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1.
《地学前缘(英文版)》2020,11(6):2083-2102
Chromitite bodies hosted in peridotites typical of suboceanic mantle (s.l. ophiolitic) are found in the northern and central part of the Loma Caribe peridotite, in the Cordillera Central of the Dominican Republic. These chromitites are massive pods of small size (less than a few meters across) and veins that intrude both dunite and harzburgite. Compositionally, they are high-Cr chromitites [Cr# = Cr/(Cr + Al) atomic ratio = 0.71–0.83] singularly enriched in TiO2 (up to 1.25 wt.%), Fe2O3 (2.77–9.16 wt.%) as well as some trace elements (Ga, V, Co, Mn, and Zn) and PGE (up to 4548 ppb in whole-rock). This geochemical signature is unknown for chromitites hosted in oceanic upper mantle but akin to those chromites crystallized from mantle plume derived melts. Noteworthy, the melt estimated to be in equilibrium with such chromite from the Loma Caribe chromitites is similar to basalts derived from different source regions of a heterogeneous Caribbean mantle plume. This mantle plume is responsible for the formation of the Caribbean Large Igneous Province (CLIP). Dolerite dykes with back-arc basin basalt (BABB) and enriched mid-ocean ridge basalt (E-MORB) affinities commonly intrude the Loma Caribe peridotite, and are interpreted as evidence of the impact that the Caribbean plume had in the off-axis magmatism of the back-arc basin, developed after the Caribbean island-arc extension in the Late Cretaceous. We propose a model in which chromitites were formed in the shallow portion of the back-arc mantle as a result of the metasomatic reaction between the supra-subduction zone (SSZ) peridotites and upwelling plume-related melts. 相似文献
2.
蛇绿岩型金刚石的特征 总被引：6，自引：2，他引：4
蛇绿岩型金刚石产在蛇绿岩地幔橄榄岩和铬铁矿中,不同于产在大陆克拉通的金伯利岩型金刚石,也不同于产在板块俯冲带中的超高压变质型金刚石和陨石撞击成因的金刚石。蛇绿岩型金刚石的主要特征是粒度普遍较小,多数在200~500μm之间,C同位素显示极低的δ13CVPDB值(-28‰~-18‰),金刚石中包裹体以含富Mn的矿物为特征,金刚石产在蛇绿岩大洋地幔橄榄岩中,其构造背景为板块缝合带。金伯利岩型金刚石粒度可达厘米级,是大颗粒宝石级金刚石的主要来源,其C同位素显示轻微低的δ13CVPDB值(-10‰~-5‰),包裹体矿物多为富Mg的矿物组合,金刚石产在克拉通和大陆岩石圈构造背景。超高压变质带中的金刚石颗粒十分细小,由数微米至100μm,C同位素为中等低的δ13CVPDB值(-15‰~-7‰),金刚石通常与碳酸岩和地壳成因矿物伴生,含有金刚石的超高压变质岩石形成的构造背景为板块深俯冲边界。形成于陨石撞击的金刚石产出和研究均较少,金刚石通常也是微米级,产出在陨石撞击形成的变质岩中,伴生矿物的成分与与撞击变质的原岩有关。以上四类金刚石无论野外和室内区分显著,由此,可以将蛇绿岩型金刚石归为地球上一种新的金刚石产出类型。 相似文献
3.
Platinum-group element (PGE) mineralisation within the Platreef at Overysel is controlled by the presence of base metal sulphides (BMS). The floor rocks at Overysel are Archean basement gneisses, and unlike other localities along the strike of the Platreef where the floor is comprised of Transvaal Supergroup sediments, the intimate PGE–BMS relationship holds strong into the footwall rocks. Decoupling of PGE from BMS is rare and the BMS and platinum-group mineral assemblages in the Platreef and the footwall are almost identical. There is minimal overprinting by hydrothermal fluids; therefore, the mineralisation style present at Overysel may represent the most ‘primary’ style of Platreef mineralisation preserved anywhere along the strike. Chondrite-normalised PGE profiles reveal a progressive fractionation of the PGE with depth into the footwall, with Ir, Ru and Rh dramatically depleted with depth compared to Pt, Pd and Au. This feature is not observed at Sandsloot and Zwartfontein, to the south of Overysel, where the footwall rocks are carbonates. There is evidence from rare earth element abundances and the amount of interstitial quartz towards the base of the Platreef pyroxenites that contamination by a felsic melt derived from partial melting of the gneissic footwall has taken place. Textural evidence in the gneisses suggests that a sulphide liquid percolated down into the footwall through a permeable, inter-granular network that was produced by partial melting around grain boundaries in the gneisses that was induced by the intrusion of the Platreef magma. PGE were originally concentrated within a sulphide liquid in the Platreef magma, and the crystallisation of monosulphide solid solution from the sulphide liquid removed the majority of the IPGE and Rh from it whilst still within the mafic Platreef. Transport of PGE into the gneisses, via downward migration of the residual sulphide liquid, fractionated out the remaining IPGE and Rh in the upper parts of the gneisses leaving a ‘slick’ of disseminated sulphides in the gneiss, with the residual liquid becoming progressively more depleted in these elements relative to Pt, Pd and Au. Highly sulphide-rich zones with massive sulphides formed where ponding of the sulphide liquid occurred due to permeability contrasts in the footwall. This study highlights the fact that there is a fundamental floor rock control on the mechanism of distribution of PGE from the Platreef into the footwall rocks. Where the floor rocks are sediments, fluid activity related to metamorphism, assimilation and later serpentinisation has decoupled PGE from BMS in places, and transport of PGE into the footwall is via hydrothermal fluids. In contrast, where the floor is comprised of anhydrous gneiss, such as at Overysel, there is limited fluid activity and PGE behaviour is controlled by the behaviour of sulphide liquids, producing an intimate PGE–BMS association. Xenoliths and irregular bands of chromitite within the Platreef are described in detail for the first time. These are rich in the IPGE and Rh, and evidence from laurite inclusions indicates they must have crystallised from a PGE-saturated magma. The disturbed and xenolithic nature of the chromitites would suggest they are rip-up clasts, either disturbed by later pulses of Platreef magma in a multi-phase emplacement or transported into the Platreef from a pre-existing source in a deeper staging chamber or conduit. 相似文献
4.
《Chemie der Erde / Geochemistry》2019,79(1):130-152
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. 相似文献
5.
西藏康金拉铬铁矿床刚玉中的包裹体研究 总被引：1，自引：1，他引：0
西藏康金拉铬铁矿石的矿物学研究中,发现大量的微粒金刚石和碳硅石等超高压异常地幔矿物,表明它们产在一个强还原的高压环境.本研究在铬铁矿石中还发现了刚玉及其中大量的矿物包裹体.电子探针等方法研究表明,包裹体的种类包括简单氧化物,如金红石;自然钛;Ti-N、Ti-Si、Ti-C、Ti-Si-P、Ti-B等合金类;含稀土元素的硅酸盐矿物,以及一些未知矿物.结合对铬铁矿石中其他矿物的研究成果,认为康金拉铬铁矿石中的刚玉及其中的强还原环境形成的矿物组合形成于深部地幔.因此,康金拉铬铁矿石中的刚玉可以认为是一种新的带有高压环境信息的标志性矿物. 相似文献
6.
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 总被引：1，自引：0，他引：1
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。 相似文献
7.
Li GuanLong Liu Fei Yang JingSui Mu XiaoPing Zhang ChenJie Bo RongZhong Zhang QiZhi 《岩石学报》2022,(11):3375-3390
The ophiolite is an ancient oceanic lithosphere remnant emplaced on the continent, and it is the direct carrier to record the ancient ocean basin from formation to demise. Therefore, the study of ophiolite can provide strong evidence for revealing and restoring the tectonic system and evolution history of ancient oceans and continents. The field geological survey and mapping for ophiolite are the basis for the research on the petrogenesis of chromite, peridotite and oceanic crust units. According to the characteristics of mineral composition, structure and mineral chemistry of harzburgites in Dingqing ophiolite, it is divided into massive, inhomogeneous, pyroxene-oriented, spherulitic and mylonitized harzburgite. Five types of harzburgite were discovered for the first time in the Laraka survey area through 1 : 50000 special geological mapping, and there were obvious lithofacies zoning characteristics in the area of five types of harzburgites. The massive harzburgite lithofacies belts are distributed in the middle of the Laraka survey area, and the inhomogeneous harzburgite lithofacies appear symmetrically on both sides. A small amount of pyroxene-oriented harzburgite lithofacies is distributed in the interior or edge of massive harzburgite facies belt in a lenticular shape, and the production of a very small amount of spheroidal harzburgite is related to basic intrusive rocks. The mylonitized harzburgite lithofacies belt is a slender strip outcropping on the southern margin of the peridotite massif. There is a certain spatial correlation between the distribution, type and scale of chromite deposits or mineralization points and the peridotite facies zoning. More than 20 high -chromium -type chromite deposits are distributed in the inhomogeneous harzburgite, but the massive harzburgite lithofacies have few chromite deposits or mineralizations. The A -A' peridotite lithofacies measured profile in the Lallaka survey area and its corresponding mineral chemical profile further confirm the existence of lithofacies zoning. Cr-spinels in different types of harzburgites show different mineral chemical characteristics. The Cr-# of Cr-spinel in massive harzburgite, in inhomogeneous harzburgite, in pyroxene oriented harzburgite and in the spheroidal harzburgite is 65. 59 similar to 69. 37, 49. 99 similar to 57. 86, 75.19 similar to 85.24 and 57.66 similar to 80.04, respectively. The Cr# of Cr-spinel in inhomogeneous -> massive -> pyroxene-oriented harzburgite gradually increases from 49. 99 to 85.24, indicating that these harzburgite have experienced 23% similar to 43% medium -high partially melted. The mineral chemistry data of massive and pyroxene-oriented harzburgite in the mineral chemistry diagram both fall in the fore -arc peridotite environment, and the inhomogeneous harzburgite falls in the overlapping area of deep-sea and fore -arc peridotite. The Fo of olivine in the spherulite harzburgite is 86.43% similar to 87.05%, and the En of orthopyroxene is 84.66% similar to 85.78%, which are bronzite, indicating magmatic origin. The mylonitized harzburgite is the result of tectonic emplacement of the massif. Different types of peridotite and their lithofacies zoning were identified in the Dingqing ophiolite, which provides a new idea for us to deeply study the multi -stage magmatism of the oceanic lithosphere and the genesis of peridotite and chromite. 相似文献
8.
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. 相似文献
9.
A. G. Dessai D. B. Arolkar D. French A. Viegas T. A. Viswanath 《Journal of the Geological Society of India》2009,73(5):697-714
The Bondla mafic-ultramafic complex is a layered intrusion that consists predominantly of peridotites and gabbronorites. A
chromitite-pyroxenite-troctolite horizon serves as a marker to subdivide the intrusion into two zones. The Lower Zone displays
gravity stratified layers of chromite that alternate with those of olivine, which up-section are followed by olivine+pyroxene-chromite
cumulates. The Upper Zone comprises gabbroic rocks that exhibit uniform layering. On the basis of modal and cryptic variation
exhibited by the minerals this zone can be subdivided in to several lithohorizons starting from the troctolites at the base
to gabbronorites and leucogabbros at the top. The junction between the two zones is marked by the distinct reversal in cryptic
variation exhibited by the chromites and pyroxenes.
The peridotite chromites contain higher Al2O3 and lower Cr2O3 than those from the chromitite above. Similarly clinopyroxenes from pyroxenite and troctolites are more magnesian that those
from the peridotites stratigraphically below them. The complex in general is characterized by a gabbroic mineral assemblage
in which both Ca-rich and Capoor pyroxenes coexist and displays a Fe-enrichment trend providing evidence of evolution from
a contaminated tholeiitic magma. The rocks are characterized by low-TiO2; Ni, Cr and V, show negative correlation with Zr whereas the large ion lithophile elements (LILE) are positively correlated
and the Nb/La ratio varies from 0.4–0.6. These characteristics are consistent with a low-TiO2 sub-alkaline tholeiitic magma that may have been modified by fractional crystallization and successive injections of more
primitive melts in the magma chamber. The complex evolved in a periodically replenished magma chamber that consisted of two
separate but interconnected sub-chambers. 相似文献
10.
F. Johan Kruger 《Mineralium Deposita》2005,40(5):451-472
“His mind was like a soup dish—wide and shallow; ...” - Irving Stone on William Jennings BryanA compilation of the Sr-isotopic stratigraphy of the Bushveld Complex, shows that the evolution of the magma chamber occurred in two major stages. During the lower open-system Integration Stage (Lower, Critical and Lower Main Zone), there were numerous influxes of magma of contrasting isotopic composition with concomitant mixing, crystallisation and deposition of cumulates. Larger influxes correspond to the boundaries of the zones and sub-zones and are marked by sustained isotopic shifts, major changes in mineral assemblages and development of unconformities. During the upper, closed system Differentiation Stage (Upper Main Zone and Upper Zone), there were no major magma additions (other than that which initiated the Upper Zone), and the thick magma layers evolved by fractional crystallisation. The Lower and Lower Critical Zones are restricted to a belt that runs from Steelpoort and Burgersfort in the northeast, to Rustenburg and Northam in the west and an outlier of the Lower and Lower Critical Zone, up to the LG4 chromitite layer, in the far western extension north of Zeerust. It is only in these areas that thick harzburgite and pyroxenite layers are developed and where chromitites of the Lower Critical Zone occur. These chromitites include the economically important c. 1 m thick LG6 and MG1 layers exposed around both the Eastern and Western lobes of the Bushveld Complex. The Upper Critical Zone has a greater lateral extent than the Lower Critical Zone and overlies but also onlaps the floor-rocks to the south of the Steelpoort area . The source of the magmas also appears to have been towards the south as the MG chromitite layers degrade and thin northward whereas the LG layers are very well represented in the North and degrade southward. Sr and Os isotope data indicate that the major chromitite layers including the LG6, MG1 and UG2 originated in a similar way. Extremely abrupt and stratigraphically restricted increases in the Sr isotope ratio imply that there was massive contamination of intruding melt which “hit the roof” of the chamber and incorporated floating granophyric liquid which forced the precipitation of chromite (Kruger 1999; Kinnaird et al. 2002). Therefore, each chromitite layer represents the point at which the magma chamber expanded and eroded and deformed its floor. Nevertheless, this was achieved by in situ contamination by roof-rock melt of the intruding Critical Zone liquids that had an orthopyroxenitic to noritic lineage. The Main Zone is present in the Eastern and Western lobes of the Bushveld Complex where it overlies the Critical Zone, and onlaps the floor-rocks to the south, and the north where it is also the basal zone in the Northern lobe. The new magma first intruded the Northern lobe north of the Thabazimbi–Murchison Lineament, interacted with the floor-rocks, incorporated sulphur and precipitated the “Platreef” along the floor-rock contact before flowing south into the main chamber. This exceptionally large influx of new magma then eroded an unconformity on the Critical Zone cumulate pile, and initiated the Main Zone in the main chamber by precipitating the Merensky Reef on the unconformity. The Upper Zone magma flowed into the chamber from the southern “Bethal” lobe as well as the TML. This gigantic influx eroded the Main Zone rocks and caused very large-scale unconformable relationships, clearly evident as the “Gap” areas in the Western Bushveld Complex. The base of this influx, which is also coincident with the Pyroxenite Marker and a troctolitic layer in the Northern lobe, is the petrological and stratigraphic base of the Upper Zone. Sr-isotope data show that all the PGE rich ores (including chromitites) are related to influxes of magma, and are thus related to the expansion and filling of the magma chamber dominantly by lateral expansion; with associated transgressive disconformities onto the floor-rocks coincident with major zone changes. These positions in the stratigraphy are marked by abrupt changes in lithology and erosional features over which succeeding lithologies are draped. The outcrop patterns and the concordance of geochemical, isotopic and mineralogical stratigraphy, indicate that during crystallisation, the Bushveld Complex was a wide and shallow, lobate, sill-like sheet, and the rock-strata and mineral deposits are quasi-continuous over the whole intrusion.
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