Extensional to compressive Mesozoic magmatism at the SE Eurasia margin as recorded from the Meratus ophiolite (SE Borneo,Indonesia)

The Meratus ophiolitic series (SE Borneo) present a specific assemblage that have recorded (1) a continental extensional episode mostly seen within the peridotites and 2) later subduction-related magmatic events marked by the emplacement of calc-alkaline magmas. These events relate the magmatic activity and geodynamic evolution of the SE Eurasia margin in Mesozoic times. The ophiolitic series comprise ultramafic rocks with minor metavolcanic rocks. The ultramafic rocks include dominant lherzolites and pyroxenites with rather scarce harzburgites and dunites. Spinel peridotite, mineral chemistry data and bulk rock Rare Earth Element (REE) abundances show that most rocks underwent a low degree of partial melting. However, a few samples display significant depletions in Light REE (LREE), which are interpreted as the result of fractional melting under shallow conditions. Plagioclase-bearing peridotites are characterized by high REE abundances which also point to a very low degree of melting followed by reequilibration in the plagioclase facies, as seen from phase chemistry data. These peridotites are locally crosscut by dikelets containing high-temperature K-and Cr-rich amphiboles. Lavas closely associated with the Meratus peridotites have REE compositions ranging from the ones typical of enriched MORB (E-MORB) to normal MORB (N-MORB) types. We believe that the Meratus peridotites represent a fragment of subcontinental lithospheric mantle that locally suffered a low degree of fractional melting during the last stages of a continental rifting phase, in agreement with the presence of metamorphic K- and Cr-rich amphiboles in the peridotites. The E-MORB basalts might result from the melting of an enriched subcontinental lithosphere thermally eroded during the rifting phase by rising asthenosphere which might have produced N-MORB volcanic rocks. Back-arc basin basalts (BABB) now associated with E-MORB and N-MORB have also been found in the metamorphic soles of the peridotites. These rocks would have formed in a back-arc basin now accreted to the eastern margin of Eurasia. The latter are partly covered by calc-alkaline magmatism (Alino Formation). The ophiolitic series was later crosscut by calc-alkaline melts (Manunggul Formation). The Meratus ophiolitic series hence displays a dual origin. They witness 1) a continental episode mostly seen within the peridotites. and 2) later subductionrelated events marked by the emplacement of calc-alkaline magmas.     

Geodynamic significance of the Cretaceous pillow basalts from North Anatolian Ophiolitic Mélange Belt (Central Anatolia,Turkey): geochemical and paleontological constraints

The most widespread blocks within the Cretaceous ophiolitic mélange (North Anatolian ophiolitic mélange) in Central Anatolia (Turkey) are pillow basalts, radiolarites, other ophiolitic fragments and Jurassic-Cretaceous carbonate blocks. The pillow basalts crop out as discrete blocks in close relation to radiolarites and ophiolitic units in Cretaceous ophiolitic mélange.

The geochemical results suggest that analyzed pillow basalts are within-plate ocean island alkali basalts. The enrichment of incompatible elements (Nb, Ta, Light REE, Th, U, Cs, Rb, Ba, K) demonstrates the ocean island environment (both tholeiites and alkali basalts) and enriched MORB. Dated calcareous intrafills and biodetrital carbonates reveal an age span of Callovian—Early Aptian. The thin-shelled protoglobigerinids, belonging to the genus Globuligerina, in the calcareous intrafills between pillow basalt lobes indicates a Callovian—Barremian age interval, most probably, Valanginian to Late Barremian. The volcanic and radiolarite detritus-bearing orbitolinid—Baccinella biodetrital carbonates dated as Late Barremian-Early Aptian in age, were probably deposited around atolls and have a close relationship with the ocean island pillow basalts.

The results collectively support the presence of a seamount on the Neo-Tethyan oceanic crust during the Valanginian—Late Barremian and atolls during the Late Barremian-Early Aptian interval. The presence of an oceanic crust older than that seamount along the Northern Branch of Neo-Tethys is conformable with the geodynamic evolution of the Tethys.     

西准噶尔洋中海山及其俯冲带处地质效应

随着研究的不断深入,在中亚造山带(CAOB)不断有不同时代的洋岛玄武岩(OIB)被识别出来。在中亚造山带西南缘的西准噶尔地区的多条蛇绿混杂岩带中,也存在具有OIB特征的玄武岩。这些玄武岩呈枕状,与超基性岩、辉长岩、块状玄武岩、灰岩及紫红色硅质岩等紧密共生。地球化学研究表明枕状玄武岩均为碱性系列,具有较高的TiO2含量(大多>2.5%)、强烈富集轻稀土元素、无明显Nb、Ta负异常,与典型的OIB极为相似,认为其可能形成于大洋板内与地幔柱有关的海山环境。通过对海山的发展阶段分析认为,西准噶尔地区海山应该发展到爆炸海山阶段,因为其中发育大量的枕状熔岩。海山中火山岩或火山碎屑沉积物富集大离子亲石元素和高场强元素,海山的俯冲将对弧及弧后地区火山岩地球化学产生明显影响,而西准噶尔地区泥盆纪-石炭纪火山岩中恰恰存在海山的信号。因此,海山俯冲模式可能能更好地解释西准噶尔地区火山岩中存在OIB特征火山岩的成因。另外,海山俯冲还存在潜在的资源效应,因此应该寻找和研究古海山及火山岛链俯冲的迹象,将对进一步找金铜等矿提供可靠依据。     

A new ophiolite occurrence in NW Sudan – constraints on Late Proterozoic tectonism

Mafic–ultramafic sequences in the Jebel Rahib fold-and-thrust belt of NW Sudan comprise the metamorphosed equivalents of obducted oceanic lithosphere. Primary features like pillow-structures, quenchtextures, igneous layering and primary minerals like chromian spinels and pyroxenes are partly preserved. Geochemical investigations confirm the field evidence and reveal the occurrence of harzburgites, ophiolitic chromites, gabbros, gabbroic cumulates and basalts of primitive tholeiitic compositions resembling transitional MORBs. The age of low-grade overprint of the basaltic rocks can be limited to 860–740 Myr (K–Ar). The existence of an ophiolite, as well as its close interfingering with highly deformed basin sediments, provides unambiguous evidence for a cycle of extensional and compressional tectonics in NW Sudan during Pan-African times. Consequently, the concept of an older, stable Proterozoic, or even Archaean, ‘African Craton’onto which the juvenile Pan-African Nubian Shield assemblages were accreted, needs revision.     

Subduction,ophiolite genesis and collision history of Tethys adjacent to the Eurasian continental margin: new evidence from the Eastern Pontides,Turkey

This paper presents several types of new information including U–Pb radiometric dating of ophiolitic rocks and an intrusive granite, micropalaeontological dating of siliceous and calcareous sedimentary rocks, together with sedimentological, petrographic and structural data. The new information is synthesised with existing results from the study area and adjacent regions (Central Pontides and Lesser Caucasus) to produce a new tectonic model for the Mesozoic–Cenozoic tectonic development of this key Tethyan suture zone.

The Tethyan suture zone in NE Turkey (Ankara–Erzincan–Kars suture zone) exemplifies stages in the subduction, suturing and post-collisional deformation of a Mesozoic ocean basin that existed between the Eurasian (Pontide) and Gondwanan (Tauride) continents. Ophiolitic rocks, both as intact and as dismembered sequences, together with an intrusive granite (tonalite), formed during the Early Jurassic in a supra-subduction zone (SSZ) setting within the ?zmir–Ankara–Erzincan ocean. Basalts also occur as blocks and dismembered thrust sheets within Cretaceous accretionary melange. During the Early Jurassic, these basalts erupted in both a SSZ-type setting and in an intra-plate (seamount-type) setting. The volcanic-sedimentary melange accreted in an open-ocean setting in response to Cretaceous northward subduction beneath a backstop made up of Early Jurassic forearc ophiolitic crust. The Early Jurassic SSZ basalts in the melange were later detached from the overriding Early Jurassic ophiolitic crust.

Sedimentary melange (debris-flow deposits) locally includes ophiolitic extrusive rocks of boninitic composition that were metamorphosed under high-pressure low-temperature conditions. Slices of mainly Cretaceous clastic sedimentary rocks within the suture zone are interpreted as a deformed forearc basin that bordered the Eurasian active margin. The basin received a copious supply of sediments derived from Late Cretaceous arc volcanism together with input of ophiolitic detritus from accreted oceanic crust.

Accretionary melange was emplaced southwards onto the leading edge of the Tauride continent (Munzur Massif) during latest Cretaceous time. Accretionary melange was also emplaced northwards over the collapsed southern edge of the Eurasian continental margin (continental backstop) during the latest Cretaceous. Sedimentation persisted into the Early Eocene in more northerly areas of the Eurasian margin.

Collision of the Tauride and Eurasian continents took place progressively during latest Late Palaeocene–Early Eocene. The Jurassic SSZ ophiolites and the Cretaceous accretionary melange finally docked with the Eurasian margin. Coarse clastic sediments were shed from the uplifted Eurasian margin and infilled a narrow peripheral basin. Gravity flows accumulated in thrust-top piggyback basins above accretionary melange and dismembered ophiolites and also in a post-collisional peripheral basin above Eurasian crust. Thickening of the accretionary wedge triggered large-scale out-of-sequence thrusting and re-thrusting of continental margin and ophiolitic units. Collision culminated in detachment and northward thrusting on a regional scale.

Collisional deformation of the suture zone ended prior to the Mid-Eocene (~45?Ma) when the Eurasian margin was transgressed by non-marine and/or shallow-marine sediments. The foreland became volcanically active and subsided strongly during Mid-Eocene, possibly related to post-collisional slab rollback and/or delamination. The present structure and morphology of the suture zone was strongly influenced by several phases of mostly S-directed suture zone tightening (Late Eocene; pre-Pliocene), possible slab break-off and right-lateral strike-slip along the North Anatolian Transform Fault.

In the wider regional context, a double subduction zone model is preferred, in which northward subduction was active during the Jurassic and Cretaceous, both within the Tethyan ocean and bordering the Eurasian continental margin.     

How many sutures in the southern Central Asian Orogenic Belt：Insights from East Xinjiang-West Gansu（NW China）？

How ophiolitic mèlanges can be defined as sutures is controversial with regard to accretionary orogenesis and continental growth.The Chinese Altay,East junggar,Tianshan,and Beishan belts of the southern Central Asian Orogenic Belt（CAOB） in Northwest China,offer a special natural laboratory to resolve this puzzle.In the Chinese Altay,the Erqis unit consists of ophiolitic melanges and coherent assemblages,forming a Paleozoic accretionary complex.At least two ophiolitic melanges（Armantai,and Kelameili） in East Junggar,characterized by imbricated ophiolitic melanges,Nb-enriched basalts,adakitic rocks and volcanic rocks,belong to a Devonian-Carboniferous intra-oceanic island arc with some Paleozoic ophiolites,superimposed by Permian arc volcanism.In the Tianshan,ophiolitic melanges like Kanggurtag,North Tianshan,and South Tianshan occur as part of some Paleozoic accretionary complexes related to amalgamation of arc terranes.In the Beishan there are also several ophiolitic melanges,including the Hongshishan,Xingxingxia-Shibangjing,Hongliuhe-Xichangjing,and Liuyuan ophiolitic units.Most ophiolitic melanges in the study area are characterized by ultramafic,mafic and other components,which are juxtaposed,or even emplaced as lenses and knockers in a matrix of some coherent units.The tectonic settings of various components are different,and some adjacent units in the same melange show contrasting different tectonic settings.The formation ages of these various components are in a wide spectrum,varying from Neoproterozoic to Permian.Therefore we cannot assume that these ophiolitic melanges always form in linear sutures as a result of the closure of specific oceans.Often the ophiolitic components formed either as the substrate of intra-oceanic arcs,or were accreted as lenses or knockers in subduction-accretion complexes.Using published age and paleogeographic constraints,we propose the presence of （1） a major early Paleozoic tectonic boundary that separates the Chinese Altay-East Junggar multiple subduction system     

班公湖—双湖—怒江—昌宁—孟连对接带时空结构——特提斯大洋地质及演化问题

班公湖—双湖—怒江（中北段）—昌宁—孟连对接带广泛出露特提斯大洋岩石圈俯冲消减过程中产生的不同时代、不同构造环境、不同变质程度、不同变形样式的洋板块构造地层系统、增生混杂的构造—岩石组合体,可识别出增生的远洋沉积岩、海沟浊积岩、古生代—中生代蛇绿岩、蛇绿混杂岩、洋岛-海山消减增生楔、洋底沉积增生杂岩,基底残块以及以蓝片岩、榴辉岩为代表的高压—超高压变质岩带,记录了青藏高原原古特提斯大洋形成演化的地质信息。班公湖—双湖—怒江—昌宁—孟连对接带是青藏高原中部一条重要的原古特提斯大洋自北向南后退式俯冲消亡的巨型增生杂岩带,构筑了冈瓦纳大陆与劳亚-泛华夏大陆分界带。     

http://www.sciencedirect.com/science/article/pii/S1674987114000401

In this paper, a new discrimination diagram using absolute measures of Th and Nb is applied to post-Archean ophiolites to best discriminate a large number of different ophiolitic basalts. This diagram ...     

Relicts of an intra-oceanic arc in the Sapi-Shergol mélange zone (Ladakh, NW Himalaya, India): implications for the closure of the Neo-Tethys Ocean

In the Ladakh–Zanskar area, relicts of both ophiolites and paleo-accretionary prism have been preserved in the Sapi-Shergol mélange zone. The paleo-accretionary prism, related to the northward subduction of the northern Neo-Tethys beneath the Ladakh Asian margin, mainly consists of tectonic intercalations of sedimentary and blueschist facies rocks. Whole rock chemical composition data provide new constraints on the origin of both the ophiolitic and the blueschist facies rocks. The ophiolitic rocks are interpreted as relicts of the south Ladakh intra-oceanic arc that were incorporated in the accretionary prism during imbrication of the arc. The blueschist facies rocks were previously interpreted as oceanic island basalts (OIB), but our new data suggest that the protolith of some of the blueschists is a calc-alkaline igneous rock that formed in an arc environment. These blueschists most likely originated from the south Ladakh intra-oceanic arc. This arc was accreted to the southern margin of Asia during the Late Cretaceous and the buried portion was metamorphosed under blueschist facies conditions. Following oceanic subduction, the external part of the arc was obducted to form the south Ladakh ophiolites or was incorporated into the Sapi-Shergol mélange zone. The incorporation of the south Ladakh arc into the accretionary prism implies that the complete closure of the Neo-Tethys likely occurred by Eocene time.     

Geotectonic evolution of the Western Cordillera of Colombia: New aspects from geochemical data on volcanic rocks

The Western Cordillera of Colombia (WCC) is part of the Basic Igneous Complex (BIC), which is one of the world's largest ophiolitic complexes, extending from Costa Rica through Panama and Colombia to Ecuador. Major and trace element data on 32 volcanic rocks from the central and northern parts of the Western Cordillera are presented; no data have been available to date for volcanic rocks from the northern parts of the Western Cordillera. Petrographical and geochemical investigations show that the rocks are altered and have undergone low-grade metamorphism. The subalkaline rocks are represented by tholeiitic basalts, calc-alkaline basic andesites, andesites, and one dacite. It is concluded that a mature oceanic island arc existed in the Cretaceous, in what is now the northern part of the Western Cordillera. The tectonics of the region, particularly the intensive imbrication of the chain, indicates the presence of a paleo-subduction zone with an oceanic island arc that accreted on the old continental margin. These new data, combined with new and previous data from the central part of the BIC of Colombia, suggest that volcanic rocks of the Western Cordillera can be interpreted as allochthonous slabs. These slabs were imbricated with back-arc and fore-arc sediments and tonalitic bodies during the closing of a back-arc basin in northwestern South America and accretion of an oceanic island arc. Oblique subduction accreted these different areas to the continental margin during Late Cretaceous and early Tertiary times. Two plate-tectonic models are proposed: a) development of the calc-alkaline volcanic rocks in the northern parts of the Western Cordillera, separated by tholeiitic rocks, formed along a transform fault represented by the tholeiitic basalts of the central and southern parts of the Western Cordillera; or b) development of an oceanic island arc along the Cretaceous continental margin of northwestern South America. In the central and southern parts of this island arc, accretion took place early and therefore only an island-arc tholeiitic suite was formed.     

东昆仑南缘布青山构造混杂带得力斯坦南MOR型玄武岩地质、地球化学特征及岩石成因

东昆仑南缘布青山构造混杂带发育有较多OIB型玄武岩, 这类玄武岩成因与地幔柱密切相关.与灰岩密切伴生的具有MOR型特征的基性火山岩亦是东昆仑南缘古特提斯洋盆一类重要的海山玄武岩.为了查明布青山构造混杂带中不同类型洋岛或海山玄武岩的岩石成因, 对得力斯坦南玄武岩进行了详细的地质、地球化学和岩石成因研究.布青山地区得力斯坦南出露的玄武岩岩石类型复杂多样, 主要由枕状玄武岩、气孔-杏仁状玄武岩、角砾状玄武岩和块状玄武岩组成.主量元素地球化学特征表明, 该套玄武岩属于深海拉斑玄武岩和洋脊拉斑玄武岩系列.得力斯坦南玄武岩∑REE介于34.51×10-6~61.60×10-6, LREE/HREE介于0.89~1.37, (La/Yb)_N介于0.30~0.56, δEu介于0.90~1.18.球粒陨石标准化稀土元素配分图呈现轻稀土元素亏损的左倾型, 与NMORB型玄武岩稀土元素配分曲线基本相同.得力斯坦南玄武岩Zr、Hf、Nb和Ta含量均相当于NMORB的相应元素的丰度值.Zr/Nb值介于24.59~57.69, Nb/La值介于0.45~0.94, Hf/Ta值介于18.29~31.94.在原始地幔标准化微量元素蛛网图上, 曲线右侧高场强元素基本未分异(Nb、Ta、Zr、Hf等), 并贴近于NMORB标准线, 具有与NMORB玄武岩相似而明显不同于EMORB和OIB型玄武岩的特征.微量元素判别表明其形成于洋中脊或由于洋脊扩张向两侧后移的洋中脊构造环境, 结合其上覆盖有深水硅泥岩及浅水厚层状碳酸盐岩的地质事实, 认为其在地形地貌上属于古海山.岩石成因研究表明该套玄武岩起源于亏损地幔(DM), 并估算其为地幔二辉橄榄岩发生约10%部分熔融的产物.      

Late Cretaceous oceanic crust and Early Tertiary foreland basin development, Euboea, Eastern Greece

In northern Euboea (Eastern Greece), Late Cretaceous platform carbonates of the Pelagonian Zone pass depositionally upwards into Maastrichtian hemipelagic limestones, possibly reflecting a rifting event in the adjacent Neotethys. This is followed by a c. 1 km-thick unit of siliciclastic turbidites, debris flows and detached limestone blocks. Thrust intercalations of ophiolitic rocks comprise altered pillow basalts and ultramafic rocks with ophicalcite. Calcite veins in sheared serpentinite contain planktonic foraminifera and the ophicalcite is directly overlain, with a depositional contact, by Globotruncana-bearing pelagic limestones and calciturbidites of Maastrichtian age. The ophiolitic rocks are interpreted as Late Cretaceous oceanic crust and mantle, that formed at a fracture zone, or rifted spreading axis within a Neotethyan, Vardar basin to the east. During the Early Tertiary (Palaeocene–Eocene), the Neotethyan basin began to close, with development of a subduction-accretion complex, mainly comprising sheared, trench-type sandstones, associated with ophiolitic slices. In response to trench/margin collision, the Pelagonian carbonate platform foundered and limestone debris flows and olistoliths were shed into a siliciclastic foreland basin. Suturing of the Neotethyan ocean basin then resulted in westwards thrusting of oceanic units over the foreland basin, thrusting of slices of inferred Late Cretaceous Pelagonian carbonate platform slope and large-scale recumbent folding.     

Geochemical discrimination of tectonic setting for Devonian basalts of the Yarrol Province of the New England Orogen,central coastal Queensland: An empirical approach *

Fault blocks and inliers of uppermost Silurian to Middle Devonian strata in the Yarrol Province of central coastal Queensland have been interpreted either as island-arc deposits or as a continental-margin sequence. They can be grouped into four assemblages with different age ranges, stratigraphic successions, geophysical signatures, basalt geochemistry, and coral faunas. Basalt compositions from the Middle Devonian Capella Creek Group at Mt Morgan are remarkably similar to analyses from the modern Kermadec Arc, and are most consistent with an intra-oceanic arc associated with a backarc basin. They cannot be matched with basalts from any modern continental arc, including those with a thin crust (Southern Volcanic Zone of the Andes) or those built on recently accreted juvenile oceanic terranes (Eastern Volcanic Front of Kamchatka). Analyses from the other assemblages also suggest island-arc settings, although some backarc basin basalt compositions could be present. Arguments for a continental-margin setting based on structure, provenance, and palaeogeography are not conclusive, and none excludes an oceanic setting for the uppermost Silurian to Middle Devonian rocks. The Mt Morgan gold–copper orebody is associated with a felsic volcanic centre like those of the modern Izu–Bonin Arc, and may have formed within a submarine caldera. The data are most consistent with formation of the Capella Creek Group as an intra-oceanic arc related to an east-dipping subduction zone, with outboard assemblages to the east representing remnant arc or backarc basin sequences. Collision of these exotic terranes with the continent probably coincided with the Middle–Upper Devonian unconformity at Mt Morgan. An Upper Devonian overlap sequence indicates that all four assemblages had reached essentially their present relative positions early in Late Devonian time. Apart from a small number of samples with compositions typical of spreading backarc basins, Upper Devonian basalts and basaltic andesites of the Lochenbar and Mt Hoopbound Formations and the Three Moon Conglomerate are most like tholeiitic or transitional suites from evolved oceanic arcs such as the Lesser Antilles, Marianas, Vanuatu, and the Aleutians. However, they also match some samples from the Eastern Volcanic Front of Kamchatka. Their rare-earth and high field strength element patterns are also remarkably similar to Upper Devonian island arc tholeiites in the ophiolitic Marlborough terrane, supporting a subduction-related origin and a lack of involvement of continental crust in their genesis. Modern basalts from rifted backarc basins do not match the Yarrol Province rocks as well as those from evolved oceanic arcs, and commonly have consistently higher MgO contents at equivalent levels of rare-earth and high field strength elements. One of the most significant points for any tectonic model is that the Upper Devonian basalts become more arc-like from east to west, with all samples that can be matched most readily with backarc basin basalts located along the eastern edge of the outcrop belt. It is difficult to account for all geochemical variations in the Upper Devonian basalts of the Yarrol Province by any simplistic tectonic model using either a west-dipping or an east-dipping subduction zone. On a regional scale, the Upper Devonian rocks represent a transitional phase in the change from an intra-oceanic setting, epitomised by the Middle Devonian Capella Creek Group, to a continental margin setting in the northern New England Orogen in the Carboniferous, but the tectonic evolution must have been more complex than any of the models published to date. Certainly there are many similarities to the southern New England Orogen, where basalt geochemistry indicates rifting of an intra-oceanic arc in Middle to Late Devonian time.     

The Quebradagrande Complex: A Lower Cretaceous ensialic marginal basin in the Central Cordillera of the Colombian Andes

The Quebradagrande Complex of Western Colombia consists of volcanic and Albian–Aptian sedimentary rocks of oceanic affinity and outcrops in a highly deformed zone where spatial relationships are difficult to unravel. Berriasian–Aptian sediments that display continental to shallow marine sedimentary facies and mafic and ultramafic plutonic rocks are associated with the Quebradagrande Complex. Geochemically, the basalts and andesites of the Quebradagrande Complex mostly display calc-alkaline affinities, are enriched in large-ion lithophile elements relative to high field strength elements, and thus are typical of volcanic rocks generated in supra-subduction zone mantle wedges. The Quebradagrande Complex parallels the western margin of the Colombian Andes’ Central Cordillera, forming a narrow, discontinuous strip fault-bounded on both sides by metamorphic rocks. The age of the metamorphic rocks east of the Quebradagrande Complex is well established as Neoproterozoic. However, the age of the metamorphics to the west – the Arquía Complex – is poorly constrained; they may have formed during either the Neoproterozoic or Lower Cretaceous. A Neoproterozoic age for the Arquía Complex is favored by both its close proximity to sedimentary rocks mapped as Paleozoic and its intrusion by Triassic plutons. Thus, the Quebradagrande Complex could represent an intracratonic marginal basin produced by spreading-subsidence, where the progressive thinning of the lithosphere generated gradually deeper sedimentary environments, eventually resulting in the generation of oceanic crust. This phenomenon was common in the Peruvian and Chilean Andes during the Uppermost Jurassic and Lower Cretaceous. The marginal basin was trapped during the collision of the Caribbean–Colombian Cretaceous oceanic plateau, which accreted west of the Arquía Complex in the Early Eocene. Differences in the geochemical characteristics of basalts of the oceanic plateau and those of the Quebradagrande Complex indicate these units were generated in very different tectonic settings.     

西藏白朗地区蛇绿岩火山岩中单斜辉石的化学特征

近年来,应用岩石的地球化学和矿物学特征区分不同构造位置的现代火山岩,已被人们所注意。同时,根据这种观测资料所确定的一般特征,亦已广泛地应用于古火山岩,并获得了一些古地质环境的概念。     

Geochemistry of the Boil Mountain ophiolitic complex,northwest Maine,and tectonic implications

A coherent ophiolitic complex of pyroxenite, serpentinite, metagabbro, mafic volcanics, felsic volcanics and sediments crops out in NW Maine, adjacent to the Chain Lakes massif. The complex (here informally referred to as the Boil Mountain ophiolitic complex) is about 500 m.y. old. The volcanic sequence is not typical of ophiolites in that it contains a large proportion of felsic volcanics. The mafic volcanics are divided into two geochemical groups. A stratigraphically lower group is depleted in Ti, Zr, Y, Cr and REE contents similar to basalts from supra-subduction zone ophiolites. An upper mafic group has trace element contents similar to normal mid-ocean ridge basalts. The felsic volcanics are mostly rhyolitic and similar to low-K rhyolites found in the forearc of the Marianas trench and in an island arc sequence in the Klamath Mountains, California. The flat REE patterns of the felsic volcanic rocks are similar to those found in siliceous rocks in the Oman ophiolite. The presence of thick sequences of felsic volcanics, the abundance of pyroxenite, the low Ti, Zr and REE contents of some mafic rocks, the flat REE patterns of the felsic volcanics, and the composition of clinopyroxene all suggest the complex was formed in the vicinity of a subduction zone. The complex may be correlated with ophiolitic fragments in the eastern part of the Dunnage Zone in Newfoundland, rather than the main ophiolite belt of the western Appalachians.     

http://www.sciencedirect.com/science/article/pii/S1674987111001125

<正>Greenstone belts of the eastern Dharwar Craton,India are reinterpreted as composite tectonostratigraphic terranes of accreted plume-derived and convergent margin-derived magmatic sequences based on new high-precision elemental data.The former are dominated by a komatiile plus Mg-tholeiitic basalt volcanic association,with deep water siliciclastic and banded iron formation(BIF) sedimentary rocks.Plumes melted at90 km under thin rifted continental lithosphere to preserve inlraoceanic and continental margin aspects.Associated alkaline basalts record subduction-recycling of Mesoarchean oceanic crust,incubated in the asthenosphere.and erupted coevally with Mg basalts from a heterogeneous mantle plume.Together.komaliites-Mg basalts-alkaline basalts plot along the Phanerozoic mantle array in Th/Yb versus Nb/Yb coordinate space,representing zoned plumes,establishing that these reservoirs were present in the Neoarchean mantle. Convergent margin magmatic associations are dominated by tholeiitic to calc-alkaline basalts eompositionally similar to recent intraoceanic arcs.As well,boninitic flows sourced in extremely depleted mantle are present,and the association of arc basalts with Mg-andesites-Nb enriched basalts-adakites documented from Cenozoic arcs characterized by subduction of young(20 Ma),hot,oceanic lithosphere. Consequently.Cenozoic style "hot" subduction was operating in the Neoarchean.These diverse volcanic associations were assembled to give composite terranes in a subduction-accretion orogen at～2.1 Ga,coevally with a global accretionary orogen at ~2.7 Ga,and associated orogenic gold mineralization. Archean lithospheric mantle,distinctive in being thick,refractory,and buoyant,formed complementary to the accreted plume and convergent margin terranes.as migrating arcs captured thick plumeplateaus. and the refractory,low density.residue of plume melting coupled with accreted imbricated plume-arc crust.     

Middle Paleozoic mafic magmatism and ocean plate stratigraphy of the South Tianshan,Kyrgyzstan

The tectonic history of the Kyrgyz South Tianshan in the western Central Asian Orogenic Belt (CAOB) remains controversial, first of all, due to the limited amount of geochemical and isotope data. Our paper presents the first results of a detailed geochemical study (major and trace elements, Sr, Nd and Pb isotopes) of Middle Paleozoic mafic volcanic and subvolcanic rocks of the Ferghana and Atbashi–Kokshaal accretionary belts of the South Tianshan orogen in Kyrgyzstan, which formed during the evolution of the Turkestan Ocean. A special focus is given to the relation between magmatic rocks and sedimentary units of marine origin, chert, siliceous shale/mudstone, volcanogenic–carbonate clastics, seamount carbonates, and turbidites, which we consider as elements of Ocean Plate Stratigraphy (OPS). The age range of marine sediments is Late Silurian to Early Carboniferous, but the age of the most volcanic rocks associated with fossil-bearing OPS sediments is Devonian. The magmatic rocks have geochemical affinity to oceanic island basalts (OIB-type) and, to a lesser extent, mid-oceanic ridge (MORB-type) basalts associated with hemipelagic siliceous mudstone and pelagic chert. The rocks with OIB-type affinity are associated with chert, siliceous shale and carbonaceous clastics and carbonates. They are enriched in TiO₂, LREE (La/Sm_n = 1.9), and Nb (Zr/Nb_av. = 10), have differentiated HREE (Gd/Yb_n = 2.0), medium to low εNd (~ 5.7) and are characterized by clear Nb positive anomalies in normalized multi-element plots (Nb/Th_pm = 1.3, Nb/La_pm = 1.1). The OIBs formed by relatively low degrees of melting (< 5%) of mantle sources in the garnet stability field and erupted in an oceanic island setting. The MORB-type samples associated with siliceous mudstone and chert are less enriched in incompatible elements, possess flat REE and multi-element pattern, and show higher εNd values (~ 9.1); they were probably produced by high-degree melting of spinel lherzolite and/or harzburgite and erupted in a mid-oceanic ridge setting. The geological, lithological and geochronological data suggest that the OPS units with dominantly OIB-type basalts formed at one or several seamount chains of the Turkestan Ocean, which were accreted to the Kazakhstan continent, and thus contribute to our understanding of the Paleozoic tectonic evolution of the western CAOB during the Serpukhovian–Bashkirian.     

冲绳海槽火山岩岩石系列的厘定及构造环境意义

冲绳海槽是一个处于弧后扩张作用早期的年轻的弧后盆地,是研究弧后扩张作用早期盆地演化和壳幔过程的天然实验室.随着调查研究工作的逐步展开和深入,也发现了一些新的、重要的、亟待解决的科学问题,而火山岩岩石系列归属的厘定又是其他研究工作的基础.在系统收集和整理迄今已有冲绳海槽火山岩资料的基础上,结合近期分析测试数据, 对冲绳海槽火山岩的岩石系列归属进行了重新厘定,探讨了火山岩的构造环境指示意义和浮岩与玄武岩之间的成因联系.研究结果表明:冲绳海槽火山岩分布具有以基性玄武岩和酸性(流纹)英安岩为主的双峰式特征,中性火山岩稀少,基性的玄武岩属于亚碱性系列的橄榄拉斑玄武岩,酸性浮岩可归属为亚碱性岩系的流纹英安岩或流纹岩;在构造环境判别上,冲绳海槽玄武岩表现出大洋中脊和岛弧构造环境的特点,既有别于大洋中脊扩张中心,也有别于成熟型弧后盆地,呈现出弧后早期扩张阶段盆地独特的构造环境特征;广泛分布于冲绳海槽的酸性浮岩表现出一定的岛弧环境的特点;酸性浮岩与玄武岩具有同源性,酸性岩是基性的玄武质岩浆经不同程度结晶分异和同化混染作用的产物.      

Origin of submarine volcanism at the eastern margin of the central atlantic: Investigation of the alkaline volcanic rocks of the carter seamount (Grimaldi Seamounts)

This paper addresses the composition, geochemistry, isotopic characteristics, and age of rocks from the Carter Seamount of the Grimaldi seamount group at the eastern margin of the Central Atlantic. The age of the seamount was estimated as 57–58 Ma. Together with other seamounts of the Grimaldi system and the Nadir Seamount, it forms a “hot line” related to the Guinea Fracture Zone, which was formed during the late Paleocene pulse of volcanism. The Carter Seamount is made up of olivine melilitites, ankaramites, and analcime-bearing nepheline tephrites, which are differentiated products of the fractional crystallization of melts similar to an alkaline ultramafic magma. The volcanics contain xenoliths entrained by melt at different depths from the mantle, layer 3 of the oceanic crust, which was formed at 113–115 Ma, and earlier magma chambers. The rocks were altered by low-temperature hydrothermal solutions. The parental melts of the volcanics of the Carter Seamount were derived at very low degrees of mantle melting in the stability field of garnet lherzolite at depths of no less than 105 km. Anomalously high Th, Nb, Ta, and La contents in the volcanics indicate that a metasomatized mantle reservoir contributed to the formation of their primary melts. The Sr, Pb, and Nd isotopic systematics of the rocks show that the composition of the mantle source lies on the mixing line between two mantle components. One of them is a mixture of prevailing HIMU and the depleted mantle, and the other is an enriched EM2-type mantle reservoir. These data suggest that the formation of the Carter Seamount volcanics was caused by extension-related decompression melting in the Guinea Fracture Zone of either (1) hot mantle plume material (HIMU component) affected by carbonate metasomatism or (2) carbonated basic enclaves (eclogites) ubiquitous in the asthenosphere, whose isotopic characteristics corresponded to the HIMU and EM2 components. In the former case, it is assumed that the melt assimilated during ascent the material of the metasomatized subcontinental mantle (EM2 component), which was incorporated into the oceanic lithospheric mantle during rifting and the breakup of Pangea.