The Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment

The Moroccan Anti-Atlas orogenic belt encloses several Precambrian inliers comprising two major Neoproterozoic ophiolitic complexes: the Sirwa and Bou Azzer ophiolites. These ophiolites expose crustal and mantle units, thrusting over fragments of a long-lived intra-oceanic arc system. We present a detailed geochronological and petro-geochemical study of three mafic/ultramafic units of these two ophiolites: the Khzama sequence (Sirwa ophiolite) and the Northern and Southern Aït Ahmane sequences (Bou Azzer ophiolite). The crystallization of layered metagabbros from the Bou Azzer ophiolite (North Aït Ahmane sequence) has been dated here at 759 ± 2 Ma (U-Pb on zircons). This new age for the Bou Azzer ophiolite is similar to the formation of the Sirwa ophiolite (762 Ma) and suggests that both units formed during the same spreading event. Metabasalts of the three units show tholeiitic signature but with variable subduction-related imprints marked by LILE enrichments, HFSE depletions and variable Ti contents, similar to modern back-arc basin basalts (BABB). Their back-arc origin is also supported by the geochemical signature of ultramafic units showing very low contents in major and trace incompatible elements (Al₂O₃: 0.12–1.53 wt%, Ti: 3.5–64.2 ppm and Nb: 0.004–0.10 ppm), attesting of a highly refractory protolith. This is in agreement with the high Cr# (0.44–0.81) and low to intermediate Mg# (0.25–0.73) of their constitutive Cr-spinels. Dynamic melting models suggest that these serpentinites experienced intense and polyphased hydrous melting events, strongly influenced by supra-subduction zone SSZ-fluid influx and subduction-related melt percolation. Being particularly affected by these SSZ-melt/rock interactions and closer to arc units to the south, the Sirwa ophiolite and the South Aït Ahmane unit of the Bou Azzer ophiolite likely represent an early stage of the arc-back-arc system, which has been more influenced by the magmatic products of the arc activity compared to the North Aït Ahmane unit of the Bou Azzer ophiolite.     

Gold and platinum group elements in cobaltarsenide ores: Hydrothermal concentration from a serpentinite source-rock (Bou Azzer,Morocco)

Summary The cobalt-arsenide ores of Bou Azzer are located along the borders of serpentinite massifs (Upper Proterozoic ophiolite complex) in carbonate-quartz lenses resulting from hydrothermal carbonate alteration of serpentinite. The cobalt ores contain an average gold content of 5–20 ppm; gold is mainly located in skutterudite (120 ppm av.), whereas the Fe-arsenide (loellingite) contains < 1 ppm Au. Similarly the highest PGE contents are found in skutterudite (up to 2 ppm total PGE). All the arsenide ores of Bou Azzer exhibit the same chondrite normalized PGE pattern displaying positive Rh and negative Pt anomalies, and a slight positive slope (Pd/Ir = 1 to 2). This uncommon PGE pattern closely resembles to that of sulphides of komatiites.In serpentinite, the PGE patterns are typical of slightly depleted mantle rocks, and the associated podiform chromitites are within the range of ophiolitic chromitites, except for Pd and Au enrichment.Horizons of sulphide-bearing serpentinites show relatively high contents of noble metals and display PGE patterns which closely resemble those of the Co-arsenide ores, although an order of magnitude lower. These sulphides probably correspond to the remobilization during serpentinization of primary magmatic sulphides. The sulphiderich horizons are a possible source-rock for the noble metals of the Bou Azzer cobaltarsenide ores.

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Gold und Platingruppen-Elemente in Kobalt-Arsenid Erzen: Hydrothermale Anreicherung aus einem Serpentinit (Bou Azzer, Marokko)

Zusammenfassung Die Kobalt-Arsenid Erze von Bou Azzer kommen entlang den Grenzen eines Serpentinit-Massifs (Oberproterozoischer Ophiolit-Komplex) in Karbonat-Quarz-Linsen vor, die auf hydrothermale Umwandlung des Serpentinits zurückgehen.Die Kobalt-Erze enthalten 5–20 ppm Gold; dieses kommt hauptsächlich in Skutterudit (120 ppm) vor, während die Fe-Arsenide (Loellingit) weniger als 1 ppm Gold enthalten. Die höchsten PGE Gehalte kommen ebenso in Skutterudit vor (bis zu 2 ppm PGE). Alle Arsenid-Erze zeigen das gleiche Verteilungsbild mit positiven Rh und negativen Pt Anomalien, und eine leicht positive Neigung (Pd/Ir = 1 bis 2). Diese ungewöhnlichen PGE Verteilungsbilder erinnern an die von Sulfiden aus Komatiiten.Die PGE Verteilung in Serpentiniten ist typisch für leicht verarmte Mantelgesteine, und die assoziierten podiformen Chromitite liegen innerhalb des Bereiches für ophiolitische Chromitite, mit Ausnahme der Anreicherung in Pd und Au.Lagen von Sulfid-führenden Serpentiniten zeigen relativ hohe Gehalte an Edelmetallen, und PGE-Verteilungsmuster die denen von Co-Arseniderzen sehr ähnlich sind, obwohl sie um eine Größenordnung niedriger liegen. Diese Sulfide dürften Produkte der Remobilisierung primärer magmatischer Sulfide während der Serpentinisierung sein. Die Sulfid-reichen Lagen sind als ein mögliches Ursprungsgestein für die Edelmetalle der Kobalt-Arsenid-Erze von Bou Azzer zu sehen.

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With 8 Figures     

Thermal decomposition of serpentine during coseismic faulting: Nanostructures and mineral reactions

This paper reports a detailed characterization of an antigorite-bearing serpentinite, deformed at seismic slip-rate (1.1 m/s) in a high-velocity friction apparatus. Micro/nanostructural investigation of the slip zone (200 μm thick) revealed a zonal arrangement, with a close juxtaposition of horizons with significantly different strength, respectively consisting of amorphous to poorly-crystalline phases (with bulk anhydrous composition close to starting antigorite) and of highly-crystalline assemblages of forsterite and disordered enstatite (200 nm in size and in polygonal-like nanotextures). The slip zone also hosts micro/nanometre sized Cr-magnetite grains, aligned at low angle with respect to the slipping surface and inherited from the host serpentinite.Overall observations suggest that frictional heating at asperities on the slipping surface induced a temperature increase up to 820–1200 °C (in agreement with flash temperature theory), responsible for serpentine complete dehydration and amorphization, followed by crystallization of forsterite and enstatite (under post-deformation, static conditions). The results of this study may provide important keys for the full comprehension of the mechanical behaviour and of the possible geodynamical role of serpentinite-hosted faults through the seismic cycle.     

In Situ Oxygen Isotope Determination in Serpentine Minerals by SIMS: Addressing Matrix Effects and Providing New Insights on Serpentinisation at Hole BA1B (Samail ophiolite,Oman)

The ability to constrain the petrogenesis of multiple serpentine generations recorded at the microscale is crucial for estimating the extent and conditions of modern versus fossil serpentinisation in ophiolites. To address matrix bias effects during oxygen isotope analysis by SIMS, we present the first investigation analysing antigorite in the compositional range Mg# = 77.5–99.5 mole %, using a CAMECA IMS‐1280 secondary ion mass spectrometer. Spot‐to‐spot homogeneity is ≤ 0.5‰ (2s) for the new antigorite reference materials. The relative bias between antigorite reference materials with different Mg/Fe ratios is described by a second‐order polynomial, and a maximum difference in bias of ~ 1.8‰ was measured for Mg# ~ 78 to 100. We observed a bias up to ~ 1.0‰ between lizardite and antigorite attributed to their different crystal structures. Orientation effects up to ~ 1‰ were observed in chrysotile. The new analytical protocol allowed the identification of oxygen isotope zoning up to ~ 7‰ in serpentine minerals from two serpentinites recovered from an area of active serpentinisation in the Samail ophiolite. Thus, in situ analysis is capable of resolving isotopic heterogeneity that may directly reflect changes in the physical and chemical conditions of multiple serpentinisation events in the Samail ophiolite.     

Platinum group minerals in podiform chromitites of the Bou Azzer ophiolite, Anti Atlas, Central Morocco

Summary The Neoproterozoic Bou Azzer ophiolite complex hosts numerous, small lenticular bodies of massive and disseminated chromite. Metallurgical-grade high-Mg and high-Cr spinels (cores with 48–62 wt% Cr₂O₃) reveal complex alteration patterns of successive Cr and Mn enrichment and loss of Al towards the rims, while the Mg# ratios [(Mg/(Mg + Fe²⁺)] remain almost constant. Concentration patterns of platinum-group elements are typical for ophiolitic chromitite poor in sulfides, with predominance of the IPGE, variable Rh, and low Pt and Pd. The most abundant platinum-group mineral is Rh-bearing laurite that occurs either included in spinel or in silicate matrix, whereas Os-Ir-Ru alloy is always included in spinel. Laurite inclusions reveal complex intergrowth textures with Rh-Ru-Pt rich alloy, and with Rh-rich sulfide. Most laurites display trends to sulfur-poor compositions leading to local formation of very fine-grained Ru-Os-Ir alloy phases. Ni-Co-Fe sulfides, arsenides and sulfarsenides devoid of PGE are associated with the alteration of chromite. Textural position and chemical composition of the base metal inclusions, as well as comparison of alteration features between chromite and accessory chromian spinel in the Co-Ni-As ores of the Bou Azzer ophiolite indicate a close connection. It is suggested that hydrothermal fluids percolated through the marginal zones of the ophiolite belt during greenschist facies metamorphism and deposited Ni-Co-Fe arsenides, sulfarsenides and minor sulfides as accessories within altered chromitites, and also in structurally favourable zones as Ni-Co-As ores. Author’s address: Dr. Frank Melcher, Federal Institute for Geosciences and Natural Resources, Stilleweg 2, 30655 Hannover, Germany     

Platinum-group elements distribution and spinel composition in podiform chromitites and associated rocks from the upper mantle section of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco

The distribution of platinum-group elements (PGEs), together with spinel composition, of podiform chromitites and serpentinized peridotites were examined to elucidate the nature of the upper mantle of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco. The mantle section is dominated by harzburgite with less abundant dunite. Chromitite pods are also found as small lenses not exceeding a few meters in size. Almost all primary silicates have been altered, and chromian spinel is the only primary mineral that survived alteration. Chromian spinel of chromitites is less affected by hydrothermal alteration than that of mantle peridotites. All chromitite samples of the Bou Azzer ophiolite display a steep negative slope of PGE spidergrams, being enriched in Os, Ir and Ru, and extremely depleted in Pt and Pd. Harzburgites and dunites usually have intermediate to low PGE contents showing more or less unfractionated PGE patterns with conspicuous positive anomalies of Ru and Rh. Two types of magnetite veins in serpentinized peridotite, type I (fibrous) and type II (octahedral), have relatively low PGE contents, displaying a generally positive slope from Os to Pd in the former type, and positive slope from Os to Rh then negative from Rh to Pd in the latter type. These magnetite patterns demonstrate their early and late hydrothermal origin, respectively. Chromian spinel composition of chromitites, dunites and harzburgites reflects their highly depleted nature with little variations; the Cr# is, on average, 0.71, 0.68 and 0.71, respectively. The TiO₂ content is extremely low in chromian spinels, <0.10, of all rock types. The strong PGE fractionation of podiform chromitites and the high-Cr, low-Ti character of spinel of all rock types imply that the chromitites of the Bou Azzer ophiolite were formed either from a high-degree partial melting of primitive mantle, or from melting of already depleted mantle peridotites. This kind of melting is most easily accomplished in the supra-subduction zone environment, indicating a genetic link with supra-subduction zone magma, such as high-Mg andesite or arc tholeiite. This is a general feature in the Neoproterozoic upper mantle.     

Serpentinites of the Zermatt-Saas ophiolite complex and their texture evolution

The Zermatt‐Saas serpentinite complex is an integral member of the Penninic ophiolites of the Central Alps and represents the mantle part of the oceanic lithosphere of the Tethys. Metamorphic textures of the serpentinite preserve the complex mineralogical evolution from primary abyssal peridotite through ocean‐floor hydration, subduction‐related high‐pressure overprint, meso‐Alpine greenschist facies metamorphism, and late‐stage hydrothermal alteration. The early ocean floor hydration of the spinel harzburgites is still visible in relic pseudomorphic bastite and locally preserved mesh textures. The primary serpentine minerals were completely replaced by antigorite. The stable assemblage in subduction‐related mylonitic serpentinites is antigorite–olivine–magnetite ± diopside. The mid‐Tertiary greenschist facies overprint is characterized by minor antigorite recrystallization. Textural and mineral composition data of this study prove that the hydrated mineral assemblages remained stable during high‐pressure metamorphism of up to 2.5 GPa and 650 °C. The Zermatt‐Saas serpentinites thus provide a well documented example for the lack of dehydration of a mantle fragment during subduction to 75 km depth.     

Seismic wave velocity and anisotropy of serpentinized peridotite in the Oman ophiolite

Shallow seismic measurements in harzburgite from the Oman ophiolite performed in a zone where the maximum horizontal anisotropy is expected (vertical foliation and horizontal lineation) point to a dominant dependence of seismic properties on fracturing.

Optical microscopy studies show that microcracks are guided by the serpentine (lizardite) penetrative network oriented subparallel to the harzburgite foliation and subperpendicular to the mineral lineation, and that serpentine (lizardite) vein filling has a maximum concentration of (001) planes parallel to the veins walls. The calculated elastic properties of the oriented alteration veins filled with serpentine in an anisotropic matrix formed by oriented crystals of olivine and orthopyroxene are compared with seismic velocities measured on hand specimens.

Laboratory ultrasonic data indicate that open microcracks are closed at 100 MPa pressure, e.g. (J. Geophys. Res. 65, (1960) 1083) and (Proc. ODP Sci. Results Leg 118, (1990) 227). Above this pressure, laboratory measurements and modeling show that P-compressional and S-shear wave velocities are mainly controlled by the mineral preferred orientation. Veins sealed with serpentine are effective in slightly lowering P and S velocities and increasing anisotropy. The penetrative lizardite network does not affect directly the geometry of seismic anisotropy, but contributes indirectly in the fact that this network controls the microcrack orientations.

Comparison between seismic measurements of peridotite and gabbro in the same conditions suggest that P- and S-waves anisotropies are a possible discriminating factor between the two lithologies in the suboceanic lithosphere.     

Deformation fabrics of glaucophane schists and implications for seismic anisotropy: the importance of lattice preferred orientation of phengite

ABSTRACT

Strong seismic anisotropy is observed in many subduction zones. This effect is attributed partly to subducting oceanic crust that is transformed into blueschist facies rocks. Because blueschist facies constituents such as glaucophane, epidote, and phengite show strong anisotropic elasticity, seismic anisotropy in subducting oceanic crust can be attributed to the lattice preferred orientation (LPO) of these minerals. We studied the deformation fabrics and seismic properties of phengite-rich epidote–glaucophane schists from the Franciscan Complex of Ring Mountain, California. The samples are composed mainly of glaucophane, epidote, and phengite. Some samples contain abundant phengite, the maximum being 40%. The LPOs of glaucophane showed that the [001] axes are aligned subparallel to lineation, and both (110) poles and [100] axes are aligned subnormal to foliation. The epidote [001] axes are aligned subnormal to foliation, with both (110) and (010) poles aligned subparallel to lineation. The LPOs of phengite are characterized by the maxima of [001] axes subnormal to foliation, and both (110) and (010) poles and [100] axes are aligned in a girdle subparallel to foliation. The phengite showed substantially strong seismic anisotropy (AV_P = 42%, max.AV_S = 37%). The glaucophane schist with abundant phengite showed significantly stronger seismic anisotropy (AV_P = 30%, max.AV_S = 23%) than the epidote–glaucophane schist (AV_P = 13%, max.AV_S = 9%). When the subduction angle of phengite-rich glaucophane schist is considered, the polarization direction of the fast S-waves for vertically propagating S-waves changed to a nearly trench-parallel direction for the subduction angle of 45?60°, and the S-wave anisotropy became stronger for vertically propagating S-waves with increasing subduction angles. Our data showed that phengite-rich blueschist facies rock can therefore contribute to the strong trench-parallel seismic anisotropy occurring at the subducting oceanic crust and at the slab–mantle interface in many subduction zones.     

Crustal anisotropy in southwest Ireland from analysis of controlled source shear-wave data

Results from a travel-time analysis of three-component shear-wave (S-wave) data recorded in southwest Ireland during a controlled source seismic experiment have been used to investigate the magnitude of crustal anisotropy. The data used were recorded from 20 in-line shots on three-component short-period stations deployed at approximately 1-km spacing along two parallel profiles. Analysis of the travel-time differences between vertically and horizontally polarised S-waves recorded on vertical, radial and transverse seismometer components was undertaken using seismic phases travelling near the Earth's surface (S_g) and reflected from the Moho (S_mS). Travel-time differences between the components for both phases scatter largely within the range ± 0.2 s, which is about the uncertainty in the measurements, with no observed coherent variation with shot-receiver offset. Synthetic S-wave seismograms were also computed from 1-D S-wave velocity models with varying degrees of anisotropy in the upper and in the lower crusts. Travel-time differences of S_g and S_mS phases picked from these synthetic seismograms confirm that for anisotropies with probable symmetries of magnitude 1–2% in either the upper or lower crust should result in an observable variation of the travel-time differences between the transverse and radial, and transverse and vertical components with source–receiver offset. The study shows that crustal anisotropy does not contribute significantly to the marked anisotropy recently deduced from SKS and SKKS measurements in Ireland, which is therefore confirmed to reside at sub-crustal and deeper mantle levels.     

Modélisation magnétique de la suture ophiolitique de Bou Azzer–El Graara (Anti-Atlas central,Maroc). Implications sur la reconstitution géodynamique panafricaine

Aeromagnetic data of the Anti-Atlas Mountains show an important magnetic anomaly along the ‘Major Anti-Atlas Fault’, produced by different mafic and ultramafic rocks of a Neoproterozoic ophiolite complex. The magnetic modelling of Bou Azzer–El Graara ophiolitic suture shows a deep-seated anomaly through the upper continental crust corresponding to a north-dipping subduction. The polarity of the Pan-African subduction in the Anti-Atlas is therefore compatible with the contemporaneous Pan-African orogenic belts, where polarity of subduction dipped away from the West African Craton during the amalgamation of Western Gondwana. To cite this article: A. Soulaimani et al., C. R. Geoscience 338 (2006).     

Shear velocity and Vp/Vs ratio structure of the crust beneath the southern margin of South China continent

We here present the results of the inverse modeling of crustal S-phases recorded from a 400-km-long seismic profile, with azimuth nearly N30W, from Lianxian, near Hunan Province, to Gangkou Island, near Guangzhou City, Guangdong Province, in the southern margin of South China continent. The finding in this case is that many shot gathers provided by this wide-angle seismic experiment show relatively strong reflected and refracted S-phases, in particular some crustal refractions (Sg waves) and Moho reflections (SmS waves or simply Sm waves). The P-wave velocity structure of the crust and uppermost mantle was already obtained through the interpretation of vertical-component shot gathers. Now, with constraints introduced by the P-wave velocity architecture and after picking up S-wave traveltime data on the seismograms, we have obtained the S-velocity model of the crust by adjusting these traveltimes but keeping the geometry of the crustal reflectors. Our results demonstrate: (1) the average crustal S-velocity is about 3.64 km/s to the northwest of the Wuchuan-Sihui fault, and 3.62 km/s to the southeast of this fault; (2) relatively constant S-velocity of about 3.42 km/s for the upper crust, 3.55 km/s for the middle crust and laterally varying shear velocity around 3.82 km/s for the lower crust; (3) correspondingly, Vp/Vs ratio is 1.73 for the upper crust, 1.71 for the middle crust and 1.74 for the lower crust. Both shear velocities and Vp/Vs ratio correlate well with the major active faults that break the study area, and show significant changes especially in the upper crust. High Poisson’s ratio (1.8) is observed at shallow depth beneath the Minzhong depression to the southeast of the Wuchuan-Sihui fault and the Huiyuan depression in the southern margin of South China continent. In contrast, a very low Vp/Vs ratio (1.68) is observed between 8 and 14 km depth beneath Huiyuan. At deeper depth, a high Vp/Vs ratio (1.76) is observed in the lower crust beneath the Minzhong depression.     

俯冲带蛇纹岩的变质过程

俯冲带蛇纹岩是俯冲带流体的重要来源,特别是其深部脱水作用对地幔动力学影响深远,是研究俯冲带约80~200km深度范围的地球动力学的关键,因此研究蛇纹岩的变质作用过程及其相关特征矿物(组合)的温压稳定范围具有重要意义。蛇纹岩具有简单的矿物(组合):蛇纹石类、硅镁石类、磁铁矿、氢氧镁石、绿泥石、橄榄石、透辉石、角闪石、滑石等,并且这些矿物(组合)对温压变化不敏感从而很难用来判定蛇纹岩所经历的变质演化轨迹。近几十年来,研究者通过实验岩石学和野外地质观察,主要研究了蛇纹石类矿物和硅镁石类矿物的温压稳定范围,并且试图使用这些特征矿物(组合)来判定俯冲带蛇纹岩的峰期变质条件。本文总结了蛇纹岩中这些主要矿物的温压稳定范围和相关变质反应,并且以中国西南天山蛇纹岩为例,展示使用特征矿物(组合)和叶蛇纹石Al等值线判定蛇纹岩峰期温压条件在实际岩石中的应用。另外,早期对叶蛇纹石的研究表明:随着温压条件的变化,叶蛇纹石的晶体结构会发生相应的调整。表现为单位晶胞内硅氧四面体的个数(m值)发生变化:温度升高,m值变小;压力升高,m值变大,这个发现在高压实验和天然样品中得到了一定程度的验证。本文利用已知峰期温压范围的叶蛇纹石样品分别采用粉末制样法和离子减薄制样法,进行透射电镜测试(TEM)样品的m值,并通过统计的方法获得叶蛇纹石的m值的峰值。结果显示叶蛇纹石的m值的峰值在一定程度上可以用以指示温压条件。本文提出可以用矿物组合、叶蛇纹石Al等值线和叶蛇纹石m值峰值相结合的方法确定蛇纹岩的变质温压条件和P-T轨迹。     

Occurrence of népouite in the serpentinite of the Manipur ophiolite belt,Northeastern India: Implication for melt-rock interaction in a supra-subduction zone

Serpentinization is pervasive in the ultramafic rocks of Manipur ophiolite belt (MOB), Northeastern India. Electron microprobe data of a serpentinite from the Ukhrul-Nungbi sector of MOB shows Ni-rich serpentine mineral (NiO = 33.4-33.9 wt %, SiO₂= 37.55-38.96 wt %, MgO= 14.83-16.89 wt %). The composition and X-ray diffraction pattern characterize this Ni-rich serpentine mineral as népouite which is suggested to be a hydrothermal alteration product of NiO-rich olivine in a fore-arc peridotite. The genesis of this NiO-rich olivine is attributed to the melt-rock interaction in a supra-subduction zone setting.     

Petrological and geochemical study of serpentinised peridotites from the southern part of Manipur Ophiolitic Complex, Northeast India

The southern Manipur Ophiolitic Complex in the Indo-Myanmar Orogenic Belt, Northeast India comprises a set of serpentinised peridotites, pelagic sediments, podiform chromitites with minor mafic and felsic intrusives. Peridotites in the study area suffered various degrees of serpentinisation and also have been highly deformed and metamorphosed. Striations and slickenside surfaces are common features seen in these rocks indicating the effect of faulting or shearing due to tectonic movement. On the basis of mineralogy and modal composition these peridotites have been identified as serpentinised harzburgite, lherzolite, serpentinite with minor wehrlite. The serpentine minerals are mainly lizardite and chrysotile with sub-ordinate antigorite, whereas, pyroxenes are enstatite and diopside. The primary composition of Cr-spinels in these serpentinised peridotites are characterized by low SiO₂ (<0.11 wt.%), Fe₂O₃ (0.02 to 2.32 wt.%), TiO₂ (<0.15 wt.%), Cr# (0.13 to 0.23) and high Al# (0.74 to 0.86), Mg# (0.68 to 0.73). Olivine is relatively uniform in composition, ranges from Fo_89.64 to Fo_90.1. The mineralogical compositions and primary Cr-spinel chemistry of these peridotites are similar to Abyssal peridotites that formed in a mid-ocean ridge setting after low degree of partial melting. Thus, it supports that the mantle peridotites of the Manipur Ophiolitic Complex initially formed in a spreading regime.     

Foliation development in serpentinites, Glenrock, New South Wales

At Glenrock, near the southern end of the Peel Fault System, two fault zones are delineated by mélanges in which serpentinite is the main rock type.Protogranular and mylonitic textures are present in relicts of the parent peridotite and in blocks of massive pseudomorphic serpentinite that are surrounded by schistose serpentinite. In schistose serpentinite, the earliest foliation (S₁) is defined, microscopically, by the parallel alignment of platy and fibrous serpentine minerals (lizardite and chrysotile) and by trains of magnetite and flattened serpentine pseudomorphs after olivine and pyroxene. It is considered that the schistosity formed perpendicular to the direction of maximum shortening, under conditions in which lizardite and chrysotile were ductile, but antigorite was not, by breakdown of pre-existing serpentine minerals and new growth of lizardite and chrysotile.Post-s₁ foliations (S₂andS₃) superficially resemble crenulation cleavages in the field but, microscopically, show evidence of shear displacement and are referred to as microshear sets. They probably originated in the ductile-brittle transitional field of serpentine behaviour (Raleigh and Paterson, 1965).     

An experimental investigation of antigorite dehydration in natural silica-enriched serpentinite

Piston cylinder experiments were performed to constrain the pressure and temperature conditions for two high-pressure antigorite dehydration reactions found in silica-enriched serpentinites from Cerro del Almirez (Nevado–Filábride Complex, Betic Cordillera, southern Spain). At 630–660°C and pressures greater than 1.6 GPa, antigorite first reacts with talc to form orthopyroxene ± chlorite + fluid. We show that orthopyroxene + antigorite is restricted to high-pressure metamorphism of silica-enriched serpentinite. This uncommon assemblage is helpful in constraining metamorphic conditions in cold subduction environments, where antigorite serpentinites have no diagnostic assemblages over a large pressure and temperature range. The second dehydration reaction leads to the breakdown of antigorite to olivine + orthopyroxene + chlorite + fluid. The maximum stability of antigorite is found at 680°C at 1.9 GPa, which also corresponds to the maximum pressure limit for tremolite coexisting with olivine + orthopyroxene. The high aluminium (3.70 wt% Al₂O₃) and chromium contents (0.59 wt% Cr₂O₃) of antigorite in the investigated starting material is responsible for the expansion of the serpentinite stability to 60–70°C higher temperatures at 1.8 GPa than the antigorite stability calculated in the Al-free system. The antigorite from our study has the highest Al–Cr contents among all experimental studies and therefore likely constraints the maximum stability of antigorite in natural systems. Comparison of experimental results with olivine–orthopyroxene–chlorite–tremolite assemblages outcropping in Cerro del Almirez indicates that peak metamorphic conditions were 680–710°C and 1.6–1.9 GPa.     

蛇纹石化和俯冲带蛇纹岩变质脱水过程中流体活动性元素的行为

蛇纹石是大洋岩石圈和俯冲带内水和流体活动性元素最重要的载体之一。研究蛇纹石化和蛇纹岩变质脱水过程中流体活动性元素的行为是认识俯冲带元素地球化学循环的关键。蛇纹岩是指主要由蛇纹石类矿物构成的岩石,包括利蛇纹石、纤蛇纹石和叶蛇纹石。蛇纹石化过程中会造成流体活动性元素(B、Li、As、Sb、Pb、Cs、U、Sr和Ba等)的显著富集,并且由于原岩性质、流体成分和氧逸度等条件的不同,大洋岩石圈蛇纹岩和弧前蛇纹岩的特征也略有不同。例如,弧前蛇纹岩具有相对高的As、Sb、B和相对低的U,这反映了俯冲沉积物来源流体的贡献。在俯冲带蛇纹岩的变质脱水过程中,利蛇纹石向叶蛇纹石的转变伴随着矿物内超过50%F和Cl的释放,以及一些流体活动性元素(如B和Li)的迁出;此外,蛇纹石分解形成的变质橄榄石中的流体包裹体指示,蛇纹石脱水分解所产生的流体具有高于原始地幔几个数量级的Cl、Cs、Pb、As、Sb、Ba、Rb、B、Sr、Li和U含量。由于利蛇纹石中的Fe~(3+)含量较叶蛇纹石高,这种矿物相转变过程中也伴随着俯冲通道内的一系列氧化还原过程,从而影响流体性质和新形成的叶蛇纹石的成分。蛇纹岩与岛弧岩浆在流体活动性元素富集规律上的相似性说明蛇纹岩在俯冲带元素循环中扮演着重要的角色。此外,蛇纹石矿物相转变过程中F、Cl、B等元素的释放,可能对于斑岩型金矿、蛇绿岩中的金矿和某些蛇纹岩作为赋矿围岩的硼矿的形成起到重要的作用。     

Three steps of serpentinization in an eclogitized oceanic serpentinization front (Lanzo Massif – Western Alps)

The Lanzo peridotite massif is a fragment of oceanic lithosphere generated in an ocean–continent transition context and eclogitized during alpine collision. Despite the subduction history, the massif has preserved its sedimentary oceanic cover, suggesting that it may have preserved its oceanic structure. It is an exceptional case for studying the evolution of a fragment of the lithosphere from its oceanization to its subduction and then exhumation. We present a field and petrological study retracing the different serpentinization episodes and their impact on the massif structure. The Lanzo massif is composed of slightly serpentinized peridotites (<20% serpentinization) surrounded by an envelope of foliated serpentinites (100% serpentinization) bordered by oceanic metabasalts and metasedimentary rocks. The limit between peridotites and serpentinites defines the front of serpentinization. This limit is sharp: it is marked by the presence of massive serpentinites (80% serpentinization) and, locally, by dykes of metagabbros and mylonitic gabbros. The deformation of these gabbros is contemporaneous with the emplacement of the magma. The presence of early lizardite in the peridotites testifies that serpentinization began during the oceanization, which is confirmed by the presence of meta‐ophicarbonates bordering the foliated serpentinite envelope. Two additional generations of serpentine occur in the ultramafic rocks. The first is a prograde antigorite that partially replaced the lizardite and the relict primary minerals of the peridotite during subduction, indicating that serpentinization is an active process at the ridge and in the subduction zone. Locally, this episode is followed by the deserpentinization of antigorite at peak P–T (estimated in eclogitized metagabbros at 2–2.5 GPa and 550–620 °C): it is marked by the crystallization of secondary olivine associated with chlorite and/or antigorite and of clinopyroxene, amphibole and chlorite assemblages. A second antigorite formed during exhumation partially to completely obliterating previous textures in the massive and foliated serpentinites. Serpentinites are an important component of the oceanic lithosphere generated in slow to ultraslow spreading settings, and in these settings, there is a serpentinization gradient with depth in the upper mantle. The seismic Moho limit could correspond to a serpentinization front affecting the mantle. This partially serpentinized zone constitutes a less competent level where, during subduction and exhumation, deformation and fluid circulation are localized. In this zone, the reaction kinetics are increased and the later steps of serpentinization obliterate the evidence of this progressive zone of serpentinization. In the Lanzo massif, this zone fully recrystallized into serpentinite during alpine subduction and collision. Thus, the serpentinite envelope represents the oceanic crust as defined by geophysicists, and the sharp front of serpentinization corresponds to an eclogitized seismic palaeo‐Moho.     

Serpentinization: a review

The common alteration assemblage produced by serpentinization of ultramafic rocks is: lizardite, chrysotile, magnetite±brucite±antigorite. Lizardite-chrysotile serpentinites are more common than antigorite; the presence of antigorite indicates that the serpentinite has undergone prograde metamorphism or that the periootite was serpentinized in a higher P,T regime than lizardite and chrysotile. The iron subsitution into serpentine minerals and brucite is a function of temperature at low f_O2, with increased temperature enhancing magnetite formation. The presence of awaruite and native Fe are strong evidence for a locally very reducing environment. Isotopic studies have shown a wide variety of origins for the fluids involved in serpentinization. The increased boron content of serpentinized rocks when compared to boron contents of the parent ultramafic body indicates a possible sea water origin for the fluids. Serpentinization takes place under both constant volume and constant chemical composition conditions. The factors in evaluating the importance of the two processes for an individual serpentinite are: (1) determination of the mineral assemblage and its paragenesis, (2) the structural and tectonic relationship of the ultramafic body to its country rock, (3) fluid access to the rock in duration and amount, and (4) timing of serpentinization - before, during or after emplacement into the crust.