Refractory- and metallurgical-type chromite ores,Zambales ophiolite,Luzon, Philippines

The Zambales ophiolite is the major source of chromite ore in the Philippines. The chromitites are concordant cumulates and are associated with distinct chromitite-bearing sequences within the mantle peridotites. Refractory and metallurgical chromite deposits are spatially separated and related to different lithologic associations, which crystallized from different parental magmas. — Refractory chromite ores (30–44 wt% Cr₂O₃; 20–30 wt% Al₂O₃) are linked with the peridotite-troctolite-olivine gabbro lineage. Two main types were found: (1) Al-rich refractory ores associated with harzburgites and feldspathic periodotites and (2) more Cr-rich varieties associated with lherzolites. — Metallurgical chromite ores (45–53 wt% Cr₂O₃; 12–18 wt% Al₂O₃) are linked with the peridotite-pyroxenite-norite lineage. Two main types were also found: (1) Cr-rich metallurgical ores associated with orthopyroxenites and (2) more Al-rich varieties related to clinopyroxenites. — The chemical composition of chromite within the deposits varies depending on the chromite/silicate ratios of the ore types and grades continuously into accessory chrome spinels in the wall-rock peridotites. — The geochemistry of accessory chrome spinels in various peridotites and mafic cumulates depends on the mineralogical composition and the stratigraphic position of their host rocks.New address: BEB Erdgas und Erdöl GmbH, Riethorst 12, D-3000 Hannover 51The terms chrome-spinels and chromite are used as follows: 1. Chrome-spinel is only used for those occuring as accessory minerals in various ultramafic and mafic rocks (= accessory chrome-spinels). Their chemical composition has been determined only by microprobe analysis. — 2. Chromite is used for ore and ore deposits (=chromitites); the chemical composition has been determined by wet chemistry (AAS) or by microprobe analysis     

Geochemistry of reduced gas related to serpentinization of the Zambales ophiolite, Philippines

Methane-hydrogen gas seeps with mantle-like C and noble gas isotopic characteristics issue from partially serpentinized ultramafic rocks in the Zambales ophiolite, Philippines. New measurements of noble gas and ¹⁴C isotope abundances, rock/mixed-volatile equilibrium calculations, and previous chemical and isotopic data suggest that these reduced gases are products of periodotite hydration. The gas seeps are produced in rock-dominated zones of serpentinization, and similar gases may be ubiquitous in ultramafic terranes undergoing serpentinization.     

The distribution of halogens and carbon in PGE-bearing ultramafics of the Acoje ophiolite block, Zambales, Philippines

Evidence for redistribution of Pt and Pd in the Acoje ultramafic rocks led to an investigation of the role of Cl, Br, F, I and C in Pt and Pd transport in hydrothermal solution. Anomalously high contents of 300–1000 ppm Cl, 3 ppm Br, up to 50 ppm F, 180–380 ppm I and 300–3300 ppm C are characteristic of the Acoje ultramafic rocks. The Cl and Br concentrations are restricted to serpentinized dunites and a positive correlation between Br and Cl indicate their common origin and their introduction during serpentinization. The ratios Br/Cl,F/Cl, and I/Cl correspond to those of sediments that contain seawater which suggests that Cl, Br and I were partly expelled from deep sea sediments during emplacement of the ophiolite. Fluorine could have been derived from mantle material.Carbon occurs in fluid inclusions in olivines as CO₂, CO or CH₄ and/or submicroscopic graphite. The high C content in serpentinized dunites suggests that C, at least in part, is also of serpentinization origin.Chlorine is mainly incorporated into Fe-rich serpentines and Ca-amphiboles. Very low F concentration in hydrous phases is common, except in serpentines from pyroxenes, pargasites and edenites. Brucite is finely dispersed in serpentines derived from olivines, indicating low CO₂-activity during brucite formation and a pH of about 11.The presence of Pt and Pd tellurides, arsenides and bismuthides and the absence of selenides, in spite of elevated Se concentrations in bulk analyses of about 5 ppm, indicates that the stability conditions for selenide formation were not obtained during alteration. The formation of Pt and Pd halogen complexes, requiring highly oxidizing (f_O2 > hematite-magnetite boundary (HM)) and acid environments is not favoured for Pt and Pd transport in Acoje ultramafics. An redistribution caused by the solubility reduction of Pt and Pd by Te, Bi and As and a precipitation of their intermetallic phases is proposed. No correlation between Cl and PGE-bearing rock units was observed, which indicates the minor role of halogens during redistribution of Pt and Pd in the Acoje ophiolite.     

Structural features of ophiolitic chromitites in the Zambales Range,Luzon, Philippines

The chromitite-bearing peridotites of the Zambales mafic-ultramafic complex form the lowermost level of the Zambales ophiolite, which exposes a complete ophiolitic sequence. The chromitites occur close to the peridotite/gabbro transition zone.The chromite orebodies are structurally classified into three major types: (1) concordant tabular deposits, (2) strings of pods and (3) pocketlike deposits.Concordant tabular deposits show a gradational transition from chromitite to host rock (modal grading) and are characterized by the parallelism of ore and host-rock structures. Primary magmatic features like inch-scale layering, size grading, glomeroporphyric chromite aggregates, skeletal chromite growth and adcumulus growth (cumulus textures) are common.The concordant chromite bodies are often tectonically disrupted and boudined forming strings of pods or fault-controlled pocketlike deposits. With increasing tectonization chromite shows pull-apart textures and lineations (plastic deformation), shearing, prismatic jointing, brecciation and mylonitization (brittle deformation). Recrystallization of cataclastic chromite occurs on a microscopic scale.Plastic deformation is caused by mantle flow and/or the volume increase of the peridotites during serpentinization. The influence of mantle flow is indicated by the orientation of the pod strings and lineations in chromitite perpendicular to the ridge axis. Brittle deformation of chromite (cataclasis) and disruption by faults is related to the emplacement of the ophiolite.     

The Zambales complex: gravity anomalies and a very strange ophiolite

In common with most of the larger ophiolite bodies that were emplaced during the Cenozoic and late Mesozoic and which have experienced relatively little post-emplacement disruption or metamorphism, the Zambales Ophiolite Complex (ZOC) of western Luzon (northern Philippines) is associated with very large gravity anomalies. Worldwide and in the ZOC, peak to trough amplitudes of more than 100 mGal are common and amplitudes in excess of 200 mGal are not unknown     

Evolution of Hydrothermal System at the Dizon Porphyry Cu-Au Deposit, Zambales, Philippines

Abstract. Evolution of hydrothermal system from initial porphyry Cu mineralization to overlapping epithermal system at the Dizon porphyry Cu‐Au deposit in western central Luzon, Zambales, Philippines, is documented in terms of mineral paragen‐esis, fluid inclusion petrography and microthermometry, and sulfur isotope systematics. The paragenetic stages throughout the deposit are summarized as follows; 1) stockwork amethystic quartz veinlets associated with chalcopyrite, bornite, magnetite and Au enveloped by chlorite alteration overprinting biotite alteration, 2) stockwork quartz veinlets with chalcopyrite and pyrite associated with Au and chalcopyrite and pyrite stringers in sericite alteration, 3) stringer quartz veinlets associated with molybdenite in sericite alteration, and 4) WNW‐trending quartz veins associated with sphalerite and galena at deeper part, while enargite and stibnite at shallower levels associated with advanced argillic alteration. Chalcopyrite and bornite associated with magnetite in quartz veinlet stockwork (stage 1) have precipitated initially as intermediate solid solution (iss) and bornite solid solution (bnss), respectively. Fluid inclusions in the stockwork veinlet quartz consist of gas‐rich inclusions and polyphase inclusions. Halite in polyphase inclusions dissolves at temperatures ranging from 360d?C to >500d?C but liquid (brine) and gas (vapor) do not homogenize at <500d?C. The maximum pressure and minimum temperature during the deposition of iss and bnss with stockwork quartz veinlets are estimated to be 460 bars and 500d?C. Fluid inclusions in veinlet stockwork quartz enveloped in sericite alteration (stage 2) consist mainly of gas‐rich inclusions and polyphase inclusions. In addition to the possible presence of saturated NaCl crystals at the time of entrapment of fluid inclusions that exhibit the liquid‐vapor homogenization temperatures lower than the halite dissolution temperatures in some samples, wide range of temperatures of halite dissolution and liquid‐vapor homogenization of polyphase inclusions from 230d?C to >500d?C and from 270d?C to >500d?C, respectively, suggests heterogeneous entrapment of gaseous vapor and hypersaline brine. The minimum pressure and temperature are estimated to be about 25 bars and 245d?C. Fluid inclusions in veinlet quartz associated with molybdenite (stage 3) are dominated by gas‐rich inclusions accompanied with minor liquid‐rich inclusions that homogenize at temperatures between 350d?C and 490d?C. Fluid inclusions in vuggy veinlet quartz associated with stibnite (stage 4) consist mainly of gas‐rich inclusions with subordinate polyphase inclusions that do not homogenize below 500d?C. Fluid inclusions in veinlet quartz associated with galena and sphalerite (stage 4) are composed of liquid‐rich two‐phase inclusions, and they homogenize into liquid phase at temperatures ranging widely from 190d?C to 300d?C (suggesting boiling) and the salinity ranges from 1.0 wt% to 3.4 wt% NaCl equivalent. A pressure of about 15 bars is estimated for the dilute aqueous solution of 190d?C from which veinlet quartz associated with galena and sphalerite precipitated. In addition to a change in temperature‐pressure regime from lithostatic pressure during the deposition of iss and bnss with stockwork quartz veinlets to hydrostatic pressure during fracture‐controlled quartz veinlet associated with galena and sphalerite, a decrease in pressure is supposed to have occurred due to unroofing or removal of the overlying piles during the temperature decrease in the evolution of hydrothermal system. The majority of the sulfur isotopic composition of sulfides ranges from ±0 % to +5 %. Sulfur originated from an iso‐topically uniform and homogeneous source, and the mineralization occurred in a single hydrothermal system.     

菲律宾Zambales蛇绿岩套的再研究：对其特提斯来源的启示

A number of geological studies have already been conducted on the Zambales Ophiolite Complex (ZOC), a north-south trending complete ophiolite sequence exposed in the western portion of Central Luzon, Philippines. Previous works recognized the ZOC as being made up of two blocks, the Acoje and the Coto, acting as an arc-back arc pair sometime during the Eocene.     

The Acoje Block Platiniferous Dunite Horizon, Zambales Ophiolite Complex, Philippines: Melt Type and Associated Geochemical Controls

Abstract: The Zambales Ophiolite Complex, a supra-subduction zone ophiolite, is made up of the mid-ocean ridge-related Coto block and the island arc-related Acoje block. This crust-mantle sequence hosts platinum-group elements (PGE) in the Acoje block. The melts responsible for the PGE-bearing nickel sulfide and chromitite deposits are of magmatic origin characterized by high-MgO basalt to boninitic composition which, being second or third-stage melts, carry higher PGE budgets. Metal ratio diagrams, utilizing base and precious metals, reveal that the distribution and deposition of the PGE in the Acoje block are affected by olivine, chromite and sulfide crystallization. The generation, accumulation and segregation of the PGE, oxide and sulfide minerals from the melts are governed by the combined factors of high degrees of partial melting, multiple melt replenishment with concomitant magma mixing and fractional crystallization. Although previous sulfide segregation events could have occurred below the PGE-bearing nickel sulfide horizon as shown by the Ni/Cu (>1), the Cu/Pd and Ni/Pd strongly suggest that the main platiniferous zone is confined within the Acoje block transition zone dunite.     

Slab rollback and microcontinent subduction in the evolution of the Zambales Ophiolite Complex(Philippines):A review

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.     

Multiple ophiolite generation preserved in the northern Philippines and the growth of an island arc complex

Although all oceanic arcs grow through the addition of subduction-generated magmas, the geology of the northern Philippines demonstrates that a major contribution to arc crustal growth can come from repeated, episodic, intra-arc, back-arc, and/or fore-arc oceanic crust generation with subsequent preservation of the basic–ultrabasic units in the arc complex. At least five episodes of oceanic crust generation are represented in the northern Philippines by preserved ophiolitic sequences and recent intra-arc seafloor spreading. Each episode is distinct in age as confirmed by modern dating techniques, with the ages ranging from pre(?)-Jurassic to Quaternary. Although the Philippines is widely regarded as an amalgamation of allochthonous terranes, a review of the available data shows that there is currently no compelling evidence that these ophiolites are of exotic origin and that they have been tectonically accreted to the Philippine arc complex. Rather, the evidence suggests that most—and possibly all—of the ophiolites were generated as back-arc, fore-arc, or intra-arc crust within the Philippine arc complex. Hence, there is a close spatial association of several ophiolitic terranes of diverse ages spanning 150 Myr that formed as part of the arc complex. Such an association may have arisen from episodic generation of oceanic crust during periods of local extension in a suprasubduction zone setting, which has experienced changing and possibly overlapping subduction from the east and west sides (in the current reference frame). Disruption of the ophiolitic basement terranes has been, and continues to be, effected primarily by wrench faulting. This style of arc growth has implications for the paleotectonic interpretation of ancient ophiolite-arc terranes in continents and the petrologic evolution of island arcs.     

Paleomagnetism of Palawan,Philippines

Paleomagnetic studies have been carried out on Palawan and on the island of Busuanga to the north. Results from the Cretaceous Espina Basalts of the Calatuigas Ophiolite in the South Palawan Block (SPB) pass a fold test, yield normal and reversed directions with a magnetic intensity and AF demagnetization characteristics consistent with a primary TRM. The mean direction is 293.9° and an inclination of 5.8°, with a k of 37.7 and an α₉₅ of 12.6°. This suggests that these ophiolites have moved northward and rotated counterclockwise by 66°±+13° with respect to the geocentric axial dipole (GAD) field. It also suggests that they were obducted from the south.Paleomagnetic directions from the Jurassic Busuanga Cherts and the Cretaceous Guinlo Formation from the island of Busuanga in the North Palawan Block (NPB) and from the Guinlo on the main island of Palawan are similar, fail regional fold tests, and have AF demagnetization characteristics consistent with secondary magnetization. Their inclinations are statistically indistinguishable at a 95% significance level, but variation in declination suggests differential local rotation about a vertical axis. The paleolatitude is comparable to that of regions of pervasive Cretaceous remagnetization in the South China borderland and may reflect similar remagnetization, consistent with the NPB’s proposed South China origin.     

Rare earth element geochemistry of the Betts Cove ophiolite,Newfoundland: complexities in ophiolite formation

The Betts Cove ophiolite includes the components of typical ocean crust: pillow lavas, sheeted dikes, gabbros and ultramafics. However, the trace element geochemistry of basaltic rocks is unusual. Three geochemical units are recognized within the lava and dike members. Within the pillow lavas, the geochemical units correspond to stratigraphic units. Upper lavas have ‘normal’ (i.e., typical for ocean floor basalts) TiO₂ contents (0.75 to 2.0 wt%), heavy rare earth elements (HREE) values in the range 6–20× chondrites and chondrite-normalized REE patterns with relative LREE depletion. Intermediate lavas have TiO₂ contents between 0.30 and 0.50 wt%, HREE contents from 4–7× chondrites and extreme relative LREE depletion. Lower lavas have anomalously low TiO₂ contents (<0.30 wt%) and unusual convex-downwards REE patterns with REE abundances around 2–5 × chondrite. These geochemical differences can be explained if the three groups were derived from different mantle sources. Independent mantle sources for the three units are consistent with their different ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratios varying at 480 m.y.B.P. from 0.51222 in a lower lava to 0.51238 in an upper lava. The upper lavas may be partial melts of a source similar in composition to that of modern MORB, the intermediate lavas may be from a very depleted oceanic mantle (second stage melt), and the lower lavas may have formed by melting an extremely depleted mantle that had been invaded by a LREE-enriched fluid. A possible tectonic environment where these different sources could be juxtaposed is a back-arc or inter-arc basin.     

Platinum-group minerals from the Troodos ophiolite,Cyprus

Summary Platinum-group minerals have been identified in chromitites from the Troodos ophiolitic complex of Cyprus. The host chromitites occur as podiform bodies within the basal harzburgite of the ophiolite, as occasional discontinuous layers in the dunites at the base of the overlying cumulate sequence and rarely as minor schlieren in clinopyroxene dunites at higher levels. Podiform chromitites are generally highly deformed and frequently brecciated while those from the cumulate sequence are well-preserved and display cumulate textures. Chromite grains from bodies at all levels host a broad and mineralogically complex assemblage of inclusions including silicates, platinum-group minerals, base metal sulphides and fluid inclusions, all of which have been studied in detail. The platinum-group minerals (PGM) and base metal sulphides (BMS), which are described here, are modally much less abundant than the silicate inclusions and generally small in size (< 50 µm). PGM comprise sulphides and alloys and are dominated by laurite (RuS₂). Other sulphides are Ru-poor. Alloys include iridosmine, osmian ruthenium and Ru-Fe alloys. Two generations of PGM are believed to be present, the first having been trapped during chromite formation, the second (including most of the alloys) having been formed during serpentinization. The base metal sulphides include common small Ni-Fe sulphides, sometimes associated with silicate inclusions, and larger Cu-rich inclusions, some of which are almost pure chalcopyrite. The origin of the latter is of genetic significance.

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Minerale der Platin-Gruppe aus denn Troodos-Ophiolith, Zypern

Zusammenfassung Minerale der Platin-Gruppe wurden in verschiedenen Chromititen des Troodos Ophiolithes, Zypern, nachgewiesen. Die Chromitite kommen als podiforme Körper in den basalen Harzburgiten, als gelegentlich unregelmäßige Lagen in den Duniten an der Basis der Kumulat-Abfolge, und selten als Schlieren in den Klinopyroxen-Duniten in höheren Bereichen vor. Podiforme Chromitite sind im allgemeinen intensiv deformiert und häufig brekziiert, während jene aus der Kumulat-Abfolge gut erhalten sind und Kumulat-Texturen zeigen. In Chromiten aus allen Niveaus des Ophiolithes kommt eine umfangreiche, und mineralogisch komplexe Assoziation von Einschlüssen vor; diese umfassen Silikate, Platingruppen-Minerale, Buntmetallsulfide und FlüssigkeitsEinschlüsse. Die Platingruppen-Minerale (PGM) und Buntmetallsulfide (BMS) die hier beschrieben werden, sind modal weniger verbreitet als die Silikateinschlüsse, und sind meist sehr feinkörnig ( < 50 µm). Die PGM umfassen Sulfide, einerseits dominiert von Laurit (RuS₂), aber auch Ruthenium-arme Sulfide, und Legierungen, bestehend aus Iridosmin, Osmium-führendes Ruthenium und Ruthenium-Eisenlegierungen. Die PGM können zwei Generationen zugeordnet werden. Die erste wurde während der Bildung der Chromite eingeschlossen, die zweite (und hierzu gehören die meisten Legierungen) wurden während der Serpentinisierung gebildet. Die Buntmetallsulfide unfassen die bekannten Nickel-Eisen-Sulfide, die manchmal mit Silikateinschlüssen vergesellschaftet sind, und größere Kupfer-reiche Einschlüsse, die zum Teil reiner Kupferkies sind. Die Entstehung dieser Einschlüsse ist von Bedeutung für die Genese.

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With 13 Figures     

新疆鄯善康古尔塔格蛇绿岩及其大地构造意义

康古尔塔格蛇绿岩的岩石组合为变质橄榄岩-堆晶橄榄岩-辉长岩-斜长花岗岩-辉绿岩-玄武岩。方辉橄榄岩（蛇纹岩）、蛇纹石化辉石岩、蚀变辉长岩与特罗多斯蛇绿岩中同类型岩石类似，岩石总体低钾。变质橄榄岩MgO／（MgO＋TFeO）为0．834～0．866，TiO，（wt％）为0．02％，为SSZ型蛇绿岩的变质橄榄岩。玄武岩的构造环境判别显示其形成于边缘海盆。放射虫硅质岩的Al2O3／（Al2O3＋Fe2O3）值平均为0．047，MnO／TiO2比值平均为0．93，Ce具负异常，Ce／Ce^＊=0．548，Lan／Cen=1．661．表明放射虫硅质岩的形成环境与洋中脊有密切关系。该蛇绿岩位于塔里木板块和哈萨克斯坦-准噶尔板块的艾比湖-康古尔塔格缝合线上，为一套无序产出的古生代北天山洋在该区的古洋壳残片。     

元古代蛇绿岩及铬铁矿

本文总结了国外典型元古代蛇绿岩的岩石组合、野外产状、地球化学资料以及成矿特征,并与显生宙蛇绿岩进行了对比,继而探讨元古代板块构造演化和铬铁矿成因。资料表明,早元古代和中-新元古代均有蛇绿岩的存在,但前者较少,仅见于Canadian地盾的Cape Smith Belt中的Prutuniq蛇绿岩（2.05~2.0Ga）和芬兰Fennoscandian地盾的Outokumpu和Jormua蛇绿岩（时代为1.97~1.96Ga）,而中-新元古代的蛇绿岩则见于世界许多地区,如埃及东部沙漠区（~750Ma）和非洲东北部地区（ca.900~800Ma）等。与显生宙蛇绿岩相比,这些老蛇绿岩具如下特征：（1）它们均为被肢解的蛇绿岩,大多与"弧火山岩"和（或）混杂岩伴生,经历不同程度的变形和变质（具绿片岩相-角闪岩相组合）;（2）岩石组合大多较齐全,壳层组合发育,以镁铁-超镁铁岩（堆晶岩）、辉长岩、镁铁质席状岩床（墙）杂岩、火山岩为代表;层状镁铁-超镁铁岩的韵律层以及矿物的隐晶变化等均提示了岩浆多期次活动及开放岩浆房的特征;（3）元古代蛇绿岩中既有高铝型铬铁矿,也有高铬型铬铁矿,且主要寄主于纯橄岩（或蛇纹岩）中;高铝型和高铬型直接受控于熔体的熔融程度及含水流体的参与,反映了铬铁矿形成于俯冲带演化的不同阶段;铬铁矿规模均较小,且均以低TiO2为特征,均为岩浆分异作用的产物,明显区别于显生宙熔融残余成因的豆荚状铬铁矿;（4）元古代蛇绿岩常伴有硫化物Cu-Co-Zn-Au矿,且铬铁矿含Zn较高（Zn=0.11%~0.18%）（如芬兰Outokumpu蛇绿岩）;橄榄岩及铬铁矿中常含较高的MnO（高达1.79%,如埃及东部的Wizer蛇绿岩）;（5）元古代蛇绿岩具多种成因：主要为俯冲带成因（如埃及蛇绿岩、北东非蛇绿岩、芬兰Outokumpu蛇绿岩）,少量为洋中脊成因（加拿大Purtuniq蛇绿岩）及裂谷成因（芬兰Jormua蛇绿岩）等。     

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。如果双沟镁铁-超镁铁岩不是蛇绿岩,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。双沟镁铁-超镁铁岩不是蛇绿岩是什么？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。

     

Seamount volcanism associated with the Xigaze ophiolite, Southern Tibet

Basaltic lavas at Renbu, Southern Tibet are associated with the Xigaze ophiolite in the Yarlung-Zangbo suture zone. They are alkaline lavas rich in large ion lithophile elements (LILE, Ba, Rb and Sr) and high field strength elements (HFSE, Nb, Ta, Zr and Hf), but poor in Cr, Co and Ni. All of the rocks have chondrite-normalized REE patterns enriched in light rare earth elements (LREE), comparable to modern basalts of the Society Islands, Kerguelen Plateau and Broken Ridge. Abundances of some immobile or moderately immobile elements (Nb, Ta, Zr, Hf, Y, Ti and REE) are also comparable to Kerguelen alkaline basalts. The Renbu basalts are geochemically similar to oceanic island basalts (OIB) and have some elemental ratios, such as Nb/Ta ratios = 15.7–18.1, Th/Nb =  0.06–0.10, La/Nb = 0.59–0.83 and Th/Ta = 1.03–1.52, similar to the primitive mantle. Their ⁸⁷Sr/⁸⁶Sr ratios (0.70453–0.70602) are relatively high, similar to OIB. In the ⁸⁷Sr/⁸⁶Sr vs. εNd(t) diagram, the Renbu basalts plot along a trend from N-MORB to EMII (enriched mantle II), suggesting the involvement of at least two mantle sources in their generation. The Renbu basalts represent seamount volcanism associated with the Xigaze ophiolite. They formed from an OIB-type mantle source within the Neo-Tethyan Ocean that had a composition similar to the modern Indian Ocean mantle.     

The Late Proterozoic ophiolite of Sol Hamed,NE Sudan

The Sol Hamed complex, a sequence from ultramafics, through gabbros, thin sheeted dykes, to pillow lavas, is an ophiolite. It was obducted, tilted nearly to vertical, eroded and covered unconformably by the Nafirdeib Series. This is a volcaniclastic series, dated at 712 ± 58 Ma, the lower part of which includes conglomerates with ophiolitic clasts, olistostromes, one with large oolitic limestone slabs, felsitic and dacitic tuffs, turbidites and black shales, with andesites above. Some of the structures in the ophiolite are attributed to sub-oceanic deformation. Deformation after the deposition of the Nafirdeib Series produced folds and cleavage trending about NE-SW and, more locally, a cleavage trending NW in a shear zone where the ophiolite was thrust northeastwards over the Nafirdeib Series. The ophiolite and the Nafirdeib Series were intruded by the batholithic granite dated elsewhere at ca. 669 Ma. The ophiolite is thought to occur on a suture with the Nafirdeib Series representing part of an island arc sequence.     

Tectonic evolution of the Brooks Range ophiolite, northern Alaska

Analysis of internal structures of the Brooks Range ophiolite at the three largest and well-exposed klippen reveals a NE–SW structural grain that may parallel the original axis of magmatism of a slow spreading marginal ocean basin. Sub-parallel directions of lattice fabrics in olivine of mantle peridotite and shape fabrics in pyroxene and plagioclase of layered gabbro indicate that asthenospheric and magmatic flow was closely coupled. These structures, including the petrologic moho, mostly dip steeply to the NW and SE, with slightly oblique flow lineations. Sedimentary and volcanic cover deposits also dip SE. The few exposures found of sheeted dike complexes generally strike parallel, but dip orthogonal to both the petrologic moho and cover deposits. These structural features are locally disturbed by syn- and post-magmatic normal faults emblematic of slow-spreading ridge processes. However, the consistent geometry of structures over a distance of 200 km demonstrates not only that the magmatic system was organized in a similar manner to an oceanic ridge, but that there was little to no rotation of individual klippe during tectonic emplacement.Ductile fabrics related to tectonic emplacement yield top-to-the NNW sense of shear indicators. The basal thrust and accompanying serpentinized shear zone is mostly flat-lying and truncates the steeply dipping ductile fabric of the ophiolite. This relationship and paleomagnetic data from the igneous sequence suggest that flow fabrics were most likely moderately inclined at the time the ophiolite formed. Similar relationships are found at diapiric centers along oceanic ridges and in other ophiolite bodies.     

Ocean floor metamorphism in the Betts Cove ophiolite,Newfoundland

The mineralogical and geochemical features of the lower Ordovician Betts Cove ophiolite of northeastern Newfoundland indicate that hydrothermal circulation of seawater near a mid-ocean ridge has been involved in the metamorphism of the complex. The degree of greenschist facies metamorphism increases with stratigraphie depth in the ophioli te. Calcite, hematite and epidote distributions show that the metamorphosing fluid penetrated downward and was reduced with depth. The mobilities of major and trace elements support the hypothesis of the interaction of seawater and basalt: Fe₂O₃, MgO, Na₂O and H₂O increase whereas CaO and Cu decrease in the rock after alteration; SiO₂, total iron, K₂O, Ba and Rb can either be depleted or enhanced in the altered material; TiO₂, P₂O₅, Zr, Y, Cr and Ni remain stable during the metamorphic episode. Finally, the occurrence of massive sulphides and incipient rodingitic gabbro is explicable in a circulatory seawater system.