajikistan: seamount volcanism and accretionary tectonics

The Fan–Karategin metamorphic belt, South Tianshan, Tajikistan, is regarded to be an ancient subduction–accretionary complex and is composed of three tectonostratigraphic units which display lithologies consistent with different tectonic settings. The mafic schists, which make up the major part of the older unit of the belt, contain both alkali and tholeiitic metabasalts. On the basis of rare-earth and other immobile element characteristics, the alkali metabasalts are akin to within-plate ocean island basalts, whereas the tholeiitic metabasalts resemble E-type MORB. The association is interpreted to have been formed on seamount-like structures under a within-plate plume. Bedded cherts and marbles in the unit are regarded as ancient pelagic sediments and carbonate caps developed upon basaltic seamounts, respectively. Dismemberment of the seamount-related basalts and pelagic sediments and the high-P/low-T prograde metamorphism of the unit rocks up to transitional blueschist/greenschist facies was the result of paleoseamount submergence into a subduction zone. This unit is tectonically overlain by arc-derived metavolcanic unit and a disrupted, mainly clastic unit of Upper Ordovician–Lower Silurian age. Metavolcanic and metasedimentary rocks of the two upper units have geochemical characteristics compatible with subduction-related origin. The lithological assemblages of the individual units and their juxtaposition suggest an origin involving collision–accretionary processes. The Fan–Karategin belt is a subduction–accretionary complex which formed during subduction of oceanic crust under a volcanic arc and was subjected to tectonic juxtaposition and imbrication of seamount, deep-sea, trench and volcanic arc sequences.     

Yakchi chert–volcanogenic Formation—fragment of the Jurassic accretionary prism in the Central Sikhote-Аlin,Russian Far East

The Yakchi chert–volcanogenic formation is differentiated at the base of the stratigraphic succession in the Khor-Tormasu subzone of the Central Sikhote-clin structural–formational zone or the Samarka terrane of the Jurassic accretionary prism. The paper considers the results of biostratigraphic study of its deposits and petrogeochemical studies of its basalts. A tectonically disrupted sequence of the Yakchi Formation is restored on the basis of fossil conodonts and radiolarians, and its Late cermian–Middle Jurassic age is determined. The authors interpret the resulting stratigraphic succession in terms of changing depositional settings on the moving oceanic plate and recognize events of the ocean history recorded in it. Chert accumulated on the oceanic plate in pelagic canthalassa/caleopacifica from the Late cermian through to the Middle Jurassic. Deposition of siliceous claystone in the Late cermian–Early Triassic reflects the decline in productivity of radiolarians and a long anoxic event in Panthalassa. Chert accumulation resumed in the Triassic and persisted in the Jurassic, and it was interrupted by the eruption of basalts of different nature. Formation of the Middle–Late Triassic oceanic intraplate basalts likely occurred on the thick and old oceanic lithosphere and that of the Jurassic basalts on the thin and newly created lithosphere. In the Middle Jurassic, chert accumulation was replaced by accumulation of tuffaceous siltstone at a subduction zone along the csian continental margin. The middle Bathonian–early Callovian age of this siltstone closely predates accretion of the Yakchi Formation. The materials of the upper layer of the oceanic plate that formed over 100 million years in different parts of the ocean and on the lithospheric fragments of different ages were accreted to the continental margin. The bulk of the accreted material consists of oceanic intraplate basalts, i.e., fragments of volcanic edifices on the oceanic floor. accretion of this western part of the Khor-Tormasu subzone occurred concurrently with accretion of the southeastern part of the Samarka subzone in Primorye, which clarifies the paleotectonic zonation of the Central Sikhote-Alin accretionary prism. The cataclastic gabbroids and granitoids, as well as the clastic rocks with shallow-marine fossils in the Khor-Tormasu subzone, are considered as possible analogues of the Okrainka-Sergeevka allochthonous complex.     

Provenance and Evolution of the Guarguariiz Complex, Cordillera Frontal, Argentina

The Guarguardz Complex, basement of the Cordillera Frontal, included in the proposed Chilenia Terrane, consists of metasedimentary rocks deposited in clastic and carbonatic platforms. Turbiditic sequences point out to slope or external platform environments. According to geochemical data, the sedimentary protoliths derived through erosion of a mature cratonic continental basement. Volcanic and subvolcanic rocks with N and E-MORB signature were interbeded in the metasedimentary rocks during basin development. A compressional stage, starting with progressive deformation and metamorphism, followed this extensional stage. Continuing deformation led to the emplacement of slices of oceanic crust, conforming an accretionary prism during Late Devonian. The Guarguardz Complex and equivalent units in western Precordillera and also in the Chilean Coastal Cordillera share common evolutional stages, widely represented along the western Gondwana margin. These evidences imply that Chilenia is not an allochthonous terrane to Gondwana, but a portion of its Early Paleozoic margin. Regional configuration indicates that the Guarguardz Complex and equivalent units represent the accretionary prism of the Famatinian arc (Middle Ordovician-Late Devonian).     

延边地区晚三叠世火山岩的岩石地球化学特征及其构造意义

延边地区晚三叠世火山岩主要由安山岩和流纹岩构成，其中包括少量玄武安山岩。火山岩的岩石化学和地球化学研究表明，晚三叠世火山岩属于亚碱性系列，并具有钙碱性系列演化特征，位于中钾和高钾钙碱性岩系列范围。岩石组合、主要元素和稀土元素(REE)等显示晚三叠世火山岩的形成与活动大陆边缘伸展环境有关，它标志着泛太平洋板块(法拉隆板块)对欧亚大陆作用的开始，同时也标志着古亚洲枸造域的结束。     

Early‐Middle Paleozoic Andes‐type Continental Margin in the Chifeng Area,Inner Mongolia: Framework,Geochronology and Geochemistry and Implications for Tectonic Evolution of the Central Asian Orogenic Belt

Three tectonic units have been recognized in the Chifeng area, Inner Mongolia, from north to south, including the Qiganmiao accretionary prism, Jiefangyingzi arc belt and Sidaozhangpeng molasse basin, which formed an Andeantype active continent margin during the early to middle Paleozoic. The Qiganmiao accretionary prism is characterized by a mélange that consists of gabbro, two-mica quartz schist and basic volcanic rock blocks and heterogeneously deformed marble matrix. Two zircon U-Pb ages of 446.0±6.3 Ma and 1104±27 Ma have been acquired and been interpreted as the metamorphic and forming ages for the gabbro and two-mica quartz schist, respectively. The prism formed during the early to middle Paleozoic southward subduction of the Paleo Asian Ocean(PAO) and represents a suture between the North China craton(NCC) and Central Asian Orogenic Belt(CAOB). The Jiefangyingzi arc belt consists of pluton complex and volcanic rocks of the Xibiehe and Badangshan Formations, and Geochronology analysis indicates that the development of it can be divided into two stages. The first stage is represented by the Xibiehe Formation volcanic rocks, which belong to the subalkaline series, enriched LREE and LILE and depleted HFSE, with negative Eu anomalies, and plot in the volcanic arc field in discrimination diagrams. These characters indicate that the Xibiehe Formation results from to the continental arc magmatic activity related to the subduction of the PAO during 400–420 Ma. Magmatism of the second stage in 380–390 Ma consists of the Badangshan Formation volcanic rocks. Geochemistry analysis reveals that rhyolite, basaltic andesite and basalt of the Badangshan Formation were developed in continental margin arc setting. Moreover, the basaltic andesite and basalt display positive Sr anomalies, and the basalt have very low Nb/La values, suggesting that fluid is involved in magma evolution and the basalts were contaminated by continental crust. The sequence of Sidaozhangpeng molasse basin is characterized by proximity, coarseness and large thickness, similar to the proximity molasses basin. According to our field investigation, geochronological and geochemical data, combined with previous research in this area, a tectonic evolutionary model for Andes-type active continental margin of the CAOB has been proposed, including a development of the subduction-free PAO before 446 Ma, a subduction of the PAO and arc-related magmatism during 446–380 Ma, and formation of a molasse basin during 380–360 Ma.     

Ophiolites of the Kamchatsky Mys Peninsula, eastern Kamchatka: Structure, composition, and geodynamic setting

Ophiolites of the Afrika Mys Block of the Kamchatsky Mys Peninsula, eastern Kamchatka, are a fragment of an accretionary prism that formed in the Late Cretaceous-Eocene on the southern side of the Kronotsky island arc as a result of its collision with the Smagino volcanic uplift that arose at the post-Neocomian time on the subducting plate. On the basis of the geologic, geochemical, and paleomagnetic data available to date, it is established that ophiolites are heterogeneous in their origin and were formed in different geodynamic settings that changed progressively with time. The heterogeneous structure of ophiolites displays the evolution of a fragment of the oceanic lithosphere, which was not submerged into subduction zone, from its origination in the spreading center via transformation under conditions of the plume-related volcanic uplift to the involvement in the structure of the Kronotsky island arc, which is currently a constituent of the accretionary system of Kamchatka. The reconstruction of ophiolites tectonically fragmented in the accretionary prism allows recognition of (1) derivatives of an ocean ridge (ultramafic-gabbro-basaltic complex of the Mount Olen’ya Massif) conjugated with a transform fault and volcanosedimentary rocks of the Smagino volcanic uplift (cover of the oceanic crust) and (2) a fragment of the lithospheric mantle (ultramafic rocks of the Lake Stolbovoe Massif) exhumed in the process of collision and genetically related to the evolution of the volcanic uplift. In the course of evolution of the Kronotsky island arc, all these elements were overlapped by tephrogenic turbidites (Pikezh Formation) and quartz-feldspar graywackes (Pikezh Sandstone) that were involved in the accretionary prism as well. The paleotectonic reconstructions broadly support the petrologic conclusions about the complementary nature of different igneous complexes and ascertain the temporal sequence of events.     

Basalts of the Pantalassa ocean in the Samarka terrane, Central Sikhote Alin

Basalts developed on the right bank of the Matai River belong to the Samarka terrane (Central Sikhote Alin), which is a fragment of the Jurassic accretionary prism. They associate with Carboniferous-Permian reef limestones, Permian pelagic cherts, Jurassic hemipelagic cherty-clayey deposits, and terrigenous rocks of the near-continental sedimentation area. The petrogeochemical features of the basalts provide insight into the character of the volcanism in different settings of the ancient Pantalassa ocean. In terms of chemistry, the Carboniferous-Permian basalts are similar to the within-plate ocean-island basalts related to plume mantle sources. They were presumably formed in an oceanic area with numerous islands and seamounts. The Permian basalts associated with cherts are tholeiitic in composition and were formed from depleted mantle in a spreading center located in the pelagic area. The Jurassic basalts are of plume origin and, in terms of geochemistry, occupy an intermediate position between OIB and E-MORB. They were presumably formed in a convergent zone in a geodynamic setting of rapid oblique subduction.     

锡铁山块状硫化物铅锌矿床成矿构造环境及矿区南部找矿潜力:来自滩间山群火山岩岩石化学、地球化学证据

近年来海底块状硫化物矿床的深入研究表明,块状硫化物矿床形成与其成矿构造环境演化期间特定阶段的构造-火山作用有关,矿床持续形成时间一般不超过几个百万年。锡铁山矿床是我国西北地区最大的海底热液块状硫化物铅锌矿床,本文通过锡铁山矿床赋矿岩系火山岩岩石化学及地球化学特征的系统研究,对锡铁山矿床赋矿火山岩系岩浆演化过程及成矿构造环境得出如下几点认识:(1)锡铁山矿区赋矿滩间山群火山岩岩石化学及地球化学特征一致表明滩间山群岩浆活动具有自酸性向基性同源岩浆演化的特点。矿区滩间山群不同岩组/段的火山岩代表了同源岩浆不同演化阶段的产物。自O1-2tna-1岩段、O1-2tnb岩组→O1-2tnd-1岩段→O1-2tnd-3岩段,矿区火山岩岩石化学及稀土与微量元素地球化学具有明显的渐变过渡关系。(2)矿区滩间山群火山岩稀土、微量元素组成特征及成岩构造环境判别图解一致揭示,从O1-2tna-1岩段、O1-2tnb岩组→O1-2tnd-1岩段→O1-2tnd-3岩段,滩间山群火山岩成岩构造环境经历了从陆缘基底岛弧→洋陆过渡型地壳→典型大洋地壳的连续过渡变化。(3)矿区滩间山群火山岩岩浆演化过程及Rb、Sr组分变化趋势与大陆边缘弧后盆地火山岩岩浆演化过程相近,与现代西太平洋冲绳海槽形成过程相似。(4)综上推断,矿区深部及外围找矿工作的重点对象是弧后盆地拉张早期形成的具有双峰式火山岩组合的下部火山-沉积组合,而矿区南部O1-2tnd岩组找寻同类矿床的可能性不大。     

Tectonic significance of mafic volcanic rocks in a Mesozoic sequence of the Menderes Massif, West Turkey

The Mesozoic platform sequence of the Menderes Massif consists of thick succession of detrital and carbonate rocks. In this sequence there are mafic metavolcanic rocks at two different levels. The first level of mafic metavolcanic intercalations is in the Late Triassic detrital-rich series located in the ÇaltayL Formation, which is the lowermost unit of the Mesozoic platform. The second level of the mafic metavolcanic rocks is located in the Late Cretaceous-(?)Paleocene Selçuk Formation laying on top of the platform sequence. The ÇaltayL Formation, which is composed of mica-schists, thinly-bedded cherts, calc-schist and mafic volcanic intercalations unconformably overlie the BayLndLr Formation, which consists of mica-schists, phyllites, and white quartzites of Palaeozoic or probably older age. The mafic volcanic rocks in the ÇaltayL Formation are alkaline basalts with within plate characteristics and are formed during an intraplate extension. The ÇaltayL Formation is conformably overlain by the KayaaltL Formation represented by calc-schists, dolomitic marbles, and rudist- and emery-bearing massive marbles in ascending order. The Selçuk Formation overlies the KayaaltL Formation and consists of a mica-schist matrix with allochthonous blocks of mafic volcanic rocks, metaperidotites, metagabbros and massive marbles. The mafic volcanic rocks in the Selçuk Formation are tholeiitic basalts and are petrologically similar to mid-oceanic basalts. The geological and geochemical characteristics of the mafic metavolcanic rocks in the ÇaltayL Formation indicate that during the Late Triassic, the Menderes platform was segmented, probably by the opening of a branch of the Neotethyan Ocean. Between the Late Triassic and the Late Cretaceous, the Menderes carbonate platform was built up. During the Latest Cretaceous-Early Paleocene, a slab of oceanic crust obducted on this platform and provided slices of mafic metavolcanic rocks into the Selçuk Formation.     

Geochemical evidence for consecutive accretion of oceanic fragments: The example of the Samarka Terrane,Sikhote-Alin

The data of geochemical study of Late Triassic cherts from tectonic–sedimentary complexes from different structural levels of the Samarka Terrane are reported. It is shown that the concentration and character of distribution of the major petrogenic oxides and minor and rare-earth elements in cherts of the upper and lower structural levels differ significantly, which results from differences in the facies environments of chert deposition. All the geochemical characteristics of cherts show that their deposition proceeded in the pelagic area of sedimentation, but in different parts. The Katen Complex composing the lower structural level is the most distant from the continental margin. The closest is the Amba-Matay Complex composing the upper structural level. Based on the geochemical and biostratigraphic data and the age of accretion of paleooceanic fragments, the length of the subducted oceanic plate (>6000 km) is calculated.     

Structure,age, and mechanism of emplacement of the Amur and Kiselevka–Manoma accretionary complexes of the lower Amur region,Russian Far East

The Amur and Kiselevka–Manoma accretionary complexes belong to the Cretaceous Khingan–Okhotsk active continental margin, which was formed in the east of Eurasia as a result of the subduction of the Pacific oceanic plates. The Kiselevka–Manoma complex is composed of oceanic pelagic and hemipelagic sedimentary rocks and intraplate oceanic basalts. It is located to the southeast, along the ocean-faced front of the Amur complex, which is predominantly composed of turbidites of the convergent boundary of lithospheric plates. The biostratigraphic study of radiolarians from rocks of the frontal part of the Amur complex allowed us to correlate them with rocks of the Kiselevka–Manoma complex and to define the period of accretion to be from the Late Aptian to the Middle Albian. The tectonostratigraphic interrelations of these two contrasting lithotectonic complexes are established and two possible models of their common emplacement are suggested. Both models suppose a continuous spatiotemporal relation of complexes with the primary paleolocation of the Kiselevka–Manoma complex in front of (on the ocean side) the Amur complex. The frontal part of the Amur complex and the Kiselevka–Manoma complex were emplaced synchronously with the western part of the East Sakhalin accretionary complex. This scenario defines the Early Cretaceous tectonic zonation of the region and the choice of the appropriate paleotectonic model. At the end of the Early Cretaceous, a single convergent boundary of the lithospheric plates is suggested with the position of the Sakhalin island arc system south of the Khingan–Okhotsk active continental margin.     

New data on the composition and age of basement rocks of the Tagil paleo-island arc system

The Tagil paleo-island arc terrane is composed of Late Ordovician-Devonian intrusive, volcanic, and volcano-sedimentary complexes. The western margin of the terrane is comprised of dunite-clinopyroxenite-gabbro massifs of the Ural platinum-bearing belt, which are fringed by rock strata of widely different metamorphic grades. Work on isotope systematics of olivine gabbros enabled us to infer a Vendian age (550–540 Ma) of homogenization of the Sm-Nd isotopic system of the Kytlym and Knyaspa massifs within the Ural platinum-bearing belt. The Sm-Nd ages for metamorphic rocks of the Belaya Gora complex surrounding the studied massifs also agree with a Vendian age (573–574 Ma). Our results suggest that metamorphites of the Belaya Gora complex (amphibolites, plagiogneisses, two-mica and biotite gneisses, schists containing garnet, cordierite, staurolite, gedrite, and sillimanite) and dunite-clinopyroxenite-gabbro intrusions of the proto-platinum-bearing belt may have been formed in a convergence setting above a mantle plume, most likely in a back-arc (?) extension region. Reactivation of this plume during the late Precambrian resulted in the opening of the Ural paleo-ocean. The Late Ordovician-Early Devonian times were marked by metamorphic reworking and tectonic transport of pre-Paleozoic complexes into an accretionary prism setting of the Tagil paleo-arc that was accompanied by generation of gabbroid and granitoid magmas. Based on the obtained results, the Tagil terrane can be now considered as part of the Paleozoic paleo-island arc system developed on a heterogeneous Proterozoic basement.     

松辽盆地东南缘晚中生代火山事件的锆石年代学与地球化学制约

松辽盆地东南缘营城组地层中发育大量的晚中生代火山岩。SHRIMP锆石U-Pb测年表明,火山岩主要形成于117~110Ma,可以划分出早、晚两期,时代上属于早白垩世Albian期-Aptian期。岩石学和地球化学研究表明火山活动早期以酸性喷发为主,晚期则以中基性的玄武粗安岩喷发为主,基本上都属高钾钙碱性系列或亚碱系列,两期火山岩具有演化的亲缘性特征。Sr-Nd同位素研究表明,早、晚两期火山岩是同源岩浆演化的产物,来源于亏损型地幔源区,但演化过程又受到地壳物质不同程度的混染。松辽盆地东南缘火山事件的精确厘定对认识覆盖区晚中生代火山活动序列及邻区岩浆事件的对比具有重要的意义。