Serpentinized Peridotites at the North Part of the Wadi Allaqi District (Egypt): Implications for the Tectono-Magmatic Evolution of Fore-arc Crust

The Neoproterozoic Allaqi-Heiani suture （800-700 Ma） in the south Eastern Desert of Egypt is the northernmost linear ophiolitic belt that defines an arc-arc suture in the Arabian- Nubian shield （ANS）. The Neoproterozoic serpentinized peridotites represent a distinct lithology of dismembered ophiolites along the Allaqi-Heiani suture zone. The alteration of peridotites varies, some contain relicts of primary minerals （Cr-spinel and olivine） and others are extremely altered, especially along thrusts and shear zones, with development of talc, talc-carbonate and quartz-carbonate. The fresh cores of the chromian spinels are rimmed by ferritchromite and Cr- magnetite. The fresh chromian spinels have high Cr# （0.62 to 0.79）, while Mg# shows wider variation （0.35-0.59）. High Cr# in the relict chromian spinels and Fo content in the primary olivines indicate that they are residual peridotites after extensive partial melting. The studied ophiolitic upper mantle peridotites are highly depleted and most probably underwent high degrees of partial melting at a supra-subduction zone setting. They can be produced by up to -20%-22% dynamic melting of a primitive mantle source. The mineralogical and geochemical features of the studied rocks reflect that the mantle peridotites of the north part of the Wadi Allaqi district are similar to the fore-arc peridotites of a supra-subduction zone.     

Geochemistry and fore-arc evolution of upper mantle peridotites in the Cryogenian Bir Umq ophiolite,Arabian Shield,Saudi Arabia

ABSTRACT

The Bir Umq ophiolite is one of the most important ophiolitic successions in the Arabian Shield, and represents an excellent case for the study of the tectonomagmatic evolution of the earliest Precambrian events in the juvenile part of the Arabian-Nubian Shield (ANS). It is a dismembered ophiolite, which includes a serpentinized peridotite with small amounts of gabbro and mélange, and is overlain by the Sumayir formation. The mantle section of the Bir Umq ophiolite has been pervasively sheared and folded during its emplacement and is extensively serpentinized, carbonated and silicified, resulting in the common development of magnesite and listwaenite along the shear zones. Listwaenite occurs in the form of upstanding ridges due to its resistance to erosion. Antigorite is the main serpentine mineral, which, however, has low amounts of lizardite and chrysotile, indicating that the present serpentinites formed by prograde metamorphism. The ophiolitic rocks of Bir Umq have undergone regional metamorphism up to the greenschist to amphibolite facies. The presence of mesh and bastite textures indicates harzburgite and dunite protoliths. The serpentinized peridotite preserves rare relicts of primary minerals such as olivine, pyroxene and Cr-spinel. The serpentinized ultramafics of Bir Umq have high Mg# [molar Mg/(Mg+Fe²⁺); 0.90–0.93), low CaO, and Al₂O₃ contents similar to that of the environment of the suprasubduction zone. Additionally, they are characterized by the depletion of some compatible trace elements (e.g., Nb, Sr, Ta, Zr, Hf and REE), but show a wide variation in the Rb and Ba. Moreover, they are enriched in some elements that have affinities for Mg-rich minerals such as Ni, Cr, V, and Co. Fresh relics of olivine have high Fo (av. 0.91) and NiO (av. 0.42) contents, similar to those in the mantle olivine. The fresh Cr-spinel has high Cr# (0.68) and low TiO₂ content (av. 0.11), similar to those in modern fore-arc peridotites. The composition of both orth- and clinopyroxenes confirms the fore-arc affinity of the studied ultramafics. The present study indicates that the protoliths of the serpentinized ultramafics of Bir Umq have high partial melt degrees, which is consistent with the characteristics of ultramafic rocks formed in a subarc environment (fore-arc) within a suprasubduction zone system.     

Geochemistry of Khor Um-Safi ophiolitic serpentinites,central Eastern desert,Egypt: Implications for neoproterozoic arc-basin system in the Arabian-Nubian shield

Serpentinites in the Eastern Desert of Egypt are the most distinctive lithological unit in the Arabian–Nubian Shield (ANS) ophiolite sequence which associated with major suture zones. Khor Um-Safi (KUS) serpentinites represent dismembered fragments of ophiolitic rocks located in the central Eastern Desert (CED) of Egypt.KUS serpentinites exhibit affinity to the typical metamorphic peridotites with harzburgitic protolith compositions. Their opaque mineral assemblage (pentlandite, heazlewoodite and magnetite) is similar to that observed in oceanic serpentinites and implies serpentinization under highly reducing conditions. They have refractory major element compositions with Al₂O₃ contents comparable to oceanic and active margin peridotites as well as Pan-African serpentinites. The Cr and TiO₂ contents reflect evolution within a supra-subduction zone (SSZ) environment. This implication is confirmed by the Al₂O₃/SiO₂ and MgO/SiO₂ ratios which akin to ANS ophiolitic peridotites in fore-arc setting. Their enrichment in compatible trace elements (Cr, Ni and Co) reveals a depleted mantle peridotite protolith.Modelling trace elements indicates that they represent the mantle residues from 15 to 20 % melting of spinel peridotite at oxygen fugacity conditions of the QFM + 1 buffer. This range of melt extraction is consistent with the typical range of SSZ peridotite. Oxygen fugacity estimation suggests evolution under more oxidizing regime similar to modern fore-arc basin system. Moreover, this implication indicates that the KUS mantle represents arc lithosphere interacted with arc melt.     

Origin of Neoproterozoic ophiolitic peridotites in south Eastern Desert, Egypt, constrained from primary mantle mineral chemistry

The ophiolitic peridotites in the Wadi Arais area, south Eastern Desert of Egypt, represent a part of Neoproterozoic ophiolites of the Arabian-Nubian Shield (ANS). We found relics of fresh dunites enveloped by serpentinites that show abundances of bastite after orthopyroxene, reflecting harzburgite protoliths. The bulk-rock chemistry confirmed the harzburgites as the main protoliths. The primary mantle minerals such as orthopyroxene, olivine and chromian spinel in Arais serpentinites are still preserved. The orthopyroxene has high Mg# [=Mg/(Mg + Fe²⁺)], ~0.923 on average. It shows intra-grain chemical homogeneity and contains, on average, 2.28 wt.% A1₂O₃, 0.88 wt.% Cr₂O₃ and 0.53 wt.% CaO, similar to primary orthopyroxenes in modern forearc peridotites. The olivine in harzburgites has lower Fo (93?94.5) than that in dunites (Fo_94.3?Fo_95.9). The Arais olivine is similar in NiO (0.47 wt.% on average) and MnO (0.08 wt.% on average) contents to the mantle olivine in primary peridotites. This olivine is high in Fo content, similar to Mg-rich olivines in ANS ophiolitic harzburgites, because of its residual origin. The chromian spinel, found in harzburgites, shows wide ranges of Cr#s [=Cr/(Cr + Al)], 0.46?0.81 and Mg#s, 0.34?0.67. The chromian spinel in dunites shows an intra-grain chemical homogeneity with high Cr#s (0.82?0.86). The chromian spinels in Arais peridotites are low in TiO₂, 0.05 wt.% and Y_Fe [= Fe³⁺/(Cr + Al + Fe³⁺)], ~0.06 on average. They are similar in chemistry to spinels in forearc peridotites. Their compositions associated with olivine’s Fo suggest that the harzburgites are refractory residues after high-degree partial melting (mainly ~25?30 % partial melting) and dunites are more depleted, similar to highly refractory peridotites recovered from forearcs. This is in accordance with the partial melting (>20 % melt) obtained by the whole-rock Al₂O₃ composition. The Arais peridotites have been possibly formed in a sub-arc setting (mantle wedge), where high degrees of partial melting were available during subduction and closing of the Mozambique Ocean, and emplaced in a forearc basin. Their equilibrium temperature based on olivine?spinel thermometry ranges from 650 to 780 °C, and their oxygen fugacity is high (Δlog ?O₂?=?2.3 to 2.8), which is characteristic of mantle-wedge peridotites. The Arais peridotites are affected by secondary processes forming microinclusions inside the dunitic olivine, abundances of carbonates and talc flakes in serpentinites. These microinclusions have been formed by reaction between trapped fluids and host olivine in a closed system. Lizardite and chrysotile, based on Raman analyses, are the main serpentine minerals with lesser antigorite, indicating that serpentines were possibly formed under retrograde metamorphism during exhumation and near the surface at low T (<400 °C).     

Supra-subduction affinity in the Neoproterozoic serpentinites in the Eastern Desert, Egypt: Evidence from mineral composition

Serpentinites in the Eastern Desert (ED) of Egypt represent integral components of the ophiolites. Metamorphic textures of the serpentinites preserve the complex mineralogical evolution from primary peridotite through metamorphism, and late-stage hydrothermal alteration. Two textural types are distinguished in the olivines of the present serpentinized peridotites, namely (a) highly-strained olivine grains with kink bands, as in the deformed mantle tectonites from ophiolites, and (b) non-strained grains. The latter may represent recrystallized crystals during later thermal metamorphic events due to the intrusion of granite. On the basis of X-ray diffraction analysis, antigorite is the main serpentine minerals with lesser chrysotile and lizardite which indicates that serpentinites were formed under prograde metamorphism. Relict primary minerals of the serpentinites are Cr-spinel, olivine and pyroxene. Chrome spinel relicts have high Cr# (0.60–0.80), whereas primary olivines are Mg-rich nature (Fo = 89–96). Geochemical compositions of serpentinites indicate that they formed not at mid-ocean ridges but at spreading centers associated with subduction zones and this could have happened in a supra-subduction zone either in the fore-arc or back-arc environments. Mineral compositions of primary chrome spinels and olivines are similar to those of modern fore-arcs. High Cr# in the relict chrome spinels and Fo in the primary olivines of serpentinites indicate that they are residual after extensive partial melting and originated by sea-floor spreading during subduction initiation.     

西南天山阔克萨彦岭地区巴雷公地幔橄榄岩成因及其地质意义

巴雷公蛇绿混杂岩位于西南天山阔克萨彦岭地区, 代表了南天山古生代洋盆的洋壳残片.通过其中的地幔橄榄岩进行详细的地球化学研究, 分析其岩石成因及其形成环境.主量和微量元素特征显示巴雷公蛇纹石化橄榄岩Ti, Al含量低, Cr (1604~3863), Ni (1719×10-6~2375×10-6) 和Mg#值(0.90~0.92) 高; 样品的稀土强烈亏损(ΣREE=0.28×10-6~0.35×10-6, Yb=0.1~0.11×CI), 呈宽缓的U型REE配分模式; 原始地幔标准化蛛网图显示, 强烈富集Rb、U和Sr, 从LILE (Cs、Rb、Ba、U、K和Sr) 到HREE, 元素含量自左向右随不相容性的逐渐降低, 呈右倾标准化微量元素配分型式; 橄榄岩熔融程度在20%~25%之间, 为俯冲带氧化条件下的熔融残留物.上述特征显示了俯冲板片流体与俯冲带上地幔楔物质的相互作用, 具有俯冲带型地幔橄榄岩特征.结合区域新获得的资料, 指示了南天山早古生代洋盆为一成熟的大洋, 早古生代曾发生过洋内俯冲作用.巴雷公蛇纹石化橄榄岩可能产生在南天山古生代洋内俯冲作用中的大洋岛弧的弧前地幔楔部位.      

Process of serpentinization in the ultramafic massif of Beni Bousera (internal Rif,Morocco)

The Beni Bousera massif forms part of the Sebtide units in the internal Rif Mountain (Morocco). It is mainly composed of mantle peridotites surrounded by crustal metamorphic rocks (kinzigites, micaschists, and schists). The serpentinization affects all of peridotite massif to various degrees. Serpentinization is concentrated at the top of the peridotites, along the mylonitized zone, and in the NE part of the massif. It is manifested by the formation of mesh and hourglass textures along the tectonic foliation in the highly serpentinized peridotites; and brecciated texture in the least serpentinized peridotites. Pyroxene minerals are still intact hosting few serpentine veins. These petrographic features are consistent with the geochemical data, marked by the increasing of LOI and decreasing of MgO and FeO toward the top of the massif and Aaraben fault. The Raman characterization of serpentine with the brecciated mesh and hourglass textures correspond to lizardite type whereas the serpentine with the vein texture is formed by lizardite + chrysotile.     

Petrological and mineralogical studies of Pan-African serpentinites at Bir Al-Edeid area, central Eastern Desert, Egypt

The studied serpentinites occur as isolated masses, imbricate slices of variable thicknesses and as small blocks or lenses incorporated in the sedimentary matrix of the mélange. They are thrusted over the associated island arc calc-alkaline metavolcanics and replaced by talc-carbonates along shear zones. Lack of thermal effect of the serpentinites upon the enveloping country rocks, as well as their association with thrust faults indicates their tectonic emplacement as solid bodies. Petrographically, they are composed essentially of antigorite, chrysotile and lizardite with subordinate amounts of carbonates, chromite, magnetite, magnesite, talc, tremolite and chlorite. Chrysotile occurs as cross-fiber veinlets traversing the antigorite matrix, which indicate a late crystallization under static conditions. The predominance of antigorite over other serpentine minerals indicates that the serpentinites have undergone prograde metamorphism or the parent ultramafic rocks were serpentinized under higher pressure. The parent rocks of the studied serpentinites are mainly harzburgite and less commonly dunite and wehrlite due to the prevalence of mesh and bastite textures. The serpentinites have suffered regional metamorphism up to the greenschist facies, which occurred during the collisional stage or back-arc basin closure, followed by thrusting over a continental margin. The microprobe analyses of the serpentine minerals show wide variation in SiO₂, MgO, Al₂O₃, FeO and Cr₂O₃ due to different generations of serpentinization. The clinopyroxene relicts, from the partly serpentinized peridotite, are augite and similar to clinopyroxene in mantle-derived peridotites. The chromitite lenses associated with the serpentinites show common textures and structures typical of magmatic crystallization and podiform chromitites. The present data suggest that the serpentinites and associated chromitite lenses represent an ophiolitic mantle sequence from a supra-subduction zone, which were thrust over the continental margins during the collisional stage of back-arc basin.     

Mineralogy and distribution of Platinum-Group Minerals (PGM) and other solid inclusions in the Faryab ophiolitic chromitites, Southern Iran

High-Cr podiform chromitites hosted by upper mantle depleted harzburgite were investigated for PGM and other solid inclusions from Faryab ophiolitic complex, southern Iran. Chemical composition of the chromian spinels, Cr#[100*Cr/(Cr+Al) = 77–85], Mg# [100*Mg/(Mg+Fe²⁺) = 56–73], TiO₂≤0.25wt%, and the presence of abundant primary hydrosilicates included in the chromian spinels indicate that the deposits were formed from aqueous melt generated by high degree of partial melting in a suprasubduction zone setting. Solid phases hosted by chromian spinel grains from the Faryab ophiolitic chromitites can be divided into three categories: PGM, base-metal minerals and silicates. Most of the studied PGM occurred as very small (generally less than 20 μm in size) primary single or composite inclusions of IPGE-bearing phases with or without silicates and base metal minerals. The PGM were divided into the three subgroups: sulfides, alloys and sulfarsenides. Spinel-olivine geothermometry gives the temperatures 1,131–1,177 °C for the formation of the studied chromitites. At those temperatures, fS₂ values ranged from 10^?3 to 10^?1 and provided a suitable condition for Ru-rich laurite formation in equilibrium with Os-Ir alloys. Progressive crystallization of chromian spinel was accompanied by increase of fS₂ in the melt. The formation of Os-rich laurite, erlichmanite and then sulfarsenides occurred by increase of fS₂ and slight decrease in temperature of the milieu. The compositional and mineralogical determinations of PGM inclusions respect to their spatial distribution in chromian spinels show that the minerals regularly distributed within the chromitites, reflecting cryptic variation consistent with magmatic evolution during host chromian spinel crystallization.     

Diamond in Oceanic Peridotites and Chromitites: Evidence for Deep Recycled Mantle in the Global Ophiolite Record

Diamonds have been discovered in mantle peridotites and chromitites of six ophiolitic massifs along the 1300 km‐long Yarlung‐Zangbo suture (Bai et al., 1993; Yang et al., 2014; Xu et al., 2015), and in the Dongqiao and Dingqing mantle peridotites of the Bangong‐Nujiang suture in the eastern Tethyan zone (Robinson et al., 2004; Xiong et al., 2018). Recently, in‐situ diamond, coesite and other UHP mineral have also been reported in the Nidar ophiolite of the western Yarlung‐Zangbo suture (Das et al., 2015, 2017). The above‐mentioned diamond‐bearing ophiolites represent remnants of the eastern Mesozoic Tethyan oceanic lithosphere. New publications show that diamonds also occur in chromitites in the Pozanti‐Karsanti ophiolite of Turkey, and in the Mirdita ophiolite of Albania in the western Tethyan zone (Lian et al., 2017; Xiong et al., 2017; Wu et al., 2018). Similar diamonds and associated minerals have also reported from Paleozoic ophiolitic chromitites of Central Asian Orogenic Belt of China and the Ray‐Iz ophiolite in the Polar Urals, Russia (Yang et al., 2015a, b; Tian et al., 2015; Huang et al, 2015). Importantly, in‐situ diamonds have been recovered in chromitites of both the Luobusa ophiolite in Tbet and the Ray‐Iz ophiolite in Russia (Yang et al., 2014, 2015a). The extensive occurrences of such ultra‐high pressure (UHP) minerals in many ophiolites suggest formation by similar geological events in different oceans and orogenic belts of different ages. Compared to diamonds from kimberlites and UHP metamorphic belts, micro‐diamonds from ophiolites present a new occurrence of diamond that requires significantly different physical and chemical conditions of formation in Earth's mantle. The forms of chromite and qingsongites (BN) indicate that ophiolitic chromitite may form at depths of >150‐380 km or even deeper in the mantle (Yang et al., 2007; Dobrthinetskaya et al., 2009). The very light C isotope composition (δ13C ‐18 to ‐28‰) of these ophiolitic diamonds and their Mn‐bearing mineral inclusions, as well as coesite and clinopyroxene lamallae in chromite grains all indicate recycling of ancient continental or oceanic crustal materials into the deep mantle (>300 km) or down to the mantle transition zone via subduction (Yang et al., 2014, 2015a; Robinson et al., 2015; Moe et al., 2018). These new observations and new data strongly suggest that micro‐diamonds and their host podiform chromitite may have formed near the transition zone in the deep mantle, and that they were then transported upward into shallow mantle depths by convection processes. The in‐situ occurrence of micro‐diamonds has been well‐demonstrated by different groups of international researchers, along with other UHP minerals in podiform chromitites and ophiolitic peridotites clearly indicate their deep mantle origin and effectively address questions of possible contamination during sample processing and analytical work. The widespread occurrence of ophiolite‐hosted diamonds and associated UHP mineral groups suggests that they may be a common feature of in‐situ oceanic mantle. The fundamental scientific question to address here is how and where these micro‐diamonds and UHP minerals first crystallized, how they were incorporated into ophiolitic chromitites and peridotites and how they were preserved during transport to the surface. Thus, diamonds and UHP minerals in ophiolites have raised new scientific problems and opened a new window for geologists to study recycling from crust to deep mantle and back to the surface.     

An early Palaeozoic supra-subduction lithosphere in the Variscides: new evidence from the Maures massif

Petrographic and geochemical studies of peridotites and melagabbros from the Maures massif (SE France) provide new constraints on the Early Palaeozoic evolution of the continental lithosphere in Western Europe. Peridotites occur as lenses along a unit rooted in the main Variscan suture zone. They are dominantly spinel peridotites and minor garnet–spinel peridotites. Spinel peridotites represent both residual mantle and ultramafic cumulates. Mantle-related dunites and harzburgites display high temperature textures, with olivine (Mg#_0.90), orthopyroxene (Mg#_0.90) and spinel (TiO₂ < 0.2%; Cr#_0.64–0.83) compositions typical of fore-arc upper mantle. Ultramafic cumulates are dunite adcumulates, harzburgite heteradcumulates and mesocumulates, melagabbro heteradcumulates and amphibole peridotites, with olivine (Mg#_0.85–0.89), orthopyroxene (Mg#_0.86–0.89) and Cr-spinel (TiO₂ = 0.5–3.3%; Cr#_0.7–0.98) compositions typical of ultramafic cumulates. Cr-spinel compositions of both spinel peridotite types suggest their genesis in a supra-subduction zone lithosphere. Core to rim zoning in spinel is related to the incomplete influence of regional metamorphism and serpentinisation. The covariation of major and minor elements with Al₂O₃ for cumulates is consistent with igneous processes involving crystal accumulation. Both mantle and cumulate dunites and harzburgites have U-shaped REE patterns and extremely low trace element contents, similar to peridotites from modern fore-arc peridotites (South Atlantic) and from ophiolites related to supra-subduction zones (Semail, Cyclops, Pindos, Troodos). Melagabbros also have U-shaped REE patterns similar to xenoliths from the Philippine island arc, but also similar to intrusive ultramafic cumulates from the Semail nappe of Oman related to a proto-subduction setting. A wehrlite has a REE pattern similar to that of amphibole peridotites reflecting metasomatism of clinopyroxene-bearing peridotites due to subduction-related fluids. The Maures spinel peridotites and melagabbros are therefore interpreted as the lowermost parts of a crustal sequence and minor residual mantle of lithosphere generated in a supra-subduction zone during Early Palaeozoic time. Garnet–spinel peridotites are chemically close to melagabbros, but have recorded high pressure metamorphism before their retrogression similar to spinel peridotites into amphibolites to greenschists facies metamorphism. They indicate burial to mantle depths of the margin of the supra-subduction lithosphere during the Early Palaeozoic continental subduction. Both peridotite types were exhumed during the Upper Palaeozoic continental collision. Comparable observations from other Variscan-related peridotites, in particular of the Speik complex of the Autroalpine basement, and a common age for the subduction stage allow extension of these regional conclusions to a broad area sharing the Cambrian suture zone, extending from the Ossa-Morena to the Bohemian massif.     

雅鲁藏布江蛇绿岩带西段错不扎地幔橄榄岩组成特征及岩石成因

错不扎蛇绿岩位于雅鲁藏布江缝合带西段北亚带,岩体呈北西-南东走向带状产出,主要由地幔橄榄岩和辉长岩脉组成。地幔橄榄岩主体为方辉橄榄岩,详细的矿物学及岩石地球化学研究表明,错不扎方辉橄榄岩中橄榄石为镁橄榄石,斜方辉石主要为顽火辉石,而单斜辉石主要为顽透辉石和透辉石,铬尖晶石具有高Al和高Mg(Mg#=60~70)特征。稀土配分图解显示其具有轻稀土亏损而重稀土富集的左倾型亏损地幔源区特征,(La/Yb)N=0.11~0.60,模拟结果显示其为经历了15%~20%部分熔融后的残余,与快速扩张大洋中脊环境下形成的深海橄榄岩的熔融程度(10%~22%)较为一致。此外,错不扎方辉橄榄岩轻稀土含量明显高于部分熔融模型中LREE的含量,而且,在微量元素原始地幔标准化图解中富集大离子亲石元素Rb、Sr和高场强元素Ta、Hf和Ti,这一特征指示错不扎方辉橄榄岩在大洋中脊环境形成后又受到后期俯冲带熔/流体的改造。结合南北两带不同蛇绿岩体构造环境的对比,笔者认为雅鲁藏布江西段南北两带蛇绿岩体具有相似的形成环境,两者在地理位置以及产状方面的差别可能是受到构造侵位的影响。     

西藏雅鲁藏布江缝合带东波地幔橄榄岩中金刚石的发现及地质意义

本文报道了雅鲁藏布江缝合带西段的东波蛇绿岩地幔橄榄岩中发现与罗布莎和普兰岩体相似的金刚石和特殊地幔矿物群。东波地幔橄榄岩体以方辉橄榄岩为主,含少量二辉橄榄岩和纯橄岩,辉石岩和辉长岩呈脉状产在方辉橄榄岩中。岩石地球化学特征表明东波岩体形成于MOR环境后受到SSZ环境的改造。通过重砂分选实验,在693 kg的地幔橄榄岩大样中发现了金刚石和碳硅石等30余种特殊矿物群,包括金刚石、自然铬、自然金、自然铁、自然硅、自然铜等自然元素矿物类;碳硅石等碳化物;铬尖晶石、金红石、铬铁矿、刚玉、黑钨矿、铌钽铁矿、磁铁矿、赤铁矿、方铁矿、锡石等氧化物;铁铬镍合金、镍铁合金、铜锌合金等合金互化物;辉钼矿、方铅矿、辉铋矿、黄铁矿、黄铜矿、毒砂和闪锌矿等硫化物;橄榄石、辉石、锆石、蓝晶石、白云母、蛇纹石、绿帘石等硅酸岩;萤石等氟化物。该矿物群与雅鲁藏布江缝合带的罗布莎和普兰两个岩体中发现的特殊矿物群非常类似,同时也为蛇绿岩型金刚石这一新类型提供了佐证。     

Petrological and Os Isotopic Characteristics of Zedong Peridotites in the Eastern Yarlung–Zangbo Suture in Tibet

The Zedong ophiolites in the eastern Yarlung–Zangbo suture zone of Tibet represent a mantle slice of more than 45 km~2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine(Fo89.7–91.2), orthopyroxene(En_(88–92)), clinopyroxene(En_(45–49) Wo_(47–51) Fs_(2–4)) and spinel [Mg~#=100×Mg/(Mg+Fe)]=49.1–70.7; Cr~#=(100×Cr/(Cr+Al)=18.8–76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%–40%, indicating that the Zedong mantle peridotites may experience a multi–stage process. The peridotites are characterized by depleted major element compositions and low REE content(0.08–0.62 ppm). Their "spoon–shaped" primitive–mantle normalized REE patterns with(La/Sm)_N being 0.50–6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites(18.19–50.74 ppb) is similar with primary mantle with Pd/Ir being 0.54–0.60 and Pt/Pd being 1.09–1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with ~(187)Os/~(188)Os=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge(MOR) and were later modified in supra–subduction zone(SSZ) environment.     

西藏雅鲁藏布江缝合带东段泽当地幔橄榄岩特征及其意义

泽当岩体位于雅鲁藏布江缝合带东段,主要由地幔橄榄岩、辉长辉绿岩和基性火山岩等组成。地幔橄榄岩主要为方辉橄榄岩和二辉橄榄岩,有少量透镜状纯橄岩。地幔橄榄岩经历了强烈的塑性变形作用。地幔橄榄岩中橄榄石的Fo值为89.6~91.8,属镁橄榄石;斜方辉石为顽火辉石,En 87.8~90.3;单斜辉石En 44.1~50.0,主要为顽透辉石和透辉石。铬尖晶石的Cr#值(=100×Cr/(Cr+Al))为17.0~93.6,其中,二辉橄榄岩和方辉橄榄岩中的铬尖晶石为富铝型尖晶石,纯橄岩中的铬尖晶石Cr#最高,为富铬型尖晶石。地幔橄榄岩的部分熔融程度为17%~34%,表明泽当地幔橄榄岩可能经历了多阶段的过程。亏损的主量元素组成和低于原始地幔的稀土元素含量(0.15×10-6~0.61×10-6)指示泽当地幔橄榄岩为经历过部分熔融和熔体抽取的亏损残余地幔岩石。REE配分型式为中稀土亏损的"V"型或"U"型,原始地幔标准化元素比值(La/Sm)N为0.5~8.0,表明泽当地幔橄榄岩经历过交代作用。矿物化学与地球化学数据表明泽当地幔橄榄岩形成于MOR环境,后受到SSZ环境的改造。     

Petrological and mineralogical studies of Pan-African serpentinites at Bir Al-Edeid area,central Eastern Desert,Egypt

The studied serpentinites occur as isolated masses, imbricate slices of variable thicknesses and as small blocks or lenses incorporated in the sedimentary matrix of the mélange. They are thrusted over the associated island arc calc-alkaline metavolcanics and replaced by talc-carbonates along shear zones. Lack of thermal effect of the serpentinites upon the enveloping country rocks, as well as their association with thrust faults indicates their tectonic emplacement as solid bodies. Petrographically, they are composed essentially of antigorite, chrysotile and lizardite with subordinate amounts of carbonates, chromite, magnetite, magnesite, talc, tremolite and chlorite. Chrysotile occurs as cross-fiber veinlets traversing the antigorite matrix, which indicate a late crystallization under static conditions. The predominance of antigorite over other serpentine minerals indicates that the serpentinites have undergone prograde metamorphism or the parent ultramafic rocks were serpentinized under higher pressure. The parent rocks of the studied serpentinites are mainly harzburgite and less commonly dunite and wehrlite due to the prevalence of mesh and bastite textures. The serpentinites have suffered regional metamorphism up to the greenschist facies, which occurred during the collisional stage or back-arc basin closure, followed by thrusting over a continental margin. The microprobe analyses of the serpentine minerals show wide variation in SiO₂, MgO, Al₂O₃, FeO and Cr₂O₃ due to different generations of serpentinization. The clinopyroxene relicts, from the partly serpentinized peridotite, are augite and similar to clinopyroxene in mantle-derived peridotites. The chromitite lenses associated with the serpentinites show common textures and structures typical of magmatic crystallization and podiform chromitites. The present data suggest that the serpentinites and associated chromitite lenses represent an ophiolitic mantle sequence from a supra-subduction zone, which were thrust over the continental margins during the collisional stage of back-arc basin.     

Geochemical make-up of oceanic peridotites from NW Turkey and the multi-stage melting history of the Tethyan upper mantle

We present the whole-rock and the mineral chemical data for upper mantle peridotites from the Harmanc?k region in NW Turkey and discuss their petrogenetic–tectonic origin. These peridotites are part of a Tethyan ophiolite belt occurring along the ?zmir-Ankara-Ercincan suture zone in northern Turkey, and include depleted lherzolites and refractory harzburgites. The Al₂O₃ contents in orthopyroxene and clinopyroxene from the depleted lherzolite are high, and the Cr-number in the coexisting spinel is low falling within the abyssal field. However, the orthopyroxene and clinopyroxene in the harzburgites have lower Al₂O₃ contents for a given Cr-number of spinel, and plot within the lower end of the abyssal field. The whole-rock geochemical and the mineral chemistry data imply that the Harmanc?k peridotites formed by different degrees of partial melting (~%10–27) of the mantle. The depleted lherzolite samples have higher MREE and HREE abundances than the harzburgitic peridotites, showing convex-downward patterns. These peridotites represent up to ~16 % melting residue that formed during the initial seafloor spreading stage of the Northern Neotethys. On the other hand, the more refractory harzburgites represent residues after ~4–11 % hydrous partial melting of the previously depleted MOR mantle, which was metasomatized by slab-derived fluids during the early stages of subduction. The Harmanc?k peridotites, hence, represent the fragments of upper mantle rocks that formed during different stages of the tectonic evolution of the Tethyan oceanic lithosphere in Northern Neotethys. We infer that the multi-stage melting history of the Harmanc?k peridotites reflect the geochemically heterogeneous character of the Tethyan oceanic lithosphere currently exposed along the ?zmir-Ankara-Erzincan suture zone.     

New Data on the Composition of Ophiolite Complexes on Karaginskii Island (Eastern Kamchatka)

The geochemistry and composition of peridotite rock-forming minerals from blocks in the serpentinite mélange of Karaginskii Island have been studied. The composition features of the rock-forming minerals are indicative of the fact that they represent abyssal peridotites of the mid-oceanic ridges that did not undergo remelting under suprasubduction conditions. According to the geochemical data, these rocks were subject to metasomatic alterations under mantle conditions in the suprasubduction setting, which were caused by metasomatizing melts and/or fluids generated in the subduction zone.     

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.     

The Bangong Lake ophiolite (NW Tibet) and its bearing on the tectonic evolution of the Bangong–Nujiang suture zone

The Jurassic Bangong Lake ophiolite, NW Tibet, is a key element within the western part of the Bangong–Nujiang suture zone, which marks the boundary between the Lhasa and Qiangtang blocks. It is a tectonic mélange consisting of numerous blocks of peridotite, mafic lavas and dikes. The mantle peridotites include both clinopyroxene-bearing and clinopyroxene-free harzburgites. The Cpx-bearing harzburgite contains Al-rich spinel with low Cr#s (20–25), resembling peridotites formed in mid-ocean ridge settings. On the other hand, the Cpx-free harzburgite is highly depleted with Cr-rich spinel (Cr# = 69–73), typical of peridotites formed in subduction zone environments. Mafic rocks include lavas of N-MORB and E-MORB affinity and boninites. The N-MORB rocks consist of pillow lavas and mafic dikes, whereas the E-MORB rocks are brecciated basalts. The boninites have high SiO₂ (53.2–57.9 wt%), MgO (6.5–12.5 wt%), Cr (166–752 ppm) and Ni (63–213 ppm) and low TiO₂ (0.22–0.37 wt%) and Y (5.34–8.10 ppm), and are characterized by having U-shaped, chondrite-normalized REE patterns. The N-MORB and E-MORB lavas probably formed by different degrees of partial melting of primitive mantle, whereas the boninites reflect partial melting of depleted peridotite in a suprasubduction zone environment. The geochemistry of the ophiolite suggests that it is a fragment of oceanic lithosphere formed originally at a mid-ocean ridge (MOR) and then trapped above an intraoceanic subduction zone (SSZ), where the mantle peridotites were modified by boninitic melts. The Bangong–Nujiang suture zone is believed to mark the boundary between two blocks within Gondwanaland rather than to separate Gondwanaland from Eurasia.