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1.
The inter- and intragrain distribution of Li and Be in the subduction-related ultrahigh-pressure (UHP) garnet peridotite from Alpe Arami, Central Swiss Alps, was studied using secondary ion mass spectrometry. The data indicate substantial Li infiltration during exhumation of this ultramafic body. Orthopyroxene porphyroclasts and neoblasts are characterised by low Li contents (0.11-0.36 µg/g) typical of depleted peridotites, whereas Li zonation profiles across porphyroclasts of garnet and clinopyroxene document a metasomatic addition of Li. Small clinopyroxene grains in the matrix contain extremely high and variable abundances of Li (4-16 µg/g). In marked contrast to the behaviour of Li, the abundances of Be (77-134 ng/g) are similar in all textural types of clinopyroxene. Olivine porphyroclasts and neoblasts are characterised by somewhat elevated Li contents (0.95-1.79 µg/g), typical of fertile lherzolites. All textural types of clinopyroxene in the Alpe Arami peridotite are enriched in Li, providing evidence for infiltration of Li-rich and Be-poor aqueous solutions after the peak of UHP metamorphism. The lack of Li enrichment in orthopyroxene is consistent with orthopyroxene dissolution and formation of secondary olivine and clinopyroxene during metasomatism. Cr-diopside pyroxenite veins and boudins within the peridotite show low abundances of Li, with 0.7-2.5 µg/g in clinopyroxene and 1.1-1.5 µg/g in olivine. These pyroxenites likely represent precipitates from aqueous solutions which infiltrated the host peridotite after Li enrichment of the peridotite. A slab-derived nature of the metasomatic agent is suggested by the general lack of Ti enrichment in the Alpe Arami rocks. 相似文献
2.
Revised Thermobarometry of Alpe Arami and other Garnet Peridotites from the Central Alps 总被引:5,自引:5,他引:5
The four, currently best constrained, independent thermobarometersfor garnet peridotites, namely Taylors (Neues Jahrbuchfür Mineralogie, Abhandlungen 172, 381408, 1998)pyroxene solvus and Kroghs (Contributions to Mineralogyand Petrology 99, 4448, 1988) clinopyroxenegarnetFeMg exchange thermometers, and Taylors (1998)Al-in-orthopyroxene and Nimis & Taylors (Contributionsto Mineralogy and Petrology, 139, 541544, 2000) Cr-in-clinopyroxenebarometers, have been applied to garnet lherzolites from theCentral Alps. Analyses from the literature, as well as new in-houseanalyses, all pertaining to core compositions of first-generation,garnet lherzolite minerals, have been selected for thermobarometriccalculations. The PT data obtained for the three knowngarnet lherzolite occurrences in the Central Alps are tightlyconstrained, consistent with one another, and summarized asfollows: Alpe Arami, 3·2 GPa and 840°C; Monte Duria,3·0 GPa and 830°C; Cima di Gagnone, 3·0 GPaand 740°C. These values are consistent with experimentaldata on pargasite stability and composition in peridotitic systems.Our PT estimates, along with microstructural and fieldgeological observations, indicate that the garnet lherzoliteparageneses form part of the prograde, Alpine, high-pressuremetamorphic sequence of the AdulaCima Lunga unit. Thermobarometryshows that the garnet lherzolites reached a maximum depth ofsubduction of 相似文献
3.
Ca-rich Garnet-Clinopyroxene Rocks at Hujialin in the Su-Lu Terrane (Eastern China): Deeply Subducted Arc Cumulates? 总被引:3,自引:0,他引:3
Layers of Ca-rich garnetclinopyroxene rocks enclosedin a serpentinite body at Hujialin, in the SuLu terraneof eastern China, preserve igneous textures, relict spinel ingarnet, and exsolution lamellae of Ca-rich garnet, ilmenite/magnetite,Fe-rich spinel, and also amphibole in clinopyroxene. In termsof their major and trace element compositions, the studied samplesform a trend from arc cumulates towards FeTi gabbros.Reconstructed augite compositions plot on the trend for clinopyroxenein arc cumulates. These data suggest that the rocks crystallizedfrom mantle-derived magmas differentiated to various extentsbeneath an arc. The Ca-rich garnet + diopside assemblage isinferred to have formed by compressing Ca-rich augite, whereasthe relatively Mg-rich cores of garnet porphyroblasts may haveformed at the expense of spinel. The protolith cumulates weresubducted from near the crustmantle boundary (c. 1 GPa)deep into the upper mantle (4·8 ± 0·6 GPaand 750 ± 50°C). Negatively sloped PT pathsfor the garnetclinopyroxene rocks and the corollary ofcorner flow induced subduction of mantle wedge peridotite arenot supported by the available data. Cooling with, or without,decompression of the cumulates after the igneous stage probablyoccurred prior to deep subduction. KEY WORDS: arc cumulates; Ca-rich garnet; garnetclinopyroxene rocks; SuLu terrane; UHP metamorphism 相似文献
4.
KONZETT JURGEN; MILLER CHRISTINE; ARMSTRONG RICHARD; THONI MARTIN 《Journal of Petrology》2005,46(4):717-747
Olivine-rich rocks containing olivine + orthopyroxene + spinel+ Ca-amphibole ± clinopyroxene ± garnet are presentin the central ÖtztalStubai crystalline basementassociated with eclogites of tholeiitic affinity. These rockscontain centimetre-sized garnet layers and lenses with garnet+ clinopyroxene ± corundum. Protoliths of the olivine-richrocks are thought to be olivine + orthopyroxene + spinel dominatedcumulates generated from an already differentiated Fe-rich () tholeiitic magma that was emplaced into shallowcontinental crust. Protoliths of the garnet-rich rocks are interpretedas layers enriched in plagioclase and spinel intercalated ina cumulate rock sequence that is devoid of, or poor in, plagioclase.UPb sensitive high-resolution ion microprobe dating ofzircons from a garnet layer indicates that emplacement of thecumulates took place no later than 517 ± 7 Myr ago. Aftertheir emplacement, the cumulates were subjected to progressivemetamorphism, reaching eclogite-facies conditions around 800°Cand >2 GPa during a Variscan metamorphic event between 350and 360 Ma. Progressive high-P metamorphism induced breakdownof spinel to form garnet in the olivine-rich rocks and of plagioclase+ spinel to form garnet + clinopyroxene ± corundum inthe garnet layers. Retrogressive metamorphism at T 650680°Cled to the formation of Ca-amphibole, chlorite and talc in theolivine-rich rocks. In the garnet layers, högbomite formedfrom corundum + spinel along with Al-rich spinel, Ca-amphibole,chlorite, aspidolitepreiswerkite, magnetite, ilmeniteand apatite at the interface between olivine-rich rocks andgarnet layers at P < 0·8 GPa. Progressive desiccationof retrogade fluids through crystallization of hydrous phasesled to a local formation of saline brines in the garnet layers.The presence of these brines resulted in a late-stage formationof Fe- and K-rich Ca-amphibole and Sr-rich apatite, both characterizedby extremely high Cl contents of up to 3·5 and 6·5wt % Cl, respectively. KEY WORDS: cumulates; Variscan metamorphism; SHRIMP dating; högbomite; saline brines 相似文献
5.
Contrasting P-T Paths in the Eastern Himalaya, Nepal: Inverted Isograds in a Paired Metamorphic Mountain Belt 总被引:1,自引:0,他引:1
Petrology and phase equilibria of rocks from two profiles inEastern Nepal from the Lesser Himalayan Sequences, across theMain Central Thrust Zone and into the Greater Himalayan Sequencesreveal a Paired Metamorphic Mountain Belt (PMMB) composed oftwo thrust-bound metamorphic terranes of contrasting metamorphicstyle. At the higher structural level, the Greater HimalayanSequences experienced high-T/moderate-P metamorphism, with ananticlockwise PT path. Low-P inclusion assemblages ofquartz + hercynitic spinel + sillimanite have been overgrownby peak metamorphic garnet + cordierite + sillimanite assemblagesthat equilibrated at 837 ± 59°C and 6·7 ±1·0 kbar. Matrix minerals are overprinted by numerousmetamorphic reaction textures that document isobaric coolingand re-equilibrated samples preserve evidence of cooling to600 ± 45°C at 5·7 ±1·1 kbar.Below the Main Central Thrust, the Lesser Himalayan Sequencesare a continuous (though inverted) Barrovian sequence of high-P/moderate-Tmetamorphic rocks. Metamorphic zones upwards from the loweststructural levels in the south are: Zone A: albite + chlorite + muscovite ± biotite; Zone B: albite + chlorite + muscovite + biotite + garnet; Zone C: albite + muscovite + biotite + garnet ± chlorite; Zone D: oligoclase + muscovite + biotite + garnet ± kyanite; Zone E: oligoclase + muscovite + biotite + garnet + staurolite+ kyanite; Zone F: bytownite + biotite + garnet + K-feldspar + kyanite± muscovite; Zone G: bytownite + biotite + garnet + K-feldspar + sillimanite+ melt ± kyanite. The Lesser Himalayan Sequences show evidence for a clockwisePT path. Peak-P conditions from mineral cores average10·0 ± 1·2 kbar and 557 ± 39°C,and peak-metamorphic conditions from rims average 8·8± 1·1 kbar and 609 ± 42°C in ZonesDF. Matrix assemblages are overprinted by decompressionreaction textures, and in Zones F and G progress into the sillimanitefield. The two terranes were brought into juxtaposition duringformation of sillimanitebiotite ± gedrite foliationseams (S3) formed at conditions of 674 ± 33°C and5·7 ± 1·1 kbar. The contrasting averagegeothermal gradients and PT paths of these two metamorphicterranes suggest they make up a PMMB. The upper-plate positionof the Greater Himalayan Sequences produced an anticlockwisePT path, with the high average geothermal gradient beingpossibly due to high radiogenic element content in this terrane.In contrast, the lower-plate Lesser Himalayan Sequences weredeeply buried, metamorphosed in a clockwise PT path anddisplay inverted isograds as a result of progressive ductileoverthrusting of the hot Greater Himalayan Sequences duringprograde metamorphism. KEY WORDS: thermobarometry; PT paths; Himalaya; metamorphism; inverted isograds; paired metamorphic belts 相似文献
6.
Mantle Xenoliths from the Southeastern Slave Craton: Evidence for Chemical Zonation in a Thick, Cold Lithosphere 总被引:7,自引:3,他引:7
We present the first data on the petrology of the mantle lithosphereof the Southeastern (SE) Slave craton, Canada. These are basedon petrographic, mineralogical and geochemical studies of mantlexenoliths in Pipe 5034 of the Cambrian Gahcho Kué kimberlitecluster. Major types of mantle xenoliths include altered eclogite,coarse garnet or spinel peridotite, and deformed garnet peridotite.The peridotites belong to the low-temperature suite and formedat T=6001300°C and P= 2580 kbar in a thick(at least 220250 km), cool lithosphere. The SE Slavemantle is cooler than the mantle of other Archaean cratons andthat below other terranes of the Slave craton. The thick lithosphereand the relatively cool thermal regime provide favourable conditionsfor formation and preservation of diamonds beneath the SE Slaveterrane. Similar to average Archaean mantle worldwide, the SESlave peridotite is depleted in magmaphile major elements andcontains olivine with forsterite content of 9193·5.With respect to olivine composition and mode, all terranes ofthe Slave mantle show broadly similar compositions and are relativelyorthopyroxene-poor compared with those of the Kaapvaal and Siberiancratons. The SE Slave spinel peridotite is poorer in Al, Caand Fe, and richer in Mg than deeper garnet peridotite. Thegreater chemical depletion of the shallow upper mantle is typicalof all terranes of the Slave craton and may be common for thesubcontinental lithospheric peridotitic mantle in general. Peridotiticxenoliths of the SE Slave craton were impregnated by kimberliticfluids that caused late-stage recrystallization of primary clinopyroxene,spinel, olivine and spinel-facies orthopyroxene, and formationof interstitial clinopyroxene. This kimberlite-related recrystallizationdepleted primary pyroxenes and spinel in Al. The kimberliticfluid was oxidizing, Ti-, Fe- and K-rich, and Na-poor, and introducedserpentine, chlorite, phlogopite and spinel into peridotitesat P < 35 kbar. KEY WORDS: kimberlite xenolith; lithosphere; mantle terrane; chemical zoning; thermobarometry; Slave craton 相似文献
7.
Primitive nephelinites and basanites from the Tertiary Hocheifelarea of Germany (part of the Central European Volcanic Province;CEVP) have high Mg-number (>0·64), high Cr and Nicontents and strong light rare earth element enrichment butsystematic depletion in Rb, K and Ba relative to trace elementsof similar compatibility in anhydrous mantle. Alkali basaltsand more differentiated magmatic rocks have lower Mg-numberand lower abundances of Ni and Cr, and have undergone fractionationof mainly olivine, clinopyroxene, FeTi oxide, amphiboleand plagioclase. Some nephelinites and basanites approach theSrNdPb isotope compositions inferred for the EAR(European Asthenospheric Reservoir) component. The NdSrPbisotope composition of the differentiated rocks indicates thatassimilation of lower crustal material has modified the compositionof the primary mantle-derived magmas. Rare earth element meltingmodels can explain the petrogenesis of the most primitive maficmagmatic rocks in terms of mixing of melt fractions from anamphibole-bearing garnet peridotite source with melt fractionsfrom an amphibole-bearing spinel peridotite source, both sourcescontaining residual amphibole. It is inferred that amphibolewas precipitated in the asthenospheric mantle beneath the Hocheifel,close to the garnet peridotitespinel peridotite boundary,by metasomatic fluids or melts from a rising mantle diapir orplume. Melt generation with amphibole present suggests relativelylow mantle potential temperatures (<1200°C); thus themantle plume is not thermally anomalous. A comparison of recentlypublished Ar/Ar ages for Hocheifel basanites with the geochemicaland isotopic composition of samples from this study collectedat the same sample sites indicates that eruption of earlierlavas with an EM signature was followed by the eruption of laterlavas derived from a source with EAR or HIMU characteristics,suggesting a contribution from the advancing plume. Thus, theHocheifel area represents an analogue for magmatism during continentalrift initiation, during which interaction of a mantle plumewith the overlying lithosphere may have led to the generationof partial melts from both the lower lithosphere and the asthenosphere. KEY WORDS: alkali basalts; continental volcanism; crustal contamination; partial melting; Eifel, Germany 相似文献
8.
Petrochemical Study of Lherzolitic Rocks from the Western Alps 总被引:4,自引:2,他引:4
An integrated geologic and petrochemical comparative study offive peridotite complexes in the western Alps has been undertaken.Investigated bodies are exposed at Alpe Arami in southern Switzerland,and at Finero, Balmuccia, Baldissero and Lanzo in northwesternItaly. The Alpe Arami mass has been tectonically emplaced withinthe Lepontine gneiss terrane of the in part subducted, morenortherly European lithospheric plate, whereas the other fourultramafic masses represent original portions of the non-subductedSouth Alpine plate. Eighty samples were examined petrographically. Most are lherzolites,but a few are clinopyroxene-bearing harzburgites, olivine websteritesor clinopyroxenite. Alpe Arami peridotites locally contain primarygarnet (± minor spinel); in contrast, the South Alpinelherzolites are spinel bearing, and in addition, the Lanzo massifcontains widespread plagioclase. All masses display strain effectssuch as bent lamellae in pyroxenes, gradational or sectoraloptical extinction, and minor recrystallization. Twenty-fourbulk XRF analyses demonstrate that the investigated rocks fairlyclosely match pyrolite composition, but are slightly impoverishedin alumina. The normative olivines of all analyzed specimenshave Fa contents ranging between 8 and 10 mole per cent. Electronmicroprobe analyses for 26 olivines, 27 orthopyroxenes, 23 calcicpyroxenes, three garnets, 18 spinels, three plagioclases, 13calcic amphiboles, two chlorites and two phlogopites are presented.Phases in a particular sample are remarkably homogeneous, anobservation consistent with an inferred close approach to chemicalequilibrium. Use of various two-pyroxene geothermometers allows the followingassignments of grand average apparent temperatures for the fiveperidotite complexes: Alpe Arami, 966±78°C; Finero,893 ± 94°C; Balmuccia, 973 ± 50°C; Baldissero,1002 ± 37°C; and Lanzo, 1069 ± 85°C. Pressureestimates, determined using the lherzolitic petrogenetic grid,Al2O3cpx and Al2O3opx isopleths, involve large uncertainties.The Alpe Arami ultramafic body evidently crystallized at a pressureof 40 ± 10 kilobars, the nominal value depending on themethod of computation. In contrast, the South Alpine spinellherzolites seem to have last equilibrated at pressures of approximately520 kb. The deep upper-mantle source region of the AlpeArami body apparently was subjected to a subcontinental-typegeothermal gradient, consistent with solid-state rise of thismass and Late Alpine tectonic insertion in the old Lepontinesialic crust of the European lithospheric plate. The mechanismand driving force of this process remain obscure. In contrast,the investigated spinel peridotite complexes of the South Alpinelithospheric plate seem to have been derived from shallow, uppermostmantle sections characterized by oceanic geothermal gradients.For this reason it is conjectured that these massifs were derivedfrom the northern margin of the southern plate where continentalcrust evidently was moderately thin or absent. Their upliftand overthrusting involved P-T paths which in most cases alloweddecompression partial recrystallization and incipient fusion,the latter indicated by the presence of transecting mafic dikesand segregations associated with the peridotites. The Finerobody appears to have reached its present position at the baseof the South Alpine continental crust in pre-Mesozoic time,whereas the Lanzo complex was tectonically involved in the EarlyAlpine orogeny. 相似文献
9.
Petrogenesis of Corundum-Bearing Mafic Rock in the Horoman Peridotite Complex, Japan 总被引:6,自引:2,他引:6
A corundum-bearing Type II mafic rock, within the Horoman peridotite,Japan, was petrologically examined in detail to obtain the PTpaths of the mafic rock as well as of the host peridotite. Ofall the mafic rocks documented from the Horoman complex, onlythe corundum-bearing mafic rock has preserved, at least partly,its high-pressure mineralogy; all of the others have been completelyrecrystallized at low pressures. The Type II mafic rocks wereinitially formed at <1·0 GPa as cumulates of olivine,plagioclase and clinopyroxene. Corundum was then formed by metamorphismand/or partial melting of the Type II protolith at higher pressures(>1·5 GPa) than the initial condition of formation.Corundum reacted with clinopyroxene during exhumation of theHoroman peridotite down to the plagioclase stability field.The field and petrographical observations of the Type II maficrocks (± corundum) coupled with published isotopic datasuggest a complicated spiral-like PT trajectory for theHoroman peridotite. The Type II protolith was formed at lowpressure within the peridotite at the time of initial formationof the Horoman peridotite as a residue from primitive mantleat 相似文献
10.
Exhumation History of a Garnet Pyroxenite-bearing Mantle Section from a Continent-Ocean Transition (Northern Apennine Ophiolites, Italy) 总被引:3,自引:0,他引:3
Garnet clinopyroxenite and garnet websterite layers occur locallywithin mantle peridotite bodies from the External Liguride Jurassicophiolites (Northern Apennines, Italy). These ophiolites werederived from an oceancontinent transition similar tothe present-day western Iberian margin. The garnet clinopyroxenitesare mafic rocks with a primary mineral assemblage of pyrope-richgarnet + sodic Al-augite (Na2O 2·5 wt %, Al2O3 12·5wt %), with accessory graphite, FeNi sulphides and rutile.Decompression caused Na-rich plagioclase (An5045) exsolutionin clinopyroxene porphyroclasts and extensive development ofsymplectites composed of secondary orthopyroxene + plagioclase(An8572) + Al-spinel ± clinopyroxene ±ilmenite at the interface between garnet and primary clinopyroxene.Further decompression is recorded by the development of an olivine+ plagioclase-bearing assemblage, locally under syn-kinematicconditions, at the expense of two-pyroxenes + Al-spinel. Mg-richgarnet has been also found in the websterite layers, which arecommonly characterized by the occurrence of symplectites madeof orthopyroxene + Al-spinel ± clinopyroxene. The enclosingperidotites are Ti-amphibole-bearing lherzolites with a fertilegeochemical signature and a widespread plagioclase-facies myloniticfoliation, which preserve in places a spinel tectonite fabric.LuHf and SmNd mineral isochrons (220 ±13 Ma and 186.0 ± 1·8 Ma, respectively) have beenobtained from a garnet clinopyroxenite layer and interpretedas cooling ages. Geothermobarometric estimates for the high-pressureequilibration have yielded T 1100°C and P 2·8 GPa.The early decompression was associated with moderate cooling,corresponding to T 950°, and development of a spinel tectonitefabric in the lherzolites. Further decompression associatedwith plagioclaseolivine growth in both peridotites andpyroxenites was nearly isothermal. The shallow evolution occurredunder a brittle regime and led to the superposition of hornblendeto serpentine veining stages. The garnet pyroxenite-bearingmantle from the External Liguride ophiolites represents a raretectonic sampling of deep levels of subcontinental lithosphereexhumed in an oceanic setting. The exhumation was probably accomplishedthrough a two-step process that started during Late Palaeozoiccontinental extension. The low-pressure portion of the exhumationpath, probably including also the plagioclase mylonitic shearzones, was related to the Mesozoic (Triassic to Jurassic) riftingthat led to continental break-up. In Jurassic times, the studiedmantle sequence became involved in an extensional detachmentprocess that resulted in sea-floor denudation. KEY WORDS: garnet pyroxenite; ophiolite; non-volcanic margin; mantle exhumation; SmNd and LuHf geochronology 相似文献
11.
Constraints on the Trace Element Composition of the Archean Mantle Root beneath Somerset Island, Arctic Canada 总被引:1,自引:6,他引:1
Peridotites that sample Archean mantle roots are frequentlyincompatible trace element enriched despite their refractorymajor element compositions. To constrain the trace element budgetof the lithosphere beneath the Canadian craton, trace elementand rare earth element (REE) abundances were determined fora suite of garnet peridotites and garnet pyroxenites from theNikos kimberlite pipe on Somerset Island, Canadian Arctic, theirconstituent garnet and clinopyroxene, and the host kimberlite.These refractory mantle xenoliths are depleted in fusible majorelements, but enriched in incompatible trace elements, suchas large ion lithophile elements (LILE), Th, U and light rareearth elements (LREE). Mass balance calculations based on modalabundances of clinopyroxene and garnet and their respectiveREE contents yield discrepancies between calculated and analyzedREE contents for the Nikos bulk rocks that amount to LREE deficienciesof 7099%, suggesting the presence of small amounts ofinterstitial kimberlite liquid (0·42 wt %) toaccount for the excess LREE abundances. These results indicatethat the peridotites had in fact depleted or flat LREE patternsbefore contamination by their host kimberlite. LREE and Sr enrichmentin clinopyroxene and low Zr and Sr abundances in garnet in low-temperatureperidotites (8001100°C) compared with high-temperatureperidotites (12001400°C) suggest that the shallowlithosphere is geochemically distinct from the deep lithospherebeneath the northern margin of the Canadian craton. The Somersetmantle root appears to be characterized by a depth zonationthat may date from the time of its stabilization in the Archean. KEY WORDS: Canada; mantle; metasomatism; peridotite; trace elements 相似文献
12.
BRUECKNER HANNES K.; VAN ROERMUND HERMAN L. M.; PEARSON NORMAN J. 《Journal of Petrology》2004,45(2):415-437
Mineralogical, isotopic, geochemical and geochronological evidencedemonstrates that the Friningen body, a garnet peridotite bodycontaining garnet pyroxenite layers in the Seve Nappe Complex(SNC) of Northern Jämtland, Sweden, represents old, certainlyProterozoic and possibly Archean, lithosphere that became incorporatedinto the Caledonian tectonic edifice during crustal subductioninto the mantle at c. 450 Ma. Both garnet peridotite and pyroxenitecontain two (M1 and M2) generations of garnet-bearing assemblagesseparated by the formation of two-pyroxene, spinel symplectitearound the M1 garnet and the crystallization of low-Cr spinel1Cin the matrix. These textures suggest initial high-pressure(HP) crystallization of garnet peridotite and pyroxenite succeededby decompression into the spinel stability field, followed byrecompression into the garnet peridotite facies. Some pyroxenitelayers appear to be characterized solely by M2 assemblages withstretched garnet as large as several centimeters. Laser ablationmicroprobeinductively coupled plasma mass spectrometryReOs analyses of single sulfide grains generally definemeaningless model ages suggesting more than one episode of Reand/or Os addition and/or loss to the body. Pentlandite grainsfrom a single polished slab of one garnet peridotite, however,define a linear array on an ReOs isochron diagram that,if interpreted as an errorchron, suggests an Archean melt extractionevent that left behind the depleted dunite and harzburgite bodiesthat characterize the SNC. Refertilization of this mantle bymelts associated with the development of the pyroxenite layersis indicated by enriched clinopyroxene SrNd isotope ratios,and by parallel large ion lithophile-enriched trace elementpatterns in clinopyroxene from pyroxenite and the immediatelyadjacent peridotite. Clinopyroxene and whole-rock model SmNdages (TDM = 1·12·2 Ga) indicate that fertilizationtook place in Proterozoic times. SmNd garnet2clinopyroxene2wholerock ± orthopyroxene2 mineral isochrons from three pyroxenitelayers define overlapping ages of 452·1 ± 7·5and 448 ± 13 Ma and 451 ± 43 Ma (2 相似文献
13.
One characteristic of many subduction-zone garnet peridotites is that they contain titanium-bearing phases not otherwise found in mantle rocks. In particular, titanoclinohumite and/or its breakdown assemblage consisting of symplectic intergrowths of olivine and ilmenite is common in many of these bodies. The Alpe Arami garnet lherzolite of the Swiss Alps, while lacking titanoclinohumite, displays instead large numbers of FeTiO3 rod-shaped precipitates in the oldest generation of olivine, amounting to approximately 1% by volume, indicating that at some time in its past, the peridotite experienced conditions under which the solubility of TiO2 in olivine was >0.6 wt.%. In order to test the hypothesis that the environment of very high solubility of TiO2 in olivine is to be found at very high pressures, we have conducted experiments on lherzolite compositions with added ilmenite at pressures between 5 and 12 GPa and temperatures of 1350–1700 K. Our results on anhydrous compositions show that whereas solubility of TiO2 was not detected in olivine at 5 GPa, 1400 K where it coexists with rutile, when rutile disappeared from the paragenesis, the solubility climbed to 0.4 wt.% at 8 GPa, 0.5 wt.% at 10 GPa and to >1.0 wt.% at 12 GPa, 1700 K. These results support our previous interpretations from titanate morphology and abundance that the Alpe Arami massif has surfaced from P=10 GPa but remove the need to suggest a deeper origin and possible precursor phase such as wadsleyite. They also support the hypothesis that garnet peridotites with unusual Ti-bearing phases reflect a unique mantle environment occurring in the mantle wedge overlying subduction zones. 相似文献
14.
Sub-solidus Oligocene zircon formation in garnet peridotite during fast decompression and fluid infiltration (Duria, Central Alps) 总被引:1,自引:0,他引:1
Summary A garnet peridotite lens from Monte Duria (Adula nappe, Central Alps, Northern Italy) contains porphyroblastic garnet and
pargasitic amphibole and reached peak metamorphic conditions of ∼830 C, ∼2.8 GPa. A first stage of near isothermal decompression
to pressures <2.0 GPa is characterised by domains where fine grained spinel, clinopyroxene, orthopyroxene and amphibole form.
The newly formed amphibole contains elevated levels of fluid mobile elements such as Rb, Ba and Pb indicating that recrystallization
was assisted by infiltration of a crustal-derived fluid. Further decompression and cooling to ∼720 °C, 0.7–1.0 GPa associated
with limited fluid influx is documented by the formation of orthopyroxene-spinel-amphibole symplectites around garnet.
Zircon separated from this garnet peridotite exhibits two distinct zones. Domain 1 displays polygonal oscillatory zoning and
high trace element contents. It contains clinopyroxene and amphibole inclusions with the same composition as the same minerals
formed during the spinel peridotite equilibration, indicating that this domain formed under sub-solidus conditions during
decompression and influx of crustal fluids. Domain 2 has no zoning and much lower trace element contents. It replaces domain
1 and is likely related to zircon recrystallization during the formation of the symplectites. SHRIMP dating of the two domains
yielded ages of 34.2 ± 0.2 and 32.9 ± 0.3 Ma, respectively, indicating fast exhumation of the peridotite within the spinel
stability field. We suggest that the Duria garnet peridotite originates from the mantle wedge above the tertiary subduction
of the European continental margin and that it was assembled to the country rock gneisses between 34 and 33 Ma.
Third author was Deceased 相似文献
15.
Krienitz M.-S.; Haase K. M.; Mezger K.; Shaikh-Mashail M. A. 《Journal of Petrology》2007,48(8):1513-1542
A geochemical and petrological study of Miocene to recent alkalibasalts, basanites, hawaiites, mugearites, trachytes, and phonoliteserupted within the Harrat Ash Shamah volcanic field was performedto reconstruct the magmatic evolution of southern Syria. Themajor element composition of the investigated lavas is mainlycontrolled by fractional crystallization of olivine, clinopyroxene,± FeTi oxides and ± apatite; feldspar fractionationis restricted to the most evolved lavas. Na2O and SiO2 variationswithin uncontaminated, primitive lavas as well as variably fractionatedheavy rare earth element ratios suggest a formation by variabledegrees of partial melting of different garnet peridotite sourcestriggered, probably, by changes in mantle temperature. The isotopicrange as well as the variable trace element enrichment observedin the lavas imply derivation from both a volatile- and incompatibleelement-enriched asthenosphere and from a plume component. Inaddition, some lavas have been affected by crustal contamination.This effect is most prominent in evolved lavas older than 3·5Ma, which assimilated 3040% of crustal material. In general,the periodicity of volcanism in conjunction with temporal changesin lava composition and melting regime suggest that the Syrianvolcanism was triggered by a pulsing mantle plume located underneathnorthwestern Arabia. KEY WORDS: 40Ar/39Ar ages; intraplate volcanism; mantle plume; partial melting; Syria 相似文献
16.
P-T Evolution of a Variscan Lower-Crustal Segment: a Study of Granulites from the Schwarzwald, Germany 总被引:2,自引:0,他引:2
Pressuretemperaturetime (PTt) pathsof orogenic granulites provide important information on thethermal and chemical structure of the lower continental crustthrough time, and constraints on tectonic processes. We presentthe first detailed petrological investigation of granulitesfrom the Variscan Schwarzwald. Pelitic granulites from the CentralSchwarzwald Gneiss Complex (CSGC) are characterized by the peakassemblage garnet + rutile + kyanite + antiperthite ±quartz. Felsic to intermediate granulites from the SouthernSchwarzwald Gneiss Complex (SSGC) exhibit different peak assemblageswith clinopyroxene, orthopyroxene, ternary feldspar, garnet,quartz and sillimanite, and manifold retrograde reaction textures.Peak PT conditions were calculated by two-feldspar thermometry,garnetorthopyroxene thermometry and various geobarometers.Minimum estimates for peak conditions are 9501010°Cand 1·41·8 GPa for the granulites of theCSGC, which followed a clockwisePT path. The retrogradepath is characterized by initial isothermal decompression, associatedwith partial melting, followed by isobaric cooling. Peak conditionsfor the SSGC are 1015°C and 1·5 GPa (minimum temperature,maximum pressure). No prograde relics are preserved, and isothermaldecompression was less pronounced than in the CSGC. Other VariscanHPHT granulites from Central Europe show similar lithologies,equilibration temperatures and ages (340335 Ma). Theheat for widespread high-temperature metamorphism in the Variscanlower crust could have been supplied by repeated intrusion ofsubduction-related basic magmas. Rapid, near-isothermal decompressionof the granulites may have been facilitated by considerablevolumes of partial melt and by orogenic extension. KEY WORDS: granulites; near-isothermal decompression; two-feldspar thermometry; HT metamorphism; Variscan Schwarzwald 相似文献
17.
Experimental Melting of Carbonated Peridotite at 6-10 GPa 总被引:2,自引:0,他引:2
Brey Gerhard P.; Bulatov Vadim K.; Girnis Andrei V.; Lahaye Yann 《Journal of Petrology》2008,49(4):797-821
Partial melting of magnesite-bearing peridotites was studiedat 6–10 GPa and 1300–1700°C. Experiments wereperformed in a multianvil apparatus using natural mineral mixesas starting material placed into olivine containers and sealedin Pt capsules. Partial melts originated within the peridotitelayer, migrated outside the olivine container and formed poolsof quenched melts along the wall of the Pt capsule. This allowedthe analysis of even small melt fractions. Iron loss was nota problem, because the platinum near the olivine container becamesaturated in Fe as a result of the reaction Fe2SiO4Ol = FeFe–Ptalloy + FeSiO3Opx + O2. This reaction led to a gradual increasein oxygen fugacity within the capsules as expressed, for example,in high Fe3+ in garnet. Carbonatitic to kimberlite-like meltswere obtained that coexist with olivine + orthopyroxene + garnet± clinopyroxene ± magnesite depending on P–Tconditions. Kinetic experiments and a comparison of the chemistryof phases occasionally grown within the melt pools with thosein the residual peridotite allowed us to conclude that the meltshad approached equilibrium with peridotite. Melts in equilibriumwith a magnesite-bearing garnet lherzolite are rich in CaO (20–25wt %) at all pressures and show rather low MgO and SiO2 contents(20 and 10 wt %, respectively). Melts in equilibrium with amagnesite-bearing garnet harzburgite are richer in SiO2 andMgO. The contents of these oxides increase with temperature,whereas the CaO content becomes lower. Melts from magnesite-freeexperiments are richer in SiO2, but remain silicocarbonatitic.Partitioning of trace elements between melt and garnet was studiedin several experiments at 6 and 10 GPa. The melts are very richin incompatible elements, including large ion lithophile elements(LILE), Nb, Ta and light rare earth elements. Relative to theresidual peridotite, the melts show no significant depletionin high field strength elements over LILE. We conclude fromthe major and trace element characteristics of our experimentalmelts that primitive kimberlites cannot be a direct productof single-stage melting of an asthenospheric mantle. They rathermust be derived from a previously depleted and re-enriched mantleperidotite. KEY WORDS: multianvil; carbonatite melt; peridotite; kimberlite; element partitioning 相似文献
18.
Pressure-Temperature Path Recorded in the Yangkou Garnet Peridotite, in Su-Lu Ultrahigh-pressure Metamorphic Belt, Eastern China 总被引:2,自引:0,他引:2
Chemical variations along with changes in microstructure ofthe principal constituent minerals make it possible to identifyat least four equilibrium stages in the evolution of the Yangkougarnet peridotite in the Su-Lu ultrahigh-pressure metamorphicbelt, eastern China: Stage Ia primary garnet lherzolitestage represented by coarse-grained (a few millimeters size)porphyroclastic aluminous pyroxenes + chromian spinel ±garnet; Stage IIan ultrahigh-pressure (UHP) stage definedby fine-grained matrix phases (0·10·3 mmsize) of garnet + extremely low-Al orthopyroxene + high-Na clinopyroxene+ chromite; Stage IIIa medium-pressure stage definedby fine-grained mineral aggregates (<0·10·2mm size) mainly composed of aluminous spinel + high-Al orthopyroxenein the matrix; Stage IVan amphibolite- to greenschist-faciesstage defined by poikiloblastic amphibole. Orthopyroxeneclinopyroxenethermometry and an empirical spinel barometer give temperaturesof around 800830°C and pressures of 1·22·9GPa for porphyroclasts of Stage I. Garnetorthopyroxene,garnetclinopyroxene and empirical spinel geothermobarometersgive relatively uniform PT conditions for the matrixgarnetorthopyroxeneclinopyroxenechromiteassemblage of Stage II ( 相似文献
19.
Transformation of Spinel Lherzolite to Garnet Lherzolite in Ultramafic Lenses of the Austridic Crystalline Complex, Northern Italy 总被引:7,自引:4,他引:7
Numerous lenticular bodies of ultramafic rocks occur withinthe upper amphibolite- to granulitefacies metamorphic terraneof the Austrides between the Non and Ultimo valleys (Nonsbergregion), northern Italy. The ultramafic rocks are divided intotwo textural types: (a) coarse-type; and (b) finetype. The coarse-typerocks have the protogranular texture and are predominantly spinellherzolite. Some coarse-type spinel lherzolites have partlytransformed to garnet lherzolite. The fine-types are consideredto be metamorphic derivatives of the former, and the observedmineral assemblages are: (1) olivine + orthopyroxene + clinopyroxene+ garnet + amphibole ? spinel, (2) olivine + orthopyroxene +garnet + amphibole + spinel; (3) olivine + orthopyroxene + amphibole+ spinel; and (4) olivine+ orthopyroxene + amphibole + chlorite.Based on the microprobe analyses of constituent minerals fromten representative peridotite samples, physical conditions ofthe metamorphism, particularly that of the spinel to garnetlherzolite transformation, are estimated. Applications of pyroxenegeothermometry yield temperature estimates of 11001300?Cfor the formation of the primary spinel lherzolite, and 700800?Cfor that of the fine-type peridotites. A pressure range of 1628kb is obtained for the garnet lherzolite crystallization dependingon the choice of geobarometers. Two alternative P-T paths, i.e.(1) isobaric cooling or (2) pressure-increase and temperaturedecrease are considered and their geodynamic implications discussed. 相似文献
20.
Ultrahigh-pressure metamorphism and exhumation of garnet peridotite in Pohorje, Eastern Alps 总被引:1,自引:0,他引:1
M. JANÁK N. FROITZHEIM M. VRABEC E. J. KROGH RAVNA J. C. M. De HOOG 《Journal of Metamorphic Geology》2006,24(1):19-31
New evidence for ultrahigh‐pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet‐bearing ultramafic rocks from the Pohorje Mountains in Slovenia. The garnet peridotites are closely associated with UHP kyanite eclogites. These rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. On the basis of petrochemical data, garnet peridotites could have been derived from depleted mantle rocks that were subsequently metasomatized by melts and/or fluids either in the plagioclase‐peridotite or the spinel‐peridotite field. At least four stages of recrystallization have been identified in the garnet peridotites based on an analysis of reaction textures and mineral compositions. Stage I was most probably a spinel peridotite stage, as inferred from the presence of chromian spinel and aluminous pyroxenes. Stage II is a UHPM stage defined by the assemblage garnet + olivine + low‐Al orthopyroxene + clinopyroxene + Cr‐spinel. Garnet formed as exsolutions from clinopyroxene, coronas around Cr‐spinel, and porphyroblasts. Stage III is a decompression stage, manifested by the formation of kelyphitic rims of high‐Al orthopyroxene, aluminous spinel, diopside and pargasitic hornblende replacing garnet. Stage IV is represented by the formation of tremolitic amphibole, chlorite, serpentine and talc. Geothermobarometric calculations using (i) garnet‐olivine and garnet‐orthopyroxene Fe‐Mg exchange thermometers and (ii) the Al‐in‐orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at conditions of around 900 °C and 4 GPa. These results suggest that garnet peridotites in the Pohorje Mountains experienced UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of continental crust, which incorporated mantle peridotites from the upper plate, in an intracontinental subduction zone. Sinking of the overlying mantle and lower crustal wedge into the asthenosphere (slab extraction) caused the main stage of unroofing of the UHP rocks during the Upper Cretaceous. Final exhumation was achieved by Miocene extensional core complex formation. 相似文献