蛇绿岩中金刚石和异常矿物的全球记录

蛇绿岩型金刚石产在地幔橄榄岩和铬铁矿中,是新建立的金刚石产出类型,不同于产在金伯利岩和超高压变质岩中的金刚石。全球已在21个蛇绿岩中发现了该类金刚石,含金刚石的蛇绿岩主要分布在特提斯造山带、乌拉尔- 中亚造山带、日高变质岩带和北美克拉马斯- 阿卡特兰造山带。本文梳理了含金刚石蛇绿岩的全球分布和地质背景以及蛇绿岩中超高压－强还原矿物与其它壳幔矿物组合的特征,讨论了已有的含金刚石铬铁矿和地幔橄榄岩的四种成因机制。金刚石和伴生的超高压－强还原矿物组合产在不同时代的造山带蛇绿岩中,不仅仅揭示了金刚石在蛇绿岩中普遍存在,需要重新思考蛇绿岩和铬铁矿的成因以及它们形成的地质构造背景,还证实了蛇绿岩地幔橄榄岩和铬铁矿是地球深部矿物重要的储存库,为认识地球深部的物质组成和物理化学环境,以及深部物质运移的轨迹和动力学过程等提供了天然样品。     

大洋地幔橄榄岩-铬铁矿中的金刚石和深地幔再循环

全球多地蛇绿岩型地幔橄榄岩和铬铁矿中发现微粒金刚石,并在中国西藏南部和俄罗斯乌拉尔北部的蛇绿岩铬铁矿中发现原位产出的金刚石,认为是地球上金刚石的一种新的产出类型,不同于金伯利岩型金刚石和超高压变质型金刚石。它们与呈斯石英假象的柯石英、高压相的铬铁矿和青松矿等高压矿物以及碳硅石和单质矿物等强还原矿物伴生,指示蛇绿岩中的这些矿物组合形成于深度150~300 km或者更深的地幔。金刚石具有很轻的C同位素组成（δ13C-18‰~-28‰）,并出现多种含Mn矿物和壳源成分包裹体。研究认为它们曾是早期深俯冲的地壳物质,达到>300 km深部地幔或地幔过渡带后,经历了熔融并产生新的流体,后者在上升过程中结晶成新的超高压、强还原矿物组合,通过地幔对流或地幔柱作用被带回到浅部地幔,由此建立了一个俯冲物质深地幔再循环的新模式。蛇绿岩型地幔橄榄岩和铬铁矿中发现金刚石等深部矿物,质疑了蛇绿岩铬铁矿形成于浅部地幔的已有认识,引发了一系列新的科学问题,提出了新的研究方向。      

蛇绿岩型金刚石和铬铁矿深部成因

地球上的原生金刚石主要有3种产出类型,分别来自大陆克拉通下的深部地幔金伯利岩型金刚石、板块边界深俯冲变质岩中超高压变质型金刚石,和陨石坑中的陨石撞击型金刚石。在全球5个造山带的10处蛇绿岩的地幔橄榄岩或铬铁矿中均发现金刚石和其他超高压矿物的基础上,我们提出地球上一种新的天然金刚石产出类型,命名为蛇绿岩型金刚石。认为蛇绿岩型金刚石普遍存在于大洋岩石圈的地幔橄榄岩中,并提出蛇绿岩型金刚石和铬铁矿的深部成因模式。认为早期俯冲的地壳物质到达地幔过渡带(410~660 km深度)后被肢解,加入到周围的强还原流体和熔体中,当熔融物质向上运移到地幔过渡带顶部,铬铁矿和周围的地幔岩石以及流体中的金刚石等深部矿物一并结晶,之后,携带金刚石的铬铁矿和地幔岩石被上涌的地幔柱带至浅部,经历了洋盆的拉张和俯冲阶段,最终在板块边缘就位。     

西藏雅鲁藏布江缝合带的普兰地幔橄榄岩中发现金刚石:蛇绿岩型金刚石分类的提出

西藏罗布莎蛇绿岩的地幔橄榄岩和铬铁矿中发现金刚石和特殊矿物群引发了新的问题,罗布莎地幔橄榄岩含特殊地幔矿物是不是一个孤立的特殊现象,或这是一个普遍存在的规律?显然,这是一个至关重要的问题.本文报道在雅鲁藏布江缝合带西段,距离罗布莎1000km以远的普兰蛇绿岩的地幔橄榄岩中发现与罗布莎类似的金刚石和特殊地幔矿物群.普兰的地幔橄榄岩体主体为方辉橄榄岩,含少量的纯橄岩和二辉橄榄岩,研究表明,属典型MOR型亏损地幔橄榄岩.通过分选,在657kg的地幔橄榄岩大样中发现了金刚石和碳硅石等30余种矿物的特殊矿物群,包括自然铬、自然铁和自然锌等强还原条件下形成的单质元素矿物.该矿物群与罗布莎地幔橄榄岩和铬铁矿中发现的特殊矿物群十分相似,表明罗布莎的地幔橄榄岩不是雅鲁藏布江缝合带中的一个特例.结合在俄罗斯乌拉尔Ray-Iz铬铁矿中发现类似的矿物群,以及世界其他地区的有关阿尔卑斯型地幔橄榄岩中金刚石的报道,认为蛇绿岩地幔橄榄岩中可能普遍含有金刚石,并将蛇绿岩地幔橄榄岩中产出的金刚石归为一种新的金刚石产出类型,即蛇绿岩型金刚石,不同于金伯利岩型金刚石和超高压变质带中产出的变质金刚石类型.     

Discovery and Significance of Diamonds and Moissanites in Chromitite within the Skenderbeu Massif of the Mirdita Zone Ophiolite, West Albania

In recent years diamonds and other unusual minerals(carbides,nitrides,metal alloys and native elements) have been recovered from mantle peridotites and chromitites(both high-Cr chromitites and high-Al chromitites) from a number of ophiolites of different ages and tectonic settings.Here we report a similar assemblage of minerals from the Skenderbeu massif of the Mirdita zone ophiolite,west Albania.So far,more than 20 grains of microdiamonds and 30 grains of moissanites(SiC) have been separated from the podiform chromitite.The diamonds are mostly light yellow,transparent,euhedral crystals,200~300 μm across,with a range of morphologies;some are octahedral and cuboctahedron and others are elongate and irregular.Secondary electron images show that some grains have well-developed striatums.All the diamond grains have been analyzed and yielded typical Raman spectra with a shift at ~1325 cm~(-1).The moissanite grains recovered from the Skenderbeu chromitites are mainly light blue to dark blue,but some are yellow to light yeUow.All the analyzed grains have typical Raman spectra with shifts at 766 cm~(-1),787 cm~(-1),and 967 cm~(-1).The energy spectrums of the moissanites confirm that the grains are composed entirely of silicon and carbon.This investigation expands the occurrence of diamonds and moissanites to Mesozoic ophiolites in the Neo-Tethys.Our new findings suggest that diamonds and moissanites are present,and probably ubiquitous in the oceanic mantle and can provide new perspectives and avenues for research on the origin of ophiolites and podiform chromitites.     

FTIR Spectroscopy Data and Carbon Isotope Characteristics of the Ophiolite‐hosted Diamonds

We report new δ¹³C ‐values data and N‐content and N‐aggregation state values for microdiamonds recovered from peridotites and chromitites of the Luobusa ophiolite (Tibet) and chromitites of the Ray‐Iz ophiolite in the Polar Urals (Russia). All analyzed microdiamonds contain significant nitrogen contents (from 108 up to 589 ± 20% atomic ppm) with a consistently low aggregation state, show identical IR spectra dominated by strong absorption between 1130 cm^?1 and 1344 cm^?1, and hence characterize Type Ib diamond. Microdiamonds from the Luobusa peridotites have δ¹³C ‐PDB‐values ranging from ‐28.7‰ to ‐16.9‰, and N‐contents from 151 to 589 atomic ppm. The δ¹³C and N‐content values for diamonds from the Luobusa chromitites are ‐29‰ to ‐15.5‰ and 152 to 428 atomic ppm, respectively. Microdiamonds from the Ray‐Iz chromitites show values varying from ‐27.6 ‰ to ‐21.6 ‰ in δ¹³C and from 108 to 499 atomic ppm in N. The carbon isotopes values bear similar features with previously analyzed metamorphic diamonds from other worldwide localities, but the samples are characterized by lower N‐contents. In every respect, they are different from diamonds occurring in kimberlites and impact craters. Our samples also differ from the few synthetic diamonds; we also analyzed showing enhanced δ¹³C ‐variability and less advanced aggregation state than synthetic diamonds. Our newly obtained N‐aggregation state and N‐content data are consistent with diamond formation over a narrow and rather cold temperature range (i.e. <950°C), and in a short residence time (i.e. within several million years) at high temperatures in the deep mantle.     

Chondritic osmium isotopic composition of Archean ophiolitic mantle, North China craton

Podiform chromitites are diagnostic but rare features of Phanerozoic ophiolites, and often contain the most pristine textural, chemical and isotopic record of convective upper mantle conditions extant during ophiolite genesis. Ophiolitic podiform chromitites, owing to their high Os concentrations and low Re/Os ratios provide the best evidence for the Os-isotopic evolution of oceanic mantle, but established records of ophiolitic chromites from bona fide Archean ophiolites are still lacking. We report Re–Os isotopic compositions of the world's oldest known ophiolitic podiform chromites from the 2.5 billion year old Dongwanzi–Zunhua ophiolite, North China. This provides the oldest Os isotope composition for the convective upper mantle yet obtained from ophiolitic podiform chromitites, and reveals a chondritic Os isotopic composition of the Archean convective upper mantle.     

Surprises from the top of the mantle transition zone

Recent studies of chromite deposits from the mantle section of ophiolites have revealed a most unusual collection of minerals present as inclusions within the chromite. The initial discoveries were of diamonds from the Luobosa ophiolite in Tibet. Further work has shown that mantle chromitites from ophiolites in Tibet, the Russian Urals and Oman contain a range of crustal minerals including zircon, and a suite of highly reducing minerals including carbides, nitrides and metal alloys. Some of the minerals found represent very high pressure phases indicating that their likely minimum depth is close to the top of the mantle transition zone. These new results suggest that crustal materials may be subducted to mantle transition zone depths and subsequently exhumed during the initiation of new subduction zones—the most likely environment for the formation of their host ophiolites. The presence of highly reducing phases indicates that at mantle transition zone depths the Earth's mantle is ‘super’‐reducing.     

古老大陆岩石圈地幔再循环与蛇绿岩中铬铁矿床成因

不同地区、不同时代蛇绿岩中不同类型铬铁矿岩的Re-Os同位素研究表明,在铬铁矿石或围岩中均存在极度亏损的具有大陆岩石圈地幔属性的物质。新疆达拉布特古生代蛇绿岩带中萨尔托海富Al铬铁矿岩的Os同位素组成为0.1109～0.1256,对应的模式年龄为3.5～0.6Ga;西藏班公湖—怒江中生代蛇绿岩带中东巧富Cr铬铁矿石及围岩Os同位素组成介于0.1175～0.1261,对应的模式年龄为1.5～0.1Ga;雅鲁藏布江中生代蛇绿岩带中罗布莎富Cr铬铁矿岩的Os同位素变化范围为0.1038～0.1266,对应的模式年龄为3.37～0.28Ga,而该带中不含矿的泽当二辉橄榄岩的Os同位素组成为0.1256～0.1261,没有古老大陆岩石圈地幔属性的物质存在,与新特提斯洋地幔Os组成较为接近。推测在蛇绿岩形成过程中,古老大陆岩石圈地幔参与循环有利于形成铬铁矿床,明确提出"熔体与古老大陆岩石圈地幔反应成矿"的假说,指出蛇绿岩带中存在的古老微陆块可能是找矿的指示标志。     

Melt Events in A Nascent Mantle Wedge: Implication from the Luobusa Ophiolite,Tibet

The compositions of minerals and whole rocks of the Luobusa ophiolite in South Tibet, a fragment of Neo‐Tethyan forearc lithosphere, is used to investigate the magmatic evolution of nascent mantle wedges in newly‐initiated subduction zones. Clinopyroxenes in the Luobusa peridotites all have diopsidic compositions, and their Al₂O₃ contents vary from ～ 2% in the dunites and refractory harzburgites to 2‐4% in the cpx‐bearing harzburgites. The REE of clinopyroxenes in the harzburgites have left‐sloping patterns with contents comparable to those in abyssal peridotites that have experienced 5‐15% partial melting. Chromites in the Luobusa chromitites have the highest Cr#s (～ 80) and TiO2 contents (0.1‐0.2%), and those in the cpx‐bearing harzburgites have the lowest Cr#s (20‐60) and TiO2 contents (0‐0.1%), whereas those in refractory harzburgites and dunites have intermediate compositions. Cpx‐bearing and refractory harzburgites show spoon‐and U‐shaped REE patterns, respectively, and their HREE distribution patterns suggest at least 15%‐ 20% partial melting. The REE patterns of dunites and high‐Cr chromitites vary from spoon‐ to U‐shaped and require 15‐30% partial melting in their mantle sources to produce their parental melts. Our dataset reveals that the nascent Luobusa mantle wedge was first infiltrated by slab‐derived fluids and later refertilized by transitional lava‐like melts, resulting in cpx‐bearing harzburgites. Partial melting in the deeper cpx‐bearing mantle generated high‐Ca boninitic to arc picritic melts, which interacted with the peridotites in the uppermost mantle to generate high‐Cr chromitites, dunites and some refractory harzburgites. Lithological variation from cpx‐bearing to refractory harzburgites in forearc ophiolites is the result of multi‐stage melt events rather than increasing degrees of partial melting. Intermittent slab rollback during subduction initiation induces asthenospheric upwelling and high heat flux in nascent mantle wedges. Elevated geothermal gradients play a more important role than slab dehydration in triggering Mg‐rich magmatism in newly‐initiated subduction zones.     

西藏罗布莎豆荚状铬铁矿床深部找矿突破与成因模式讨论

中国铬铁矿资源的瓶颈状态已持续多年。最近,在西藏罗布莎蛇绿岩地幔橄榄岩的深部勘探发现200万t致密块状铬铁矿床,这是中国近50年来铬铁矿找矿的重大突破,对今后继续寻找同类型的铬铁矿床具有重要指导意义。蛇绿岩地幔橄榄岩中产出的豆荚状铬铁矿床是工业需求铬的重要来源。研究豆荚状铬铁矿的成矿作用和矿体围岩地幔橄榄岩地质特征,建立铬铁矿的成矿模型和找矿标志,是开展寻找同类型矿床的重要保证。随着近些年在豆荚状铬铁矿及围岩地幔橄榄岩中金刚石等深部矿物的不断发现和深入研究,人们对蛇绿岩型铬铁矿的物质来源和形成过程,有了新的认识,提出了铬铁矿的深部成因模式。研究认为深部成因铬铁矿床主要经历了4个阶段:(1)早期俯冲到地幔过渡带(410~660 km)的陆壳和洋壳物质被脱水和肢解,过渡带产生的热和流体促成了地幔的熔融和Cr的释放和汇聚;(2)铬铁矿浆在地幔柱驱动下,运移到过渡带顶部冷凝固结,并有强还原的流体进入,后者携带了深部形成的金刚石、斯石英等高压矿物,进入"塑性—半塑性地幔橄榄岩"中;(3)随着物质向上移动,深度降低,早期超高压相矿物发生相变,如斯石英转变成柯石英,高压相的铬铁矿中出溶成柯石英和单斜辉石;(4)在侵位过程和俯冲带环境下,含水熔体与方辉橄榄岩反应形成了不含超高压矿物的规模相对较小的浸染状铬铁矿及纯橄岩岩壳。进一步研究表明,同处雅鲁藏布江缝合带西段的几个大型地幔橄榄岩岩体与罗布莎岩体可以对比,经历了相同的构造背景和豆荚状铬铁矿的成矿作用,存在较大的找矿空间。     

High‐ and low‐Cr chromitite in a Tibetan Ophiolite: Evolution from Mature Subduction System to Incipient Forearc in the Neo‐Tethyan Ocean

The microstructures, major‐ and trace‐element compositions of minerals and electron backscattered diffraction (EBSD) maps of high‐ and low‐Cr# [spinel Cr# = Cr³⁺/(Cr³⁺+Al³⁺)] chromitites and dunites from the Zedang ophiolite in the Yarlung Zangbo Suture (South Tibet) have been used to reveal their genesis and the related geodynamic processes in the Neo‐Tethyan Ocean. The high‐Cr# (0.77‐0.80) chromitites (with or without diopside exsolution) have chromite compositions consistent with initial crystallization by interaction between boninitic magmas, harzburgite and reaction‐produced magmas in a shallow, mature mantle wedge. Some high‐Cr# chromitites show crystal‐plastic deformation and grain growth on previous chromite relics that have exsolved needles of diopside. These features are similar to those of the Luobusa high‐Cr# chromitites, possibly recycled from the deep upper mantle in a mature subduction system. In contrast, mineralogical, chemical and EBSD features of the Zedang low‐Cr# (0.49‐0.67) chromitites and dunites and the silicate inclusions in chromite indicate that they formed by rapid interaction between forearc basaltic magmas (MORB‐like but with rare subduction input) and the Zedang harzburgites in a dynamically extended, incipient forearc lithosphere. The evidence implies that the high‐Cr# chromitites were produced or emplaced in an earlier mature arc (possibly Jurassic), while the low‐Cr# associations formed in an incipient forearc during the initiation of a new episode of Neo‐Tethyan subduction at ～130‐120 Ma. This two‐episode subduction model can provide a new explanation for the coexistence of high‐ and low‐Cr# chromitites in the same volume of ophiolitic mantle.     

Petrological and geochemical characteristics of the Samar Ophiolite ultramafic section: implications on the origins of the ophiolites in Samar and Leyte islands,Philippines

Cretaceous ophiolites and ophiolitic fragments occur in the Samar and Leyte islands in eastern central Philippines. The Samar Ophiolite is a complete crust–mantle sequence exposed in southern Samar, whereas the Tacloban and Malitbog ophiolite complexes are, respectively, located in the northeastern and southwestern portions of the nearby Leyte island. Despite the close proximity of these islands, the genetic relationship of these ophiolites and ophiolitic complexes, if any, remains to be elucidated. We present here new petrographic and geochemical data on the harzburgites and dunites of the ultramafic section of the Samar Ophiolite. These mantle peridotites are highly depleted residues which have low modal pyroxene content, high spinel Cr# (=0.62–0.79), and slightly enriched light rare earth element abundance with depletion in Zr and Ti. Such characteristics are typical of supra-subduction zone peridotites and strongly contrast with the abyssal signatures of the Tacloban and Malitbog ophiolite complexes. The absence of a structure between these adjacent ophiolite fragments initially hints that they form a single oceanic crust. However, with our new results, we suggest other possible mechanisms that could explain the relationship of these ophiolites.     

内蒙古贺根山蛇绿岩的铬铁矿中发现金刚石等深部地幔矿物

前人报道在西藏中生代和俄罗斯极地乌拉尔早古生代蛇绿岩地幔橄榄岩铬铁矿中发现了金刚石等深部地幔矿物,认为需重新考虑铬铁矿浅部成因的传统认识。为了查明不同造山带蛇绿岩的铬铁矿中金刚石等深部矿物的分布规律和豆荚状铬铁矿的成因,笔者开展了内蒙古贺根山晚古生代蛇绿岩中的铬铁矿床的人工重砂矿物学研究,本研究获得约2000 kg的内蒙古贺根山蛇绿岩铬铁矿石样品,对所采样品开展人工重砂选矿,表明该铬铁矿矿石样品中至少有金刚石、碳硅石及其他自然元素类、金属互化物类、氧化物类、硫化物类、硅酸盐类等30余种矿物。内蒙—大兴安岭造山带晚古生代的内蒙古贺根山蛇绿岩带铬铁矿石中,发现金刚石等深部地幔矿物表明,贺根山铬铁矿可能为深部成因。     

Neoproterozoic ophiolitic peridotites along the Allaqi-Heiani suture, South Eastern Desert, Egypt

The Wadi Allaqi ophiolite along the Egyptian-Sudanese border defines the southernmost ophiolitic assemblage and suture zone in the Eastern Desert. Ophiolite assemblages comprise nappes composed mainly of mafic and ultramafic rocks that were tectonically emplaced and replaced by serpentine and carbonates along shear zones probably due to CO₂-metasomatism. Serpentinites, altered slices of the upper mantle, represent a distinctive lithology of dismembered ophiolites of the western YOSHGAH suture. Microscopically, they are composed of more than 90 % serpentine minerals with minor opaque minerals, carbonate, brucite and talc. The mineral chemistry and whole-rock chemical data reported here indicate that the serpentinized peridotites formed as highly-depleted mantle residues. They show compositions consistent with formation in a suprasubduction zone environment. They are depleted in Al₂O₃ and CaO similar to those in fore-arc peridotites. Also, high Cr# (Cr/ (Cr+Al)) in the relict chrome spinels (average ~0.72) indicates that these are residual after extensive partial melting, similar to spinels in modern fore-arc peridotites. Therefore, the studied serpentinites represent fragments of an oceanic lithosphere that formed in a fore-arc environment, which belongs to an ophiolitic mantle sequence formed in a suprasubduction zone.     

Geochemical Characteristics of Mid-oceanic Ridge Volcanic Glass from Lau Basin: Evidence from the Major Elements Variation

Orientale size craters are not recognized on Earth nor expected for Phanerozoic and Proterozoic eons from conventional crater size frequency distributions (Ivanov et al., 2002). Here suggested are three such Phanerozoic craters, modified by plate tectonics, and tentatively correlated with extinction and “ophiolite obduction” events. Hypothesis testing is proposed and plate tectonics implications are discussed. Such basins might manifest:

• circular to elliptical rims (or rim segments), with exposed lithospheric mantle, as strain markers for plate boundary motion;
• thick ejecta near rim expressed as “ophiolitic melange”;
• power law decay of ejecta thickness with radial distance from rim (McGetchin et al., 1973) and/or systematic azimuthal variation of ejecta thickness for low angle impacts (Schultz, 1999);
• weathering resistant shocked mantle minerals (Bohor et al., 1990) in ejecta;? global spherule layer with PGE anomalies (Alvarez et al., 1980);
• rim structures consistent with cratering mechanics (Melosh, 1989; Kenkmann, 2014);
• impact melt basement (Grieve et al., 1992; Pierazzo et al. 2000) recording uniform cooling age and Earth's magnetic polarity of the time. Tentatively suggested Phanerozoic impact basins:
• Yucatan Basin: Greater Antilles ophiolite rim – KPg Boundary? Maastrichtian ophiolite obduction in southeast Cuba (Iturralde‐Vinent et al., 2006).
• Sulu Sea Basin: Palawan, Sabah etc. ophiolite rim – Middle Miocene Disruption? MM ophiolitic mélange emplacement in Sabah (Clennell, 1991).
• Loyalty Basin: New Caledonia ophiolite and d'Entrecasteaux ridge rim – EO Boundary? EO ophiolite obduction in New Caledonia (Cluzel et al., 2012).

     

Geochemistry and petrogenesis of the Late Cretaceous Haji‐Abad ophiolite (Outer Zagros Ophiolite Belt,Iran): implications for geodynamics of the Bitlis–Zagros suture zone

The Haji‐Abad ophiolite in SW Iran (Outer Zagros Ophiolite Belt) is a remnant of the Late Cretaceous supra‐subduction zone ophiolites along the Bitlis–Zagros suture zone of southern Tethys. These ophiolites are coeval in age with the Late Cretaceous peri‐Arabian ophiolite belt including the Troodos (Cyprus), Kizildag (Turkey), Baer‐Bassit (Syria) and Semail (Oman) in the eastern Mediterranean region, as well as other Late Cretaceous Zagros ophiolites. Mantle tectonites constitute the main lithology of the Haji‐Abad ophiolite and are mostly lherzolites, depleted harzburgite with widespread residual and foliated/discordant dunite lenses. Podiform chromitites are common and are typically enveloped by thin dunitic haloes. Harzburgitic spinels are geochemically characterized by low and/or high Cr number, showing tendency to plot both in depleted abyssal and fore‐arc peridotites fields. Lherzolites are less refractory with slightly higher bulk REE contents and characterized by 7–12% partial melting of a spinel lherzolitic source whereas depleted harzburgites have very low abundances of REE and represented by more than 17% partial melting. The Haji‐Abad ophiolite crustal sequences are characterized by ultramafic cumulates and volcanic rocks. The volcanic rocks comprise pillow lavas and massive lava flows with basaltic to more‐evolved dacitic composition. The geochemistry and petrology of the Haji‐Abad volcanic rocks show a magmatic progression from early‐erupted E‐MORB‐type pillow lavas to late‐stages boninitic lavas. The E‐MORB‐type lavas have LREE‐enriched patterns without (or with slight) depletion in Nb–Ta. Boninitic lavas are highly depleted in bulk REEs and are represented by strong LREE‐depleted patterns and Nb–Ta negative anomalies. Tonalitic and plagiogranitic intrusions of small size, with calc‐alkaline signature, are common in the ophiolite complex. The Late Cretaceous Tethyan ophiolites like those at the Troodos, eastern Mediterranean, Oman and Zagros show similar ages and geochemical signatures, suggesting widespread supra‐subduction zone magmatism in all Neotethyan ophiolites during the Late Cretaceous. The geochemical patterns of the Haji‐Abad ophiolites as well as those of other Late Cretaceous Tethyan ophiolites, reflect a fore‐arc tectonic setting for the generation of the magmatic rocks in the southern branch of Neotethys during the Late Cretaceous. Copyright © 2012 John Wiley & Sons, Ltd.     

Oceanic lithosphere and ophiolites: Birth,life and final resting place of related ore deposits

In this contribution an overview of oceanic lithosphere, associated ore deposits (sulphides, Fe and Mn oxides, chromitites) and their final destination in ophiolitic rocks are presented. This is followed by a discussion on massive sulphide mineralisation formed at mid-ocean ridges (MOR) and/or supra-subduction zones (SSZ). The geological characteristics and the genesis of the Cu-rich massive sulphide deposits of Cyprus and of the Oman ophiolite are discussed based on an extensive review of the published literature. This is followed by a synopsis of the ophiolitic terranes and associated mineral system in the Urals. We also present an overview of the ophiolitic belts and sutures of the Tethyan orogens, focussing on the podiform chromite deposits that they typically host, with a special focus on the ophiolitic chromitites of Turkey. A final section deals with possible ophiolites of Proterozoic and Archaean ages and, where applicable, associated chromitites. In the concluding remarks a brief note is made of some specific ancient seafloor hydrothermal constructs that have been interpreted as black chimneys in volcanogenic massive sulphide (VMS) deposits now hosted in ophiolitic sequences.     

蛇绿岩地幔岩中自由SiO2的发现及其地质意义

自由SiO_2系指石英及其同质多型物(polymorphs)柯石英、斯石英等。石英广泛分布于地壳中的各种岩石中,柯石英和斯石英只存在于超高压岩石和陨石坑中。由于石英和非饱和SiO_2的橄榄石不能共生,因此在地幔橄榄岩和超镁铁岩中不存在原生石英。最近笔者在西藏罗布莎蛇绿岩的地幔岩(方辉橄榄岩)的豆荚状铬铁矿中发现了自由SiO_2和柯石英相。根据高温高压相平衡实验资料,橄榄石、辉石这样的硅酸盐矿物在地幔深部的压力条件下可以分解成简单氧化物,如FeO(方铁矿)、MgO(方镁石)以及SiO_2(斯石英)等。由此推测,西藏蛇绿岩地幔岩中自由SiO_2可能是来自于下地幔的矿物,是地幔柱作用将其搬运到上地幔浅部。     

The tectonic-setting of ophiolites in the western Qinghai-Tibet Plateau,China

The study of geology, geochemistry, rare earth elements, trace elements, Pb and Sr isotopes of representative ophiolite bodies from four ophiolitic belts in the western Qinghai-Tibetan Plateau, shows that the mantle peridotites of these ophiolites are mainly harzburgite in composition, with minor dunite. They are characterized by high magnesium (MgO) and low aluminum, calcium and alkali oxide contents. Enrichment of light rare earth elements in mantle peridotites may be due to two geological processes: relatively strong partial melting; and later metasomatism by the liquids released during the subduction of oceanic crust. Mantle peridotites are characterized by low contents of the trace elements Sr, Ti and Y and relatively high contents of Rb, Nb, Zr, Hf and Th, similar to metasomatic pyrolite. The isotopic compositions of Sr and Pb show evidence of contamination by a crustal component. All the evidence indicates that the four ophiolite belts in the western Qinghai-Tibetean Plateau have undergone metasomatism by liquids released during the subduction of oceanic crust, suggesting that they were formed in a supra-subduction zone (SSZ) tectonic setting. The mantle peridotites in ophiolite belts located in eastern Qinghai-Tibetan Plateau, e.g. Sanjiang and West Kunlun, may be compared with the Troodos, which is regarded as a typical SSZ complex, having the same geochemical characteristics, i.e. high MgO and LREE-rich. The geochemistry, combined with the occurrence of boninite and adakite rocks, which are associated with subduction magmatism, suggest that ophiolites formed at different times in Qinghai-Tibetan Plateau, including Sanjiang and West Kunlun, are all SSZ-type ophiolites formed in a supra-subduction zone tectonic setting.