Structures in podiform chromite from the Maqsad district (Sumail ophiolite,Oman)

About 15 chromite bodies have been recognized in the Maqsad area of the Oman ophiolite. The occurrence in this area of three chromite bodies within the cumulate sequence must be integrated into the classification of Cassard et al. (1981) which presently explains only those pods lying in the uppermost mantle sequence (plastically deformed harzburgites and dunites). The occurrence of chromite bodies within the cumulates and the abundance of chromite in the Maqsad area are related to the exceptional magmatic activity and the unusual plastic-flow pattern particular to this area. It was probably a feeding zone along the oceanic spreading center sitting on top of a mantle diapir.     

Thick chromitite of the Jacurici Complex (NE Craton São Francisco,Brazil): Cumulate chromite slurry in a conduit

The Jacurici Complex, located in the NE part of the São Francisco Craton, hosts the largest chromite deposit in Brazil. The mineralized intrusion is considered to be a single N-S elongated layered body, disrupted into many segments by subsequent deformation. The ore is hosted in a thick, massive layer. Two segments, Ipueira and Medrado, have been previously studied. We provide new geological information, and chromite composition results from the Monte Alegre Sul and Várzea do Macaco segments located farther north, and integrate these with previous results. The aim of this study is to determine and discuss the magma chamber process that could explain the formation of the thick chromitite layer. All segments exhibit similar stratigraphic successions with an ultramafic zone (250 m thick) hosting a 5–8 m thick main chromitite layer (MCL), and a mafic zone (40 m thick). The chromite composition of the MCL, Mg-numbers (0.48–0.72) and Cr-numbers (0.59–0.68), is similar to chromites from layered intrusions and other thick chromitites. Previous work concluded that the parental magma of the mineralized intrusion was very primitive based on olivine composition (up to Fo₉₃) and orthopyroxene composition (up to En₉₄) from harzburgite samples, and that it originated from an old subcontinental lithospheric mantle. We estimate that the melt from which the massive chromitite layer crystallized was similar to a boninite, or low siliceous high-Mg basalt, with a higher Fe/Mg ratio. The petrologic evidence from the mafic-ultramafic rocks suggests that a high volume of magma flowed through the sill, which acted as a dynamic conduit. Crustal contamination has previously been considered as the trigger for the chromite crystallization, based on isotope studies, as the more radiogenic signatures correlate with an increase in the volumetric percentage of amphibole (up to 20%). The abundant inclusions of hydrous silicate phases in the chromites from the massive ore suggest that the magma was hydrated during chromite crystallization. Fluids may have played an important role in the chromite formation and/or accumulation. However, the trigger for chromite crystallization remains debatable. The anomalous thickness of the chromitite is a difficult feature to explain. We suggest a combined model where chromite crystallized along the margins of the magma conduit, producing a semi-consolidated chromite slurry that slumped through the conduit forming a thick chromitite layer in the magma chamber where layered ultramafic rocks were previously formed. Subsequently, the conduit was obstructed and the resident magma fractionated to produce a more evolved composition.     

On the formation mode of the Guleman chromite deposits (Turkey)

Kefdag and Soridag chromite pods occur in upper mantle residual peridotites, which consist of harzburgite and dunites. The peridotites represent the residual of multistage, depleted upper-mantle peridotites. The chromitite bodies were formed during the uprising of chromium-rich picritic melts, through the residual upper mantle diapir, along the magma conduits. Chromitite grains were deposited in the caves of the magma conduits under the control of the convection currents.     

Jadeite, albite and nepheline as inclusions in spinel of chromitite from Hess Deep, equatorial Pacific: their genesis and implications for serpentinite diapir formation

Polymineralic inclusions which consist of a few grains of diopside, enstatite, jadeite, nepheline, albite, pargasite, phlogopites and olivine were found in chromian spinel in a chromitite pod and in troctolite from Hess Deep, equatorial Pacific. The inclusion mineral suite in chromitite is characterized by Na-Al silicates, such as jadeite, nepheline and albite. Jadeite and nepheline commonly coexist with enstatite, and tend to occur as interstitial grains between subhedral enstatite (or other minerals) and host spinel. Albite, diopside and enstatite occur as equant inclusions. The mafic minerals in the inclusions have similar chemistry to those found in the troctolite and dunite. The modes of occurrence and mineral chemistry of the inclusions are controlled by magmatic precipitation, and subsequent reequilibration due to decrease of temperature in the uppermost mantle. The mafic minerals in spinel inclusions were crystallized from a melt enriched in Cr and some incompatible components formed by melt-mantle interaction process mixed to various extent with subsequently supplied more primary melt. Albite and nepheline could also be formed at the magmatic stage. Jadeite was formed by a subsolidus reaction of albite and nepheline at low temperatures (250–300 °C) at slightly less than 3 kbar. This requires a remarkable temperature decrease, at least locally, of the uppermost mantle and crust. The Hess Deep rocks were formed in the uppermost mantle beneath a spreading-ridge axis at more than 1000 °C, and were transposed outwards from the axis by corner flow. At the off-ridge conditions, the rocks were cooled and serpentinized by circulation of sea water at the mantle depth to form jadeite in chromitite. The serpentinized portion could have risen as a kind of serpentinite diapir through the thin crust up to the ocean floor. Received: 24 January 1997 / Accepted: 6 November 1997     

Fe-Mg同位素在蛇绿岩中铬铁矿床成因研究中的应用潜力

蛇绿岩中铬铁矿床成因一直存在较大争议,其主要原因可归结为:寄主蛇绿岩存在成因争议、产出状态不清、矿石及围岩矿物组合单一以及主要矿物成分简单但矿物包裹体复杂多样.针对这些研究瓶颈,率先对西藏普兰和罗布莎、土耳其K?z?lda?和Kop蛇绿岩中的地幔橄榄岩和铬铁岩进行了全岩和单矿物Fe-Mg同位素的探索性研究工作.结果表明:（1）蛇绿岩中的地幔橄榄岩具有较均一的Fe-Mg同位素组成,与世界上其他地区的地幔橄榄岩相似;（2）铬铁岩中铬铁矿和橄榄石之间存在明显的Fe-Mg同位素分馏,铬铁矿多具有比共存橄榄石轻的Fe同位素组成,与地幔橄榄岩中的尖晶石和橄榄石相反,Mg同位素变化较大;（3）铬铁矿和橄榄石的Fe-Mg同位素主要受控于结晶分异和Fe-Mg交换,且这两个过程造成的同位素变化趋势明显不同.因此,Fe-Mg同位素在揭示铬铁矿母岩浆来源、性质及成矿过程方面具有较大的应用潜力.      

Origin of phlogopite-orthopyroxene inclusions in chromites from the Merensky Reef of the Bushveld Complex,South Africa

About 30% of the chromite grains of variable sizes in a chromitite seam at the base of the Merensky Reef of the Bushveld Complex on the farm Vlakfontein contain abundant composite mineral inclusions. The inclusions are polygonal to circular with radial cracks that protrude into the enclosing chromite. They vary from a few microns to several millimeters in diameter and are concentrated in the cores and mantles of chromite crystals. Electron backscattered patterns indicate that the host chromites are single crystals and not amalgamations of multiple grains. Na-phlogopite and orthopyroxene are most abundant in the inclusions. Edenitic hornblende, K-phlogopite, oligoclase and quartz are less abundant. Cl-rich apatite, rutile, zircon and chalcopyrite are present at trace levels. Na-phlogopite is unique to the inclusions; it has not been found elsewhere in the Bushveld Complex. Other minerals in the inclusions are also present in the matrix of the chromitite seam, but their compositions are different. The Mg/(Mg+Fe²⁺) ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix. K-phlogopite in the inclusions contains more Na than in the matrix. The average compositions of the inclusions are characterized by high MgO (26 wt%), Na₂O (2.4 wt%) and H₂O (2.6 wt%), and low CaO (1.1 wt%) and FeO (4.4 wt%). The δ¹⁸O value of the trapped melt, estimated by analysis of inclusion-rich and inclusion-poor chromites, is ∼7‰. This value is consistent with the previous estimates for the Bushveld magma and with the δ¹⁸O values of silicate minerals throughout the reef. The textural features and peculiar chemical compositions are consistent with entrapment of orthopyroxene with variable amounts of volatile-rich melts during chromite crystallization. The volatile-rich melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and Na-K-rich fluids expelled from the underlying crystal pile. The addition of fluid to the magma is thought to have caused dissolution of orthpyroxene, leaving the system saturated only in chromite. Both oxygen and hydrogen isotopic values are consistent with the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution. Most of the Cr and Al in the inclusions was contributed through wall dissolution of the host chromite. Dissolution of minor rutile trapped along with orthopyroxene provided most of the Ti in the inclusions. The Na- and K-rich hydrous silicate minerals in the inclusions were formed during cooling by reaction between pyroxene and the trapped volatile-rich melts.     

Thick sections of layered ultramafic cumulates in the Oman ophiolite revealed by an airborne hyperspectral survey: Petrogenesis and relationship to mantle diapirism

Using the HyMap instrument, we have acquired visible and near infrared hyperspectral data over the Maqsad area of the Oman ophiolite (~ 15 × 60 km). This survey allowed us to identify and map the distribution of clinopyroxene-rich cumulates (inter-layered clinopyroxenites and wehrlites) whose occurrence was previously undocumented in this area. The cumulates reach several hundred meters in thickness and crop out at distances exceeding 15 km on both sides of the Maqsad former spreading centre. They occur either in mantle harzburgites, as km-sized layered intrusions surrounded by fields of pegmatitic dykes consisting of orthopyroxene-rich pyroxenite and gabbronorites, or at the base of the crustal section where they are conformably overlain by cumulate gabbros. These ultramafic cumulates crystallized from silica- and Mg-rich melts derived from a refractory mantle source (e.g. high Cr#, low [Al₂O₃], low [TiO₂]). These melts are close to high-Ca boninites, although, strictly speaking, not perfect equivalents of present-day, supra-subduction zone, boninites. Chemical stratigraphy reveals cycles of replenishment, mixing and fractional crystallization from primitive (high Mg#) melts, typical of open magma chambers and migration of inter-cumulus melts. The TiO₂ content of clinopyroxene is always low (≤ 0.2 wt.%) but quite variable compared to the associated pegmatites that are all derived from a source ultra-depleted in high field strength elements (HFSE). This variability is not caused by fractional crystallization alone, and is best explained by hybridization between the ultra-depleted melts (parent melts of the pegmatites) and the less depleted mid-ocean ridge basalts (MORB) parent of the dunitic–troctolitic–gabbroic cumulates making up the crustal section above the Maqsad diapir.We propose that, following a period of magma-starved spreading, the Maqsad mantle diapir, impregnated with tholeiitic melts of MORB affinity, reached shallow depths beneath the ocean ridge. This diapir induced melting of the formerly accreted and hydrothermally altered lithosphere. At this stage, these boninitic-like lithospheric melts crystallized as pegmatitic dykes. As the diapir continued to rise, the amount of MORB reaching shallow depths increased, together with the surrounding temperature, leading to the formation of magma chambers where the crystallization of layered cumulates became possible. These cumulates remained rich in pyroxene and devoid of plagioclase as long as the contribution of MORB-derived melts was moderate relative to the lithospheric-derived melts. As the contribution of MORB to the refilling of the magma chamber increased, gabbroic cumulates started to crystallize.     

Origin of the UG2 chromitite layer, Bushveld Complex

Chromitite layers are common in large mafic layered intrusions.A widely accepted hypothesis holds that the chromitites formedas a consequence of injection and mixing of a chemically relativelyprimitive magma into a chamber occupied by more evolved magma.This forces supersaturation of the mixture in chromite, whichupon crystallization accumulates on the magma chamber floorto form a nearly monomineralic layer. To evaluate this and othergenetic hypotheses to explain the chromitite layers of the BushveldComplex, we have conducted a detailed study of the silicate-richlayers immediately above and below the UG2 chromitite and anotherchromitite layer lower in the stratigraphic section, at thetop of the Lower Critical Zone. The UG2 chromitite is well knownbecause it is enriched in the platinum-group elements and extendsfor nearly the entire 400 km strike length of the eastern andwestern limbs of the Bushveld Complex. Where we have studiedthe sequence in the central sector of the eastern Bushveld,the UG2 chromitite is embedded in a massive, 25 m thick plagioclasepyroxenite consisting of 60–70 vol. % granular (cumulus)orthopyroxene with interstitial plagioclase, clinopyroxene,and accessory phases. Throughout the entire pyroxenite layerorthopyroxene exhibits no stratigraphic variations in majoror minor elements (Mg-number = 79·3–81·1).However, the 6 m of pyroxenite below the chromitite (footwallpyroxenite) is petrographically distinct from the 17 m of hangingwall pyroxenite. Among the differences are (1) phlogopite, K-feldspar,and quartz are ubiquitous and locally abundant in the footwallpyroxenite but generally absent in the hanging wall pyroxenite,and (2) plagioclase in the footwall pyroxenite is distinctlymore sodic and potassic than that in the hanging wall pyroxenite(An_45–60 vs An_70–75). The Lower Critical Zone chromititeis also hosted by orthopyroxenite, but in this case the rocksabove and below the chromitite are texturally and compositionallyidentical. For the UG2, we interpret the interstitial assemblageof the footwall pyroxenite to represent either interstitialmelt that formed in situ by fractional crystallization or chemicallyevolved melt that infiltrated from below. In either case, themelt was trapped in the footwall pyroxenite because the overlyingUG2 chromitite was less permeable. If this interpretation iscorrect, the footwall and hanging wall pyroxenites were essentiallyidentical when they initially formed. However, all the modelsof chromitite formation that call on mixing of magmas of differentcompositions or on other processes that result in changes inthe chemical or physical conditions attendant on the magma predictthat the rocks immediately above and below the chromitite layersshould be different. This leads us to propose that the Bushveldchromitites formed by injection of new batches of magma witha composition similar to the resident magma but carrying a suspendedload of chromite crystals. The model is supported by the commonobservation of phenocrysts, including those of chromite, inlavas and hypabyssal rocks, and by chromite abundances in lavasand peridotite sills associated with the Bushveld Complex indicatingthat geologically reasonable amounts of magma can account foreven the massive, 70 cm thick UG2 chromitite. The model requiressome crystallization to have occurred in a deeper chamber, forwhich there is ample geochemical evidence. KEY WORDS: Bushveld complex; chromite; crystal-laden magma; crustal contamination; magma mixing; UG2 chromitite     

Morphology and microstructure of chromite crystals in chromitites from the Merensky Reef (Bushveld Complex, South Africa)

The Merensky Reef of the Bushveld Complex consists of two chromitite layers separated by coarse-grained melanorite. Microstructural analysis of the chromitite layers using electron backscatter diffraction analysis (EBSD), high-resolution X-ray microtomography and crystal size distribution analyses distinguished two populations of chromite crystals: fine-grained idiomorphic and large silicate inclusion-bearing crystals. The lower chromitite layer contains both populations, whereas the upper contains only fine idiomorphic grains. Most of the inclusion-bearing chromites have characteristic amoeboidal shapes that have been previously explained as products of sintering of pre-existing smaller idiomorphic crystals. Two possible mechanisms have been proposed for sintering of chromite crystals: (1) amalgamation of a cluster of grains with the same original crystallographic orientation; and (2) sintering of randomly orientated crystals followed by annealing into a single grain. The EBSD data show no evidence for clusters of similarly oriented grains among the idiomorphic population, nor for earlier presence of idiomorphic subgrains spatially related to inclusions, and therefore are evidence against both of the proposed sintering mechanisms. Electron backscatter diffraction analysis maps show deformation-related misorientations and curved subgrain boundaries within the large, amoeboidal crystals, and absence of such features in the fine-grained population. Microstructures observed in the lower chromitite layer are interpreted as the result of deformation during compaction of the orthocumulate layers, and constitute evidence for the formation of the amoeboid morphologies at an early stage of consolidation. An alternative model is proposed whereby silicate inclusions are incorporated during maturation and recrystallisation of initially dendritic chromite crystals, formed as a result of supercooling during emplacement of the lower chromite layer against cooler anorthosite during the magma influx that formed the Merensky Reef. The upper chromite layer formed from a subsequent magma influx, and hence lacked a mechanism to form dendritic chromite. This accounts for the difference between the two layers.     

Chromite-bearing spessartites from Kasuga-mura,Japan, and their bearing on possible mantle origin andesite

An unusual association of chromite and hornblende was found in the spessartites of andesite composition, occurring as a dike swarm associated with a Cretaceous granite batholith. The spessartites are largely porphyritic with phenocrysts of either hornblende or augite. One dike, comprising a finegrained spessartite, exhibits distinct chilled selvages of aphanitic facies. The chromites in the fine-grained and augite-spessartites are significantly higher in Cr/ (Cr+Al) than those occurring rarely as inclusions in the phenocrystic hornblendes in the hornblende spessartite, although both are similar in Mg/ (Mg+Fe), Fe₂O₃, and TiO₂. The phenocrystic hornblendes are titaniferous pargasite with high Mg/ (Mg+Fe), and differ in their higher octahedral Al from the groundmass hornblendes including those in the fine-grained spessartite. The crystallization sequence in the phenocrystic hornblende-bearing spessartites is Al-rich chromite, phenocrystic hornblende, and plagioclase without pyroxene, suggesting a high water content in the magma and the start of the crystallization at relatively high pressures. The finegrained spessartite from which the porphyritic spessartites have been derived by fractionation of dominant mafic minerals, has the high Mg-value and Cr content equivalent to those in primitive, undifferentiated basalts, although still andesitic in SiO₂ content. Chemically similar magnesian andesites, although uncommon, found in some orogenic calc-alkalic suites may represent a magma composition in equilibrium with mantle peridotite under the condition of high water pressures.     

Crystallization conditions of the alkaline-basic dike from the Yllymakh Massif,Central Aldan: Evidence from melt inclusion data in minerals

Alkaline-basic dike from the Yllymakh Massif (Central Aldan) has been studied. Its partially crystallized matrix contains corroded phenocrysts of olivine and hypidiomorphic phenocrysts of clinopyroxene and pseudo-, epileucite. It was found that phenocrysts of clinopyroxene contain abundant primary inclusions, Ti-magnetite and apatite bear only single inclusions, whereas olivine is enriched in secondary inclusions, which are confined to the cleavage of host mineral (along second and third pinacoids) and its cracks. The homogenization temperatures of the primary inclusions in clinopyroxene and secondary inclusions in olivine are approximately equal and lie within 1260–1240°C. The compositions of melt inclusions in olivine and clinopyroxene are also similar and corresponded to the malignite-pseudoleucite phonolite-monzonite pulaskites, which are developed at the Yllymakh Massif. Unheated inclusions in apatite and Ti-magnetite compositionally approach monzonites and nepheline syenites—tinguaites, respectively. It was concluded that the alkaline basaltoid magma was presumably parental magma for the entire rock complex of the Yllymakh Massif. Its crystallization and differentiation presumably provided all observed rock variety from ultrabasics (early derivatives located at depth) and malignites (later derivatives) to leucite phonolites, monzonites, and alkaline pulaskites, which were obtained during subsequent stages of the melt evolution. The parental magma, and especially its derivatives, were enriched in BaO (0.8–0.1 wt %), Cl (0.1–0.3 wt %) and trace elements (primarily, LREE and MREE), which are several times higher than mantle values. At the same time, ion microprobe (SIMS) study showed that derivative melts were dry: contained only 0.01–1.13 wt % H₂O. The trend of melts conserved in the minerals and the massif rocks corresponds to the evolution of alkalinebasaltoid magma with increase in Si, Al, alkalis and decrease in Mg, Ca, and Fe, i.e. the Bowen trend. The considered alkaline-basic dike was presumably formed from the derivative of leucite-phonolite melt, which during emplacement captured olivine xenocrysts from previously fractionated ultrabasic rocks. The parental magma was presumably derived by high-degree melting of garnet-spinel-facies depleted mantle at some influence of crustal material.     

胶东早白垩世中-酸性脉岩造岩矿物微区地球化学特征研究

胶东地区广泛发育早白垩世中-酸性脉岩群,但是其成因演化及成岩地质背景至今仍存在诸多争论。本文利用电子探针(EMPA)与激光剥蚀电感藕合等离子质谱(LA-ICP-MS)技术分析了胶东早白垩世石英闪长脉岩与闪长脉岩中主要造岩矿物(斜长石和黑云母)的主、微量元素组成;并结合岩石地球化学特征,对两者的岩浆源区和岩浆演化进行了研究探讨。石英闪长脉岩与闪长脉岩中黑云母低于检测线的Ca O含量与斜长石主量元素之间良好的线性关系指示两者为未受到后期变质作用影响的原生矿物,进一步说明胶东中生代石英闪长脉岩与闪长脉岩岩浆形成后,在上涌成岩过程中未受到变质作用的影响。石英闪长脉岩中壳源黑云母矿物成分基本一致的,以及斜长石正环带中核边部线性变化的An值与Fe、Mg、Sr、Ba等不相容元素特征指示石英闪长脉岩源于华北克拉通东部古老的加厚下地壳部分熔融作用,并在岩浆演化早期和晚期有一定幔源镁铁质岩浆混入,整个岩浆演化过程并未受到大气、俯冲、变质流体混入或构造作用的影响。闪长脉岩中黑云母矿物较大的Fe2+/(Fe2++Mg)比值范围,Al含量与结晶压力高度正相关以及斜长石中不相容元素特征指示本次研究中胶东闪长脉岩源自俯冲的板片来源的流体或沉积物混入所形成富集岩石圈地幔源区。胶东早白垩世石英闪长脉岩与闪长脉岩形成的大地构造动力学背景为古太平洋板俯冲-回撤引起热-机械侵蚀,进而导致岩石圈地幔减薄。在此情况下软流圈地幔上涌加热导致胶东富集岩石圈地幔部分熔融形成地幔熔体。这些幔源熔体经历分离结晶形成早白垩世闪长脉岩。此外,幔源镁铁质岩浆持续加热导致加厚下地壳部分熔融,形成了石英闪长脉岩。     

PGE and PGM in the Luanga mafic-ultramafic intrusion in Serra dos Carajás (Pará State, Brazil)

The late Archean, Luanga mafic-ultramafic complex intrudes an Archean greenstone belt, that is mainly composed of ultramafic and mafic metavolcanics. The Luanga intrusion consists of dunite, peridotite, gabbro and norite; chromitite seams and layers are present in the ultramafic rocks.A metamorphic overprint transformed the primary paragenesis into a serpentine-talc-chlorite-tremolite and magnetite association. The magnetite is commonly altered to Fe-hydroxides. Unaltered chromite commonly displays atoll-like textures and a chemical composition typical of stratiform chromites (Cr₂O₃ below 45 wt%).Base-metal sulfides, base-metal alloys, platimum-group minerals and platinum group element bearing phases are present in the form of inclusions in the silicate assemblages and in or on the edges of chromite grains. The main minerals detected are pentlandite, pyrrhotite, millerite, chalcopyrite and mackinawite, Fe---Ni alloy, braggite, sperrylite and platinum group elements (PGE) bearing sulfo-arsenides. Braggite is associated with the chromite, whereas sperrylite lies on the edges of or is included in silicates. The PGE content of the massive and disseminated chromities is dominated by Pt (up to 8900 ppb) and the chondrite-normalized PGE profile shows a cuspidal shape with a Pt peak.The main hypothesis for the source of the PGE-rich magma, which fractionated the chromitite-bearing ultramafic magma, consists of a relatively primitive mantle that partially melted in the late Archean.     

西藏罗布莎不同类型铬铁矿的特征及成因模式讨论

蛇绿岩地幔橄榄岩中产出的豆荚状铬铁矿是铬的主要来源。已有的研究表明,豆荚状铬铁矿形成于洋中脊或俯冲带的浅部地幔环境。但随着近些年在豆荚状铬铁矿及围岩地幔橄榄岩中不断发现金刚石等深部矿物,人们也开始质疑豆荚状铬铁矿的浅部成因理论。本文系统研究了西藏雅鲁藏布江蛇绿岩带东段的罗布莎豆荚状铬铁矿床,识别出两类铬铁矿,一类以方辉橄榄岩为围岩的致密块状铬铁矿(Cr1#),另一类是以纯橄岩壳为围岩的浸染状铬铁矿(Cr2#)。两类铬铁矿在铬尖晶石的矿物化学成分、PGE和Re-Os同位素特征上存在较大差别,属不同演化过程的结果。地幔橄榄岩的地球化学特征指示罗布莎橄榄岩中存在由低铬且轻稀土亏损和高铬且轻稀土富集的两类方辉橄榄岩。在此基础上,提出豆荚状铬铁矿为多阶段形成的新认识,经历了早期俯冲至地幔过渡带(410~660km)的陆壳和洋壳物质被脱水和肢解,过渡带产生的热和流体促成了地幔的熔融和Cr的释放和汇聚;铬铁矿浆在地幔柱/地幔对流驱动下,运移至过渡带顶部冷凝固结,并有强还原性的流体进入,后者携带了深部形成的金刚石、斯石英等高压矿物,并进入"塑性-半塑性地幔橄榄岩"中;随着物质向上移动,深度降低,早期超高压相矿物发生相变,如斯石英转变成柯石英,高压相的铬铁矿中出溶成柯石英和单斜辉石;在侵位过程和俯冲带环境,含水熔体与方辉橄榄岩反应形成了不含超高压矿物的规模相对较小的浸染状铬铁矿(Cr2#)及纯橄岩壳。     

Crystallization of Podiform Chromitites from Silicate Magmas and the Formation of Nodular Textures

Abstract: Podiform chromite deposits consist of numerous individual accumulations of chromite in the mantle sequences of ophiolites, suggesting formation in separate, mini-magma conduits in the upper mantle. They may show unique nodular and orbicular textures. Simple mixing of two distinct magmas, invoked for chromite deposits in layered intrusions, is inadequate to explain the formation of podiform chromite deposits. More likely, melt/rock interaction triggers the precipitation of chromite by addition of newly-formed droplets of melt to the main body of magma passing through a conduit, a process similar to that of magma mingling but involving a turbulent, moving magma so that newly-formed melt droplets behave like snowballs. These droplets concentrate chromite to form an outer shell and, while the magma is moving upwards, less dense silicate melts are squeezed out of the droplets as the shell collapses to form a nodule. Upon cooling, both orbicular and nodular textures are preserved in the chromitite.     

The significance of magmatic erosion for bifurcation of UG1 chromitite layers in the Bushveld Complex

One of the most puzzling features of the UG1 chromitite layers in the famous exposures at Dwars River, Eastern Bushveld Complex, is the bifurcation, i.e. convergence and divergence of layers along strike that isolate lenses of anorthosite. The bifurcations have been variously interpreted as resulting from: (1) the intermittent accumulation of plagioclase on the chamber floor as lenses, terminated by crystallization of continuous chromitite layers (the depositional model); (2) late-stage injections of chromite mush or chromite-saturated melt along anastomosing fractures that dismembered semi-consolidated plagioclase cumulates (the intrusive model); (3) post-depositional deformation of alternating plagioclase and chromite cumulates, resulting in local amalgamation of chromitite layers and anorthosite lenses that wedge out laterally (the deformational model). None of these hypotheses account satisfactorily for the following field observations: (a) wavy and scalloped contacts between anorthosite and chromitite layers; (b) abrupt lateral terminations of thin anorthosite layers within chromitite; (c) in situ anorthosite inclusions with highly irregular contacts and delicate wispy tails within chromitite; many of these inclusions are contiguous with footwall and hanging wall cumulates; (d) transported anorthosite fragments enclosed by chromitite; (e) disrupted anorthosite and chromitite layers overlain by planar chromitite; (f) protrusions of chromitite into underlying anorthosite; (g) merging of chromitite layers around anorthosite domes. We propose a novel hypothesis that envisages basal flows of new dense and superheated magma that resulted in intense thermo-chemical erosion of the temporary floor of the chamber. The melting and dissolution of anorthosite was patchy and commonly inhibited by chromitite layers, resulting in lens-like remnants of anorthosite resting on continuous layers of chromitite. On cooling, the magma crystallized chromite on the irregular chamber floor, draping the remnants of anorthosite and merging with pre-existing chromitite layers excavated by erosion. With further cooling, the magma crystallized chromite-bearing anorthosite. Emplacement of multiple pulses of magma led to repetition of this sequence of events, resulting in a complex package of anorthosite lenses and bifurcating chromitite layers. This hypothesis is the most satisfactory explanation for most of the features of this enigmatic igneous layering in the Bushveld Complex.     

Formation and upwelling of mantle diapirs through the cratonic lithosphere: Numerical thermomechanical modeling

This paper reports the results of the numerical modeling of gravitationally instable processes in the lithospheric mantle of ancient cratons. The gravitational instability is considered as a result of melting at the lithosphere base owing to its local heating by anomalous mantle. Modeling was based on a finite element method in 2D formulation and took into account the geological structure and thermomechanical parameters of the lithosphere of the Siberian platform. Numerical results revealed the main tendencies in the mantle diapirisim of the mafic and ultramafic magma ascending through the “cold” high-viscosity lithosphere. It was shown that the shape of diapiric magmatic bodies is controlled by realistic visco-elastic-plastic rheology of lithosphere. The ascent of diapir in lithosphere was modeled for diverse regimes differing in duration, temperature field, and upwelling depth. It was concluded that the ascent of melt through lithosphere to the crust-mantle boundary is mainly controlled by rheology, and conditions of oscillatory diapirism with recurrent magma replenishments were modeled. Modeling results may shed light on some features related to the trap magmatism of the Siberian igneous province. The duration and rate of magma upwelling as well as the parameters of periodical magma upwelling were estimated and attempt was made to explain the high-velocity seismic anomalies that were recorded in the subcrustal regions of the Siberian platform.     

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

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

西藏罗布莎铬铁矿中的原位铂族矿物研究:铬铁矿结晶环境的指示

本文对罗布莎三个矿区的铬铁矿进行了详细的原位PGM研究,发现罗布莎各个矿区的铬铁矿中PGM组合和显微结构不同,暗示PGM能够记录铬铁矿形成与演化过程。罗布莎矿区的PGM显微特征显示铬铁矿结晶于高温、低硫逸度的环境中,可能系岩石/熔体反应和结晶分异双重作用下的产物;康金拉矿区的原位PGM主要为组合型包裹体,有少量产于铬铁矿裂隙之间的贱金属硫化物和合金矿物,为不同来源的熔体混合作用的结果,并暗示铬铁矿成矿后还受到热液流体的改造;香卡山矿区的PGM表明铬铁矿成矿之后遭受到还原性流体的交代作用,铬铁矿中早期结晶出来的硫化物或者铂族矿物被还原改造,形成铁镍矿等次生矿物,保存于铬铁矿粒间或者铬铁矿的裂隙中,这个过程可能与蛇纹石化或者晚期构造流体改造作用有关。罗布莎原位PGM研究表明,PGM矿物贯穿于铬铁矿结晶成矿过程的始终,PGM的矿物及其组合能够记录铬铁矿结晶时母熔体的物理化学条件,甚至还能反映铬铁矿成矿后所经历的后期构造热液事件。因此,结合单矿物分选和原位调查两种方法,查明铬铁矿中PGM的赋存类型及微观结构,对全面理解铬铁矿的成矿过程有重要意义。     

Hydrous aluminosilicate metasomatism in an intra-oceanic subduction zone: Implications from the Kurancali (Turkey) ultramafic-mafic cumulates within the Alpine Neotethys Ocean

The Kurancali ultramafic-mafic cumulate body, an allochthonous ophiolitic sliver in central Anatolia, is characterized by the presence of abundant hydrous phases (phlogopite, pargasite) besides augitic diopside, plagioclase, and accessory amounts of rutile, sphene, apatite, zircon, and calcite. Based on modes of the essential minerals, the olivine-orthopyroxene-free cumulates are grouped as clinopyroxenite, hydrous clinopyroxenite, phlogopitite, hornblendite, layered gabbro, and diorite. Petrographical, mineralogical and geochemical features of the rocks infer crystallization from a hydrous magma having high-K calc-alkaline affinity with slightly alkaline character, and point to metasomatised mantle as the magma source. Our evidence implies that the metasomatising component, which modified the composition of the mantle wedge source rock in an intra-oceanic subduction zone, was a H₂O, alkali and carbonate-rich aluminosilicate fluid and/or melt, probably derived from a subducted slab. We suggest that the metasomatic agents in the subarc mantle led to the generation of a hydrous magma, which produced the Kurancali cumulates in an island-arc basement in a supra-subduction-zone setting during the closure of the Izmir-Ankara-Erzincan branch of the Alpine Neotethys Ocean.