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1.
The Luobusa ophiolite in the Indus—Yarlung Zangbo sutureof southern Tibet hosts the largest known chromite deposit inChina. The podiform chromitites occur in a well-preserved mantlesequence consisting of harzburgite with abundant lenses of dunite.The harzburgites have relatively uniform bulk-rock compositionswith mg-numbers [100 Mg/(Mg + Fe)] ranging from 89 to 91 andshow flat, unfractionated, chondrite-normalized platinum groupelement (PGE) patterns. Their accessory chromite varies widelyin cr-number [100Cr/(Cr + Al)] (18–66). These rocks areessentially residua left after extraction of mid-ocean ridgebasalt (MORB)-type magmas. The podiform chromitites displaynodular, massive, disseminated and banded textures and typicallyhave dunite envelopes that grade into the surrounding harzburgiteand diopsidic harzburgite with increasing pyroxene contents.They consist of relatively uniform chromite with high cr-numbers(74–82), have strongly fractionated, chondrite-normalizedPGE patterns with enrichment in Os, Ir and Ru relative to Rh,Pt and Pt, and are believed to have formed from a boniniticmagma produced by a second stage of melting. Dunites containaccessory chromite intermediate in composition between thoseof harzburgite and chromitite and are believed to be the productsof reaction between new boninitic magmas and old MORB-type peridotites.The melt-rock reaction removed pyroxene from the peridotitesand precipitated oli-vine, forming dunite envelopes around thechromitite pods. The melts thus became more boninitic in compositionand chromite saturated, leading to precipitation of chromitealone. The interplay of melt-rock interaction, chromite fractionationand magma mixing should lead to many fluctuations in melt composition,producing both massive and disseminated chromitites and phaselayering within individual podiform bodies observed in the Luobusaophiolite. KEY WORDS: boninitic magmas; dunite envelope; melt—rock interaction; MORB peridotities; podiform chromitites *Corresponding author. Present address: Department of Geology, Laurentian University, Sudbury, Ont, Canada P3E 2C6.  相似文献   

2.
KUBO  K. 《Journal of Petrology》2002,43(3):423-448
Dunite formation processes in highly depleted peridotites arediscussed based upon a detailed study of the Iwanaidake peridotite,Hokkaido, Japan, which consists mainly of harzburgite with asmall amount of dunite. In the harzburgites, the Mg# [= 100x Mg/(Mg + Fe2+)] of olivine ranges from 91·5 to 92·5,and the Cr# [= 100 x Cr/(Cr + Al)] of spinel from 30 to 70;in the dunites, the Mg# of olivine ranges from 92·5 to94 and the Cr# of spinel from 60 to 85, respectively. The NiOwt % of olivine in harzburgites ranges from 0·38 to 0·44,and in dunites from 0·35 to 0·37. The Mg# andCr# are higher and NiO wt % is lower in the dunites than inthe harzburgites surrounding the dunites. The Mg# and Cr# exhibitnormal depletion trends expected from simple partial melting,whereas the NiO wt % shows an abnormal trend. On the basis ofmass balance calculations, dunites are considered to be derivedfrom the harzburgites by a process involving incongruent meltingof orthopyroxene (orthopyroxene olivine + Si-rich melt). Hydrousconditions were necessary to lower the solidus, and thus meltingof harzburgite was probably triggered by the introduction ofhydrous silicate melt. The dunite in this massif may have formedin the mantle wedge above a subduction zone. KEY WORDS: depleted peridotite; hydrous melt; incongruent melting; residual dunite; Iwanaidake peridotite  相似文献   

3.
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  相似文献   

4.
The ultramafic tectonite of the Miyamori ophiolitic complex is divided into two types, one bearing aluminous spinel (Cr/(Cr+Al)< 0.4) and the other, chromian spinel(Cr/(Cr + Al)<0.4) (denoted ASPP and CSPP respectively). ASPP consists mainly of harzburgite and lherzolite and occurs as isolated kilometric patches in CSPP, which can be subdivided into massive and layered types. Massive CSPP consists mainly of magnesian harzburgite and dunite, whereas layered CSPP commonly is stratified and consists of less magnesian harzburgite, dunite, wehrlite, lherzolite, websterite, and clinopyroxenite. The 2 km thick layered CSPP occurs within the massive CSPP, and their lithologies are transitional. The structural and lithologic features of CSPP and the chemical variations of its olivine and spinel suggest that the layered CSPP crystallized from segregated partial melt, leaving the massive CSPP as a strongly depleted residue. Hornblende is invariably present in both the ASPP and CSPP, whereas phlogopite ispresent only in CSPP. The hornblende in CSPP is distinctly richer in K2O (0.4–1.0 wt%) than that in ASPP(<0.1 wt%), but residual peridotite of CSPP is more depleted in major elements than that of ASPP. The low TiO2/K2O ratio of hornblende and the presence of TiO2 poor phlogopite suggest that partial melting, melt segregation, and crystallization to form CSPP took place in the upper mantle beneath an island arc. By contrast, ASPP could be the source material of CSPP which formed as slightly depleted residue beneath a back-arc basin.  相似文献   

5.
阿尔巴尼亚布尔其泽纯橄岩壳非常新鲜,主要由橄榄石、尖晶石和单斜辉石等矿物组成.其中橄榄石存在单斜辉石和铬尖晶石（磁铁矿）共生包裹体现象,包裹体矿物粒度在1~10 μm,有些甚至为纳米级200~500 nm.纯橄岩橄榄石的Fo值为94.7~96.0,铬尖晶石的Cr#为76.5~82.4,远高于蛇绿岩地幔橄榄岩中常见纯橄岩的铬值（Cr#>60）.基于前人研究结果,提出这种现象是由于亏损方辉橄榄岩与含钛、铬、铁熔体发生交代作用,从而形成橄榄石的固溶体并存在Ti4+、Al3+、Ca2+、Fe3+,而部分Cr3+进入铬尖晶石结晶.后期由于岩体在抬升过程中降温,橄榄石中混溶的组分析出包裹体形成磁铁矿和铬尖晶石.并且依据铬尖晶石-橄榄石的矿物化学成分,识别出岩体内方辉橄榄岩相对较低的部分熔融程度约为30%~40%,纯橄岩部分熔融程度约为40%,表明不同岩相间其形成背景存在明显差异.因此,认为布尔奇泽蛇绿岩具有多阶段的过程,首先是在洋中脊环境下经历部分熔融作用形成了方辉橄榄岩,后受到俯冲环境（SSZ）的岩石-熔体反应生成更富Mg、Si和Cr等的熔体,致使地幔橄榄岩高度部分熔融,形成此类纯橄岩.   相似文献   

6.
Wadi Sifein podiform chromite deposits, Central Eastern Desert of Egypt, are hosted by fully serpentinized peridotite that is a part of the dismembered Pan‐African ophiolite complexes. Relics of primary minerals and the chemical characters indicate that the ophiolitic rocks were derived from depleted mantle peridotite of harzburgite and subordinate dunite compositions. The mantle rocks were initially formed at a mid‐oceanic ridge and subsequently thrust at a supra‐subduction zone. The chromite mineralization at Wadi Sifein area displays either pod‐shaped bodies with massive and lumpy chromitite appearance or dissemination of chromian‐spinel in serpentinite matrix. The podiform chromitite exhibits a very limited compositional range in terms of Cr# [Cr/(Cr + Al) atomic ratio] and Mg# [Mg/(Mg + Fe) atomic ratio]. The chromian‐spinel, however, frequently displays optical and geochemical zoning. Four zones can be identified from core to edge: inner core representing the original composition of the chromian‐spinel; narrow Cr‐rich ferritchromit zone; wide ferritchromit zone; and outer Cr‐magnetite/magnetite zone. The zonation of chromian‐spinel is interpreted to be a result of serpentinization rather than magmatic or metamorphic processes. The geochemical data obtained from the chromitite and chromian‐spinel was statistically processed using discriminant and R‐mode factor analyses. Two trends, minor and major, were achieved considering the formation of ferritchromit. The minor trend is controlled by the redistribution of trivalent cations, where Cr2O3 increased on the expense mainly of Al2O3 and to less extent Fe2O3 to form zone II during the peak of serpentinization. The major trend of alteration, however, is explained by the exchange between Mg‐Fe2+ rather than Cr, Al, and Fe3+ to form zone III. Kammererite formation was accompanied the formation of zones III and IV at a 314°C temperature of formation.  相似文献   

7.
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.  相似文献   

8.
西藏罗布莎不同类型铬铁矿的特征及成因模式讨论   总被引：6，自引：2，他引：4  
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。  相似文献   

9.
Phlogopite has been recognized for the first time in ultramaficxenoliths from the Canadian Cordillera. The phlogopite-bearingxenoliths are hosted in post-glacial basanitoid flows and ejectaof the Kostal Lake volcanic center, British Columbia. The xenolithassemblage consists of 60% cumulate-textured wehrlites, and40% coarse-textured lherzolites, harzburgites, dunites, andolivine websterites. The phlogopite occurs: (1) as sub-euhedral grains along grainboundaries in dunite and lherzolite xenoliths; or (2) alongorthopyroxene lamellae exsolved from intercumulus clinopyroxenein the wehrlite xenoliths; or (3) as grains hosted in 10–100pm diameter fluid inclusions in clinopyroxene of all xenoliths.The phlogopites do not show any reaction relationships withother phases in any of the xenoliths studied. Phlogopites ina given xenolith have Mg/Mg + Fe2+ similar to that of coexistingolivine, clinopyroxene, and orthopyroxene. The partitioningof Fe and Mg between phlogopite and coexisting olivine and clinopyroxeneis similar to that observed in other phlogopite-bearing mantlexenoliths, and in high-pressure melting experiments on rockswith similar bulk compositions. This indicates that the phlogopitesin xenoliths from Kostal Lake have equilibrated with these coexistingphases. The occurrence of phlogopites in fluid inclusions containingNa, K, Cl, P, and S, suggests that incompatible element-enrichedhydrous fluids/melts fluxed this part of the upper mantle beneatheastern British Columbia. Metasomatism of the upper mantle beneathKostal Lake probably occurred prior to Quaternary alkaline magmatism(7550–400 B.P.) and after the initial volcanism whichformed the wehrlite cumulates (3–5 Ma). Metasomatism causedoverall oxidation of the upper mantle beneath this area butwas not responsible for the anomalously Fe-rich nature of somexenoliths from the Kostal Lake eruptive center.  相似文献   

10.
The Wenquan ultramafic rocks, located in the East Kunlun Orogenic belt in the northeastern part of the Qinghai‐Tibet Plateau, consist of dunite, wehrlite, olivine‐clinopyroxenite and clinopyroxenite, and exhibit cumulate textures. Olivine from dunite has high Fo (forsterite, 90–92) and NiO (0.15–0.42 wt%) contents. Cr‐spinels from all of the rocks in this suite are characterized by high Cr# (100×[Cr/(Cr+Al)], 67–91), low Mg# (100×[Mg/(Mg+Fe2+)], 17–35) and low TiO2 contents (mostly < 0.5 wt%). Clinopyroxenes display high Mg# (92–98) and low TiO2 contents (0.002–0.099 wt%), similar to those in ophiolitic cumulates. Geochemically, the Wenquan ultramafic rocks show enrichment of LILE, Sr, and Ba, and depletion of Nb and Th. Mineral chemistry and geochemistry indicate that the Wenquan cumulates were generated from a depleted mantle and likely evolved from high‐Mg basaltic magmas (Mg#=78) that underwent fractional crystallization and crustal contamination. Zircons from clinopyroxenites yield a U–Pb weighted mean age of 331 ± 2 Ma, which is nearly coeval with the formation age of the Buqingshan and A'nyemaqen Carboniferous ophiolites. The Wenquan Carboniferous ophiolites are confirmed to exist in the Central East Kunlun Fault zone, whereas previous studies have considered them to be the Proterozoic ophiolites. The Wenquan ultramafic rocks might be an arc cumulates of the Paleo‐Tethyan ocean, indicating that there were two cycles of oceanic–continental evolution along the Central East Kunlun Fault zone. Keywords Ultramafic rock, Cumulate, Ophiolite, East Kunlun Orogenic belt.  相似文献   

11.
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.  相似文献   

12.
菲律宾迪纳加特岛汇洋镍铬矿区发育板块碰撞成因的蛇绿岩,主要岩性为纯橄岩和方辉橄榄岩,与铬铁矿化关系密切,在纯橄岩和方辉橄榄岩的过渡带中铬铁矿化较为集中,铬铁矿多呈薄层块状或浸染状与围岩岩性条带呈"互层状"产出,块状、浸染状的铬铁矿重复出现;矿区内原生铬铁矿化非常普遍,虽然未构成原生铬铁矿体,但为次生红土型铬铁矿的成矿提供了丰富的物质来源。矿区内第四系中赋存有残坡积红土型铬铁砂矿,并与同类型镍矿共生;铁质红土层是最佳的含矿层位,松散状红土层次之;铬铁矿体呈层状、似层状,有3个铬铁砂矿化区,矿体厚度5~7m,铬铁矿品位w(Cr2O3)=2.0%~4.1%。  相似文献   

13.
T. Agata 《Lithos》1994,33(4):241-263
The Asama igneous complex comprises layered mafic and ultramafic plutonic rocks exposed over about 500×6000 m in the Mikabu greenstone belt, Sambagawa metamorphic terrain of Mie Prefecture; its margins terminate by faults, and there is no trace of chilled rocks. The exposed layered sequence is about 460 m thick, and includes dunite, plagioclase wehrlite, olivine gabbro and two-pyroxene gabbro. The crystallization sequence of essential cumulus minerals is olivine, followed by plagioclase and clinopyroxene together, and finally the appearance of orthopyroxene. Olivine systematically varies in composition from Fo89 to Fo78 with stratigraphic height in the lower to middle portion of the layered sequence. The composition of clinopyroxene changes from Ca49Mg46Fe5 to Ca40Mg47Fe13 upward in the layered sequence; cumulus orthopyroxene, which occurs at the top of the exposed layered sequence, has a composition of Ca2Mg74Fe24. Cumulus chromite occurs as disseminated grains in peridotitic rocks, and tends to increase its Fe3+/(Cr+Al+Fe3+) ratio with stratigraphic height. The most aluminous chromite [Cr/(Cr+Al) = 0.48] occurs in dunite that crystallized shortly before plagioclase began to separate as an essential phase. The Cr/(Cr+Al) ratio of the most aluminous chromite, coupled with the crystallization order of essential minerals, suggests that the Asama parental magma was moderately enriched in plagioclase and clinopyroxene components in the normative mineral diagram plagioclase-clinopyroxene-orthopyroxene. It was similar to a Hawaiian tholeiite and different from the Bushveld and Great “Dyke” parental magmas that were more enriched in orthopyroxene component; it also differed from mid-oceanic ridge basalts that are more depleted in the orthopyroxene component. The Asama clinopyroxene and chromite show characteristically high TiO2 contents and are also similar to those in Hawaiian tholeiites. The Asama igneous complex probably resulted from the crystallization of a magma of a Hawaiian (oceanic-island) tholeiite composition and formed in an oceanic island regime.  相似文献   

14.
The Red Hills peridotite in the Dun Mountain ophiolite of SouthIsland, New Zealand, is assumed to have been produced in a paleo-mid-oceanridge tectonic setting. The peridotite is composed mostly ofharzburgite and dunite, which represent residual mantle andthe Moho transition zone (MTZ), respectively. Dunite channelswithin harzburgite blocks of various scales represent the MTZcomponent. Plagioclase- and clinopyroxene-bearing dunites occursporadically within common dunites. These dunites representproducts of melt–wall-rock interaction. Chondrite-normalizedrare earth element (REE) patterns of MTZ clinopyroxenes showa wide compositional range. Clinopyroxenes in plagioclase dunitesare extremely depleted in light REE (LREE) ([Lu/La]N >100),and are comparable with clinopyroxenes in abyssal peridotitesfrom normal mid-ocean ridges. Interstitial clinopyroxenes inthe common dunite have flatter patterns ([Lu/La]N 2) comparablewith those for dunite in the Oman ophiolite. Clinopyroxenesin the lower part of the residual mantle harzburgites are evenmore strongly depleted in LREE ([Lu/La]N = 100–1000) thanare mid-ocean ridge peridotites, and rival the most depletedabyssal clinopyroxenes reported from the Bouvet hotspot. Incontrast, those in the uppermost residual mantle harzburgiteand harzburgite blocks in the MTZ are less LREE depleted ([Lu/La]N= 10–100), and are similar to those in plagioclase dunite.Clinopyroxenes in the clinopyroxene dunite in the MTZ are similarto those reported from mid-ocean ridge basalt (MORB) cumulates,and clinopyroxenes in the gabbroic rocks have compositions similarto those reported from MORB. Strong LREE and middle REE (MREE)depletion in clinopyroxenes in the harzburgite suggests thatthe harzburgites are residues of two-stage fractional melting,which operated initially in the garnet field, and subsequentlycontinued in the spinel lherzolite field. The early stage meltingproduced the depleted harzburgite. The later stage melting wasresponsible for the gabbroic rocks and dunite. Strongly LREE–MREE-depletedclinopyroxene in the lower harzburgite and HREE-enriched clinopyroxenein the upper harzburgite and plagioclase dunite were formedby later reactive melt migration occurring in the harzburgite. KEY WORDS: clinopyroxene REE geochemistry; Dun Mountain ophiolite; Moho transition zone; orogenic peridotite; Red Hills  相似文献   

15.
鲁西莱芜晚中生代高镁闪长岩中含丰富的超镁铁质捕虏体,主要为纯橄岩(>80%),少量方辉橄榄岩(<5%)和橄榄辉石岩(<15%)。测定了代表性捕虏体的亲铁、亲铜元素含量,根据其矿物化学成分计算了捕虏体的氧化-还原状态。方辉橄榄岩遭受了不同程度富Si熔体的交代,全岩富含Cr、Co、Ni和IPGE,高IPGE/PPGE值,是地幔深度部分熔融的残留。纯橄岩由铬铁矿和高Mg橄榄石组成,全岩富含Cr、Co、Ni,贫IPGE,低IPGE/PPGE值,可能为一种镁铁质熔体的堆晶岩。纯橄岩中橄榄石成分表现出一定的变化范围,局部镁橄榄石（Fo）成分高达94,可能是堆晶中橄榄石与足够的铬铁矿反应的结果,原始熔体可能为玻镁安山质岩浆。方辉橄榄岩和纯橄岩都显示高fO2值,FMQ+1.4~+2.4,与研究区早古生代相对还原的陆下岩石圈地幔(fO2低于C-CO2缓冲反应)形成鲜明对比。数据表明,中生代扬子大陆和华北克拉通碰撞之前,特提斯大洋板块和扬子大陆边缘相继俯冲到华北克拉通东缘之下,导致当时岩石圈地幔的fO2陡升。  相似文献   

16.
We found fine-grained Fe-rich orthopyroxene-rich xenoliths (mainly orthopyroxenite) containing partially digested dunite fragments of Group I from Takashima, Southwest Japan. Orthopyroxenite veinlets, some of which contain plagioclase at the center, also replace olivine in dunite and wehrlite xenoliths of Group I. This shows high reactivity with respect to olivine of the melt involved in orthopyroxenite formation, indicating its high SiO2 activity. The secondary orthopyroxene of this type is characterized by low Mg# [= Mg/(Mg + total Fe) atomic ratio] (down to 0.73) and high Al2O3 contents (5–6 wt%). It is different in chemistry from other secondary orthopyroxenes found in peridotite xenoliths derived from the mantle wedge. Clinopyroxenes in the Fe-rich orthopyroxenite show a convex-upward REE pattern with a crest around Sm. This pattern is strikingly similar to that of clinopyroxenes of Group II pyroxenite xenoliths and of phenocrystal and xenocrystal clinopyroxenes, indicating involvement of similar alkali basaltic melts. The Fe-rich orthopyroxenite xenoliths from Takashima formed by reaction between evolved alkali basalt melt and mantle olivine; alkali basalt initially slightly undersaturated in silica might have evolved to silica-oversaturated compositions by fractional crystallization at high-pressure conditions. The Fe-rich orthopyroxenites occur as dikes within the uppermost mantle composed of dunite and wehrlite overlying pockets of Group II pyroxenites. The orthopyroxene-rich pyroxenites of this type are possibly common in the uppermost mantle beneath continental rift zones where alkali basalt magmas have been prevalent.  相似文献   

17.
The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt,and they share similar geological tectonic setting and age.Thus,an understanding of their origins is very important for discussion of the evolution of the Eastern Tethys Ocean.There is no complete ophiolite assemblage in the Zedang ophiolite.The Zedang ophiolite is mainly composed of mantle peridotite and a suite of volcanic rocks as well as siliceous rocks,with some blocks of olivinepyroxenite.The mantle peridotite mainly consists of Cpx-harzburgite,harzburgite,some lherzolite,and some dunite.A suite of volcanic rocks is mainly composed of caic-aikaline pyroclastic rocks and secondly of tholeiitic pillow lavas,basaltic andesites,and some boninitic rocks with a lower TiO2 content （TiO2 ＜ 0.6％）.The pyroclastic rocks have a LREE-enriched REE pattern and a LILE-enriched （compared to HFSE） spider diagram,demonstrating an island-arc origin.The tholeiitic volcanic rock has a LREE-depleted REE pattern and a LILE-depleted （compared to HFSE） spider diagram,indicative of an origin from MORB.The boninitic rock was generated from fore-arc extension.The Luobusa ophiolite consists of mantle peridotite and mafic-ultramaflc cumulate units,without dike swarms and volcanic rocks.The mantle peridotite mainly consists of dunite,harzburgite with low-Opx （Opx ＜ 25％）,and harzburgite （Opx ＞ 25％）,which can be divided into two facies belts.The upper is a dunite-harzburgite （Opx ＜ 25％） belt,containing many dunite lenses and a large-scale chromite deposit with high Cr203; the lower is a harzburgite （Opx ＞25％） belt with small amounts of dunite and lherzolite.The Luobusa mantle peridotite exhibits a distinctive vertical zonation of partial melting with high melting in the upper unit and low melting in the lower.Many mantle peridotites are highly depleted,with a characteristic U-shaped REE pattern peculiar to fore-arc peridotite.The Luobusa cumulates are composed of wehrlite and olivine-pyroxenite,of the P-P-G ophiolite series.This study indicates that the Luobusa ophiolite was formed in a fore-arc basin environment on the basis of the occurrence of highly depleted mantle peridotite,a high-Cr2O3 chromite deposit,and cumulates of the P-P-G ophiolite series.We conclude that the evolution of the Eastern Tethys Ocean involved three stages：the initial ocean stage （formation of MORB volcanic rock and dikes）,the forearc extension stage （formation of high-Cr203 chromite deposits and P-P-G cumulates）,and the islandarc stage （formation of caic-alkaline pyroclastic rocks）.  相似文献   

18.
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.  相似文献   

19.
北秦岭松树沟橄榄岩与铬铁矿矿床的成因关系   总被引：2，自引：1，他引：1  
李犇  朱赖民  弓虎军  郭波  杨涛  王飞  王伟  徐奥 《岩石学报》2010,26(5):1487-1502
松树沟橄榄岩体是秦岭造山带中规模最大的赋存铬铁矿床的超基性岩体。松树沟橄榄岩主要由细粒橄榄岩质糜棱岩和中粗粒橄榄岩组成。本文通过对松树沟橄榄岩的岩相学、主微量、稀土元素地球化学的系统研究,认为松树沟细粒方辉橄榄岩为洋脊扩张过程中地幔岩减压-近分离熔融产生的残留体,细粒纯橄岩主要由地幔橄榄岩熔融残留橄榄石、消耗辉石的减压熔融反应:aCpx+bOpx+cSpl=dOl+1Melt生成的橄榄石和少量的地幔方辉橄榄岩残留体组成,但均受到了后期渗滤熔体的再富集作用;中粗粒纯橄岩和方辉橄榄岩主要为上述反应产生的渗滤熔体被圈闭在迁移通道或减压扩容带内在热边界层(TBL)通过反应:MeltA=Ol+MeltB冷凝结晶而成,属堆晶橄榄岩。Pb-Sr-Nd同位素地球化学的证据显示,松树沟橄榄岩与基性岩具有共同的地幔源区,二者同为松树沟蛇绿岩的重要组成部分。通过矿床地质特征及铬铁矿电子探针测试研究,认为松树沟铬铁矿床是产于中粗粒堆晶纯橄岩中的层状铬铁矿床,形成于格林威尔期松树沟洋盆的扩张过程中,是中粗粒纯橄岩在热边界层(TBL)的冷凝结晶过程中岩浆分异作用的产物。  相似文献   

20.
Crystallization of Chromite and Chromium Solubility in Basaltic Melts   总被引：6，自引：3，他引：6  
The equilibrium between chromite and melt has been determinedon four basalts at temperatures of 1200–1400?C over arange of oxygen fugacity (fo2) and pressures of 1 atm and 10kb. The Cr content of chromite-saturated melts at 1300?C and1 atm ranges from 0?05 wt.% Cr2O3 at a log fo2= –3 to1?4 wt.% at a log fo2=–12?8. The Cr2+/Cr3+ of melt increaseswith decreasing fo2 and is estimated by assuming a constantpartitioning of Cr3+ between chromite and melt at constant temperature.The estimated values of Cr2+/Cr3+ in the melt are at fo2 valuesof 4–5 orders of magnitude lower than the equivalent Fe2+/Fe3+values. The Cr/(Cr+Al) of chromite coexisting with melt at constanttemperature changes little with variation of fo2 below log fo2=–6.Five experiments at 10 kb indicate that Cr2O3 dissolved in themelt is slightly higher and the Cr/(Cr + Al) of coexisting chromiteis slightly lower than experiments at 1 atm pressure. Thus variationin total pressure cannot explain the large variations of Cr/(Cr+ Al) that are common to mid-ocean ridge basalt (MORB) chromite. Experiments on a MORB at 1 atm at fo2 values close to fayalite-magnetite-quartz(FMQ) buffer showed that the Al2O3 content of melt is highlysensitive to the crystallization or melting of plagioclase,and consequently coexisting chromite shows a large change inCr/(Cr + Al). It would appear, therefore, that mixing of a MORBmagma containing plagioclase with a hotter MORB magma undersaturatedin plagioclase may give rise to the large range of Cr/(Cr +Al) observed in some MORB chromite.  相似文献   

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