REE and PGE Geochemical Constraints on the Formation of Dunites in the Luobusa Ophiolite, Southern Tibet

The Luobusa ophiolite, Southern Tibet, lies in the Indus–YarlungZangbo suture zone that separates Eurasia to the north fromthe Indian continent to the south. The ophiolite contains awell-preserved mantle sequence consisting of harzburgite, clinopyroxene(cpx)-bearing harzburgite and dunite. The harzburgite containsabundant pods of chromitite, most of which have dunite envelopes,and the cpx-bearing harzburgites host numerous dunite dykes.Dunite also exists as a massive unit similar to those of themantle–crust transition zones in other ophiolites. Allof the dunites in the ophiolite have a similar mineralogy, comprisingmainly olivine with minor orthopyroxene and chromite and tracesof clinopyroxene. They also display similar chemical compositions,including U-shaped chondrite-normalized REE patterns. Mantle-normalizedPGE patterns show variable negative Pt anomalies. Detailed analysisof a chromite-bearing dunite dyke, which grades into the hostcpx-bearing harzburgite, indicates that LREE and Ir decrease,whereas HREE, Pd and Pt increase away from the dunite. Thesefeatures are consistent with formation of the dunite dykes byinteraction of MORB peridotites with boninitic melts from whichthe chromitites were formed. Because the transition-zone dunitesare mineralogically and chemically identical to those formedby such melt–rock reaction, we infer that they are ofsimilar origin. The Luobusa ultramafic rocks originally formedas MORB-source upper mantle, which was subsequently trappedas part of a mantle wedge above a subduction zone. Hydrous meltsgenerated under the influence of the subducted slab at depthmigrated upward and reacted with the cpx-bearing harzburgitesto form the dunite dykes. The modified melts ponded in smallpockets higher in the section, where they produced podiformchromitites with dunite envelopes. At the top of the mantlesection, pervasive reaction between melts and harzburgite producedthe transition-zone dunites. KEY WORDS: melt–rock interaction; REE; PGE; hydrous melt; mantle; ophiolite; Tibet     

西藏罗布莎蛇绿岩中不同产出的纯橄岩及成因探讨

罗布莎蛇绿岩中的纯橄岩有三种产出情况,除了与豆荚状铬铁矿伴生的薄壳状纯橄岩外,还有产在方辉橄榄岩底部被认为是堆晶岩的厚层状纯橄岩和方辉橄榄岩中的透镜状纯橄岩。厚层状纯橄岩约700～1000m厚,以橄榄石富镁(Fo93～95),单斜辉石低铝富镁(Al2O30.47%～0.85%,Mg#95～97),铬尖晶石高铬低镁(Cr#值平均77,Mg#平均51)为特征。该纯橄岩中的浸染状铬铁矿也是高铬低镁型,但Mg#值(平均59)高于厚层状纯橄岩的副矿物铬尖晶石。薄壳状纯橄岩与厚层状纯橄岩成分相近,其橄榄石Fo92～94,单斜辉石Al2O3<1%和Mg#95～97;铬尖晶石的Cr#值平均71,Mg#值平均52。与薄壳状纯橄岩伴生的块状铬铁矿为高镁高铬型,但Mg#值(平均68)相对更高些,Cr#值平均79。透镜状纯橄岩的特征是橄榄石Fo(91～92)和铬尖晶石Cr#(60左右)均低于前两类纯橄岩,但单斜辉石的Al2O3(1.41%～1.71%)则高于前两者。透镜状纯橄岩的矿物成分与方辉橄榄岩重叠,两者为渐变过渡关系。研究对比表明,罗布莎厚层状纯橄岩不同于经典的蛇绿岩的超镁铁质堆晶岩,认为将其成因解释为拉斑玄武质熔体与地幔橄榄岩的反应较为合理。透镜状纯橄岩与方辉橄榄岩存在成生联系,可能是地幔橄榄岩高度部分熔融的产物,或熔体和方辉橄榄岩在原位发生反应的产物;薄壳状纯橄岩成因与厚层状纯橄岩相同,但与其相伴的块状铬铁矿是否由拉斑玄武质熔体与方辉橄榄岩反应形成,值得商榷。     

雅鲁藏布江缝合带西段达机翁地幔橄榄岩及铬铁矿的铂族元素特征

达机翁蛇绿岩位于雅鲁藏布江缝合带的西段北亚带,该蛇绿岩主要由地幔橄榄岩、玄武岩以及硅质岩组成,其中地幔橄榄岩以方辉橄榄岩为主,同时含有少量的纯橄榄岩,纯橄岩主要呈不规则透镜状或团块状分布于方辉橄榄岩中。在达机翁地幔橄榄岩中产出有3种不同类型的铬铁矿,分别为块状,豆状以及浸染状铬铁矿。文章主要对达机翁地幔橄榄岩的方辉橄榄岩及豆荚状铬铁矿进行了研究,结合岩石的主量元素和铂族元素,对地幔橄榄岩和豆荚状铬铁矿的成因以及雅鲁藏布江缝合带的找矿远景进行了探讨。达机翁地幔橄榄岩具有较高的Mg O含量以及较低的Al2O3和Ca O等含量,这种亏损的全岩成分指示了达机翁地幔橄榄岩经历了较高的部分熔融作用,同时方辉橄榄岩的PGEs的总量为23.68×10-9~31.02×10-9,高于原始地幔的值,Pd和Cu 2个元素的含量较为分散明显偏离部分熔融曲线,指示了达机翁方辉橄榄岩可能遭受了熔体的改造,在熔体-岩石反应的过程中,导致了富含PPGE的硫化物的加入。达机翁豆荚状铬铁矿为高Cr型铬铁矿,具有IPGE和Rh明显富集以及Pt,Pd明显亏损的特征,不同类型的铬铁矿之间具有一致的PGEs的分配模式。雅鲁藏布江缝合带内大量分布的超镁铁岩体在岩石组合、地球化学特征、成因以及形成时代等方面,均具有相似性,是中国铬铁矿找矿的有利远景区。     

Early Palaeozoic sub-arc chromitite-bearing peridotite in the Kudi ophiolite on the westernmost Tibetan Plateau

ABSTRACT

A chromite deposit was discovered in the Kudi ophiolite in the Palaeozoic western Kunlun orogenic belt. Chromite forms elongated (<2 m in width) and banded chromitite bodies (<0.1 m in width for each band) in dunite and podiform chromitite bodies (<1.5 m in width) in harzburgite. Dunite is classified into two types. Type I dunite hosting massive and banded chromitites shows low Fo in olivine (88.1–90.9), moderate Cr^# [=Cr/(Cr + Al), 0.47–0.56] in chromite, and a positively sloped primitive mantle-normalized platinum group elements (PGE) pattern, suggesting that it is a cumulate of a mafic melt. Harzburgite and type II dunite show olivine with high Fo (>91.1) and chromite with moderate to high Cr^# (0.44–0.61), and flat to negatively sloped primitive mantle-normalized PGE patterns, indicating that they are residual mantle peridotite after partial melting. Chromite in all three types of chromitites has relatively uniform moderate values Cr^# ranging from 0.43 to 0.56. Massive chromitite contains euhedral chromite with high TiO₂ (0.40–0.43 wt.%) and has a positively sloped primitive mantle-normalized PGE pattern, suggesting that it represents a cumulate of a melt. Rocks containing disseminated and banded chromite show overall low total PGE, < 117 ppb, and a negatively sloped primitive mantle-normalized PGE pattern. Chromite grains in these two types of occurrences are irregular in shape and enclose olivine grains, suggesting that chromite formed later than olivine. We suggest that chromite-oversaturated melt penetrated into the pre-existing dunite and crystallized chromite. The oxygen fugacity (fO₂ values of chromitites and peridotites are high, ranging from FMQ+0.8 (0.8 logarithmic unit above the fayalite-magnetite-quartz buffer) to FMQ+2.3 for chromitites and from FMQ+0.9 to FMQ+2.8 for peridotites (dunite and harzburgite). The mineral compositions and high fO₂ values as well as estimated parental magma compositions of the chromitites suggest that the Kudi ophiolite formed in a sub-arc setting.     

西昆仑库地蛇绿岩富Al型豆荚状铬铁矿床成因

西昆仑库地蛇绿岩发育小规模的铬铁矿床,矿体呈豆荚状和层状、似层状,均与纯橄岩紧密伴生。这些纯橄岩主要由橄榄石和副矿物尖晶石组成,与方辉橄榄岩相比,橄榄岩中的橄榄石粒径粗(平均2.5mm),Mg#(88~90)低,这与它们全岩低Mg#(90)值,富Al_2O_3、TiO_2、Cr_2O_3、Fe_2O_3相吻合,与熔融残余成因的纯橄岩明显不同,反映了其很可能是由熔体与方辉橄榄岩反应而成。矿体主要由块状、浸染状及脉状铬铁矿石组成;铬铁矿石中的尖晶石具有低而相对稳定的Cr#(43~56),低于富铬型铬铁矿矿床中的铬铁矿(Cr#60)。块状矿石与纯橄岩呈突变接触,矿石中的尖晶石呈浑圆状,包裹有较多橄榄石、辉石等硅酸盐矿物及角闪石等含水硅酸盐矿物;浸染状铬铁矿石中的尖晶石与橄榄石颗粒构成交织结构,或呈云朵状,沿橄榄石颗粒边界相互连接,矿石的结构构造显示了熔/岩反应成因特征。通过计算分析,我们认为该区富铝型铬铁矿石是由拉斑玄武质熔体与地幔橄榄岩反应而成,由于熔体中含有较高的H_2O,参与反应的熔体可能源于弧后扩张脊环境。     

Platinum‐Group Elements Geochemistry and Chromian Spinel Composition in Podiform Chromitites and Associated Peridotites from the Cheshmeh‐Bid Deposit,Neyriz, Southern Iran: Implications for Geotectonic Setting

Dunite and serpentinized harzburgite in the Cheshmeh-Bid area, northwest of the Neyriz ophiolite in Iran, host podiform chromitite that occur as schlieren-type, tabular and aligned massive lenses of various sizes. The most important chromitite ore textures in the Cheshmeh-Bid deposit are massive, nodular and disseminated. Massive chromitite, dunite, and harzburgite host rocks were analyzed for trace and platinum-group elements geochemistry. Chromian spinel in chromitite is characterized by high Cr~#(0.72-0.78), high Mg~#(0.62–0.68) and low TiO_2(0.12 wt%-0.2 wt%) content. These data are similar to those of chromitites deposited from high degrees of mantle partial melting. The Cr~# of chromian spinel ranges from 0.73 to 0.8 in dunite, similar to the high-Cr chromitite, whereas it ranges from 0.56 to 0.65 in harzburgite. The calculated melt composition of the high-Cr chromitites of the Cheshmeh-Bid is 11.53 wt%–12.94 wt% Al_2O_3, 0.21 wt%–0.33 wt% TiO_2 with FeO/MgO ratios of 0.69-0.97, which are interpreted as more refractory melts akin to boninitic compositions. The total PGE content of the Cheshmeh-Bid chromitite, dunite and harzburgite are very low(average of 220.4, 34.5 and 47.3 ppb, respectively). The Pd/Ir ratio, which is an indicator of PGE fractionation, is very low(0.05–0.18) in the Cheshmeh-Bid chromitites and show that these rocks derived from a depleted mantle. The chromitites are characterized by high-Cr~#, low Pd + Pt(4–14 ppb) and high IPGE/PPGE ratios(8.2–22.25), resulting in a general negatively patterns, suggesting a high-degree of partial melting is responsible for the formation of the Cheshmeh-Bid chromitites. Therefore parent magma probably experiences a very low fractionation and was derived by an increasing partial melting. These geochemical characteristics show that the Cheshmeh-Bid chromitites have been probably derived from a boninitic melts in a supra-subduction setting that reacted with depleted peridotites. The high-Cr chromitite has relatively uniform mantle-normalized PGE patterns, with a steep slope, positive Ru and negative Pt, Pd anomalies, and enrichment of PGE relative to the chondrite. The dunite(total PGE = 47.25 ppb) and harzburgite(total PGE =3 4.5 ppb) are highly depleted in PGE and show slightly positive slopes PGE spidergrams, accompanied by a small positive Ru, Pt and Pd anomalies and their Pdn/Irn ratio ranges between 1.55–1.7 and 1.36-1.94, respectively. Trace element contents of the Cheshmeh-Bid chromitites, such as Ga, V, Zn, Co, Ni, and Mn, are low and vary between 13–26, 466–842, 22-84, 115–179, 826–-1210, and 697–1136 ppm, respectively. These contents are compatible with other boninitic chromitites worldwide. The chromian spinel and bulk PGE geochemistry for the Cheshmeh-Bid chromitites suggest that high-Cr chromitites were generated from Cr-rich and, Ti-and Al-poor boninitic melts, most probably in a fore-arc tectonic setting related with a supra-subduction zone, similarly to other ophiolites in the outer Zagros ophiolitic belt.     

Multi‐stage Melting Processes of Ophiolitic Chromitites from Purang Peridotite,the Western Yarlung‐Zangbo Suture Zone,Tibet

The Purang ophiolite, which crops out over an area of about 600 km2 in the western Yarlung‐Zangbo suture zone, consists chiefly of mantle peridotite, pyroxenite and gabbro. The mantle peridotites are mostly harzburgite and minor lherzolite that locally host small pods of dunite. Some pyroxenite and gabbro veins of variable size occur in the peridotites, and most of them strike NW. On the basis of their mineral chemistry podiform chromitites are divided into high‐alumina (Cr# = 20‐60) (Cr# = 100*Cr/(Cr+Al)) and high‐chromium (Cr# = 60‐80) varieties (Thayer, 1970). Typically, only one type occurs in a given peridotite massif, although some ophiolites contain several massifs which can have different chromitite compositions. However, the Purang massif contains both high chrome and high alumina chromitites within a single mafic‐ultramafic body. Seven small, lenticular bodies of chromitite ore have been found in the harzburgite, with ore textures ranging from massive to disseminated to sparsely disseminated; no nodular ore has been observed. Individual ore bodies are 2‐6 m long, 0.5‐2 m wide and strike NW, parallel to the main structure of the ophiolite. Ore bodies 1 and 6 consist of Al‐rich chromitite (Cr# = 52‐55), whereas orebodies 2, 3, 4 and 5 are Cr‐rich varieties (Cr # = 63 to 89). In addition to magnesiochromite, all of the orebodies contain minor olivine, amphibole and serpentine. Mineral structures show that the peridotites experienced plastic deformation and partial melting. On the basis of magnesiochromite and olivine/clinopyroxene compositions two stages of partial melting are identified in the Purang peridotites, an early low‐partial melting event (about 8%), and a later high‐partial melting event (about 40%). We interpret the Al‐rich chromitites as the products of early MORB magmas, whereas the Cr‐rich varieties are thought to have been generated by the later SSZ melts..     

Textural Evidence for the Chromite‐Oversaturated Character of the Melt Involved in Podiform Chromitite Formation

Well‐preserved oval‐shaped dunite clots occur within the exceptionally fresh massive podiform chromitites from the Coto Block of the Zambales Ophiolite Complex, the Philippines. The dunite/chromitite boundary shows an interlocking texture; olivine inclusions in chromites in the podiform chromitites show the same optical extinction with larger adjacent olivines in the dunite clots. This texture was formed by the reaction between chromite‐oversaturated melt and its dunite inclusions. The existence of such type of melt was previously only hypothesized to explain the origin of layered and podiform chromitites but is now confirmed by this discovery.     

蛇绿岩型铬铁矿床包壳纯橄榄岩中的流体过程印记:来自西藏雅鲁藏布江缝合带罗布莎和泽当岩体的地质学、岩石学和橄榄石晶体化学证据

岩浆型矿床一般认为是岩浆分异的产物,因为这类矿床通常缺乏强烈的近矿围岩蚀变。蛇绿岩中的豆荚状铬铁矿被认为是一种典型的岩浆型矿床,流行的成因模型包括岩浆通道模型和熔体-岩石反应模型。深部晶体群的大量发现,表明铬铁矿成矿系统不是一种理想系统,而是至少由两类子系统组成的复杂性动力系统。因此,流行模型不再适用,必须构建能够整合新证据的成因模型。这类矿床的典型地质特征是具有从方辉橄榄岩围岩经包壳纯橄榄岩到铬铁矿石的分带,且包壳纯橄榄岩与铬铁矿之间为渐变接触关系,包壳纯橄榄岩与方辉橄榄岩之间既可以为渐变接触关系,也可以为截然接触关系或侵入接触关系。因此,阐明纯橄榄岩的成因是理解豆荚状铬铁矿形成机制的关键。西藏雅鲁藏布江缝合带中罗布莎和泽当两个代表性超镁铁质杂岩体的新观察揭示:(1)包壳纯橄榄岩的出露宽度变化于厘米级到百米级,但岩石具有均匀的细粒结构,流行模型难以解释;(2)包壳纯橄榄岩可以划分为至少两种构造类型:块状纯橄榄岩和片理化纯橄榄岩,暗示了纯橄榄岩形成过程的多阶段特点;(3)包壳纯橄榄岩主要由变晶橄榄石组成,仅含有少量由熔体或流体析出的橄榄石晶体;(4)与方辉橄榄岩相比,包壳纯橄榄岩中的橄榄石具有高MgO、Cr2O3、CaO和低MnO、Al2O3的特点,展示了矛盾的晶体化学特征;(5)邻近铬铁矿体的纯橄榄岩中常见反豆状结构,类似于多相稀释流体流体制中紊流产生的中尺度结构。上述看似矛盾的证据表明包壳纯橄榄岩的形成过程有大量深部流体的参与,因而流体过程可以作为构建一个新的整合模型的基础。据此,文中提出一个熔体-流体流模型,其基本机制是溶解-沉淀反应Opx+Fluid→Ol±Sp±Cpx±Pl±SiO2(fluid),而基本前提则是深部还原流体的持续供给和熔体-流体流的快速上升。此外,文中还表明,依据火成岩地质学、岩石学和名义无水矿物晶体化学证据也可以再造岩浆系统的流体过程。     

Geochemistry of platinum-group elements and mineral composition in chromitites and associated rocks from the Abdasht ultramafic complex,Kerman, Southeastern Iran

The Abdasht complex is a major ultramafic complex in south-east Iran (Esfandagheh area). It is composed mainly of dunite, harzburgite, podiform chromitites, and subordinate lherzolite and wehrlite. The podiform chromitites display massive, disseminated, banded and nodular textures. Chromian spinels in massive chromitites exhibit a uniform and restricted composition and are characterized by Cr# [= Cr / (Cr + Al)] ranging from 0.76 to 0.77, Mg# [= Mg/(Mg + Fe^2 +)] from 0.63 to 0.65 and TiO₂ < 0.2 wt.%. These values may reflect crystallization of the chromian spinels from boninitic magmas. Chromian spinels in peridotites exhibit a wide range of Cr# from 0.48 to 0.86, Mg# from 0.26 to 0.56 and very low TiO₂ contents (averaging 0.07 wt.%). The Fe^3 +# is very low, (< 0.08 wt.%) in the chromian spinel of chromitites and peridotites of the Abdasht complex which reflects crystallization under low oxygen fugacities.The distribution of platinum group elements (PGE) in Abdasht chromitites displays a high (Os + Ir + Ru)/(Rh + Pt + Pd) ratio with strongly fractionated chondrite-normalized PGE patterns typical of ophiolitic chromitites. Moreover, the Pd/Ir value, which is an indicator of PGE fractionation, is very low (< 0.1) in the chromitites.The harzburgite, dunite and lherzolite samples are highly depleted in PGE contents relative to chondrites. The Pd_N/Ir_N ratios in dunites are unfractionated, averaging 0.72, whereas the harzburgites and lherzolites show slightly positive slopes PGE spidergrams, together with a small positive Ru anomaly, and their Pd_N/Ir_N ratio averages 2.4 and 2.3 respectively. Moreover, the PGE chondrite and primitive mantle normalized patterns of harzburgite, dunite and lherzolite are relatively flat which are comparable to the highly depleted mantle peridotites.The mineral chemistry data and PGE geochemistry indicate that the Abdasht chromitites and peridotites were generated from a melt with boninitic affinity under low oxygen fugacity in a supra-subduction zone setting. The composition of calculated parental melts of the Abdasht chromitites is consistent with the differentiation of arc-related magmas.     

The chromite deposits associated with ophiolite complexes, Southeastern Desert, Egypt： Petrological and geochemical characteristics and mineralization

1 Introduction The association of massive Fe-Ni-Cu sulfides andchromite is a very unusual feature of podiformchromitites occurring in mantle tectonites of ophioliticcomplexes. It has only been described in theSoutheastern Desert, Egypt, where sulfides a…     

Petrogenesis and PGE Abundances of High‐Cr and High‐Al Chromitites and Peridotites from the Bulqiza Ultramafic Massif,Eastern Mirdita Ophiolite,Albania

The Bulqiza ultmafic massif, which belongs to the eastern Mirdita ophiolite of northern Albania, is world renowned for its high‐Cr chromite resource. The high‐Cr chromitites commonly host in the mantle section, while high‐Al chromitites also present in massive dunite of the mantle‐crust transition zone (MTZ) in this massif. Chromian‐spinel in the MTZ high‐Al chromitites and MTZ dunites have much lower Cr# values [Cr/(Cr+Al)×100] (47.7–55.1 and 46.5–51.7, respectively) than those of chromian‐spinel in the high‐Cr chromitites (78.2–80.4), harzburgites (72.6–77.9) and mantle dunites (79.4–84.3). The high‐Cr chromitites are rich in IPGE relative to PPGE with 0.10–0.45 PPGE/IPGE ratios, whereas the high‐Al chromitites have higher PPGE/IPGE ratios between 1.20 and 7.80. The partial melting degrees of parental magmas for the high‐Cr chromitites are beyond the critical interval (> 25%) and thus prevented sulfide saturation and diluted Pt and Pd in melts, producing high‐Cr chromitites barren of Pt and Pd. However, the degrees for the high‐Al chromitites just enter the critical interval (20–25%) for the effective extraction of PGE from mantle sulfides, which may account for the enrichments of PPGE in high‐Al chromitites. The parental melts of the high‐Cr chromitites have Al₂O₃ and TiO₂ contents of ～10.6–11.4 wt.% and 0.14–0.31 wt.%, whereas the calculated Al₂O₃ and TiO₂ for the high‐Al chromitites are ～14.9–15.9 wt.% and 0.07–0.61 wt.%, respectively. The calculated melts in equilibrium with the high‐Cr chromitites are boninitic‐like, and those with high‐Al chromitites are MORB‐like but with hydrous, oxidized and TiO₂‐poor affinities. To make a compromise between the inconsistence above, we proposed that coexistence of both types of chromitites in the Bulqiza ultramafic massif may reflect that their magma compositions transited from MORB‐like to boninitic‐like in a proto‐forearc setting during subduction initiation. Key words: Chromian‐spinel, Platinum‐group elements, high‐Cr and high‐Al chromitite, Mirdita ophiolite, Albania.     

Chromite composition and platinum-group mineral assemblage in the Uktus Uralian-Alaskan-type complex (Central Urals,Russia)

Chromitite segregations in dunites of the Uktus Uralian-Alaskan-type complex (Central Urals, Russia) display large variation of the chromite composition: Cr/(Cr+Al)=0.46-0.77, Fe²⁺/(Fe²⁺+Mg)=0.28-0.66, and Fe³⁺/(Fe³⁺+Fe²⁺)=0.23-0.59. Three types of PGM assemblages have been recognized, varying in accordance with chromite composition: type I, dominated by Ru-Os-Ir (sulfides), is associated with magnesiochromite having Fe³⁺/(Fe³⁺+Fe²⁺)<0.30, in the southern dunite body. Type II, containing abundant Pt-Ir (alloys, minor sulfides), is found in magnesiochromite with Fe³⁺/(Fe³⁺+Fe²⁺)=0.40-0.44; type III, consisting of Ir-Rh-Pt-Pd (alloys, sulfarsenides, antimonides) in Fe-rich chromite having Fe²⁺/(Fe²⁺+Mg)=0.66 and Fe³⁺/(Fe³⁺+Fe²⁺)=0.59. Positive anomalies of Ir and Pt, and a negative peak of Ru characterize the PGE patterns of chromitites with type II and III PGM assemblages, whereas a positive Pt anomaly is observed in their dunite host. Intensive fractionation of Pt-Fe alloys in the Uktus chromitites reflects the anomalous behavior of Pt which is decoupled from Rh and Pd. Among other factors, the high iron activity and oxygen fugacity in the parent melt appear to exert a major control on precipitation of Pt-Fe alloys, below sulfur saturation. The strong Pt anomaly in chromitites from Uktus may indicate that Uralian-Alaskan-type magmas were derived from a Pt-rich mantle source.     

Alteration of chromitites from the Voidolakkos and Xerolivado mines,Vourinos ophiolite complex,Greece: implications for deformation‐induced metamorphism

Chromitites, associated with upper mantle spinel peridotites from the Voidolakkos and Xerolivado districts, located in the Vourinos ophiolite complex, northwestern continental Greece, were re‐investigated with respect to their structural and textural mode of occurrence, as well as their compositional signatures. They include variably deformed banded and podiform chromitite bodies made up of massive, semi‐massive, nodular, anti‐nodular, schlieren and disseminated chromian spinel. Chromitites have suffered intense shearing that was more severe in all but disseminated textured ore. Deformation has partly produced elongation of chromian spinel nodules and widespread protocataclastic zones within chromitites. The examined deposits are composed of magnesiochromite that shows a quite restricted range of Cr# [Cr/(Cr + Al)] values varying between 0.76 and 0.83, whereas Mg# [Mg/(Mg + Fe²⁺)] ranges from 0.55 to 0.67 accompanied by relatively low content in TiO₂ (<0.15 wt.% on average). Compositional data indicate that these high‐Cr chromitite bodies crystallized from melts of boninitic affinities that have been compositionally modified after reaction with depleted harzburgite, followed by interaction with relatively undifferentiated low SiO₂ melts within an intertwined system of dunite channels in the mantle wedge below the fore‐arc region of a supra‐subduction zone (SSZ). Magnesiochromite displays limited textural modification, being scarcely transformed to an opaque spinel phase along grain boundaries and fracture walls. The opaque spinel phase is characterized by elevated Cr# (0.76–0.97), relatively low Mg# (0.33–0.63) and low Fe³⁺# (≤0.14) and corresponds to ferrian chromite. Microscopic studies revealed that ferrian chromite is paragenetically associated with clinochlore even in unaltered chromitite specimens and the degree of serpentinization does not correlate with the frequency and abundance of alteration effects on magnesiochromite. Therefore, it is supported that regional metamorphism prior to serpentinization was responsible for the formation of the ferrian chromite–clinochlore association. In addition, magnesiochromite alteration was systematically recorded only in variably sheared chromitites displaying protocataclastic brecciation, thus it can be claimed that metamorphism was mainly governed by deformation mechanisms, which took place during the transition from ductile to semi‐brittle conditions. Ferrian chromite can be locally erratically enriched in MnO and ZnO, which is attributed to a Mn‐ and Zn‐bearing, slightly acid post‐magmatic fluid that invaded the chromitites across weakness zones. Overall, the data suggest that after magnesiochromite equilibration with the intergranular olivine, both phases were partly replaced by ferrian chromite and clinochlore, respectively, during a brief, fluid assisted episode of retrogade metamorphism that took place within a temperature interval between 700 and 300 °C, before ocean‐floor alteration. Copyright © 2014 John Wiley & Sons, Ltd.     

西藏依拉山蛇绿岩中铬铁矿特征及构造背景

依拉山蛇绿岩位于班公湖-怒江缝合带中部,主要由蚀变较强的方辉橄榄岩和纯橄岩及豆荚状铬铁岩组成。铬铁矿矿体集中分布在依拉山岩体北部,围岩以纯橄岩为主,少量为方辉橄榄岩。铬铁岩中铬尖晶石的电子探针分析结果表明Cr~#值为64.2～73.9,Mg~#值为46.9～71.6,TiO_2为0.03%～0.31%,Al_2O_3为4.5%～18.7%,指示依拉山铬铁矿为高铬型铬铁矿。方辉橄榄岩的稀土元素及微量元素配分模式指示其具有深海地幔橄榄岩的特征,铬铁矿的铂族元素具有IPGE富集而PPGE亏损的特点,呈现出右倾的配分模式,且Pd/Ir与Pt/Pt~*之间不存在明确的相关性,反映出依拉山岩体经历了岩石-熔体反应的演化过程。结合其他岩体内铬铁矿的对比研究,提出依拉山铬铁矿可能是在俯冲带环境下,由玻安质熔体与岩石反应形成,并经历了多阶段的演化过程,即早期的洋中脊(MORB)环境以及后期的俯冲带(SSZ)的改造。     

Chemical homogeneity of high-Cr chromitites as indicator for widespread invasion of boninitic melt in mantle peridotite of Bir Tuluha ophiolite,Northern Arabian Shield,Saudi Arabia

The Bir Tuluha ophiolite is one of the most famous chromitite-bearing occurrences in the Arabian Shield of Saudi Arabia, where chromitite bodies are widely distributed as lensoidal pods of variable sizes surrounded by dunite envelopes, and are both enclosed within the harzburgite host. The bulk-rock geochemistry of harzburgites and dunites is predominately characterized by extreme depletion in compatible trace elements that are not fluid mobile (e.g., Sr, Nb, Ta, Hf, Zr and heavy REE), but variable enrichment in the fluid-mobile elements (Rb and Ba). Harzburgites and dunites are also enriched in elements that have strong affinity for Mg and Cr such as Ni, Co and V. Chromian spinels in all the studied chromitite pods are of high-Cr variety; Cr-ratio (Cr/(Cr + Al) atomic ratio) show restricted range between 0.73 and 0.81. Chromian spinels of the dunite envelopes also show high Cr-ratio, but slightly lower than those in the chromitite pods (0.73–0.78). Chromian spinels in the harzburgite host show fairly lower Cr-ratio (0.49–0.57) than those in dunites and chromitites. Platinum-group elements (PGE) in chromitite pods generally exhibit steep negative slopes of typical ophiolitic chromitite PGE patterns; showing enrichment in IPGE (Os, Ir and Ru), over PPGE (Rh, Pt and Pd). The Bir Tuluha ophiolite is a unimodal type in terms of the presence of Ru-rich laurite, as the sole primary platinum-group minerals (PGM) in chromitite pods. These petrological features indicates that the Bir Tuluha ophiolite was initially generated from a mid-ocean ridge environment that produced the moderately refractory harzburgite, thereafter covered by a widespread homogeneous boninitic melt above supra-subduction zone setting, that produced the high-Cr chromitites and associated dunite envelopes. The Bir Tuluha ophiolite belt is mostly similar to the mantle section of the Proterozoic and Phanerozoic ophiolites, but it is a “unimodal” type in terms of high-Cr chromitites and PGE-PGM distribution.     

Petrology and geochemistry of high Cr# podiform chromitites of Bulqiza,Eastern Mirdita Ophiolite (EMO), Albania

The ultramafic massif of Bulqiza, which belongs to the eastern ophiolitic belt of Albania, is a major source of metallurgical chromitite ore. The massif consists of a thick (> 4 km) sequence, composed from the base upward of tectonized harzburgite with minor dunite, a transitional zone of dunite, and a magmatic sequence of wehrlite, pyroxenite, troctolite and gabbro. Only sparse, refractory chromitites occur within the basal clinopyroxene-bearing harzburgites, whereas the upper and middle parts of the peridotite sequence contain abundant metallurgical chromitites. The transition zone dunites contain a few thin layers of metallurgical chromitite and sparse bodies are also present in the cumulate section. The Bulqiza Ophiolite shows major changes in thickness, like the 41–50 wt.% MgO composition similar with forearc peridotite as a result of its complex evolution in a suprasubduction zone (SSZ) environment. The peridotites show abundant evidence of mantle melt extraction at various scales as the orthopyroxene composition change from core to rim, and mineral compositions suggest formation in a forearc, as Fo values of olivine are in 91.1–93.0 harzburgite and 91.5–91.9 in dunite and 94.6–95.9 in massive chromitite. The composition of the melts passing through the peridotites changed gradually from tholeiite to boninite due to melt–rock reaction, leading to more High Cr# chromitites in the upper part of the body. Most of the massive and disseminated chromitites have high Cr# numbers (70–80), although there are systematic changes in olivine and magnesiochromite compositions from harzburgites, to dunite envelopes to massive chromitites, reflecting melt–rock reaction. Compositional zoning of orthopyroxene porphyroblasts in the harzburgite, incongruent melting of orthopyroxene and the presence of small, interstitial grains of spinel, olivine and pyroxene likewise attest to modification by migrating melts. All of the available evidence suggests that the Bulqiza Ophiolite formed in a suprasubduction zone mantle wedge.     

Refractory chromitites recovered from the Eretria mine,East Othris massif (Greece): Implications for metallogeny and deformation of chromitites within the lithospheric mantle portion of a forearc-type ophiolite

The chrome ores of the abandoned Eretria mine of the East Othris ophiolite occur within a pervasively serpentinized and sheared harzburgite body. They consist of massive chromitites with mylonitic fabric in imbricate shaped pods. Modal analyses of these ores average at about 90–95% chromian spinel (Cr-spinel) and 5–10% secondary silicates. Chromian spinel compositions vary in Cr# [Cr/(Cr + Al) × 100] and Mg# [Mg/(Mg + Fe²⁺) × 100] from 44 to 62 and from 59 to 81, respectively. Trace element (Ti, Ni, V, Mn, Zn, Sc, Co and Ga) contents in Cr-spinel do not show significant variations from grain cores to grain boundaries. However, Cr-spinel compositions show depletions in Ti, Zn and Sc when compared to the composition of accessory Cr-spinel from typical mid-ocean ridge basalts (MORB). Mineral inclusions hosted in Cr-spinel comprise a range of (hydrous and anhydrous) silicate and base metal (BM) minerals occasionally intergrown with phosphate minerals and rare intermetallic compounds. A number of these inclusions have Cr-spinel rims with higher Cr# (63–68) than those of the enclosing Cr-spinel grains.The absence of dunite sheaths around chromitites is interpreted as an artifact of dunite structural obliteration during prolonged ductile shearing within harzburgite. The microtextural characteristics of a number of inclusions in Cr-spinel imply that they were initially fully molten. Furthermore, primary hydrosilicate (amphibole, phlogopite) inclusions in Cr-spinel indicate that chromitites crystallized from a water-bearing melt. Chromian spinel rims around silicate inclusions probably represent early crystals generated from a primitive magma produced by melting of a depleted mantle source.Geochemical calculations demonstrate that the parental melts of chromitites had intermediate affinity between MORB and arc-related magmas. Our preferred hypothesis for the genesis of the Eretria chromitites is that they were formed from a melt originated within the hydrated mantle wedge beneath a nascent forearc basin during subduction initiation.     

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

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

The feature and tectonic setting of Dongqiao podiform chromitite in the central segment of the Bangong Lake-Nujiang suture zone in TibetSCIEI北大核心CSCD

早侏罗世东巧蛇绿岩位于班公湖-怒江缝合带(班怒带)东段,蕴含较为丰富的豆荚状铬铁矿资源。东巧地幔橄榄岩主体由方辉橄榄岩组成,铬铁矿赋存在其内部的纯橄岩脉中。方辉橄榄岩和纯橄岩均显示出弧前橄榄岩的特征。方辉橄榄岩中橄榄石的Fo值为89.8~92.2,斜方辉石的和单斜辉石的Mg^(#)值分别变化于89.7~92.0和92.7~95.1,铬尖晶石的Cr^(#)值(Cr^(#)=100×Cr/(Cr+Al))为60.8~75.9;纯橄岩中橄榄石的Fo值为91.7~92.5,斜方辉石Mg^(#)值变化于91.7~92.1,单斜辉石的Mg^(#)值变化于94.0~94.6,铬尖晶石的Cr^(#)值为69.0~83.1。铬铁矿主要呈致密块状和浸染状构造,其中铬尖晶石的矿物包裹体有橄榄石、斜方辉石、单斜辉石、角闪石和铂族矿物等。矿石中的铬尖晶石与橄榄岩中的铬尖晶石相比,具有较高的Cr^(#)值(72.5~86.9)和Mg^(#)值(52.8~70.5),较低的Al_(2)O_(3)(6.25%~13.6%)、TiO_(2)(0.06%~0.16%)和Zn(518×10^(-6)~714×10^(-6)),属于高铬型铬铁矿,平衡熔体与玻安质熔体有亲缘性。方辉橄榄岩中铂族元素(PGE)总含量(14.01×10^(-9)~32.81×10^(-9))近似于原始地幔,IPGE(Os、Ir和Ru)/PPGE(Rh、Pt和Pd)的比值均大于1;纯橄岩的PGE总量(13.36×10^(-9)~16.08×10^(-9))略低于原始地幔,IPGE和PPGE富集程度近似;铬铁矿的铂族元素总量(108.4×10^(-9)~645.7×10^(-9))远远高于原始地幔和地幔橄榄岩中PGE的含量,且IPGE以及Rh相对原始地幔富集,而Pt和Pd相对亏损,具明显右倾特征的配分模式,指示东巧地幔橄榄岩和铬铁矿形成过程经历了熔体抽取和交代作用。通过与全球典型豆荚状铬铁矿矿床的特征对比,认为班怒带的蛇绿岩应该有良好的铬铁矿成矿背景。