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1.
早中生代的扬子大陆向华北大陆的深俯冲碰撞作用以及中、新生代的华北东部岩石圈减薄作用是国际大陆动力学问题研究的两个热点。然而,把它们有机联系起来,探讨深部岩石圈演化动力学过程的研究还很少。报道了中国大陆科学钻探先导孔(CCSD-PP1)橄榄岩的矿物岩石化学分析结果。这些橄榄岩亏损玄武质组分(如低w(CaO+Al2O3)、高Mg#等),并经历了中元古代来自软流圈的碳酸岩熔体的交代作用和早中生代的超高压再平衡过程。结合已发表的华北捕虏体橄榄岩资料,对这一橄榄岩的原始地幔属性进行了分析。发现CCSD-PP1橄榄岩与华北古老岩石圈地幔组成相似,是早中生代来自华北岩石圈的构造侵入体。早中生代华北岩石圈的伸展减薄与苏鲁超高压变质岩石的折返提供了早期软流圈物质上涌的构造体制。  相似文献   

2.
Upper-mantle xenoliths in volcanic pipes cutting the axis ofthe Sierra Nevada batholith contain predominantly spinel-bearingperidotites (with sporadic garnet) and garnet websterites. Inspite of the enormous thickness of the Sierran crust, the Sierranupper mantle has not attained the garnet peridotite stabilityfield. The peridotites have forsteritic (Fo88–92) olivines,Cr-diopsides, Cr-spinels, and magnesian orthopyroxenes (En88–92).Their texture and compositional characteristics of the coexistingphases indicate that these are fragments of the upper mantlethat had undergone various degrees of partial fusion. The Pconditions of reequilibration and mineralogical characteristicssuggest that the partial fusion was accompanied by diapiricuprise. The REE distribution patterns are nearly chondritic.Garnet websterite xenoliths also contain magnesian and Cr-richphases. Their bulk chemical compositions are like pyroxenitecumulates. The garnet websterites from Big Creek differ fromthose occurring at Pick and Shovel in having more Fe-rich phasesand occasional hydrous minerals. The Pick and Shovel garnetwebsterites are interpreted to be pyroxene-rich, garnet-freecumulates formed by fractional crystallization of melts generatedby partial melting of subcontinental lithosphere at depth 60km. The REE abundance of these xenoliths is consistent withthis mode of origin. Presence of jadeitic clinopyroxenes andF-rich phlogopites, and the LREE- and 87Sr/86Sr-enriched characterof the garnet websterites from Big Creek may suggest their originas metasomatized upper-mantle garnet peridotites. The latestP-T conditions of equilibration of all garnet-bearing samplesshow that they lie along a nearly adiabatic gradient in therange of 900–1000 C and 18–32 kbar. An isotopically heterogeneous, old (1 b.y.) subcontinental lithosphere,characterized by high 87Sr/86Sr (0.7044–0.7082), radiogenic206Pb/204Pb (18.86–20.04), 207Pb/204Pb (15.64–15.69)and 208Pb/204Pb (38.69–39.11), and moderate 143Nd/144Nd(0.51234–0.51260; ENd–0.35 to –5.8) is consideredto be the source of these rocks. There was a fluid influx froma subducted slab carrying Ba, K, Rb, U, Th, and radiogenic Pbinto the overlying ancient lithosphere.  相似文献   

3.
Petrology and Geochemistry of Mantle Peridotite Xenoliths from SE China   总被引：11，自引：2，他引：9  
Geochemical data on Type I spinel peridotite and garnet peridotitexenoliths in Cenozoic basalts from SE China demonstrate thatthe lithospheric mantle under this region is heterogeneous.The depletion and enrichment shown by these peridotite xenolithsare not related to their locations as suggested earlier. Samplesfrom individual localities, at the continental margin or thecontinental interior, show large variational ranges from depletedharzburgite to fertile Iherzolite. The measured Nd and Sr isotopiccompositions of clinopyroxene separates range from Nd 49 to160 and from 87Sr/86Sr 070256 to 070407, respectively. Thedepleted signatures of Sr and Nd isotopic compositions and major-elementcontents (low CaO and Al2O3 in most xenoliths require an olddepletion event, probably mid-Proterozftic, and the enrichmentof LREE in the depleted peridotites implies a young metasomaticevent shortly before Cenozoic magmatism. Major-element compositionsof the peridotite xenoliths are controlled largely by the degreeof partial melting, and the extra fertile peridotites (highCaO and Al2O3) are probably the products of interaction betweenperidotites and a basaltic component. The equilibrium P–Tconditions, determined from coexisting mineral phases, indicatethat these xenoliths equilibrated over a wide P–T range,from 770 to 1250 C and from 10 to 27 kbar. Calculated oxygenfugacities for most spinel peridotites range from near the FMQbuffer to 25 log units below. The late-stage metasomatism didnot change the redox state in the upper mantle. *Corraponding author  相似文献   

4.
Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO2 = 43.7–45.7 wt.per cent, A12O3 = 1.6O–8.21 wt. per cent, CaO = 0.70–8.12wt. per cent, alk = 0.10–0.90 wt. per cent and Mg/(Mg+Fe2+)= 0.94–0.85) have been investigated in the hypersolidusregion from 800? to 1250?C with variable activities of H2O,CO2, and H2. The vapor-saturated peridotite solidi are 50–200?Cbelow those previously published. The temperature of the beginningof melting of peridotite decreases markedly with decreasingMg/(Mg+SFe) of the starting material at constant CaO/Al2O3.Conversely, lowering CaO/Al2O3 reduces the temperature at constantMg/(Mg+Fe) of the starting material. Temperature differencesbetween the solidi up to 200?C are observed. All solidi displaya temperature minimum reflecting the appearance of garnet. Thisminimum shifts to lower pressure with decreasing Mg/(Mg + Fe)of the starting material. The temperature of the beginning ofmelting decreases isobarically as approximately a linear functionof the mol fraction of H2O in the vapor (XH2Ov). The data alsoshow that some CO2 may dissolve in silicate melts formed bypartial melting of peridotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H2O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or co-exist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable aHjo conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. Itis suggested that komatiite in Precambrian terrane could formby direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of XH2Ov = 0.5–0.25 (XCO2v= 0.5–0.75). Such activities of H2O result in meltingat depths ranging between 125 and 175 km in the mantle. Thisrange is within the minimum depth generally accepted for theformation of kimberlite.  相似文献   

5.
大别山碧溪岭和山东荣成分别发育壳源和幔源的石榴二辉橄榄岩。它们在扬子板块向华北板块的俯冲过程中与俯冲板片一起经历了超高压变质作用。这两种原岩来源不同的石榴橄榄岩中的橄榄石具有相似的结晶学优选方位(LPO):[100]轴主板密垂直于面理面,[010]轴主极密平行于线理,这种橄榄石结晶学优选方位明显不同于高温地幔橄榄岩包体中的橄榄石组构,也不同于最近在超高压变质地体中发现的水导致的橄榄石组构。我们认为碧溪岭和荣成石榴二辉橄榄岩中的这种橄榄石组构可能是在相对较干的超高压变质条件下形成的。本研究为大陆深俯冲超高压变质石榴橄榄岩橄榄石组构特征提供了第一手资料。  相似文献   

6.
刘利双  刘福来  王伟 《岩石学报》2017,33(9):2899-2924
苏鲁超高压变质带是扬子板块与华北板块在三叠纪俯冲-碰撞的产物。变基性岩是苏鲁超高压变质带内出露最广泛的岩石类型之一,研究其岩石学、年代学、地球化学属性及成因机制,对于揭示扬子板块与华北板块之间的俯冲-碰撞-折返的动力学过程具有重要的科学意义。以(退变)榴辉岩为代表的超高压变质岩石广泛出露在威海-荣成一带,少量出露在乳山地区。锆石LA-ICP-MS U-Pb定年结果显示,(退变)榴辉岩的原岩时代为792~760Ma,峰期榴辉岩相变质时代为243~226Ma,后期角闪岩相退变质时代为221~207Ma。非榴辉岩相变质的基性岩(麻粒岩和斜长角闪岩)主要出露在乳山地区,其原岩形成时代应不晚于古元古代(1939Ma),峰期麻粒岩相变质时代为1895~1870Ma,后期角闪岩相退变质时代为1848~1806Ma,与胶北地体变基性岩的原岩时代和变质时代十分相似。全岩地球化学研究结果表明,(退变)榴辉岩的原岩显示高Fe拉斑玄武岩的特点,根据其稀土和微量元素特征,可将(退变)榴辉岩进一步划分为A、B和C三组。在球粒陨石标准化稀土配分模式和原始地幔均一化蛛网图解上,A、B和C三组样品分别具有轻稀土弱亏损、轻稀土弱富集和轻稀土富集的特点。轻稀土富集或弱富集型(退变)榴辉岩的原岩地球化学性质与岛弧或大陆玄武岩相似,它们的源区可能与深部富集地幔或受流体交代的地幔楔存在密切的成因关系;而轻稀土亏损型(退变)榴辉岩的原岩可能来自于亏损地幔的部分熔融。由此可见,(退变)榴辉岩的原岩具有成因多样性的特点。乳山地区的基性麻粒岩和斜长角闪岩的原岩也具有高Fe拉斑玄武岩的地球化学属性,Al2O3与Mg O呈正相关变化,TiO_2、P_2O_5与MgO表现出一定程度的负相关性。绝大多数非榴辉岩相变质基性岩的球粒陨石标准化稀土配分模式和原始地幔均一化蛛网配分曲线具有微右倾或明显右倾的特点。上述特征表明,研究区绝大多数非榴辉岩相变质的基性岩原岩来自于富集地幔,少数来自于原始地幔或亏损地幔,并经历了斜长石和辉石的分离结晶以及不同程度的部分熔融过程。由此可见,乳山地区出露的非超高压变质基性岩的原岩具有与胶北地体(高压)基性麻粒岩相近的成因特点。岩石学、同位素年代学和地球化学特征的综合对比研究结果表明,在苏鲁超高压变质带东北端的威海-荣成-乳山地区,既存在与华北板块古老变质基底相关的变基性岩,也存在与华南板块北缘新元古代变质基底相关的超高压榴辉岩,表明三叠纪时期华北板块东南缘胶北地体的部分古老变质基底曾卷入到扬子板块与华北板块之间的俯冲-碰撞造山过程,随后与超高压岩石一起抬升折返,形成当今的构造混杂岩带。  相似文献   

7.
The Tertiary to Recent basalts of Victoria and Tasmania havemineralogical and major element characteristics of magmas encompassingthe range from quartz tholeiites to olivine melilitites. Abundancesof trace elements such as incompatible elements, including therare earth elements (REE), and the compatible elements Ni, Coand Sc, vary systematically through this compositional spectrum.On the basis of included mantle xenoliths, appropriate 100 Mg/Mg+ Fe+2 (68–72) and high Ni contents many of these basaltsrepresent primary magmas (i.e., unmodified partial melts ofmantle peridotite). For fractionated basalts we have derivedmodel primary magma compositions by estimating the compositionalchanges caused by fractional crystallization of olivine andpyroxene at low or moderate pressure. A pyrolite model mantlecomposition has been used to establish and evaluate partialmelting models for these primary magmas. By definition and experimentaltesting the specific pyrolite composition yields parental olivinetholeiite magma similar to that of KilaeauIki, Hawaii (1959–60)and residual harzburgite by 33 per cent melting. It is shownthat a source pyrolite composition differing only in having0.3–0.4 per cent TiO2 rather than 0.7 per cent TiO2, isable to yield the spectrum of primary basalts for the Victorian-Tasmanianprovince by 4 per cent to 25 per cent partial melting. The mineralogiesof residual peridotites are consistent with known liquidus phaserelationships of the primary magmas at high pressures and thechemical compositions of residual peridotite are similar tonatural depleted or refractory lherzolites and harzburgites.For low degrees of melting the nature of the liquid and of theresidual peridotite are sensitively dependent on the contentof H2O, CO2 and the CO2/H2O in the source pyrolite. The melting models have been tested for their ability to accountfor the minor and trace element, particularly the distinctivelyfractionated REE, contents of the primary magmas. A single sourcepyrolite composition can yield the observed minor and traceelement abundances (within at most a factor of 2 and commonlymuch closer) for olivine melilitite (4–6 per cent melt),olivine nephelinite, basanite (5–7 per cent melt), alkaliolivine basalt (11–15 per cent melt), olivine basalt andolivine tholeiite (20–25 per cent melt) provided thatthe source pyrolite was already enriched in strongly incompatibleelements (Ba, Sr, Th, U, LREE) at 6–9 x chondritic abundancesand less enriched (2.5–3 x chondrites) in moderately incompatible(Ti, Zr, Hf, Y, HREE) prior to the partial melting event. Thesources regions for S.E. Australian basalts are similar to thosefor oceanic island basalts (Hawaii, Comores, Iceland, Azores)or for continental and rift-valley basaltic provinces and verydifferent in trace element abundances from the model sourceregions for most mid-ocean ridge basalts. We infer that thismantle heterogeneity has resulted from migration within theupper mantle (LVZ or below the LVZ) of a melt or fluid (H2O,CO2-enriched) with incompatible element concentrations similarto those of olivine melilitite, kimberlite or carbonatite. Asa result of this migration, some mantle regions are enrichedin incompatible elements and other areas are depleted. Although it is possible, within the general framework of a lherzolitesource composition, to derive the basanites, olivine nephelinitesand olivine melilitites from a source rock with chondritic relativeREE abundances at 2–5 x chondritic levels, these modelsrequire extremely small degrees of melting (0.4 per cent forolivine melilitite to 1 per cent for basanite). Furthermore,it is not possible to derive the olivine tholeiite magmas fromsource regions with chondritic relative REE abundances withoutconflicting with major element and experimental petrology argumentsrequiring high degrees (15 per cent) of melting and the absenceof residual garnet. If these arguments are disregarded, andpartial melting models are constrained to source regions withchondritic relative REE abundances, then magmas from olivinemelilitites to olivine tholeiites can be modelled if degreesof melting are sufficiently small, e.g., 7 per cent meltingfor olivine tholeiite. However, the source regions must be heterogenousfrom 1 to 5 x chondritic in absolute REE abundances and heterogerieousin other trace elements as well. This model is rejected in favorof the model requiring variation in degree of melting from 4per cent to 25 per cent and mantle source regions ranging fromLREE-enriched to LREE-depleted relative to chondritic REE abundances.  相似文献   

8.
The Baikal Rift is a zone of active lithospheric extension adjacentto the Siberian Craton. The 6–16 Myr old Vitim VolcanicField (VVF) lies approximately 200 km east of the rift axisand consists of 5000 km3 of melanephelinites, basanites, alkaliand tholeiitic basalts, and minor nephelinites. In the volcanicpile, 142 drill core samples were used to study temporal andspatial variations. Variations in major element abundances (e.g.MgO = 3·3–14·6 wt %) reflect polybaric fractionalcrystallization of olivine, clinopyroxene and plagioclase. 87Sr/86Sri(0·7039–0·7049), 143Nd/144Ndi (0·5127–0·5129)and 176Hf/177Hfi (0·2829–0·2830) ratiosare similar to those for ocean island basalts and suggest thatthe magmas have not assimilated significant amounts of continentalcrust. Variable degrees of partial melting appear to be responsiblefor differences in Na2O, P2O5, K2O and incompatible trace elementabundances in the most primitive (high-MgO) magmas. Fractionatedheavy rare earth element (HREE) ratios (e.g. [Gd/Lu]n > 2·5)indicate that the parental magmas of the Vitim lavas were predominantlygenerated within the garnet stability field. Forward major elementand REE inversion models suggest that the tholeiitic and alkalibasalts were generated by decompression melting of a fertileperidotite source within the convecting mantle beneath Vitim.Ba/Sr ratios and negative K anomalies in normalized multi-elementplots suggest that phlogopite was a residual mantle phase duringthe genesis of the nephelinites and basanites. Relatively highlight REE (LREE) abundances in the silica-undersaturated meltsrequire a metasomatically enriched lithospheric mantle source.Results of forward major element modelling suggest that meltingof phlogopite-bearing pyroxenite veins could explain the majorelement composition of these melts. In support of this, pyroxenitexenoliths have been found in the VVF. High Cenozoic mantle potentialtemperatures (1450°C) predicted from geochemical modellingsuggest the presence of a mantle plume beneath the Baikal RiftZone. KEY WORDS: Baikal Rift; mafic magmatism; mantle plume; metasomatism; partial melting  相似文献   

9.
A suite of dolerite dykes from the Ahlmannryggen region of westernDronning Maud Land (Antarctica) forms part of the much moreextensive Karoo igneous province of southern Africa. The dykecompositions include both low- and high-Ti magma types, includingpicrites and ferropicrites. New 40Ar/39Ar age determinationsfor the Ahlmannryggen intrusions indicate two ages of emplacementat 178 and 190 Ma. Four geochemical groups of dykes have beenidentified in the Ahlmannryggen region based on analyses of60 dykes. The groups are defined on the basis of whole-rockTiO2 and Zr contents, and reinforced by rare earth element (REE),87Sr/86Sr and 143Nd/144Nd isotope data. Group 1 were intrudedat 190 Ma and have low TiO2 and Zr contents and a significantArchaean crustal component, but also evidence of hydrothermalalteration. Group 2 dykes were intruded at 178 Ma; they havelow to moderate TiO2 and Zr contents and are interpreted tobe the result of mixing of melts derived from an isotopicallydepleted source with small melt fractions of an enriched lithosphericmantle source. Group 3 dyke were intruded at 190 Ma and formthe most distinct magma group; these are largely picritic withsuperficially mid-ocean ridge basalt (MORB)-like chemistry (flatREE patterns, 87Sr/86Sri 0·7035, Ndi 9). However, theyhave very high TiO2 (4 wt %) and Zr (500 ppm) contents, whichis not consistent with melting of MORB-source mantle. The Group3 magmas are inferred to be derived by partial melting of astrongly depleted mantle source in the garnet stability field.This group includes several high Mg–Fe dykes (ferropicrites),which are interpreted as high-temperature melts. Some Group3 dykes also show evidence of contamination by continental crust.Group 4 dykes are low-K picrites intruded at 178 Ma; they havevery high TiO2–Zr contents and are the most enriched magmagroup of the Karoo–Antarctic province, with ocean-islandbasalt (OIB)-like chemistry. Dykes of Group 1 and Group 3 aresub-parallel (ENE–WSW) and both groups were emplaced at190 Ma in response to the same regional stress field, whichhad changed by 178 Ma, when Group 2 and Group 4 dykes were intrudedalong a dominantly NNE–SSW strike. KEY WORDS: flood basalt; depleted mantle; enriched mantle; Ahlmannryggen; Karoo dyke  相似文献   

10.
This comment addresses the interpretation of oxygen fugacitydata for spinel peridotite xenoliths from five Mexican volcanicfields presented by Luhr & Aranda-Gomez (Journal of Petrology,38, 1075–1112, 1997). The postulated east–west increaseof the FMQ (‘relative oxygen fugacity’, where FMQis fayalite–magnetite–quartz) values is inherentto the method and therefore of questionable geological significance.Increases in FMQ do not necessarily mirror oxidation processesin the mantle controlled by subduction-related fluids. KEY WORDS: mantle metasomatism; Mexico; peridotite xenoliths; relative oxygen fugacity  相似文献   

11.
Neogene basanite lavas of Kozákov volcano, located alongthe Lusatian fault in the northeastern Czech Republic, containabundant anhydrous spinel lherzolite xenoliths that providean exceptionally continuous sampling of the upper two-thirdsof central European lithospheric mantle. The xenoliths yielda range of two-pyroxene equilibration temperatures from 680°Cto 1070°C, and are estimated to originate from depths of32–70 km, based on a tectonothermal model for basalticunderplating associated with Neogene rifting. The sub-Kozákovmantle is layered, consisting of an equigranular upper layer(32–43 km), a protogranular intermediate layer that containsspinel–pyroxene symplectites after garnet (43–67km), and an equigranular lower layer (67–70 km). Negativecorrelations of wt % TiO2, Al2O3, and CaO with MgO and clinopyroxenemode with Cr-number in the lherzolites record the effects ofpartial fusion and melt extraction; Y and Yb contents of clinopyroxeneand the Cr-number in spinel indicate 5 to 15% partial melting.Subsequent metasomatism of a depleted lherzolite protolith,probably by a silicate melt, produced enrichments in the largeion lithophile elements, light rare earth elements and highfield strength elements, and positive anomalies in primitivemantle normalized trace element patterns for P, Zr, and Hf.Although there are slight geochemical discontinuities at theboundaries between the three textural layers of mantle, theretends to be an overall decrease in the degree of depletion withdepth, accompanied by a decrease in the magnitude of metasomatism.Clinopyroxene separates from the intermediate protogranularlayer and the lower equigranular layer yield 143Nd/144Nd valuesof 0·51287–0·51307 (Nd = +4·6 to+8·4) and 87Sr/86Sr values of 0·70328–0·70339.Such values are intermediate with respect to the Nd–Srisotopic array defined by anhydrous spinel peridotite xenolithsfrom central Europe and are similar to those associated withthe present-day low-velocity anomaly in the upper mantle beneathEurope. The geochemical characteristics of the central Europeanlithospheric mantle reflect a complex evolution related to Devonianto Early Carboniferous plate convergence, accretion, and crustalthickening, Late Carboniferous to Permian extension and gravitationalcollapse, and Neogene rifting, lithospheric thinning, and magmatism. KEY WORDS: xenoliths; lithospheric mantle; REE–LILE–HFSE; Sr–Nd isotopes; Bohemian Massif  相似文献   

12.
Compositions of the principal minerals and Pb, Nd, and Sr isotopeanalyses of clinopyroxene (cpx) separates are reported for TypeI spinel peridotite xenoliths from the Peridot Mesa vent ofthe San Carlos Volcanic Field. The principal phases are in chemicalequilibrium within each inclusion. Systematic changes in mineralcomposition accompany lithological changes from fertile lherzolitesto infertile harzburgites. These changes are consistent witha fusion residue origin for the major element component of thexenoliths, as noted previously by Frey & Prinz (1978). ExcessFe is additionally present in some inclusions. Pyroxene equilibrationtemperatures calculated using the Wells (1977) geothermometerfall in the narrow range of 1022?34?C (1 s.d.). Equilibrationpressures poorly limit corresponding depths to anywhere between30 and 65 km within the lithospheric mantle. The geotherm is‘advective’ and elevated by 500?C at the depth ofsampling over a reference conductive shield geotherm. The highheat flow measured at the surface results from a combinationof extension and magmatism, with the temperature perturbationextending into the lithospheric mantle. 143Nd/144Nd ratios (0?51251–0?51367) and 87Sr/86Sr ratios(0?70190–0?70504) in cpx demonstrate gross isotopic heterogeneitybeneath the Peridot Mesa vent. This largely overlaps the oceanicmantle array, although four inclusions have Nd greater thanmid-ocean ridge basalts (MORB). PM-228J with Nd = +20 is themost extreme yet reported for a spinel Iherzolite. Pb abundancesin cpx (generally <0?03ppm) are far lower than previouslyreported values. 206Pb/204Pb ratios (17?5–19?1) overlapoceanic basalts and do not correlate with 87Sr/86Sr ratio. However,some of the inclusions exhibit MORB-like 206Pb/204Pb ratiosbut much higher 87Sr/86Sr ratios, which suggests a possiblegenetic link of detached lithospheric mantle with certain oceanicislands. Metasomatic trace element enrichment processes are most widespreadin the infertile (Al-poor, Cr-rich) inclusions, as noted byFrey & Prinz (1978). This systematic relationship is a localfeature of the mantle and suggests that some degree of meltingoccurs commensurately with incompatible element addition. Inparticular, anhydrous peridotite above its volatile-presentsolidus that was flushed with C-O-H fluids containing incompatibleelements would melt and form an enriched infertile fusion residue.The ascending magmas responsible for forming Type II peridotiteveins are the most probable source of the volatiles and mayin some cases react to produce chemical gradients in the wall-rock.Prior metasomatism is also evident isotopically in some inclusions.Overall, the lithospheric mantle beneath Peridot Mesa has suffereda multi-stage history of enrichment, depletion and melting atvarious times since it became attached to the crust above.  相似文献   

13.
At Kabbaldurga, infiltration of carbonic fluids along a systemof ductile shears and foliation planes has led to partial transformationof Archaean grey biotite–hornblende gneiss to coarse-grainedmassive charnockite at about 2.5 b.y. ago. The dehydration ofthe gneiss assemblage was induced by a marked metasomatic changeof the reacting system from granodioritic to granitic, and obviouslytook place under conditions of an open system at 700–750?C and 5–7 kb. Extensive replacement of plagioclase (An16–30)by K-feldspar through Na, Ca–K exchange reactions withthe ascending carbonic fluids led to strong enrichment in K,Rb, Ba, and SiO2, and to a depletion in Ca. Progressive dissolutionof hornblende, biotite, magnetite, and the accessory mineralsapatite and zircon resulted in a marked depletion in Fe, Mg,Ti, Zn, V, P, and Zr. Most important is the recognition of REEmobility: with advancing charnockitization, the moderately fractionatedREE distribution patterns of the grey gneisses (LaN270; LaN/YbN= 5–20; EuN27; Eu/Eu* = 0.6–0.3) give way to stronglyfractionated REE patterns with a positive Eu-anomaly (LaN200;LaN/YbN = 20–80; EuN22; Eu/Eu* = 0.6–1.8). The systematicdepletion especially in the HREE is due to the progressive dissolutionof zircon, apatite (and monazite), which strongly concentratethe REE. Stable isotope data (18O of 6.9–8.0 per mille for gneissesand charnockites; 13C of –8.5 and –6.5 per millefor late carbonate) indicate a magmatogenic source for the carbonicfluids. In contrast to the currently favoured derivation ofcarbonic fluids by decarbonation of the upper mantle or degassingof underplated basaltic intrusions, it is discussed here thatabundant fluid inclusions in lower crustal charnockites providedan extensive reservoir of ‘fossil’ carbonic fluids.Shear deformation has tapped this reservoir and generated thechannel-ways for fluid ascent. Charnockitization of the Kabbaldurgatypethus appears to be a metasomatic process which is tectonicallycontrolled and restricted to the crustal level of the amphiboliteto granulite transition.  相似文献   

14.
Laser ablation microprobe data are presented for olivine, orthopyroxeneand clinopyroxene in spinel harzburgite and lherzolite xenolithsfrom La Palma, Hierro, and Lanzarote, and new whole-rock trace-elementdata for xenoliths from Hierro and Lanzarote. The xenolithsshow evidence of strong major, trace element and Sr isotopedepletion (87Sr/86Sr 0·7027 in clinopyroxene in themost refractory harzburgites) overprinted by metasomatism. Thelow Sr isotope ratios are not compatible with the former suggestionof a mantle plume in the area during opening of the AtlanticOcean. Estimates suggest that the composition of the originaloceanic lithospheric mantle beneath the Canary Islands correspondsto the residues after 25–30% fractional melting of primordialmantle material; it is thus significantly more refractory than‘normal’ mid-ocean ridge basalt (MORB) mantle. Thetrace element compositions and Sr isotopic ratios of the mineralsleast affected by metasomatization indicate that the upper mantlebeneath the Canary Islands originally formed as highly refractoryoceanic lithosphere during the opening of the Atlantic Oceanin the area. During the Canarian intraplate event the uppermantle was metasomatized; the metasomatic processes includecryptic metasomatism, resetting of the Sr–Nd isotopicratios to values within the range of Canary Islands basalts,formation of minor amounts of phlogopite, and melt–wall-rockreactions. The upper mantle beneath Tenerife and La Palma isstrongly metasomatized by carbonatitic or carbonaceous meltshighly enriched in light rare earth elements (REE) relativeto heavy REE, and depleted in Zr–Hf and Ti relative toREE. In the lithospheric mantle beneath Hierro and Lanzarote,metasomatism has been relatively weak, and appears to be causedby high-Si melts producing concave-upwards trace element patternsin clinopyroxene with weak negative Zr and Ti anomalies. Ti–Al–Fe-richharzburgites/lherzolites, dunites, wehrlites and clinopyroxenitesformed from mildly alkaline basaltic melts (similar to thosethat dominate the exposed parts of the islands), and appearto be mainly restricted to magma conduits; the alkali basaltmelts have caused only local metasomatism in the mantle wall-rocksof such conduits. The various metasomatic fluids formed as theresults of immiscible separations, melt–wall-rock reactionsand chromatographic fractionation either from a CO2-rich basalticprimary melt, or, alternatively, from a basaltic and a siliceouscarbonatite or carbonaceous silicate melt. KEY WORDS: mantle xenoliths; mantle minerals; trace elements; depletion; carbonatite metasomatism  相似文献   

15.
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   

16.
Minerals, fluid inclusions and stable isotopes have been studiedin ultrahigh-pressure (UHP) OH-rich topaz–kyanite quartzitesfrom Hushan (west of Dongai), in southern Sulu (China). Thequartzites underwent a metamorphic evolution characterized bya peak stage (3·5 GPa and 730–820°C) with theanhydrous assemblage coesite + kyanite I, followed by an earlynear-isothermal decompression stage (2·9 GPa and 705–780°C)with growth of kyanite II, muscovite, and OH-rich topaz, andby decompression-cooling stages, represented by paragonite (1·9GPa and 700–780°C) and pyrophyllite (0·3 GPaand 400°C) on kyanite (I and II) and OH-rich topaz, respectively.These rocks may exhibit unusually low 18O and D values acquiredbefore undergoing UHP metamorphism. Five distinct fluid generationsare recognized. Type I: concentrated peak solutions rich inSi, Al, and alkalis, present within multiphase inclusions inkyanite I. Type II: CaCl2-rich brines present during the growthof early retrograde OH-rich topaz. Type III, IV, and V: lateaqueous fluids of variable salinity, and rare CO2 present duringamphibolite- and late greenschist-facies conditions. A numberof conclusions may be drawn from these relationships that havean effect on fluid evolution in deeply subducted continentalrocks. (1) At a pressure of about 3·5 GPa alkali–alumino-silicateaqueous solutions, with compositions intermediate between H2Ofluid and melt (H2O > 25 and 50 wt %) evolved from quartzites,probably generated by dehydration reactions. (2) During earlydecompression stages, at the transition from UHP to high-pressure(2·9 GPa) conditions, brines of external origin withhigher water contents (82 wt % H2O) initiated the growth ofOH-rich topaz and muscovite. (3) The subsequent decompression,at P <2 GPa, was defined by a limited circulation of NaClaqueous fluids, and CO2 infiltration. Overall, fluid inclusionsand stable isotopes highlight a metamorphic fluid–rockinteraction characterized by internally derived intermediateaqueous solutions at UHP, followed by infiltration of Cl-richbrines with higher water activities. KEY WORDS: ultrahigh-pressure metamorphism; OH-rich topaz; fluid inclusions; stable isotopes; supercritical liquids  相似文献   

17.
The Wrangellia terrane of North America contains a large volumeof Middle to Late Triassic oceanic flood basalts which wereemplaced on top of a preexisting island arc. Nd-, Sr-, and Pb-isotopiccompositions reflect derivation from a plume source with Nd(T)+6 to + 7, 87Sr/86Sri0•7034, and 206Pb/204Pbi19•0.Major and trace element compositions suggest the Wrangelliaflood basalts (WFB) formed through relatively small degreesof partial melting at greater depths than estimated for otheroceanic plateaux such as Ontong Java. It appears that the WFBdid not form in a rifting environment, and that preexistingarc lithosphere limited the ascent and decompression meltingof the source plume. Rocks from the preexisting arc are stronglydepleted in high field strength elements (HFSEs) relative tolarge ion lithophile elements (LILEs), but the WFB are not.Assimilation of arc lithospheric mantle or crust was thereforegenerally minor. However, some contamination by arc componentsis evident, particularly in basalts erupted in the early stagesof volcanism. Minor isotopic shifts, to lower Nd(T) and 206Pb/204Pbiand higher 87Sr/86Sri, are accompanied by shifts in trace elementratios towards more arclike signatures, e.g. low Nb/Th and Nb/La.Arc contamination is greatest in the most evolved basalts, indicatingthat assimilation was coupled with fractional crystallization.A comparison of the WFB with other continental and oceanic floodbasalts reveals that continental flood basalts generally formthrough smaller degrees of melting than oceanic flood basaltsand that the contribution of material from the crust and litho-sphericmantle is significantly greater. KEY WORDS: oceanic flood basalts; Wrangellia terrane; petrogenesis; Sr-Nd-Pb isotopes *Corroponding author  相似文献   

18.
Komatiites from the 2 Ga Jeesiörova area in Finnish Laplandhave subchondritic Al2O3/TiO2 ratios like those in Al-depletedkomatiites from Barberton, South Africa. They are distinct inthat their Al abundances are higher than those of the Al-depletedrocks and similar to levels in Al-undepleted komatiites. Moderatelyincompatible elements such as Ti, Zr, Eu, and Gd are enriched.Neither majorite fractionation nor hydrous melting in a supra-subductionzone setting could have produced these komatiites. Their highconcentrations of moderately incompatible elements may haveresulted from contamination of their parental melt through interactionwith metasomatic assemblages in the lithospheric mantle or enrichmentof their mantle source in basaltic melt components. Re–Osisotope data for chromite from the Jeesiörova rocks yieldan average initial 187Os/188Os of 0·1131 ± 0·0006(2), Os(I) = 0·1 ± 0·5. These data, coupledwith an initial Nd of +4, indicate that melt parental to thekomatiites interacted minimally with ancient lithospheric mantle.If their mantle source was enriched in a basaltic component,the combined Os–Nd isotopic data limit the enrichmentprocess to within 200 Myr prior to the formation of the komatiites.Their Os–Nd isotopic composition is consistent with derivationfrom the contemporaneous convecting upper mantle. KEY WORDS: Finnish Lapland; Jeesiörova; komatiites; mantle geochemistry; petrogenesis; redox state; Re/Os isotopes; Ti enrichment  相似文献   

19.
Late Tertiary post-orogenic alkaline basalts erupted in theextensional Pannonian Basin following Eocene-Miocene subductionand its related calc-alkaline volcanism. The alkaline volcaniccentres, dated between 11•7 and 1•4 Ma, are concentratedin several regions of the Pannonian Basin. Some are near thewestern (Graz Basin, Burgenland), northern (Ngrd), and eastern(Transylvania) margins of the basin, but the majority are concentratednear the Central Range (Balaton area and Little Hungarian Plain).Fresh samples from 31 volcanic centres of the extension-relatedlavas range from slightly hy-normative transitional basaltsthrough alkali basalts and basanites to olivine nephelinites.No highly evolved compositions have been encountered. The presenceof peridotite xenoliths, mantle xenocrysts, and high-pressuremegacrysts, even in the slightly more evolved rocks, indicatesthat differentiation took place within the upper mantle. Rare earth elements (REE) and 87Sr/86Sr, 143Nd/144Nd, 18O, D,and Pb isotopic ratios have been determined on a subset of samples,and also on clinopyroxene and amphibole megacrysts. Sr and Ndisotope ratios span the range of Neogene alkali basalts fromwestern and central Europe, and suggest that the magmas of thePannonian Basin were dominantly derived from asthenosphericpartial melting, but Pb isotopes indicate that in most casesthey were modified by melt components from the enriched lithosphericmantle through which they have ascended. 18O values indicatethat the magmas have not been significantly contaminated withcrustal material during ascent, and isotopic and trace-elementratios therefore reflect mantle source characteristics. Incompatible-elementpatterns show that the basic lavas erupted in the Balaton areaand Little Hungarian Plain are relatively homogeneous and areenriched in K, Rb, Ba, Sr, and Pb with respect to average oceanisland basalt, and resemble alkali basalts of Gough Island.In addition, 207Pb/204Pb is enriched relative to 2O6Pb/204Pb.In these respects, the lavas of the Balaton area and the LittleHungarian Plain differ from those of other regions of Neogenealkaline magmatism of Europe. This may be due to the introductionof marine sediments into the mantle during the earlier periodof subduction and metasomatism of the lithosphere by slab-derivedfluids rich in K, Rb, Ba, Pb, and Sr. Lavas erupted in the peripheralareas have incompatible-element patterns and isotopic characteristicsdifferent from those of the central areas of the basin, andmore closely resemble Neogene alkaline lavas from areas of westernEurope where recent subduction has not occurred.  相似文献   

20.
NIU  YAOLING 《Journal of Petrology》2004,45(12):2423-2458
This paper presents the first comprehensive major and traceelement data for 130 abyssal peridotite samples from the Pacificand Indian ocean ridge–transform systems. The data revealimportant features about the petrogenesis of these rocks, mantlemelting and melt extraction processes beneath ocean ridges,and elemental behaviours. Although abyssal peridotites are serpentinized,and have also experienced seafloor weathering, magmatic signaturesremain well preserved in the bulk-rock compositions. The betterinverse correlation of MgO with progressively heavier rare earthelements (REE) reflects varying amounts of melt depletion. Thismelt depletion may result from recent sub-ridge mantle melting,but could also be inherited from previous melt extraction eventsfrom the fertile mantle source. Light REE (LREE) in bulk-rocksamples are more enriched, not more depleted, than in the constituentclinopyroxenes (cpx) of the same sample suites. If the cpx LREErecord sub-ridge mantle melting processes, then the bulk-rockLREE must reflect post-melting refertilization. The significantcorrelations of LREE (e.g. La, Ce, Pr, Nd) with immobile highfield strength elements (HFSE, e.g. Nb and Zr) suggest thatenrichments of both LREE and HFSE resulted from a common magmaticprocess. The refertilization takes place in the ‘cold’thermal boundary layer (TBL) beneath ridges through which theascending melts migrate and interact with the advanced residues.The refertilization apparently did not affect the cpx relicsanalyzed for trace elements. This observation suggests grain-boundaryporous melt migration in the TBL. The ascending melts may notbe thermally ‘reactive’, and thus may have affectedonly cpx rims, which, together with precipitated olivine, entrappedmelt, and the rest of the rock, were subsequently serpentinized.Very large variations in bulk-rock Zr/Hf and Nb/Ta ratios areobserved, which are unexpected. The correlation between thetwo ratios is consistent with observations on basalts that DZr/DHf< 1 and DNb/DTa < 1. Given the identical charges (5+ forNb and Ta; 4+ for Zr and Hf) and essentially the same ionicradii (RNb/RTa = 1·000 and RZr/RHf = 1·006–1·026),yet a factor of 2 mass differences (MZr/MHf = 0·511 andMNb/MTa = 0·513), it is hypothesized that mass-dependentD values, or diffusion or mass-transfer rates may be importantin causing elemental fractionations during porous melt migrationin the TBL. It is also possible that some ‘exotic’phases with highly fractionated Zr/Hf and Nb/Ta ratios may existin these rocks, thus having ‘nugget’ effects onthe bulk-rock analyses. All these hypotheses need testing byconstraining the storage and distribution of all the incompatibletrace elements in mantle peridotite. As serpentine containsup to 13 wt % H2O, and is stable up to 7 GPa before it is transformedto dense hydrous magnesium silicate phases that are stable atpressures of 5–50 GPa, it is possible that the serpentinizedperidotites may survive, at least partly, subduction-zone dehydration,and transport large amounts of H2O (also Ba, Rb, Cs, K, U, Sr,Pb, etc. with elevated U/Pb ratios) into the deep mantle. Thelatter may contribute to the HIMU component in the source regionsof some oceanic basalts. KEY WORDS: abyssal peridotites; serpentinization; seafloor weathering; bulk-rock major and trace element compositions; mantle melting; melt extraction; melt–residue interaction; porous flows; Nb/Ta and Zr/Hf fractionations; HIMU mantle sources  相似文献   

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