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Island arc elements and arc-related ophiolites 总被引：1，自引：0，他引：1
Evan C. Leitch 《Tectonophysics》1984,106(3-4)
All major structural elements in island arc systems, fore-arc, magmatic arc, back-arc basins and remnant arcs, are potential ophiolite sources, and those features that allegedly characterise ophiolites of ocean-ridge origin, sheeted dyke complexes, mantling pelagic rocks, hydrothermal metamorphism and associated mineralization, can also arise within arc settings. Age relations are critical in the interpretation of arc-related ophiolites. Remnants of oceanic lithosphere, identified by a pre-arc initiation age, are restricted to fore-arc, magmatic arc and remnant arc elements, as are ophiolite masses generated at the inception of underthrusting. The latter, apparently common in ancient fore-arc terrains, form in nascent arc systems in which the rate of role back of the subduction hinge exceeds the rate of convergence. Spreading occurs above a foundering slab resulting in some arc-like compositional features. In simple arc systems later ophiolitic rocks have formed either in the active back-arc basin or the magmatic arc. Only those ophiolites that have resided within or very close behind magmatic arcs should show calcalkaline or arc tholeiite magmatic affinities, or be intruded or overlain by these rocks. Volcanic-derived sediment or pelagic material may mantle ophiolites from all arc settings, but pelagic rocks will generally dominate in stratigraphic sequences above remnant arcs and on back-arc basin floors except adjacent to the magmatic arc. Ophiolites generated at major ocean ridges are unlikely to be immediately overlain by sediment with a significant volcanic component whereas such detritus may lie directly on arc-inception, arc and back-arc ophiolites. Some arc-derived ophiolites are preserved in their original tectonic position, others can be identified from their internal features, their relationship to other tectonic elements, and the nature of associated rocks. 相似文献
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Plate kinematics, origin and tectonic emplacement of supra-subduction ophiolites in SE Asia 总被引：2，自引：0，他引：2
A unique feature of the Circum Pacific orogenic belts is the occurrence of ophiolitic bodies of various sizes, most of which display petrological and geochemical characteristics typical of supra-subduction zone oceanic crust. In SE Asia, a majority of the ophiolites appear to have originated at convergent margins, and specifically in backarc or island arc settings, which evolved either along the edge of the Sunda (Eurasia) and Australian cratons, or within the Philippine Sea Plate. These ophiolites were later accreted to continental margins during the Tertiary. Because of fast relative plate velocities, tectonic regimes at the active margins of these three plates also changed rapidly. Strain partitioning associated with oblique convergence caused arc-trench systems to move further away from the locus of their accretion. We distinguish “relatively autochthonous ophiolites” resulting from the shortening of marginal basins such as the present-day South China Sea or the Coral Sea, and “highly displaced ophiolites” developed in oblique convergent margins, where they were dismantled, transported and locally severely sheared during final docking. In peri-cratonic mobile belts (i.e. the Philippine Mobile Belt) we find a series of oceanic basins which have been slightly deformed and uplifted. Varying lithologies and geochemical compositions of tectonic units in these basins, as well as their age discrepancies, suggest important displacements along major wrench faults.We have used plate tectonic reconstructions to restore the former backarc basins and island arcs characterized by known petro-geochemical data to their original location and their former tectonic settings. Some of the ophiolites occurring in front of the Sunda plate represent supra-subduction zone basins formed along the Australian Craton margin during the Mesozoic. The Philippine Sea Basin, the Huatung basin south of Taiwan, and composite ophiolitic basements of the Philippines and Halmahera may represent remnants of such marginal basins. The portion of the Philippine Sea Plate carrying the Taiwan–Philippine arc and its composite ophiolitic/continental crustal basement might have actually originated in a different setting, closer to that of the Papua New Guinea Ophiolite, and then have been displaced rapidly as a result of shearing associated with fast oblique convergence. 相似文献
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准噶尔、天山和北山52个蛇绿岩的地质特征、地球化学性质和同位素年代学资料系统集成研究表明它们可以分为14条蛇绿(混杂)岩带。绝大多数蛇绿岩呈"岩块+基质"的混杂岩型式沿重要断裂带(构造线)线状分布,少数蛇绿岩以构造岩片叠置方式面状产出。混杂岩的基质有蛇纹岩(碳酸盐化蛇纹岩)和糜棱岩化细碎屑岩两类,岩块既有地幔橄榄岩、基性杂岩和基性火山岩等蛇绿岩组分,也有其它非蛇绿岩组分岩石。堆晶岩出露局限,典型席状岩墙群没有发育。这些蛇绿岩可归类为SSZ(Supra-Subduction Zone)和MORB(Mid-Ocean Ridge)两种类型,前者玄武岩具大离子亲石元素(LILE)富集和高场强元素(HFS)亏损特征,后者不显示该特点;洋岛玄武岩(OIB)既可出现在SSZ型蛇绿混杂岩中,也可为MORB型的组成部分;SSZ型蛇绿混杂岩辉长岩和玄武岩比MORB型具有相对更富集的Sr-Nd同位素组成,但部分形成于弧后(间)盆地的SSZ型蛇绿岩与MORB型一致,具有近亏损地幔的Sr-Nd同位素组成。已确认的最老蛇绿岩为西准噶尔572 Ma玛依勒,次之为北山542~527 Ma月牙山—洗肠井和西准噶尔531 Ma唐巴勒,最年轻蛇绿岩为325 Ma北天山巴音沟和321 Ma北山芨芨台子。根据蛇绿岩证据,结合近年来中亚造山带古地磁、岩浆岩、高压—超高压变质岩和构造地质方面的进展,可以推断埃迪卡拉纪末期—早寒武世,古亚洲洋已达到一定规模宽度,发育洋岛和洋内弧;早古生代时期,多岛洋格局发育至鼎盛期,一系列弧地体分别归属哈萨克斯坦微陆块周缘的科克切塔夫—天山—北山线性弧、成吉思弧、巴尔喀什—西准噶尔弧体系和西伯利亚南部大陆边缘弧体系;晚古生代时期,古亚洲洋于石炭纪末期闭合,增生杂岩和弧地体组成哈萨克斯坦拼贴体系和蒙古拼贴体系两个巨型山弯构造。 相似文献
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The paper reviews and integrates geological, geochronological, geochemical and isotope data from 21 intra-oceanic arcs (IOA) of the Paleo-Asian Ocean (PAO), which have been identified in the Central Asian Orogenic belt, the world largest accretionary orogeny. The data We discuss structural position of intra-oceanic arc volcanic rocks in association with back-arc terranes and accretionary complexes, major periods of intra-oceanic arc magmatism and related juvenile crustal growth, lithologies of island-arc terranes, geochemical features and typical ranges of Nd isotope values of volcanic rocks. Four groups of IOAs have been recognized: Neoproterozoic – early Cambrian, early Paleozoic, Middle Paleozoic and late Paleozoic. The Neoproterozoic – early Cambrian or Siberian Group includes eleven intra-oceanic arcs of eastern and western Tuva-Sayan (southern Siberia, Russia), northern and southwestern Mongolia and Russian Altai. The Early Paleozoic or Kazakhstan Group includes Selety-Urumbai, Bozshakol-Chingiz and Baydaulet-Aqastau arc terranes of the Kazakh Orocline. The Middle Paleozoic or Southern Group includes six arc terranes in the Tienshan orogen, Chinese Altai, East-Kazakhstan-West Junggar and southern Mongoia. Only one Late Paleozoic intra-oceanic arc has been reliably identified in the CAOB: Bogda in the Chinese Tienshan, probably due to PAO shrinking and termination. The lithologies of the modern and fossil arcs are similar, although the fossil arcs contain more calc-alkaline varieties suggesting either their more evolved character or different conditions of magma generation. Of special importance is identification of back-arc basins in old accretionary orogens, because boninites may be absent in both modern and fossil IOAs. The three typical scenarios of back-arc formation - active margin rifting, intra-oceanic arc rifting and fore-arc rifting were reconstructed in fossil intra-oceanic arcs. Some arcs might be tectonically eroded and/or directly subducted into the deep mantle. Therefore, the structural and compositional records of fossil intra-oceanic arcs in intracontinental orogens allow us to make only minimal estimations of their geometric length, life span, and crust thickness. 相似文献
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The Altai-Salair area in southern Siberia is a Caledonian folded area containing fragments of Vendian–Early Cambrian island arcs. In the Vendian–Early Cambrian, an extended system of island arcs existed near the Paleo-Asian Ocean/Siberian continent boundary and was located in an open ocean realm. In the present-day structural pattern of southern Siberia, the fragments of Vendian–Early Cambrian ophiolites, island arcs and paleo-oceanic islands occur in the accretion–collision zones. We recognized that the accretion–collision zones were mainly composed of the rock units, which were formed within an island-arc system or were incorporated in it during the subduction of the Paleo-Asian Ocean under the island arc or the Siberian continent. This system consists of accretionary wedge, fore-arc basin, primitive island arc and normal island arc. The accretionary wedges contain the oceanic island fragments which consist of OIB basalts and siliceous—carbonate cover including top and slope facies sediments. Oceanic islands submerged into the subduction zone and, later were incorporated into an accretionary wedge. Collision of oceanic islands and island arcs in subduction zones resulted in reverse currents in the accretionary wedge and exhumation of high-pressure rocks. Our studies of the Gorny Altai and Salair accretionary wedges showed that the remnants of oceanic crust are mainly oceanic islands and ophiolites. Therefore, it is important to recognize paleo-islands in folded areas. The study of paleo- islands is important for understanding the evolution of accretionary wedges and exhumation of subducted high-pressure rocks. 相似文献
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祁连山蛇绿岩带和原特提斯洋演化 总被引：2，自引：1，他引：1
位于阿拉善地块和柴达木地块之间的祁连造山带记录原特提斯洋扩张、俯冲、闭合、大陆边缘增生和碰撞造山的完整过程。从南向北,祁连造山带发育有三条平行排列、不同类型的蛇绿岩带:(1)南部南祁连洋底高原-洋中脊-弧后蛇绿岩混杂带;(2)中部托勒山洋中脊型蛇绿岩带;(3)北部走廊南山SSZ型蛇绿岩带。南部南祁连蛇绿混杂岩带以拉脊山-永靖蛇绿岩为代表,为典型的洋底高原型蛇绿岩,是大洋板内地幔柱活动的产物,形成年龄为525～500Ma;中部托勒山蛇绿岩带沿熬油沟-玉石沟-冰沟-永登一线分布,为大洋中脊型蛇绿岩,蛇绿岩形成年龄为550～495Ma;北部蛇绿岩带包括弧前和弧后两种类型,弧前蛇绿岩以大岔大阪蛇绿岩为代表,形成时代为517～487Ma,反映初始俯冲/弧前扩张到弧后盆地的过程;弧后蛇绿岩以九个泉-老虎山蛇绿岩为代表,为典型的SSZ型蛇绿岩,是弧后扩张的产物,形成时代为奥陶纪(490～445Ma)。三个蛇绿岩带分别代表了新元古代-早古生代祁连洋演化历史不同环境的产物,对了解秦祁昆构造带原特提斯洋的构造演化过程有重要意义。蛇绿岩及弧火山岩的时空分布特征限定了原特提斯洋的俯冲极性为向北消减俯冲。 相似文献
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Geochronological, geochemical, and structural studies of magmatic and metamorphic complexes within the Kyrgyz North Tianshan (NTS) revealed an extensive area of early Palaeozoic magmatism with an age range of 540–475 Ma. During the first episode at 540–510 Ma, magmatism likely occurred in an intraplate setting within the NTS microcontinent and in an oceanic arc setting within the Kyrgyz-Terskey zone in the south. During the second episode at 500–475 Ma, the entire NTS represented an arc system. These two phases of magmatism were separated by an episode of accretionary tectonics of uncertain nature, which led to obduction of ophiolites from the Kyrgyz-Terskey zone onto the microcontinent. The occurrence of zircon xenocrysts and predominantly negative whole-rock ɛNd(t) values and ɛHf(t) values of magmatic zircons suggest a continental setting and melting of Precambrian continental sources with minor contributions of Palaeozoic juvenile melts in the generation of the magmatic rocks. The late Cambrian to Early Ordovician 500–475 Ma arc evolved mainly on Mesoproterozoic continental crust in the north and partly on oceanic crust in the south. Arc magmatism was accompanied by spreading in a back-arc basin in the south, where supra-subduction ophiolitic gabbros yielded ages of 496 to 479 Ma. The relative position of the arc and active back-arc basin implies that the subduction zone was located north of the arc, dipping to the south. Variably intense metamorphism and deformation in the NTS reflect an Early Ordovician orogenic event at 480–475 Ma, resulting from closure of the Djalair-Naiman ophiolite trough and collision of the Djel'tau microcontinent with the northern margin of NTS. Comparison of geological patterns and episodes of arc magmatism in the NTS and Chinese Central Tianshan indicate that these crustal units constituted a single early Palaeozoic arc and were separated from the Tarim Craton by an oceanic basin since the Neoproterozoic. 相似文献
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The Philippine archipelago resulted from a complex series of geologic events that involved continental rifting, oceanic spreading, subduction, ophiolite obduction, arc-continent collision, intra-arc basin formation and strike-slip faulting. It can be divided into two tectono-stratigraphic blocks, namely; the Palawan–Mindoro Continental Block (PCB) and the Philippine Mobile Belt (PMB). The PCB was originally a part of the Asian mainland that was rifted away during the Mesozoic and drifted in the course of the opening of the South China Sea (SCS) during Late Paleogene. On the other hand, the PMB developed mainly from island arcs and ophiolite terranes that started to form during the Cretaceous. At present, the PMB collides with the PCB in the Visayas in the central-western Philippines. This paper discusses recent updates on Philippine geology and tectonics as contribution to the establishment of the International Geologic Map of Asia at 1:5 M scale (IGMA5000). 相似文献
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在进行1：25万墨脱幅地质调查中，笔者首次在波密地区发现和填绘出了帕隆藏布残留蛇绿混杂岩带。帕窿藏布残留蛇绿混杂岩呈串珠状产出于花岗岩类侵入岩中，其岩石组合为橄揽辉石岩、辉石岩、辉长岩、辉长辉绿岩、辉绿岩、石英岩和大理岩，局部可见条带状硅质岩。上述组分之间的相互关系表明，蛇绿岩在花岗岩类岩石侵入之前发生过构造混杂和变形。根据沉积岩所记录的盆地演化过程、蛇绿岩的Rb-Sr年龄值以及残留蛇绿混杂岩带两刨花岗岩类岩石的特征和生成时代综合分析认为：帕隆藏布残留蛇绿混杂岩带形成于石炭-二叠纪的弧间盆地中，至少在晚三叠纪之前出现洋壳，在消减过程中向北俯冲并在中侏罗世之前闭合(弧-弧碰撞)。 相似文献
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《Russian Geology and Geophysics》2007,48(1):107-119
Based on a comparative study of geochemistry of metavolcanics and metasediments of two large terranes, Baikal-Muya and Khamar-Daban-Ol’khon, as well as of the Baikal-Patom passive margin and Olokit accretionary wedge, we have recognized volcanosedimentary series accumulated in the settings of island arcs of different maturities and fragments of volcanosedimentary complexes of back-arc and fore-arc basins. Metabasalts of the Medvezhy and Tyya Formations in the basement of passive-margin sequence and the Olokit Group are similar in geochemistry to plateau basalts and mark the beginning of rifting on the platform periphery. The abundance of metavolcanics and turbidites in the Olokit Group permits this structure to be considered an accretionary wedge of the Baikal-Muya island arc. According to the metavolcanics composition, the Baikal-Muya terrane formed in the environment of oceanic ensimatic island arcs and back-arc and inter-arc basins with the minimum amounts of sediments and contains ophiolite slices. The geochemistry of metavolcanics and metasediments of the Ol’khon, Talanchan, and Slyudyanka complexes evidences their formation in the environment of ensialic back-arc sediment-rich basin (Slyudyanka, Ol’khon, and Svyatoi Nos series), mature island arc (Anga-Talanchan paleoarc, Anga and Talanchan Groups), and fore-arc basin (Khangarul’ Group). According to chemistry and evolution history, all these complexes must be assigned to the Khamar-Daban-Ol’khon terrane. 相似文献
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弯岛湖蛇绿混杂岩带是金沙江缝合带西段的重要组成部分。蛇绿岩混杂于上三叠统变质碎屑岩夹变质火山岩中,成份主要为镁铁质-超镁铁质杂岩,岩石类型有变质橄榄岩、变质堆晶质辉长岩及其伴生的岛弧型花岗岩系。岩石化学及地球化学特征表明:蛇绿岩主要为低Ti(岛弧-弧后)型、富集型洋中脊(E-MORB)拉斑玄武岩;与之共(伴)生的基性喷出岩、辉绿岩脉属板内洋岛型裂谷型碱性玄武岩及其过渡类型系列。在变质辉长岩获得全岩Sm-Nd等时线年龄值为232±11Ma,代表了镁铁质-超镁铁质杂岩的形成年龄,可能为洋壳初始俯冲变质的时间。在蛇绿岩上覆的硅质岩中发现有中三叠世拉丁晚期至晚三叠世卡宁早期的放射虫化石,表明弯岛湖镁铁质-超镁铁质杂岩可能形成于中三叠世多岛洋盆或弧后盆地构造环境。 相似文献
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Graciano P.Yumul Jr. Carla B.Dimalanta Ricky C.Salapare Karlo L.Queano Decibel V.Faustino-Eslava Edanjarlo J.Marquez Noelynna T.Ramos Betchaida D.Payot Juan Miguel R.Guotana Jillian Aira S.Gabo-Ratio Leo T.Armada Jenielyn T.Padrones Keisuke Ishida Shigeyuki Suzuki 《地学前缘(英文版)》2020,11(1):23-36
New radiolarian ages show that the island arc-related Acoje block of the Zambales Ophiolite Complex is possibly of Late Jurassic to Early Cretaceous age.Radiometric dating of its plutonic and volcanichypabyssal rocks yielded middle Eocene ages.On the other hand,the paleontological dating of the sedimentary carapace of the transitional mid-ocean ridge-island arc affiliated Coto block of the ophiolite complex,together with isotopic age datings of its dikes and mafic cumulate rocks,also yielded Eocene ages.This offers the possibility that the Zambales Ophiolite Complex could have:(1)evolved from a Mesozoic arc(Acoje block)that split to form a Cenozoic back-arc basin(Coto block),(2)through faulting,structurally juxtaposed a Mesozoic oceanic crust with a younger Cenozoic lithospheric fragment or(3)through the interplay of slab rollback,slab break-off and,at a later time,collision with a microcontinent fragment,caused the formation of an island arc-related ophiolite block(Acoje)that migrated trench-ward resulting into the generation of a back-arc basin(Coto block)with a limited subduction signature.This Meso-Cenozoic ophiolite complex is compared with the other oceanic lithosphere fragments along the western seaboard of the Philippines in the context of their evolution in terms of their recognized environments of generation. 相似文献
14.
Emilio Saccani 《地学前缘(英文版)》2015,6(4)
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 ... 相似文献
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V. M. Grannik 《Doklady Earth Sciences》2012,445(2):934-938
It has been established that volcanic rocks of the Schmidt, Rymnik, and Terpeniya terranes are fragments of the compound Early to Late Cretaceous-Paleogene East Sakhalin island arc system of the Sea of Okhotsk region. This island arc paleosystem was composed of back-arc volcano-plutonic belt, frontal volcanic island arc, fore-arc, inter-arc, and back-arc basins, and the Sakhalin marginal paleobasin. The continental volcanic rocks dominate in the back-arc volcano-plutonic belt and frontal volcanic island arc. The petrochemical composition of basalts, basaltic andesites, andesites, and trachytes from the frontal island arc formed in submarine conditions are typical of oceanic island arc or marginal sea rocks (IAB). The petrochemical composition of volcanic rocks from the island arc structures indicates its formation on the heterogeneous basement including the continental and oceanic blocks. 相似文献
17.
R. M. Shackleton 《International Journal of Earth Sciences》1994,83(3):537-546
The distribution, structural relations and ages of ophiolites and ophiolitic melanges in the Eastern Deserts of Egypt and north-east Sudan, and their relation to arc and post-arc magmatism, are used to infer the probable positions of sutures and the directions of obduction and subduction. The ages of the successive tectonic stages in the terranes decrease northwards, by about 100 Ma in the region discussed. Obduction was north-westwards. The obducted sheets of ophiolite and ophiolitic melange, extending about 500 km north from the Onib-Sol Flamed suture, may all represent a single back-arc basin. The direction of subduction is still not clearly proved: more geochemical evidence of arc-magmatic polarity is needed. The late tectonic north-westward thrusting, the NW - SE stretching lineation, the transpressional north-south Hamisama Shear Zone and the NW-SE sinistral Najd faults are all attributed to north-westward movement (present coordinates) of terranes towards the older craton west of the Nile. The Najd faults are interpreted as intracontinental transform faults. 相似文献
18.
James M. Mattinson L. Kenneth Fink Jr. Clifford A. Hopson 《Contributions to Mineralogy and Petrology》1980,71(3):237-245
La Désirade, a small island east of Guadeloupe, is underlain by the only exposed pre-Tertiary basement rocks in the Lesser Antilles. The basement complex comprises spilitic and keratophyric flows and pillow lavas (with interbedded and overlying radiolarian cherts), swarms of mafic to silicic dikes, and subjacent plagiogranite. These features, and the absence of carbonates, terrigenous clastic sediments, or tuffaceous sediments from the complex indicate that it developed in a deep marine environment beyond the reach of terrigenous sedimentation or emergent island arc pyroclastic deposition. Previous workers have suggested that the Désirade basement complex originated either as oceanic crust or during an early (tholeiitic) stage of island arc growth. The isotopic compositions of Sr and Pb from the complex, and previously reported rare earth data (Johnston and Schilling, 1974) do not provide a clear distinction between these two possibilities. Nor does the presence of siliceous keratophyre in the complex rule out an oceanic crustal origin-such rocks are common in well studied ophiolites that originated as oceanic crust. Hence we turn to the age relationships of the complex, the surrounding ocean floor, and adjacent island arcs in an attempt to resolve this problem. The age of the complex strongly supports an oceanic crustal (ophiolitic) origin. The ages of zircons and a previously reported K-Ar age indicate that the complex is 145±5 m.y. old. The complex predates the next oldest volcanic rocks of the Lesser Antilles arc by ca. 110 m.y., and the oldest known rocks of the Aves Ridge, a possible Mesozoic precursor of the Lesser Antilles arc, by 50–60 m.y. This makes it unlikely that the Désirade complex is related to an early phase of either of these arcs. Instead, the age of the complex falls in the range of ages expected for oceanic crust in the vicinity of the Lesser Antilles prior to the development of any subduction zone and resulting arc. Thus we interpret the Désirade complex to be an uplifted segment of oceanic crust that represents the basement on which the later island arcs grew: first the Aves Ridge, an arc that was active in middle to late Cretaceous time (but whose exact mode of origin is enigmatic, and is considered in four alternate tectonic models), then the Eocene to Recent Lesser Antilles arc. 相似文献
19.
造山的高原——青藏高原巨型造山拼贴体和造山类型 总被引：29，自引：0，他引：29
青藏高原是一个巨型碰撞造山拼贴体,它的形成与始特提斯、古特提斯和新特提斯洋盆的先后开启、消减、闭合以及古大陆的裂解、诸地体的移动、会聚和拼合有关。造山类型形成于不同时期海(洋)盆俯冲、地体碰撞和陆内会聚的不同阶段。多地体/多岛弧/多弧前海的构架表明,诸多的俯冲型山链可以产生在地体边界的活动陆缘一侧,古特提斯南、北两洋盆的双向俯冲构筑了双向俯冲型山链;碰撞型山链由于地体边界与块体驱动方向的几何学关系形成“正向碰撞型”和“斜向碰撞型”造山类型。“斜向碰撞型山链”与走滑断裂的形成、规模及其运动学直接相关。50~60Ma印度/亚洲碰撞不仅形成青藏高原造山拼贴体的最后成员———喜马拉雅山链,而且在拼贴体的北缘由于陆内俯冲作用使早期形成的山链在整修后又一次崛起。青藏高原的周缘山链铸成屏障与外侧的克拉通相隔。青藏高原巨型碰撞造山拼贴体的形成是亚洲大陆自北往南的增生和造山迁移的生长结果,其所反映的活动长期性、非原地性、俯冲/碰撞/陆内造山类型的多样性、碰撞造山的多期性以及造山的复合叠置性比世界上任何一个复合山链(或造山拼贴体)来得复杂、多彩。 相似文献
20.
Jean-Franois Parrot 《Tectonophysics》1977,41(4):251-268
The Baër-Bassit area of northwestern Syria is composed of an ophiolite suite and a Triassic to Lower Cretaceous volcano-sedimentary formation. This area is believed to represent the front of sheets overthrust in the Maestrichtian on the Arabian Platform. The roots of the sheets are found to the north, in Turkey. The Baër-Bassit area could thus correspond to the southern margin of the northern part of the obducted Tethyan oceanic crust. Formation of the ocean started in the Late Triassic or at the beginning of the Jurassic.The subduction of Tethyan crust under the northern oceanic margin would have led to a change in the magmatic processes which would have produced the upper levels of hypertholeiitic pillow lavas on the southern rim of the northern district. In fact, those lavas are present all around the Arabian Platform, in the most ‘meridional’ ophiolitic complexes: Cyprus, Baër-Bassit and Oman.The change in magmatism would by definition occur in a relatively narrow zone; this would explain the differences observed when comparing the lavas and the sheeted intrusive complex on both the ‘meridional’ ophiolites and the more ‘internal’ Turkish massifs. Although belonging to the same oceanic crust, the differences in the ophiolitic assemblages would correspond to a different stage of its formation; the Turkish one would probably be a portion of oceanic crust formed at an earlier stage.Moreover, the volcano-sedimentary series associated with the ophiolites of Cyprus and Baër-Bassit would have been formed at the southern margin of the Tethyan region. A part of the volcano-sedimentary sequence has been subducted and metamorphosed. Amphibolites formed in this way would have been extracted from the subduction zone during the last movements when oceanic crust overthrust the Arabian—African Platform. The unmetamorphosed volcano-sedimentary series would have been folded and thrust towards the obducted oceanic crust during the same period. However, whereas the volcano-sedimentary formation of Syria is tectonically overthrust by the ophiolite, it is possible that the similar formation in Cyprus has been deposited from the south over the Troodos Massif. 相似文献