Tectonics of the Beja-Acebuches Ophiolite: a major suture in the Iberian Variscan Foldbelt

The Beja-Acebuches Ophiolitic Complex (BAOC) (south Portugal/Spain) corresponds to a high grade metamorphic belt along the boundary between Ossa-Morena and the South Portuguese Zones and comprises a lithostratigraphic sequence including (from top to bottom) metabasalts, (metamorphosed) multiple dyke intrusions in gabbro, flasergabbros and metaserpentinites. It is affected by three deformation phases. D₁ affects the ophiolite lower stratigraphic units and is represented by a mylonitic cleavage with a stretching lineation where shear criteria indicates the sense of shear to be towards the north-north-east; this deformation event can be related to the ophiolite emplacement above the crystalline footwall of the Serpa antiform, affecting Precambrian basement and Cambrian cover rocks. The obduction polarity ist north-eastwards, similar to the subduction polarity that generates the Beja Gabbroic Complex (BGC), implying a flake geometry. The second deformation phase, D₂, is represented by an intense WNW-ESE sinistral shear event which is responsible for the shattered appearance of the suture; D₂ is reactivated later by a more brittle D₃ event involving thrusting to the south-west, again with a sinistral component. ⁴⁰Ar/³⁹Ar isotopic ages were obtained for (metamorphosed) multiple dyke intrusions in the BAOC's gabbro (342.6 ± 1.4 Ma), for metagabbroic cumulates (340.7 ± 1.9 Ma), and for the undeformed/unmetamorphosed BGC (341.1 ± 1.3 Ma) occurring to the north of the ophiolitic suture. These ages reflect a last regional cooling event in the area which post-dates the ophiolite emplacement and the intrusion of the BGC through this oceanic sequence. Correspondence to: P. Fonseca     

赣东北蛇绿岩的离子探针锆石U－Pb年龄及其构造意义

离子探针锆石Ｕ－Ｐｂ年龄分析结果表明，赣东北蛇绿岩套的高度分异岩浆的结晶年龄为９６８±２３Ｍａ。重新计算的蛇绿岩Ｓｍ－Ｎｄ等时线年龄９５５±４４Ｍａ，与锆石Ｕ－Ｐｂ年龄在误差范围内一致。１５个样品的εＮｄ（Ｔ）值（Ｔ＝９７０Ｍａ）为＋４．３－＋６．７，表明蛇绿岩来源于强亏损地幔源。少数样品的Ｓｍ－Ｎｄ体系可能受到后期变质、变形或蚀变作用的影响。结合已发表的４０Ａｒ－３９Ａｒ年龄资料，可以初步确定赣东北晚元古代碰撞带发育的时限为０．９７－０．８０Ｇａ。     

蛇绿岩研究之检讨与反思:以“双沟蛇绿岩”为例

"双沟蛇绿岩"是笔者最早研究的蛇绿岩之一,位于云南哀牢山带。双沟出露的岩石有二辉橄榄岩、辉长岩、辉绿岩、斜长花岗岩、玄武岩、硅质岩等。辉长岩亏损LREE,锆石U-Pb年龄为362～328Ma。玄武岩具N-MORB和E-MORB的特征,锆石U-Pb年龄为249Ma。研究认为,"双沟蛇绿岩"可能产于陆间小洋盆或裂谷或裂陷槽背景。但是,双沟没有可信的深海沉积和混杂堆积的记录,虽然岩石组合类似蛇绿岩,地球化学也具有MORB的特征,暗示双沟可能不是一个典型的蛇绿岩。如果双沟不是蛇绿岩,则晚古生代的哀牢山带就不存在一个有一定规模的洋盆,也不可能存在大陆碰撞的记录。双沟不是蛇绿岩是什么?可能是造山橄榄岩(Orogenic peridotite)。造山橄榄岩与蛇绿岩的岩石组合类似,蛇绿岩的橄榄岩产于洋壳之下;造山橄榄岩产于陆壳之下。检讨双沟蛇绿岩的研究,反思蛇绿岩的概念。笔者认为,斯泰因曼的"三位一体"概念是合适的,1972年彭罗斯会议的决议是正确的,1996年怀柔会议构造学家对蛇绿岩概念的理解是对的。考虑到混杂堆积对于蛇绿岩的重要性,建议将混杂堆积也作为与蛇绿岩相伴的一个重要指标加进来。如果这个想法合适,则一个完整的蛇绿岩组合将由三个要素组成:1)岩浆岩(包括地幔岩、堆晶岩、侵入岩和火山岩,代表大洋岩石圈的物质组成); 2)深海沉积(代表洋盆顶部的物质组成); 3)混杂堆积(代表洋盆消失、陆块碰撞的构造产物)。蛇绿岩不同于其他岩浆岩,其研究需要特殊的方法和思路,明白这一点,蛇绿岩研究才能走上正轨。双沟蛇绿岩研究遇到危机,中国其他一些蛇绿岩也可能需要重新审视。因此,检讨双沟蛇绿岩,对反思蛇绿岩的研究具有一定的意义。     

Late Paleozoic-Mesozoic subduction and accretion of the Paleo-Pacific Plate:Insights from ophiolitic rocks in the Wandashan accretionary complex,NE China

The Wandashan accretionary complex(AC),consisting of the Raohe and Yuejinshan complexes,is located on the continental margin of Northeast Asia and represents an excellent source of information about Paleo-Pacific subduction and accretion.However,the protolith nature and tectonic evolution of the Wandashan AC are under debate.This contribution reports new geochronological,geochemical,and Sr-Nd-Pb-Hf isotopic data for ophiolitic rocks from the Wandashan AC.The 169-166 Ma plagioclasites and homogeneous gabbros from the Raohe complex are OIBs while 228-214 Ma homogeneous gabbros are continental VABs.Cumulate gabbros from the Yuejinshan complex formed at 280-278 Ma and～220 Ma and have similar characteristics with E-MORB and N-MORB,respectively.They are BABBs and their primary magma was derived from a source region between EMI and EMII that was affected by con-tinental crustal contamination as well as subduction-zone metasomatism.Combined with previous stud-ies,we suggest that the onset of subduction of the Paleo-Pacific Plate was in the Early Permian.Subsequently,a back-arc basin,whose present suture is on the eastern margin of the Jiamusi Massif,formed and widened during 280-232 Ma,after which the basin closed and BABBs were emplaced to form the Yuejinshan complex during 210-180 Ma.The formation of VABs of the Raohe complex is coincident with the closure of the back-arc basin,and together with the 169-166 Ma OIBs,they constitute a major part of the Raohe complex.The accretionary process was completed during 133-131 Ma.Taken together,the ophiolitic rocks indicating multistage magmatism in the Paleo-Wandashan region recorded the formation-closure process of back-arc basin and the accretionary process of the Wandashan AC,during the westward subduction of the Paleo-Pacific plate.The back-arc basin identified in our study sheds new lights on geodynamic evolution model of subduction and accretion of the Paleo-Pacific Plate on the continental margin of NE Asia.     

祁连山蛇绿岩带和原特提斯洋演化

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。     

Estimating original thickness and extent of the Semail Ophiolite in the eastern Oman Mountains by paleothermal indicators

The Oman Mountains preserve Permo-Mesozoic sedimentary rocks of the Arabian passive margin that were overridden during Late Cretaceous time by deep-water sediments of the Hawasina units and by the Semail Ophiolite, a portion of the Neo-Tethyan oceanic crust and upper mantle. Passive margin sequences are exposed in the Jabal Akhdar Culmination (JAC) and in the Jabal Salakh Range at the Oman Mountains thrust front. Samples of these sequences were investigated by X-ray diffraction of the clay size fraction to evaluate the thermal evolution of the subophiolite rocks and estimate the thickness and extent of the obducted ophiolites.The sedimentary succession from the northern flank of the JAC shows a clay mineral assemblage characterized by long-range ordered mixed layer I-S with an illite content between 85% and 92% and the occurrence of pyrophyllite and/or paragonite, suggesting maximum paleotemperatures between 150° and 200 °C in deep diagenetic conditions. On the southern flank of the JAC, temperature dependent clay minerals indicate maximum paleotemperatures, ranging between 120° and 150 °C, indicating a reduced ophiolite thickness towards the south. Ooid strain analyses of the subophiolite rocks from the northern flank of JAC show a component of flattening and stretching in the z-x plane as a result of plastic deformation and pressure solution. On the southern flank, such ductile deformation is absent, suggesting a brittle rheology for the subophiolite carbonates and a reduced overburden. 1D thermal modeling reveals that the sub-ophiolite units of the JAC were overthrust by 4.5 km-thick Semail Ophiolite and Hawasina units during the Coniacian, and exhumed since the Campanian. The subophiolite rocks of the Jabal Salakh Range were buried under 1.35 km of synobduction clastics and overthrust by 2 km-thick Hawasina units, suggesting a decrease of the thickness of allochthonous units from NE to SW, consistent with strain analysis and their direction of emplacement.     

Masirah—the other Oman ophiolite:A better analogue for mid-ocean ridge processes?

Oman has two ophiolites – the better known late Cretaceous northern Oman (or Semail) ophiolite and the lesser known and smaller, Jurassic Masirah ophiolite located on the eastern coast of the country adjacent to the Indian Ocean. A number of geological, geochronological and geochemical lines of evidence strongly suggest that the northern Oman ophiolite did not form at a mid-ocean ridge but rather in a supra-subduction zone setting by fast spreading during subduction initiation. In contrast the Masirah ophiolite is structurally part of a series of ophiolite nappes which are rooted in the Indian Ocean floor. There are significant geochemical differences between the Masirah and northern Oman ophiolites and none of the supra-subduction features typical of the northern Oman ophiolite are found at Masirah. Geochemically Masirah is MORB, although in detail it contains both enriched and depleted MORB reflecting a complex source for the lavas and dykes. The enrichment of this source predates the formation of the ophiolite. The condensed crustal section on Masirah (ca. 2 km) contains a very thin gabbro sequence and is thought to reflect its genesis from a cool mantle source associated with the early stages of sea-floor spreading during the early separation of eastern and western Gondwana. These data suggest that the Masirah ophiolite is a suitable analogue for an ophiolite created at a mid-ocean ridge, whereas the northern Oman ophiolite is not. The stratigraphic history of the Masirah ophiolite shows that it remained a part of the oceanic crust for ca. 80 Ma. The chemical variability and enrichment of the Masirah lavas is similar to that found elsewhere in Indian Ocean basalts and may simply reflect a similar provenance rather than a feature fundamental to the formation of the ophiolite.     

Early central American forearc follows the subduction initiation rule

The “subduction initiation rule” (SIR) (Whattam and Stern, 2011) advocates that proto-arc and forearc complexes preserved in ophiolites and forearcs follow a predictable chemotemporal and/or chemostratigraphic vertical progression. This chemotemporal evolution is defined by a progression from bottom to top, from less to more depleted and slab-metasomatized sources. This progression has been recently documented for other igneous suites associated with subduction initiation. The Sona-Azuero forearc complex of southern Panama represents the earliest magmatic arc activity at the Central American Volcanic Arc system. Comparison of new and existing geochemical data for the circa 82-40 Ma Sona-Azuero Proto-Arc/Arc, its underlying 89-85 Ma “oceanic plateau” of SW Panama and the 72-69 Ma Golfito Proto-Arc of southern Costa Rica with the 70-39 Ma Chagres-Bayano Arc of eastern Panama exhibits a chemotemporal progression as described above and which follows the SIR. Sona-Azuero lavas are predominantly MORB-like, whereas those of the younger Chagres-Bayano complex are mostly VAB-like; lavas of the Golfito Proto-Arc typically show characteristics intermediate to that of the Sona-Azuero and Chagres-Bayano proto-arc/arc complexes. On the basis of isotope evidence as shown in other studies, lava types of all three complexes are clearly derived from a source contaminated by the Caribbean Large Igneous Province plume; we term these “plume-contaminated” forearc basalts and volcanic arc basalts, respectively. Apart from a plume-induced subduction initiation origin for the Panamanian forearc, these insights suggest otherwise similar petrogenetic origins and tectonic setting to lavas comprising earliest-formed forearc crust, and most ophiolites, which follow the SIR.     

The East Anatolia-Lesser Caucasus ophiolite:An exceptional case of large-scale obduction,synthesis of data and numerical modelling

We present new,geological,metamorphic,geochemical and geochronological data on the East Anatolian-Lesser Caucasus ophiolites.These data are used in combination with a synthesis of previous data and numerical modelling to unravel the tectonic emplacement of ophiolites in this region.All these data allow the reconstruction of a large obducted ophiolite nappe,thrusted for>100 km and up to 250 km on the Anatolian-Armenian block.The ophiolite petrology shows three distinct magmatic series,highlighted by new isotopic and trace element data:(1)The main Early Jurassic Tholeiites(ophiolite s.s.)bear LILEenriched,subduction-modified,MORB chemical composition.Geology and petrology of the Tholeiite series substantiates a slow-spreading oceanic environment in a time spanning from the Late Triassic to the Middle-Late Jurassic.Serpentinites,gabbros and plagiogranites were exhumed by normal faults,and covered by radiolarites,while minor volumes of pillow-lava flows infilled the rift grabens.Tendency towards a subduction-modified geochemical signature suggests emplacement in a marginal basin above a subduction zone.(2)Late Early Cretaceous alkaline lavas conformably emplaced on top of the ophiolite.They have an OIB affinity.These lavas are featured by large pillow lavas interbedded a carbonate matrix.They show evidence for a large-scale OIB plume activity,which occurred prior to ophiolite obduction.(3)Early-Late Cretaceous calc-alkaline lavas and dykes.These magmatic rocks are found on top of the obducted nappe,above the post-obduction erosion level.This series shows similar Sr-Nd isotopic features as the Alkaline series,though having a clear supra-subduction affinity.They are thus interpreted to be the remelting product of a mantle previously contaminated by the OIB plume.Correlation of data from the Lesser Caucasus to western Anatolia shows a progression from back-arc to arc and fore-arc,which highlight a dissymmetry in the obducted oceanic lithosphere from East to West.The metamorphic P-T-t paths of the obduction sole lithologies define a southward propagation of the ophiolite:(1)P-T-t data from the northern Sevan-Akera suture zone(Armenia)highlight the presence and exhumation of eclogites(1.85±0.02 GPa and 590±5℃)and blueschists below the ophiolite,which are dated at ca.94 Ma by Ar-Ar on phengite.(2)Neighbouring Amasia(Armenia)garnet amphibolites indicate metamorphic peak conditions of 0.65±0.05 GPa and 600±20 C with a U-Pb on rutile age of 90.2±5.2 Ma and Ar-Ar on amphibole and phengite ages of 90.8±3.0 Ma and 90.8±1.2 Ma,respectively.These data are consistent with palaeontological dating of sediment deposits directly under(Cenomanian,i.e.>93.9 Ma)or sealing(Coniacian-Santonian,i.e.,≤89.8 Ma),the obduction.(3)At Hinis(NE Turkey)PT-t conditions on amphibolites(0.66±0.06 GPa and 660±20℃,with a U-Pb titanite age of80.0±3.2 Ma)agree with previous P-T-t data on granulites,and highlight a rapid exhumation below a top-to-the-North detachment sealed by the Early Maastrichtian unconformity(ca.70.6 Ma).Amphibolites are cross-cut by monzonites dated by U-Pb on titanite at 78.3±3.7 Ma.We propose that the HT-MP metamorphism was coeval with the monzonites,about 10 Ma after the obduction,and was triggered by the onset of subduction South of the Anatolides and by reactivation or acceleration of the subduction below the Pontides-Eurasian margin.Numerical modelling accounts for the obduction of an"old"~80 Myr oceanic lithosphere due to a significant heating of oceanic lithosphere through mantle upwelling,which increased the oceanic lithosphere buoyancy.The long-distance transport of a currently thin section of ophiolites(<1 km)onto the Anatolian continental margin is ascribed to a combination of northward mantle extensional thinning of the obducted oceanic lithosphere by the Hinis detachment at ca.80 Ma,and southward gravitational propagation of the ophiolite nappe onto its foreland basin.     

Compositional and geochronological signatures of metamafic dykes from the Sangsang peridotites,South Tibet: Evidence for magma-starved forearc rifting during Neo-Tethyan subduction re-initiation

In this study we present new mineral chemistry, whole-rock geochemical and zircon U–Pb geochronological data for 12 metamafic dykes in the mantle sequence of the Sangsang ophiolite in South Tibet (China). Modal analyses of these dykes gave averages of ~40%–65% plagioclase and ~35%–60% amphibole and small amounts of (igneous) clinopyroxene, epidote and opaque minerals. This mineral assemblage resembles that of typical orthoamphibolites. Nevertheless, due to the absence of foliation the investigated rocks are described as metamafic lithologies. These rocks have primitive mantle (PM)-normalized multi-element patterns with negative Nb and Ta anomalies as well as weak, negative Ti anomalies. In addition, they have initial ⁸⁷Sr/⁸⁶Sr ratios [(⁸⁷Sr/⁸⁶Sr)_i] of 0.702844–0.703581, initial ¹⁴³Nd/¹⁴⁴Nd ratios [(¹⁴³Nd/¹⁴⁴Nd)_i] of 0.512891–0.512959 and high ε_Nd(t) values (+7.9 to +9.3). Uranium-Pb ages of magmatic zircons separated from the investigated metamafic dykes indicate that the parental melts of their protoliths intruded the Sangsang mantle at ~119.0–118.5 ​Ma.The metamorphic mineral assemblages recognized in the investigated dykes are suggestive of a retrograde metamorphic process, from (epidote-)amphibolite facies (~470–610 ​°C, ~1.9–4.3 ​kbar) and to prehnite-pumpellyite facies (≤280 ​°C, ≤ 3 ​kbar), active within a rift-produced oceanic lithosphere. Microtextural and geochemical data suggest that the protoliths of the dykes were most likely massive gabbros. Compositional data show that the parental magmas of the gabbroic protoliths were generated by melting of a depleted mantle (DM) source that had been weakly modified by fluids emanating from a subducted oceanic lithospheric slab. The age of the gabbroic protoliths is slightly younger than the existing ages for ophiolites from the central Yarlung-Zangbo Suture Zone (YZSZ) in the literature (~129–123 ​Ma). We, therefore, suggest that the gabbroic protoliths of the Sangsang metamafic dykes were formed in an incipient forearc setting during Neo-Tethyan subduction re-initiation (Aptian). Our tectonomagmatic model provides insights into the igneous accretion and post-solidification evolution of the oceanic lithosphere in South Tibet.