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1.
The Big Jim complex is a concentrically zoned ultramafic to felsic plutonic complex which intruded the pelitic Chiwaukum schist. Most of the major plutonic rock types (from websterite through hornblendite, gabbronorite, hornblende gabbro and diorite, to granodiorite) enclose harzburgite and metaperidotite xenoliths similar to foliated metaperidotite lenses included in the Chiwaukum schist. The larger xenoliths preserve tectonite fabrics. All have Mg#'s (mole fraction MgO/(MgO+FeO*)) from 0.90 to 0.89, the same as those of Chiwaukum metaperidotites, and distinctly different from undeformed Big Jim dunite (Mg#'s 0.84 to 0.82) and websterite (0.82 to 0.78). Contact relations indicate widespread, stepwise replacement of harzburgite by pyroxenite, hornblendite, gabbro and diorite. Thermodynamic modelling using an expanded regular solution model for silicate liquids (Ghiorso 1985; Ghiorso and Carmichael 1985) predicts that reaction between olivine (Fo90) and a liquid with the composition of Big Jim diorite +1.5 wt% H2O, at 1,100° C and 3 kb, would produce websterite (Mg#'s 0.75 to 0.81) and dunite (0.79 to 0.82). This process is exothermic and results in a negative change in volume, since it increases total solid mass. Under conditions of decreasing temperature, modelled crystal fractionation with assimilation of olivine reproduces important features of the chemical variation observed in the Big Jim complex where crystal fractionation alone fails. The Big Jim complex has affinities with other ultramafic to felsic plutonic complexes such as the Bear Mountain complex (Snoke et al. 1981, 1982) and the Emigrant Gap complex (James 1971). The latter have wehrlite and clinopyroxenite, rather than websterite, but both have concentric zoning, with olivine-bearing rock types surrounded by successively more felsic pyroxenite, gabbro and diorite. In general, concentrically zoned complexes of this type may form where magma reacts with mantle-derived wall rock or ultramafic cumulates. Assimilation of peridotite in fractionating magma may be important in subduction-related magmatic arcs.  相似文献   

2.
Ultrabasic Xenoliths and Lava from the Lashaine Volcano, Northern Tanzania   总被引：1，自引：3，他引：1  
The Lashaine tuff-ring consists of carbonatite tuff and glassyscoria of ankaramitic composition. The pyroclastics encloseejected blocks of country-rock metamorphic rocks and a suiteof ultramafic blocks which are divisible into two groups. Thefirst group, characterized by xenomorphic granular textures,contains rocks comprising varying combinations of pyrope garnet,spinel, magnesian olivine and orthopyroxene, chromiferous diopside,and phlogopite. Analyses are given for garnet lherzolite, lherzolite,harzburgite, and wehrlite and their separate phases. The chemistryof the garnet lherzolite and its phases resembles that of garnetperidotite nodules in kimberlite diatremes, and the A12O2 contentand Ca/Ca+Mg ratio of the clinopyroxenes in the lherzolite andwehrlite indicate more affinities with those in mantle-derivedrocks rather than with peridotites derived by accumulation froma basaltic melt. The phlogopite in a mica garnet lherzolite,that otherwise resembles other mantle garnet peridotites, isan unusual variety containing > 9 per cent TiO2. The othergroup of ultramafic xenoliths, characterized by cumulate andidiomorphic textures, comprises pyroxenite, with or withoutolivine, mica and amphibole, and mica dunite. Analyses are givenfor a mica dunite and its separate phases. The pressure andtemperature of formation of the various rock-types are estimated,and the relationship of the rocks to each other and to the hostlava is discussed. The chemistry of the host lava is discussedin the light of current experimental data and also in relationto the Northern Tanzania volcanic province. The significanceof the presence of mica in the upper mantle is also discussed.  相似文献   

3.
Chromite-bearing peridotites of the Ordovician Miyamori ophiolitecomplex exhibit spatial mineralogical variations on scales rangingfrom several centimeters to a few kilometers. The largest variationscorrespond to the entire structure of the complex, which featuresa layered zone of interstratified harzburgite, wehrlite, andvarious pyroxenites sandwiched between zones of unlayered harzburgiteand dunite containing only minor pyroxenite bands. All zonesexhibit the same deformation microstructures, tabular equigranularto porphyroclastic textures, and strong mineral aggregate lineation.Harzburgite from the unlayered zones is characterized by olivinevalues of 100Mg/(Mg+Fe)=91–93.5 and chromite values of100Cr/(Cr+Al+Fe3+)=40–75. These variables exhibit a positivecorrelation, which is typical of harzburgites and lherzolitesfrom the basal units of ophiolites and from xenoliths in alkalibasalts and kimberlites. The harzburgite is therefore interpretedas a residue from partial melting in the mantle. By contrast,harzburgite in the interlayered zone features olivine valuesof 100Mg/(Mg+Fe)=88–92 and chromite values of 100Cr/(Cr+Al+Fe3+)=40–60,and in this case the variables tend to show a negative correlationin any given locality and they partly overlap data from theintercalated wehrlite and dunite. The harzburgite of the layeredzone is interpreted as residual mantle that reacted with evolvedmelts that then crystallized as wehrlite and dunite. The harzburgitein the unlayered zones is more refractory than that in the layeredzone, even after removing effects of reaction. This differencecan be explained either by enhanced partial melting and meltextraction in the unlayered zones, possibly owing to the preferentialintroduction of a waterrich fluid, or by melt segregation fromthe unlayered zones and transfer to the layered zone in responseto a piezometric pressure gradient and compaction of a solidresidual matrix. Mineralogical evidence suggests that evolvedmelts migrated through conduits formed in the layered zone byfracturing or diapirism.  相似文献   

4.
Hualalai Volcano, Hawaii, is best known for the abundant and varied xenoliths included in the historic 1800 Kaupulehu alkalic basalt flow. Xenoliths, which range in composition from dunite to anorthosite, are concentrated at 915-m elevation in the flow. Rare cumulate ultramafic xenoliths, which include websterite, olivine websterite, wehrlite, and clinopyroxenite, display complex pyroxene exsolution textures that indicate slow cooling. Websterite, olivine websterite, and one wehrlite are spinel-bearing orthopyroxene +olivine cumulates with intercumulus clinopyroxene +plagioclase. Two wehrlite samples and clinopyroxenite are spinel-bearing olivine cumulates with intercumulus clinopyroxene+orthopyroxene + plagioclase. Two-pyroxene geothermometry calculations, based on reconstructed pyroxene compositions, indicate that crystallization temperatures range from 1225° to 1350° C. Migration or unmixing of clinopyroxene and orthopyroxene stopped between 1045° and 1090° C. Comparisons of the abundance of K2O in plagioclase and the abundances of TiO2 and Fe2O3in spinel of xenoliths and mid-ocean ridge basalt, and a single 87Sr/ 86Sr determination, indicate that these Hualalai xenoliths are unrelated to mid-ocean ridge basalt. Similarity between the crystallization sequence of these xenoliths and the experimental crystallization sequence of a Hawaiian olivine tholeiite suggest that the parental magma of the xenoliths is Hualalai tholeiitic basalt. Xenoliths probably crystallized between about 4.5 and 9 kb. The 155°–230° C of cooling which took place over about 120 ka — the age of the youngest Hualalai tholeiitic basalt — yield maximum cooling rates of 1.3×10–3–1.91×10–3 °C/yr. Hualalai ultramafic xenoliths with exsolved pyroxenes crystallized from Hualalai tholeiitic basalt and accumulated in a magma reservoir located between 13 and 28 km below sealevel. We suspect that this reservoir occurs just below the base of the oceanic crust at about 19 km below sealevel.  相似文献   

5.
云南金宝山超镁铁岩原始岩浆成分反演   总被引：14，自引：7，他引：14  
金宝山岩体赋存有我国最大的铂族元素矿床,位于云南省弥渡县,大地构造位置处于杨子地台西缘南端。岩体是呈似层状(岩席)产出的小型超镁铁岩,侵位于泥盆系金宝山组白云岩、泥灰岩、砂板岩中,由橄榄岩、辉橄岩及微辉岩、辉石岩异离体组成,没有完整的相带分异,缺乏分异堆积特征。但岩体铂族元素成矿以铂钯为特征(Pd/Ir>10),硫化物Ni/Cu比值低(0.5～5),岩石稀土元素富集,配分型式为轻稀土富集型,体现基性岩浆衍生产物的岩石地球化学特点。元素含量的组合变化反映出橄榄石对岩石成分的控制作用,成岩母体可视为橄榄石与熔体两部分组成的混合体系,分析表明岩浆经历了11％的橄榄石结晶和22％左右的斜长石结晶分异,大部分熔体相携带斜长石先期离开岩浆房或被压滤出去,残余熔体携带橄榄石结晶颗粒在构造挤压作用下向上运移侵位成岩。根据岩浆演化过程反演,计算得到金宝山超镁铁岩原始岩浆组成,表明为低钛拉斑玄武岩岩浆,MgO含量为12.93％,其形成可能与裂谷作用早期的地幔热柱作用有关,原始岩浆起源于未亏损地幔的部分熔融。  相似文献   

6.
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   

7.
Near liquidus experiments on peridotite and other olivine normative compositions from 1.7 to 6 GPa confirm the applicability of exchange-based empirical models of Ni and Co partitioning between olivine and silicate liquids with compositions close to the liquidus of peridotite. Given that most estimates of lunar bulk composition are peridotitic, the partitioning models thus lend themselves to calculation of olivine compositions produced during the early stages of magma ocean crystallization. Calculation of olivine compositions produced by fractional crystallization of a model lunar magma ocean, initially 700 km deep, reveals a prominent maximum in Ni concentration versus fraction crystallized or Mg’ (molar MgO/(MgO + FeO)), but a pattern of monotonically increasing Co concentration. These patterns qualitatively match the puzzling patterns of Ni and Co concentrations observed in lunar rocks in which forsteritic olivines in magnesian suite cumulates have lower Ni and Co abundances than do less magnesian olivines from low-Ti mare basalts, and olivines from the ferroan anorthosite suite (FAS) have lower Ni, but similar Co to mare basalt olivines.The Ni and Co abundances in olivines from the magnesian suite cumulates can be reconciled in terms of fractional crystallization of a deep magma ocean which initially produces a basal dunite comprised of the hottest and most magnesian olivine overlain by an olivine-orthopyroxene (harzburgite) layer that is in turn overlain by an upper zone of plagioclase-bearing cumulates. The ultramafic portion of the cumulate pile overturns sending the denser harzburgite layer, which later becomes a portion of the green glass source region, to the bottom of the cumulate pile with Ni- and Co-rich olivine. Meanwhile, the less dense, but hottest, most magnesian olivines with much lower Ni and Co abundances are transported upward to the base of the plagioclase-bearing cumulates where subsequent heat transfer leads to melting of mixtures of primary dunite, norite, and gabbronorite with KREEP (a K-REE-P enriched component widely believed to be derived from the very latest stage magma ocean liquid). These hybrid melts have Al2O3, Ni, and Co abundances and Mg’ appropriate for parent magmas of the magnesian suite. Ni and Co abundances in the FAS are consistent with either direct crystallization from the magma ocean or crystallization of melts of primary dunite-norite mixtures without KREEP.  相似文献   

8.
A basanitoid dredged from near St. Paul's Rocks represents the first reported occurrence of an abyssal lava containing ultramafic xenoliths and high-pressure xenocrysts. The xenoliths include three separate populations. A suite of clinopyroxene-poor spinel-harzburgite and spinel-lherzolite inclusions have tectonite textures and highly refractory mineral compositions; these nodules probably represent residua from partial melting events unrelated to the host basanitoid. One harzburgite inclusion apparently accumulated from a tholeiitic magma and is also an accidental inclusion. Ultrabasic mylonites comprise a third xenolith group whose members are comparable to lithologies exposed on St. Paul's Rocks. A diverse xenocryst assemblage including labradoritebytownite, andesine, high-Ca pyroxene, low-Ca pyroxene, aluminous Cr-spinel, Cr-free aluminous spinel and Fe-Ti oxides, suggests at least two petro-genetic affinities. Fe-rich low-Ca pyroxene and Ca-rich plagioclase, together with the cumulate harzburgite xenolith, indicate the presence of a differentiated tholeiitic suite in the area. Most xenocrysts, including Ca-rich pyroxene, spinel and andesine have alkalic affinities consistent with crystallization at elevated pressure over a wide range in temperature. Both spinel populations are interpreted as cognate precipitates with Cr-free Al-spinels crystallizing above about 10 kb and aluminous Cr-spinels co-precipitating with plagioclase at lower pressures. Compared to the olivine-dominated, low-pressure phenocryst assemblage, extraction of the generally basaltic, aluminous clino-pyroxene-dominated assemblage at high pressure should have a minor effect on the major element differentiation of the host basanitoid magma.  相似文献   

9.
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   

10.
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   

11.
The G?ksun(Kahramanmaras)ophiolite(GKO),cropping out in a tectonic window bounded by the Malatya metamorphic unit on both the north and south,is located in the EW-trending lower nappe zone of the southeast Anatolian orogenic belt(Turkey).It exhibits a complete oceanic lithospheric section and overlies the Middle Eocene Maden Group/Complex with a tectonic contact at its base.The ophiolitic rocks and the tectonically overlying Malatya metamorphic(continental)unit were intruded by I-type calc-alkaline Late Cretaceous granitoid(~81-84 Ma).The ultramafic to cumulates in the GKO are represented by wehrlite,plagioclase wehrlite,olivine gabbro and gabbro.The crystallization order for the cumulate rocks is as follows:olivine±chromian spinel→clinopyroxene→plagioclase.The major and trace element geochemistry as well as the mineral chemistry of the ultramafic to mafic cumulate rocks suggest that the primary magma generating the GKO is compositionally similar to that observed in the modern island-arc tholeiitic sequences.The mineral chemistry of the ultramafic to mafic cumulates indicates that they were derived from a mantle source that was previously depleted by earlier partial melting events.The highly magnesian olivine(Fo77-83),clinopyroxene(Mg#of 82-90)and the highly Ca-plagioclase(An81-89)exhibit a close similarity to those,which formed in a supra-subduction zone(SSZ)setting.The field and the geochemical evidence suggest that the GKO formed as part of a much larger sheet of oceanic lithosphere,which accreted to the base of the Tauride active continental margin,including the ispendere,K?mürhan and the Guleman ophiolites.The latter were contemporaneous and genetically/tectonically related within the same SSZ setting during the closure of the Neotethyan oceanic basin(Berit Ocean)between the Taurides to the north and the Bitlis-Pütürge massif to the south during the Late Cretaceous.  相似文献   

12.
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   

13.
Summary ?Gabbro Akarem is a Late-Precambrian concentrically-zoned mafic-ultramafic intrusion located along a major fracture zone trending NE-SW in the Eastern Desert of Egypt. It intruded low-grade metasedimentary rocks, and has a contact metamorphic aureole a few meters wide. This intrusion comprises a dunite core enveloped by clinopyroxene hornblende-bearing lherzolite, olivine-hornblende clinopyroxenite and plagioclase hornblendite. The contacts between the rock types are gradational. They have cumulate textures and the observed crystallization sequence is: olivine ( + cotectic spinel)-orthopyroxene (Opx)-clinopyroxene (Cpx)-hornblende. Mafic minerals from the core of the intrusion are highly magnesian, a consistent increase in the Mg# of olivine (from 69 to 87), Opx (from 62 to 89), Cpx (from 85 to 96) and hornblends (from 62 to 88) is observed from the mafic to the ultramafic units. Spinel has a wide range of Cr# and Mg# ratios. The various rock units define a fractionation trend. The mafic rocks are slightly LREE-enriched relative to the ultramafic units and chondrites. In many aspects, the Gabbro Akarem intrusion is similar to Alaskan-type complexes. Mineralogical and geochemical data suggest that the different rock units were fractionated from a hydrous picritic magma with no apparent crustal contamination. A petrogenetic model involving a rapid rise of hydrous mantle magma along a major fracture zone is proposed. Extensive fractional crystallization led to magma chamber stratification; internal circulation and strong vertical stretching up the center of the rapidly rising diapir increased the rate of magma ascent towards the core. Due to cooling and high viscosity the marginal mafic magma was partly crystallized while the unsolidified core ultramafic magma continued its ascent. As a result, different mineral phases crystallized at different pressure-temperature paths. Field relations, geophysical, petrological and experimental studies support this model which explains many of the characteristics of the Gabbro Akarem and some other concentrically zoned mafic-ultramafic intrusions. Received April 24, 2001; revised version accepted November 20, 2001  相似文献   

14.
The Marum ophiolite complex in northern Papua New Guinea includesa thick (3–4 km) sequence of ultramafic and mafic cumulates,which are layered on a gross scale from dunite at the base upwardsthrough wehrlite, lherzolite, plagioclase lherzolite, pyroxenite,olivine norite-gabbro and norite-gabbro to anorthositic gabbroand ferrogabbro at the top. Igneous layering and structures,and cumulus textures indicate an origin by magmatic crystallizationin a large magma chamber(s) from magma(s) of evolving composition.Most rocks however show textural and mineralogical evidenceof subsolidus re-equilibration. The cumulate sequence is olivine and chrome spinel followedby clinopyroxene, orthopyroxene and plagioclase, and the layeredsequence is similar to that of the Troodos and Papuan ophiolites.These sequences differ from ophiolites such as Vourinos by thepresence of cumulus magnesian orthopyroxene, and are not consistentwith accumulation of low pressure liquidus phases of mid-oceanridge-type olivine tholeiite basalts. The cumulus phases show cryptic variation from Mg- and Ca-richearly cumulates to lower temperature end-members, e.g. olivineMg93–78, plagioclase An94–63. Co-existing pyroxenesdefine a high temperature solidus with a narrower miscibilitygap than that of pyroxenes from stratiform intrusions. Re-equilibratedpyroxene pairs define a low-temperature, subsolidus solvus.Various geothermometers and geobarometers, together with thermodynamiccalculations involving silica buffers, suggest the pyroxene-bearingcumulates crystallized at 1200 °C and 1–2 kb pressureunder low fO2. The underlying dunites and chromitites crystallizedat higher temperature, 1300–1350 °C. The bulk of thecumulates have re-equilibrated under subsolidus conditions:co-existing pyroxenes record equilibration temperatures of 850–900°C whereas olivine-spinel and magnetite-ilmenite pairs indicatefinal equilibration at very low temperatures (600 °C). Magmas parental to the cumulate sequence are considered to havebeen of magnesian olivine-poor tholeiite composition (>50per cent SiO2, 15 per cent MgO, 100 Mg/(Mg + Fe2+) 78) richin Ni and Cr, and poor in TiO2 and alkalies. Fractionated examplesof this magma type occur at a number of other ophiolites withsimilar cumulate sequences. Experimental studies show that suchlavas may result from ial melting of depleted mantle lherzoliteat shallow depth. The tectonic environment in which the complexformed might have been either a mid-ocean ridge or a back-arebasin.  相似文献   

15.
The Chilas Complex in the Kohistan Terrane, Pakistan, is a huge basic intrusion, about 300 km long and up to 40 km wide, which is regarded as tilted island-arc type crust. It has been interpreted as the magma chamber root zone of the Kohistan Island Arc. The Chilas Complex is composed mainly of gabbronorite (main facies) and several masses of ultramafic–mafic–anorthosite (UMA) association. The UMA association consists mainly of olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite), with minor amount of pyroxene-dominant cumulate (clinopyroxenite, websterite).The major element geochemistry of the gabbronorite (main facies) and rocks of the UMA association, plotted on Harker diagrams, are explained by a cumulate and a non-cumulate model, respectively. Namely, the UMA association is explained as variable crystal cumulates from a primary magma and the gabbronorite of the main facies is explained as due to the fractionation of the residual melt. Chemical variations of major, trace and rare earth elements for the gabbronorite of the main facies in the Chilas Complex are explained by fractional crystallization and accumulation of plagioclase, orthopyroxene and clinopyroxene from the residual melt of the primary magma.  相似文献   

16.
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.  相似文献   

17.
 Mafic and ultramafic rocks sampled in the Garrett transform fault at 13°28′S on the East Pacific Rise (EPR) provide insight on magmatic processes occurring under a fast-spreading ridge system. Serpentinized harzburgite from Garrett have modal, mineral and bulk chemical compositions consistent with being mantle residue of a high degree of partial melting. Along with other EPR localities (Terevaka transform fault and Hess Deep), these harzburgites are among the most residual and depleted in magmatophile elements of the entire mid-ocean ridge system. Geothermometric calculations using olivine-spinel pairs indicate a mean temperature of 759 ± 25 °C for Garrett residual harzburgite similar to the average of 755 °C for tectonite peridotites from slow-spreading ridges. Results of this study show that mid-ocean ridge peridotites are subject to both fractional melting and metasomatic processes. Evidence for mantle metasomatism is ubiquitous in harzburgite and is likely widespread in the entire Garrett peridotite massif. Magma-harzburgite interactions are very well preserved as pyroxenite lenses, plagioclase dunite pockets or dunitic wall rock to intrusive gabbros. Abundant gabbroic rocks are found as intrusive pockets and dikes in harzburgite and have been injected in the following sequence: olivine-gabbro, gabbro, gabbronorite, and ferrogabbro. The wide variety of magmas that crystallized into gabbros contrast sharply with present-day intratransform basalts, which have a highly primitive composition. Ferrogabbro dikes have been intruded at the ridge-transform intersection and as they represent the last event of a succession of gabbros intrusive into the peridotite, they likely constrain the origin of the entire peridotite massif to the same location. In peridotite massifs from Pacific transform faults (Garrett and Terevaka), primitive to fractionated basaltic magmas have flowed and crystallized variable amounts of dunite (±plagioclase) and minor pyroxenite, followed by a succession of cumulate gabbroic dikes which have extensively intruded and modified the host harzburgitic rocks. The lithosphere and style of magmatic activity within a fast-slipping transform fault (outcrops of ultramafic massif, discontinuous gabbro pockets intrusive in peridotite, magnesian and phyric basalts) are more analogous to slow-spreading Mid-Atlantic Ridge type than the East Pacific Rise. Received: 13 October 1997 / Accepted: 5 February 1999  相似文献   

18.
Two picrite flows from the SW rift zone of Mauna Loa containxenoliths of dunite, harzburgite, lherzolite, plagioclase-bearinglherzolite and harzburgite, troctolite, gabbro, olivine gabbro,and gabbronorite. Textures and olivine compositions precludea mantle source for the xenoliths, and rare earth element concentrationsof xenoliths and clinopyroxene indicate that the xenolith sourceis not old oceanic crust, but rather a Hawaiian, tholeiitic-stagemagma. Pyroxene compositions, phase assemblages and texturalrelationships in xenoliths indicate at least two different crystallizationsequences. Calculations using the pMELTS algorithm show thatthe two sequences result from crystallization of primitive MaunaLoa magmas at 6 kbar and 2 kbar. Independent calculations ofolivine Ni–Fo compositional variability in the plagioclase-bearingxenoliths over these crystallization sequences are consistentwith observed olivine compositional variability. Two parentsof similar bulk composition, but which vary in Ni content, arenecessary to explain the olivine compositional variability inthe dunite and plagioclase-free peridotitic xenoliths. Xenolithsprobably crystallized in a small magma storage area beneaththe rift zone, rather than the large sub-caldera magma reservoir.Primitive, picritic magmas are introduced to isolated rift zonestorage areas during periods of high magma flux. Subsequenteruptions reoccupy these areas, and entrain and transport xenolithsto the surface. KEY WORDS: xenolith; Hawaii; volcano plumbing; mineral composition; picrite  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.
鄂西黄陵背斜南部元古宙庙湾蛇绿岩的发现及其构造意义   总被引：2，自引：1，他引：1  
对鄂西黄陵背斜南部宜昌太平溪、邓村一带崆岭岩群中的元古宙庙湾岩组强烈变形变质超镁铁—镁铁质岩的研究表明,镁铁质岩主要为似层状细粒斜长角闪岩,变辉长岩岩体、岩脉及辉绿岩岩脉,超镁铁质岩则主要为蛇纹石化纯橄榄岩、方辉橄榄岩,呈构造岩片、岩块分布于斜长角闪岩之中。细粒斜长角闪岩TiO2=1.14%～1.48%,稀土元素配分型式为略亏损—平坦型,无明显的Eu异常,(La/Yb)N=0.87～1.12,La/Nb、Ce/Zr、Zr/Nb、Zr/Y、Ti/Y平均值分别为1.04、0.15、18.78、2.53、290.51,Nb/Th平均为9.88,显示为大洋中脊构造环境形成的N-MORB型拉斑玄武岩;变辉长岩具典型的堆晶结构特征,稀土元素配分型式为平坦型,具明显的Eu正异常;蛇纹石化纯橄榄岩的稀土元素配分型式具中稀土元素略亏损的U形特征,显示为LREE略富集的地幔岩。上述特征表明,黄陵背斜南部崆岭岩群中的元古宙庙湾岩组实际上是一套混杂堆积的古大洋蛇绿岩残片。元古宙庙湾蛇绿岩的发现为华南扬子克拉通存在中元古代洋盆和哥伦比亚超大陆聚合、裂解构造事件提供了重要的证据。  相似文献   

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